ALBERT

Description: © The Author(s), 2021. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Colson, B. C., & Michel, A. P. M. Flow-through quantification of microplastics using impedance spectroscopy. ACS Sensors, 6(1), (2021): 238–244, doi:10.1021/acssensors.0c02223.

Description: Understanding the sources, impacts, and fate of microplastics in the environment is critical for assessing the potential risks of these anthropogenic particles. However, our ability to quantify and identify microplastics in aquatic ecosystems is limited by the lack of rapid techniques that do not require visual sorting or preprocessing. Here, we demonstrate the use of impedance spectroscopy for high-throughput flow-through microplastic quantification, with the goal of rapid measurement of microplastic concentration and size. Impedance spectroscopy characterizes the electrical properties of individual particles directly in the flow of water, allowing for simultaneous sizing and material identification. To demonstrate the technique, spike and recovery experiments were conducted in tap water with 212–1000 μm polyethylene beads in six size ranges and a variety of similarly sized biological materials. Microplastics were reliably detected, sized, and differentiated from biological materials via their electrical properties at an average flow rate of 103 ± 8 mL/min. The recovery rate was ≥90% for microplastics in the 300–1000 μm size range, and the false positive rate for the misidentification of the biological material as plastic was 1%. Impedance spectroscopy allowed for the identification of microplastics directly in water without visual sorting or filtration, demonstrating its use for flow-through sensing.

Description: The authors thank the Richard Saltonstall Charitable Foundation and the National Academies Keck Futures Initiative (NAKFI DBS13) for their funding support.

Description: Submitted in partial fulfillment of the requirements for the degree of Master of Science in Aeronautics and Astronautics at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution June 2021.

Description: Global temperature rise and increased atmospheric carbon dioxide (CO2) levels have affected the health of the world’s ocean and water ecosystems, impacting the balances of natural carbon cycling and causing ocean acidification. Additionally, as global temperatures rise, thawing permafrost has stimulated increased release of methane (CH4), a gas with a shorter lifetime in the atmosphere but with even more heat trapping ability than CO2. In situ analysis of dissolved gas content in surface waters is currently performed with large, expensive instruments, such as spectrometers, which are coupled with gas equilibration systems, which extract dissolved gas from water and feed it to the sensor. Accurate, low cost, and portable sensors are needed to measure the dissolved CH4 and CO2 concentration in water systems to quantify their release and understand their relationship to the global carbon budget. At the same time, while greenhouse gases are well established threats to water ecosystems, the ubiquity and potential consequences of microplastics in aqueous environments are just beginning to be recognized by the environmental research community. Microplastics (MPs) are small particles of polymer debris, commonly defined as being between 1 μm and 1000 μm. Despite the pervasiveness of MPs, our ability to characterize MPs in the environment is limited by the lack of technologies for rapidly and accurately identifying and quantifying MPs. This thesis is concerned with the engineering challenges prompted by the need for high quality and quantity environmental data to better study and the impact, cycling, and prevalence of these pollutants in aqueous environments. Three distinct investigations are presented here. First, the design of the Low-Cost Gas Extraction and Measurement System (LC-GEMS) for dissolved CO2 is presented. At just under $600 dollar to build, the LC-GEMS is an ultra-portable, toolbox-sized instrument for dissolved gas sensing in near-surface waters. The LCGEMS was characterized in the lab and demonstrated linear relationships with dissolved CO2 as well as temperature. Lab calibrations and subsequent field testing in the Little Sippewissett Marsh, in Falmouth, Massachusetts showed that the LCGEMS captures both diurnal and minute-time scale trends in dissolved CO2. Second, this thesis presents the novel design of three simple and low-cost planar nanophotonic and plasmonic structures as optical transducers for measuring dissolved CH4. Through simulations, the sensitivity of the structures are evaluated and found to exhibit superior performance in the reflectance intensity readout mode to that of the standard surface-plasmon-polariton-mode Spreeta sensor. A practical, small, and low-cost implementation of this chip with a simple intensity-based measurement scheme is proposed. This design is novel in the space of dissolved gas monitoring because it shows potential to measure directly in the water phase while being robust and low-cost to implement. Finally, this thesis presents a literature review and perspective to motivate the development of field-deployable microplastic sensing techniques. A framework for field-deployable microplastic sensing is presented and seeks to inform the MP community of the potential in both traditional MP analysis techniques and unconventional methods for creating rapid and automated MP sensors. The field-deployabilty framework addresses a full scope of practical/technological trade-offs to be considered for portable MP detection.

Description: Submitted in partial fulfillment of the requirements for the degree of Doctor of Science at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution June 2019.

Description: Ocean surface transport is at the core of many environmental disasters, including the spread of marine plastic pollution, the Deepwater Horizon oil spill and the Fukushima nuclear contamination. Understanding and predicting flow transport, however, remains a scientific challenge, because it operates on multiple length- and time-scales that are set by the underlying dynamics. Building on the recent emergence of Lagrangian methods, this thesis investigates the present-day abilities to describe and understand the organization of flow transport at the ocean surface, including the abilities to detect the underlying key structures, the regions of stirring and regions of coherence within the flow. Over the past four years, the field of dynamical system theory has adapted several algorithms from unsupervised machine learning for the detection of Lagrangian Coherent Structures (LCS). The robustness and applicability of these tools is yet to be proven, especially for geophysical flows. An updated, parameter-free spectral clustering approach is developed and a noise-based cluster coherence metric is proposed to evaluate the resulting clusters. The method is tested against benchmarks flows of dynamical system theory: the quasi-periodic Bickley jet, the Duffng oscillator and a modified, asymmetric Duffing oscillator. The applicability of this newly developed spectral clustering method, along with several common LCS approaches, such as the Finite-Time Lyapunov Exponent, is tested in several field studies. The focus is on the ability to predict these LCS in submesoscale ocean surface flows, given all the uncertainties of the modeled and observed velocity fields, as well as the sparsity of Lagrangian data. This includes the design and execution of field experiments targeting LCS from predictive models and their subsequent Lagrangian analysis. These experiments took place in Scott Reef, an atoll system in Western Australia, and off the coast of Martha's Vineyard, Massachusetts, two case studies with tidally-driven channel flows. The FTLE and spectral clustering analyses were particularly helpful in describing key transient flow features and how they were impacted by tidal forcing and vertical velocities. This could not have been identified from the Eulerian perspective, showing the utility of the Lagrangian approach in understanding the organization of transport.

Description: Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy in Geochemistry at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution September 2019.

Description: Mineral dust is generated in continental interiors and exported by winds to ocean basins, providing a sedimentary archive which is one of the few direct indicators we have of atmospheric circulation in the past. This archive can be utilized in regions of dust transport also affected by monsoons to examine how different climate forcing mechanisms impact the monsoon regions over glacialinterglacial, orbital, and millennial timescales. This thesis generates new eolian dust records from two monsoon regions to reconstruct changes in atmospheric circulation in response to forcing by high-latitude insolation and boundary condition change. In Chapters 2 and 3 I use 230Thxsnormalization to construct high-resolution eolian dust flux records from sedimentary archives downwind from the West African and East Asian Monsoon regions respectively. The West African margin dust records show variability associated with an interplay between Northern Hemisphere summer insolation forcing and North Atlantic cooling. The longest record at ODP Site 658, stretching back to 67 ka, shows evidence for a “Green Sahara” interval from 60-50 ka and a skipped precessional “beat” from 35-20 ka. This record also shows evidence for abrupt increases in dust flux associated with Greenland stadials. The Shatsky Rise record at ODP Site 1208, downwind of East Asian dust sources, shows variability associated with glacial-interglacial boundary conditions over the last 330 ka, exhibiting high dust during glacial times. The record also exhibits variability associated with a Northern Hemisphere summer insolation control at times overriding the glacialinterglacial signal. In Chapter 4 I demonstrate the feasibility of using radiogenic neodymium isotopes (143Nd/144Nd) at IODP Site U1430 in the Sea of Japan to fingerprint the provenance of eolian material at the core site from Asian dust sources. I then generate a 143Nd/144Nd record from isolated eolian material over the last 200 ka to examine Westerly Jet behavior in the Asian interior, which shows resolvable orbital-scale variability from 200 to 100 ka, and muted variability from 100 to 0 ka. The findings imply a quicker shift of the Westerly Jet to the north of the Tibetan Plateau during times of high Northern Hemisphere summer insolation and a strong Asian monsoon.

Description: This thesis research was supported by the National Science Foundation under Grant Nos. OCE-1030784 and EAR-1434138 and a Post-expedition activity award through the U.S. Science Support Program, Integrated Ocean Drilling Program. Kinsley was supported by fellowships from the MIT Department of Earth, Atmospheric, and Planetary Sciences in his first year of graduate school and the WHOI Academic Programs Office in his sixth year of graduate school.

Description: Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution June, 1982

Description: Oceanic fluctuations are dependent on geographical location. Near intense currents, the eddy field is highly energetic and has broad meridional extent. It is likely that the energy arises from instabilities of the intense current. However, the meridional extent of the linearly most unstable modes of such intense jets is much narrower than the observed region of energetic fluctuations. It is proposed here that weaker instabilities, in the linear sense, which are very weakly trapped to the current, may be the dominant waves in the far field. As a preliminary problem, the (barotropic) instability of parallel shear flow on the beta plane is discussed. An infinite zonal flow with a continuous cross-stream velocity gradient is approximated with segments of uniform flow, joined together by segments of uniform potential vorticity. This simplification allows an exact dispersion relation to be found. There are two classes of linearly unstable solutions. One type is trapped to the source of energy and has large growth rates. The second type are weaker instabilities of the shear flow which excite Rossby waves in the far field: the influence of these weaker instabilities extends far beyond that of the most unstable waves. The central focus of the thesis i: the linear stability of thin, twolayer, zonal jets on the beta plane, with both horizontal and vertical shear. The method used for the parallel shear flow is extended to the two-layer flow. Each layer of the jet has uniform velocity in the center, bordered by shear zones with zero potential vorticity gradient. The velocity in each layer outside the jet is constant in latitude. Separate linearly unstable modes arise from horizontal and vertical shear. The energy source for the vertical shear modes is nearly all potential while the source for the horizontal shear modes is both kinetic and potential. The most unstable waves are tightly trapped to the jet, within two or three deformation radii for small but nonzero beta. Rossby waves and baroclinically unstable waves (in the presence of vertical shear) exist outside the jet because of a nonzero potential vorticity gradient there. Weakly growing jet instabilities can force these waves when their phase speeds and wavelengths match. In particular, westward jets and any jets with vertical shear exterior to the jet can radiate in this sense. The radiating modes influence a large region, their decay scales inversely proportional to the growth rate. Two types of radiating instability are found: (1) a subset of the main unstable modes near marginal stability and (2) modes which appear to be destabilized neutral modes. Westward jets have more vigorously unstable radiating modes. Applications of the model are made to the eddy field south of the Gulf Stream, using data from the POLYMODE settings along 55°W and farther into the gyre at MODE. The energy decay scale and the variation of vertical structure with latitude in different frequency bands can be roughly explained by the model. The lower frequency disturbances decay more slowly and become more surface intensified in the far field. These disturbances are identified with the weak, radiating instabilities of the model. The higher frequency disturbances are more trapped and retain their vertical structure as they decay, and are identified with the trapped, strongly unstable modes of the jet.

Description: This work was supported by a grant from the National Science Foundation, Office of Atmospheric Science.

Description: Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution February 2017

Description: This thesis investigates the evolution of the oceanic lithosphere in a broad sense from formation to subduction, in a focused case at the ridge, and in a focused case proximal to subduction. In general, alteration of the oceanic lithosphere begins at the ridge through focused and diffuse hydrothermal flow, continues off axis through low temperature circulation, and may occur approaching subduction zones as bending related faulting provides fluid pathways. In Chapter 2 I use a dataset of thousands of microearthquakes recorded at the Rainbow massif on the Mid-Atlantic Ridge to characterize the processes which are responsible for the long-term, high-temperature, hydrothermal discharge found hosted in this oceanic core complex. I find that the detachment fault responsible for the uplift of the massif is inactive and that the axial valleys show no evidence for faulting or active magma intrusion. I conclude that the continuous, low-magnitude seismicity located in diffuse pattern in a region with seismic velocities indicating ultramafic host rock suggests that serpentinization may play a role in microearthquake generation but the seismic network was not capable of providing robust focal mechanism solutions to constrain the source characteristics. In Chapter 3 I find that the Juan de Fuca plate, which represents the young/hot end-member of oceanic plates, is lightly hydrated at upper crustal levels except in regions affected by propagator wakes where hydration of lower crust and upper mantle is evident. I conclude that at the subduction zone the plate is nearly dry at upper mantle levels with the majority of water contained in the crust. Finally, in Chapter 4 I examine samples of cretaceous age serpentinite sampled just before subduction at the Puerto Rico Trench. I show that these upper mantle rocks were completely serpentinized under static conditions at the Mid-Atlantic Ridge. Further, they subsequently underwent 100 Ma of seafloor weathering wherein the alteration products of serpentinization themselves continue to be altered. I conclude that complete hydration of the upper mantle is not the end point in the evolution of oceanic lithosphere as it spreads from the axis to subduction.

Description: Funding was provided by the National Science Foundation through grants OCE-1029305 and OCE-0961680, the Deep Ocean Exploration Institute - Ocean Ridge Initiative, and by the WHOI Academic Programs office

Description: Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution February 2017

Description: The daily heating of the ocean by the sun can create a stably stratified near-surface layer when the winds are slight and solar insolation is strong. This type of shallow stable layer is called a Diurnal Warm Layer (DWL). This thesis examines the physics and dynamics of DWLs from observations of the subtropical North Atlantic Ocean associated with the Salinity Processes in the Upper ocean Regional Study (SPURS-I). Momentum transferred from the atmosphere to the ocean through wind stress becomes trapped within the DWL, generating shear across the layer. During SPURS-I, strong diurnal shear across the DWL was coincident with enhanced turbulent kinetic energy (TKE) dissipation (𝜖, 𝜖 〉 10−5 W/kg) observed from glider microstructure profiles of the near-surface. However, a scale analysis demonstrated that surface forcing, including diurnal shear, could not be the sole mechanism for the enhanced TKE dissipation. High-frequency internal waves (𝜔 ≫ 𝑓) were observed in the upper ocean during the daytime within the DWL. Internal waves are able to transfer energy from the deep ocean into the DWL through the unstratified remnant mixed layer, which is the intervening layer between the DWL and seasonal thermocline. As the strength of the stratification of the DWL increases, so does the shear caused by the tunneling internal waves. The analysis demonstrates that internal waves can generate strong enough shear to cause a shear-induced instability, and are a plausible source of the observed enhanced TKE dissipation. Vertically-varying horizontal transport across the upper ocean occurs because a diurnal current exists within the DWL, but not in the unstratified remnant mixed layer below. Therefore, when a DWL is present, the water within DWL is horizontally transported a different distance than the water below. Coupled with nocturnal convection that mixes the DWL with the unstratified layer at night, this cycle is a mechanism for submesoscale (1-10 km) lateral diffusion across the upper ocean. Estimates of a horizontal diffusion coefficient are similar in magnitude to current estimates of submesoscale diffusion based on observations, and are likely an important source of horizontal diffusion in the upper ocean.

Description: Supported by the Department of Defense (DoD) through the National Defense Science & Engineering Graduate Fellowship (NDSEG) Program and the National Science Foundation under Grant No. OCE-1129646. The collection and analysis of data from the SPURS-I central mooring were supported under National Aeronautics and Space Administration (NASA) Grant No. NNX11AE84G and NNX14AH38G.

Description: Submitted in partial fulfillment of the requirements for the degree of Master of Science at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution September 2015

Description: Recent advances in autonomous underwater vehicle (AUV) technology have led to their wide-spread acceptance and adoption for use in scientific, commercial, and defence applications in the underwater domain. At the same time, research progress in swarm robotics has seen swarm intelligence algorithms in use with greater effect on real-world robots in the field. A group of AUVs utilizing swarm intelligence concepts has the potential to address issues more effectively than a single AUV, and such a group can potentially open up new areas of application. Examples include the monitoring and tracking of highly dynamic oceanographic phenomena such as phytoplankton blooms and the use of an AUV swarm as a virtual acoustic receiver for sea-bottom seismic surveying or the monitoring of naturally occurring acoustic radiation from cracking ice. However, the limitations of the undersea environment places unique constraints on the use of existing swarm robotics approaches with AUVs. In particular, algorithms must be distributed and robust in the face of localization error and degraded communications. This work presents an investigation into one particular swarm strategy for a group of AUVs, termed formation control, with consideration to the constraints of the underwater domain. Four formation control algorithms, each developed and tested within the MOOS-IvP framework, are presented. In addition, a 'formation quality' metric is introduced. This metric is used in conjunction with a measure of formation energy expenditure to compare the efficacy of each behaviour during construction of a desired formation, and formation maintenance while it drifts in ocean currents. This metric is also used to compare robustness of each algorithm in the presence of vehicle failure and changing communication rate.

Description: This research was supported by APS under contract number N66001-11-C-4115 and award numbers N66001-13-C-4006 and N66001-14-C-4031.

Description: Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution September 2016

Description: Since the Last Glacial Maximum (LGM, ~ 20,000 years ago) air temperatures warmed, sea level rose roughly 130 meters, and atmospheric concentrations of carbon dioxide increased. This thesis combines global models and paleoceanographic observations to constrain the ocean’s role in storing and transporting heat, salt, and other tracers during this time, with implications for understanding how the modern ocean works and how it might change in the future. • By combining a kinematic ocean model with “upstream” and “downstream” deglacial oxygen isotope time series from benthic and planktonic foraminifera, I show that the data are in agreement with the modern circulation, quantify their power to infer circulation changes, and propose new data locations. • An ocean general circulation model (the MITgcm) constrained to fit LGM sea surface temperature proxy observations reveals colder ocean temperatures, greater sea ice extent, and changes in ocean mixed layer depth, and suggests that some features in the data are not robust. • A sensitivity analysis in the MITgcm demonstrates that changes in winds or in ocean turbulent transport can explain the hypothesis that the boundary between deep Atlantic waters originating from Northern and Southern Hemispheres was shallower at the LGM than it is today.

Description: Support for this work came from an MIT Presidential Fellowship, an NSF Graduate Research Fellowship, and grants NASA NNX12AJ93G – Gravity data for ocean circulation and climate studies, NSF OCE-0961713 – Collaborative Research: The Physics and Statistics of Global Sea Level Change, NSF OCE-1060735 – Collaborative Research: Beyond the Instrumental Record - the Ocean Circulation at the last Glacial maximum and the deglacial sequence, and NASA NNX08AR33G – Application of Satellite Altimetry Gravity Winds and in Situ Data to Problems of the Ocean Circulation.

Description: Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution February 2015

Description: The capabilities of autonomous underwater vehicles (AUVs) and their ability to perform tasks both autonomously and adaptively are rapidly improving, and the desire to quickly and efficiently sample the ocean environment as Earth's climate changes and natural disasters occur has increased significantly in the last decade. As such, this thesis proposes to develop a method for single and multiple AUVs to collaborate autonomously underwater while autonomously adapting their motion to changes in their local environments, allowing them to sample and track various features of interest with greater efficiency and synopticity than previously possible with preplanned AUV or ship-based surveys. This concept is demonstrated to work in field testing on multiple occasions: with a single AUV autonomously and adaptively tracking the depth range of a thermocline or acousticline, and with two AUVs coordinating their motion to collect a data set in which internal waves could be detected. This research is then taken to the next level by exploring the problem of adaptively and autonomously tracking spatiotemporally dynamic underwater fronts and plumes using individual and autonomously collaborating AUVs.

Description: Government support under and awarded by DoD, Air Force Office of Scientific Research, NDSEG Fellowship, 32 CFR 168a, U.S. Office of Naval Research (ONR) GOATS ’11 (N00014-11-1-0097) and GOATS ’14 (N00014-14-1-0214) projects, ONR TechSolutions Program Office (Lightweight NSW UUV program)—Technical Support, Naval Undersea Warfare Center (NUWC) Division, Newport (Code 25)—Technical Support and Logistics

Description: Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution February 2015

Description: Real-time cooperation between autonomous vehicles can enable time-critical missions such as tracking and pursuit of a dynamic target or environmental feature, but relies on wireless communications. Underwater communication is almost exclusively accomplished through acoustics, which bring challenges such as delays, low data rates, packet loss, and scheduling constraints. To address these challenges, this thesis presents contributions towards multi-vehicle feedback control in the presence of severe communication constraints. The first major area of work considers the formulation and solution of new multi-vehicle tracking and pursuit problems using closed-loop control. We first describe field experiments in target pursuit at high tracking bandwidths in a challenging shallow-water environment. Next, we present a methodology for pursuit of dynamic ocean features such as fronts, which we validate using ocean model data. The primary innovation is a linearization of ocean model forecast dynamics and uncertainty directly in vehicle coordinates. The second area of work presents a unified formalism for multi-vehicle control and estimation with measurement, control, and acknowledgment packets all subject to scheduling, delays and packet loss. We develop a modular framework that includes a novel technique for estimation using delayed and lossy control acknowledgments. Simulations and field experiments demonstrate the effectiveness of our approach.

Description: Office of Naval Research, Grant N00014-09-1-0700, and National Science Foundation, Contract CNS-1212597

Description: Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution June 2015

Description: One of the long term goals of Autonomous Underwater Vehicle (AUV) minehunting is to have multiple inexpensive AUVs in a harbor autonomously classify hazards. Existing acoustic methods for target classification using AUV-based sensing, such as sidescan and synthetic aperture sonar, require an expensive payload on each outfitted vehicle and expert image interpretation. This thesis proposes a vehicle payload and machine learning classification methodology using bistatic angle dependence of target scattering amplitudes between a fixed acoustic source and target for lower cost-per-vehicle sensing and onboard, fully autonomous classification. The contributions of this thesis include the collection of novel high-quality bistatic data sets around spherical and cylindrical targets in situ during the BayEx’14 and Massachusetts Bay 2014 scattering experiments and the development of a machine learning methodology for classifying target shape and estimating orientation using bistatic amplitude data collected by an AUV. To achieve the high quality, densely sampled 3D bistatic scattering data required by this research, vehicle broadside sampling behaviors and an acoustic payload with precision timed data acquisition were developed. Classification was successfully demonstrated for spherical versus cylindrical targets using bistatic scattered field data collected by the AUV Unicorn as a part of the BayEx’14 scattering experiment and compared to simulated scattering models. The same machine learning methodology was applied to the estimation of orientation of aspect-dependent targets, and was demonstrated by training a model on data from simulation then successfully estimating the orientations of a steel pipe in the Massachusetts Bay 2014 experiment. The final models produced from real and simulated data sets were used for classification and parameter estimation of simulated targets in real time in the LAMSS MOOS-IvP simulation environment.

Description: National Science Foundation Graduate Research Fellowship under Grant No. 0645960, the U.S. Office of Naval Research (ONR) under the GOATS’08 (N00014-08- 1-0013), GOATS ’11 (N00014-11-1-0097), SWAMSI (N00014-08-1-0011), and GOATS ’14 (N00014-14-1-0214) projects, and APS under DSOP Subtasks 1.3 (11-15-3352-005) and 2.3 (11-15-3352-215).

Description: Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution September 2014

Description: This thesis explores the buoyancy-driven circulation in the Red Sea, using a combination of observations, as well as numerical modeling and analytical method. The first part of the thesis investigates the formation mechanism and spreading of Red Sea Overflow Water (RSOW) in the Red Sea. The preconditions required for open-ocean convection, which is suggested to be the formation mechanism of RSOW, are examined. The RSOW is identified and tracked as a layer with minimum potential vorticity and maximum chlorofluorocarbon-12. The pathway of the RSOW is also explored using numerical simulation. If diffusivity is not considered, the production rate of the RSOW is estimated to be 0.63 Sv using Walin’s method. By comparing this 0.63 Sv to the actual RSOW transport at the Strait of Bab el Mandeb, it is implied that the vertical diffusivity is about 3.4 x 10-5m2 s-1 . The second part of the thesis studies buoyancy-forced circulation in an idealized Red Sea. Buoyancy-loss driven circulation in marginal seas is usually dominated by cyclonic boundary currents on f-plane, as suggested by previous observations and numerical modeling. This thesis suggests that by including β-effect and buoyancy loss that increases linearly with latitude, the resultant mean Red Sea circulation consists of an anticyclonic gyre in the south and a cyclonic gyre in the north. In mid-basin, the northward surface flow crosses from the western boundary to the eastern boundary. The observational support is also reviewed. The mechanism that controls the crossover of boundary currents is further explored using an ad hoc analytical model based on PV dynamics. This ad hoc analytical model successfully predicts the crossover latitude of boundary currents. It suggests that the competition between advection of planetary vorticity and buoyancy-loss related term determines the crossover latitude. The third part of the thesis investigates three mechanisms that might account for eddy generation in the Red Sea, by conducting a series of numerical experiments. The three mechanisms are: i) baroclinic instability; ii) meridional structure of surface buoyancy losses; iii) cross-basin wind fields.

Description: This work is supported by Award Nos. USA 00002, KSA 00011 and KSA 00011/02 made by King Abdullah University of Science and Technology (KAUST) , National Science Foundation OCE0927017, and WHOI Academic Program Office.

Description: Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution September 1996

Description: This thesis addresses the question of how a highly energetic eddy field could be generated in the interior of the ocean away from the swift boundary currents. The energy radiation due to the temporal growth of non-trapped (radiating) disturbances in such a boundary current is thought to be one of the main sources for the described variability. The problem of stability of an energetic current, such as the Gulf Stream, is formulated. The study then focuses on the ability of the current to support radiating instabilities capable of significant penetration into the far-field and their development with time. The conventional model of the Gulf Stream as a zonal current is extended to allow the jet axis to make an angle to a latitude circle. The linear stability of such a nonzonal flow, uniform in the along-jet direction on a beta-plane, is first studied. The stability computations are performed for piece-wise constant and continuous velocity profiles. New stability properties of nonzonal jets are discussed. In particular, the destabilizing effect of the meridional tilt of the jet axis is demonstrated. The radiating properties of nonzonal currents are found to be very different from those of zonal currents. In particular, purely zonal flows do not support radiating instabilities, whereas flows with a meridional component are capable of radiating long and slowly growing waves. The nonlinear terms are then included in the consideration and the effects of the nonlinear interactions on the radiating properties of the solution are studied in detail. For these purposes, the efficient numerical code for solving equation for the QG potential vorticity with open boundary conditions of Orlanski's type is constructed. The results show that even fast growing linear solutions, which are trapped during the linear stage of developement, can radiate energy in the nonlinear regime if the basic current is nonzonal. The radiation starts as soon as the initial fast exponential growth significantly slows. The initial trapping of those solutions is caused by their fast temporal growth. The new mechanism for radiation is related to the nonzonality of a current.

Description: This work was supported by NSF Grant OCE 9301845.

Description: Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution May 1997

Description: Navigation is a key technology for autonomous underwater vehicles (AUVs), and currently, it limits potential and existing vehicle capabilities and applications. This thesis presents a terrain-relative navigation system for AUVs that does not require the deployment of acoustic beacons or other navigational aids, but instead depends on a supplied digital bathymetric map and the ability of the vehicle to image the seafloor. At each time step, a bathymetric profile is measured and compared to a local region of the supplied map using a mean absolute difference criterion. The region size is determined by the current navigation uncertainty. For large regions, a coarse-to-fine algorithm with a modified beam search is used to intelligently search for good matches while reducing the computational requirements. A validation gate is defined around the position estimate using the navigation uncertainty, which is explicitly represented through a covariance matrix. A probabilistic data association filter with amplitude information (PDAFAI), grounded in the Kalman Filter framework, probabilistically weights each good match that lies within the validation gate. Weights are a function of both the match quality and the size of the innovation. Navigation updates are then a function of the predicted position, the gate size, all matches within the gate, and the uncertainties on both the prediction and the matches. The system was tested in simulation on several terrain types using a deep-ocean bathymetry map of the western flank of the Mid-Atlantic Ridge between the Kane and Atlantis Transforms. Results show more accurate navigation in the areas with greater bathymetric variability and less accurate navigation in flatter areas with more gentle terrain contours. In most places, the uncertainties assigned to the navigation positions reflect the ability of the system to follow the true track. In no case did the navigation diverge from the true track beyond the point of recovery.

Description: Funding was provided by Office of Naval Research Grants N00014-96-1-5028, N00014-94-1- 0466 and N00017-92-J-1714; Naval Research Underwater Warfare Center Grant N0014-90-D- 1979, and Naval Research Laboratory N00014-92-C-6028.

Description: Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution February 2013

Description: Studying oceanic eddies is important for understanding and predicting ocean circulation and climate variability. The central focus of this dissertation is the energy exchange between eddies and mean flow and banded structures in the low-frequency component of the eddy field. A combination of a realistic eddy-permitting ocean state estimate and simplified theoretical models is used to address the following specific questions. (1) What are the major spatial characteristics of eddy-mean flow interaction from an energy perspective? Is eddy-mean flow interaction a local process in most ocean regions? (2) The banded structures in the low-frequency eddy field are termed striations. How much oceanic variability is associated with striations? How does the time-mean circulation, for example a subtropical gyre or constant mean flow, influence the origin and characteristics of striations? How much do striations contribute to the energy budget and tracer mixing?

Description: This research was supported by the National Aeronautics and Space Administration contracts NNX09AI87G and NNX08AR33G.

Description: Submitted in partial fulfillment of the requirements for the degree of Master of Science at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution September 2013

Description: Between 2002 and 2011 a single mooring was maintained in the core of the Pacific Water boundary current in the Alaskan Beaufort Sea near 152° W. Using velocity and hydrographic data from six year-long deployments during this time period, we examine the interannual variability of the current. It is found that the volume, heat, and freshwater transport have all decreased drastically over the decade, by more than 80%. The most striking changes have occurred during the summer months. Using a combination of weather station data, atmospheric reanalysis fields, and concurrent shipboard and mooring data from the Chukchi Sea, we investigate the physical drivers responsible for these changes. It is demonstrated that an increase in summertime easterly winds along the Beaufort slope is the primary reason for the drop in transport. The intensification of the local winds has in turn been driven by a strengthening of the summer Beaufort High in conjunction with a deepening of the summer Aleutian Low. Since the fluxes of mass, heat, and freshwater through Bering Strait have increased over the same time period, this raises the question as to the fate of the Pacific water during recent years and its impacts. We present evidence that more heat has been fluxed directly into the interior basin from Barrow Canyon rather than entering the Beaufort shelfbreak jet, and this is responsible for a significant portion of the increased ice melt in the Pacific sector of the Arctic Ocean.

Description: The majority of the data for this project was funded by grant # ARC-0856244 from the O ce of Polar Programs of the National Science Foundation. My time at WHOI was funded by the United States Navy, the National Science Foundation Graduate Research Fellowship Program and the WHOI Academic Programs O ffice.

Description: Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution September 1996

Description: A convection experiment was done with a rotating rectangular tank as a model of oceanic meridional overturning circulation. Heat flux was fixed at one bottom end of the tank using an electrical heater. Temperature was fixed at the other end using a cooling plate. All other boundaries were insulated. The cross sections of temperature field were made at several locations. In equilibrium, the heat input to the fluid H was the same as the meridional heat flux (heat flux from the source to the sink), so it was possible to find a scaling law relating H to the temperature difference across the tank ΔT and rotation rate f. The experimental result suggests that the meridional heat transport in the experiment was mostly due to geostrophic flows with a minor correction caused by the bottom friction. If there was no friction, the scaling law from the experiment resembles the one verified in part in the numerical model by Bryan and Cox (1967). Flow visualization and temperature sections showed that there were meridional geostrophic currents that transported heat. When the typical values of the North Atlantic are introduced, the geostrophic scaling law predicts meridional heat flux comparable to that estimated in the North Atlantic when the vertical eddy diffusivity of heat is about 1cm2s-1. Naturally, this experiment is a only crude model of the oceanic convective circulation. We do not claim that the geostrophic scaling applies in detail to the oceans, however, it may have some important use in climate modeling. For example, almost all existing box models and two-dimensional numerical models of ocean circulation use a frictional scaling law for buoyancy transport. A box model with the geostrophic scaling law is shown to be more robust to a change in the boundary forcing so that it is less likely to have a thermohaline catastrophic transition under the present conditions. It is also shown that a restoring boundary condition for salinity introduces stability to a thermal mode circulation, unless the restoring time for salinity is several orders of magnitude larger than that for temperature.

Description: This study has been funded by NSF grant number OCE92-01464 and Korean Government Overseas Scholarship Grant.

Description: Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution February 2013

Description: Large-scale thermal forcing and freshwater fluxes play an essential role in setting temperature and salinity in the ocean. A number of recent estimates of the global oceanic freshwater balance as well as the global oceanic surface net heat flux are used to investigate the effects of heat- and freshwater forcing at the ocean surface. Such forcing induces changes in both density and density-compensated temperature and salinity changes (’spice’). The ratio of the relative contributions of haline and thermal forcing in the mixed layer is maintained by large-scale surface fluxes, leading to important consequences for mixing in the ocean interior. In a stratified ocean, mixing processes can be either along lines of constant density (isopycnal) or across those lines (diapycnal). The contribution of these processes to the total mixing rate in the ocean can be estimated from the large-scale forcing by evaluating the production of thermal variance, salinity variance and temperature-salinity covariance. Here, I use new estimates of surface fluxes to evaluate these terms and combine them to generate estimates of the production of density and spice variance under the assumption of a linear equation of state. As a consequence, it is possible to estimate the relative importance of isopycnal and diapycnal mixing in the ocean. While isopycnal and diapycnal processes occur on very different length scales, I find that the surface-driven production of density and spice variance requires an approximate equipartition between isopycnal and diapycnal mixing in the ocean interior. In addition, consideration of the full nonlinear equation of state reveals that surface fluxes require an apparent buoyancy gain (expansion) of the ocean, which allows an estimate of the amount of contraction on mixing due to cabbeling in the ocean interior.

Description: The author would like to acknowledge support from the National Aeronautics and Space Administration, grant #NNX12AF59G and the National Science Foundation, grant #OCE-0647949.

Description: Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution December 1996

Description: The transformation of potential vorticity within and stability of nonlinear deep western boundary currents in an idealized tropical ocean are studied using a shallowwater model. Observational evidence indicates that the potential vorticity of fluid parcels in deep western boundary currents must change sign as they cross the equator, but this evidence is otherwise unable to clarify the process. A series of numerical experiments investigate this transformation in a rectangular basin straddling the equator. A mass source located in the northwestern corner feeds fluid into the domain where it is constrained to cross the equator to reach a distributed mass sink. Dissipation is included as momentum diffusion. The Reynolds number, defined as the ratio of the mass source per unit depth to the viscosity, determines the nature of the flow, and a critical value, Rec, divides its possible behavior into two regimes. For Re 〈 Rec, the flow is laminar and well described by linear theory. For Re just above the critical value, the flow is time-dependent, with cyclonic eddies forming in the western boundary current near the equator. For still larger Reynolds number, eddies of both signs emerge and form a complicated, interacting network that extends into the basin several deformation radii from the western boundary, as well as north and south of the equator. The eddy field is established as the mechanism for potential vorticity transformation in nonlinear cross-equatorial flow. The analysis of vorticity fluxes follows from the flux-conservative form of the absolute vorticity equation. It is shown that the zonally integrated meridional flux of vorticity across the equator using no slip boundary conditions is virtually zero even in the strongly nonlinear limit suggesting that the eddies are extremely efficient vorticity transfer agents. A decomposition of the vorticity fluxes into components due to mean advection, eddy transport, and friction, reveals the growth with Reynolds number of a turbulent boundary layer that exchanges vorticity between the inertial portion of the boundary current and a frictional sub-layer where modification is straightforward. A linear stability analysis of the shallow-water system in the tropical ocean examines the initial formation of the eddy field. The formulation assumes that the basic state is purely meridional and on a local f-plane. Realistic western boundary current profiles undergo a horizontal shear instability that is partially stabilized by viscosity. Calculations at several latitudes indicate that the instability is enhanced in the tropics where the internal deformation radius is a maximum. The linear stability analysis predicts a length scale of the disturbance, a location for its origin, and a critical Reynolds number that agree well with numerical results.

Description: Financial support for this research was provided by NSF grant number OCE- 9115915 and ONR ASSERT grant number N00014-94-1-0844.

Description: Submitted in partial fulfillment of the requirements for the degree of Master of Science at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution February 1992

Description: The Autonomous Benthic Explorer (ABE) is an unmanned underwater vehicle being developed for scientific study of the deep ocean sea:floor. ABE will be completely autonomous from the surface which means that the lifetime of the mission will depend largely on how the vehicle is controlled. An accurate system model is critical for the controller development and trajectory planning. A model of the ABE vehicle dynamics is formulated for surge, heave and pitch motions. These motions in the lon,gitudinal plane are particularly important for the basic ABE trajectories of forward flight, depth changes and maneuvers involving both. A scale model of the ABE vehicle was towed to determine the lift/drag relationships to nonzero angles of attack. The experimental results are used in conjunction with traditional analytical techniques to generate a model of the longitudinal dynamics. The ABE model was studied in simulation over anticipated vehicle trajectories. A proportional plus derivative controller and a sliding mode controller were developed for tracking control. The power consumptions for different controllers and trajectories are examined. The results of this study will be incorporated in the final ABE design.

Description: The Office of Naval Research is gratefully acknowledged for its financial support of my graduate education. In addition, this work has been sponsored in part by the National Science Foundation, grant number OCE 8820227.

Description: Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution February 2013

Description: Autonomous marine vehicles are increasingly used in clusters for an array of oceanographic tasks. The effectiveness of this collaboration is often limited by communications: throughput, latency, and ease of reconfiguration. This thesis argues that improved communication on intelligent marine robotic agents can be gained from acting on knowledge gained by improved awareness of the physical acoustic link and higher network layers by the AUV’s decision making software. This thesis presents a modular acoustic networking framework, realized through a C++ library called goby-acomms, to provide collaborating underwater vehicles with an efficient short-range single-hop network. goby-acomms is comprised of four components that provide: 1) losslessly compressed encoding of short messages; 2) a set of message queues that dynamically prioritize messages based both on overall importance and time sensitivity; 3) Time Division Multiple Access (TDMA) Medium Access Control (MAC) with automatic discovery; and 4) an abstract acoustic modem driver. Building on this networking framework, two approaches that use the vehicle’s “intelligence” to improve communications are presented. The first is a “non-disruptive” approach which is a novel technique for using state observers in conjunction with an entropy source encoder to enable highly compressed telemetry of autonomous underwater vehicle (AUV) position vectors. This system was analyzed on experimental data and implemented on a fielded vehicle. Using an adaptive probability distribution in combination with either of two state observer models, greater than 90% compression, relative to a 32-bit integer baseline, was achieved. The second approach is “disruptive,” as it changes the vehicle’s course to effect an improvement in the communications channel. A hybrid data- and model-based autonomous environmental adaptation framework is presented which allows autonomous underwater vehicles (AUVs) with acoustic sensors to follow a path which optimizes their ability to maintain connectivity with an acoustic contact for optimal sensing or communication.

Description: I wish to acknowledge the sponsors of this research for their generous support of my tuition, stipend, and research: the WHOI/MIT Joint Program, the MIT Presidential Fellowship, the Office of Naval Research (ONR) # N00014-08-1-0011, # N00014-08-1-0013, and the ONR PlusNet Program Graduate Fellowship, the Defense Advanced Research Projects Agency (DARPA) (Deep Sea Operations: Applied Physical Sciences (APS) Award # APS 11-15 3352-006, APS 11-15-3352-215 ST 2.6 and 2.7)

Description: Submitted in partial fulfillment of the requirements for the degree of Master of Science at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution May 1996

Description: The water mass distribution in the southwestern Barents Sea, the thermohaline structure of the western Barents Sea Polar Front, and the formation of local water masses are described based on an analysis of historical hydrographic data and a recent process-oriented field experiment. This study concentrated on the frontal region between Bj0rn0ya and Hopen Island where Arctic water is found on the Spitzbergen Bank and Atlantic Water in the Bear Island Trough and Hopen Trench. Distributions of Atlantic, Arctic, and Polar Front waters are consistent with topographic control of Atlantic water circulation. Seasonal buoyancy forcing disrupts the topographic control in the surface layer, altering the frontal structure, and affecting local water mass formation. In the winter, the topographic control is firmly established and both sides of the front are vertically well-mixed. Winter cooling creates sea-ice over Spitzbergen Bank and convectively formed Modified Atlantic Water in the Bear Island Trough and Hopen Trench. In the summer, heating melts the sea-ice, producing a surface meltwater pool that can cross the polar front, disrupting topographic control and substantially increasing the vertical thermohaline gradients in the frontal region. The meltwater pool produces the largest geostrophic shear in the region.

Description: Support for this work was provided by a Department of Defense National Defense Science and Engineering Graduate Fellowship and Office of Naval Research grant N00014- 90-J-1359.

Description: Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution September 2013

Description: A fundamental problem in autonomous underwater robotics is the high latency between the capture of image data and the time at which operators are able to gain a visual understanding of the survey environment. Typical missions can generate imagery at rates hundreds of times greater than highly compressed images can be transmitted acoustically, delaying that understanding until after the vehicle has been recovered and the data analyzed. While automated classification algorithms can lessen the burden on human annotators after a mission, most are too computationally expensive or lack the robustness to run in situ on a vehicle. Fast algorithms designed for mission-time performance could lessen the latency of understanding by producing low-bandwidth semantic maps of the survey area that can then be telemetered back to operators during a mission. This thesis presents a lightweight framework for processing imagery in real time aboard a robotic vehicle. We begin with a review of pre-processing techniques for correcting illumination and attenuation artifacts in underwater images, presenting our own approach based on multi-sensor fusion and a strong physical model. Next, we construct a novel image pyramid structure that can reduce the complexity necessary to compute features across multiple scales by an order of magnitude and recommend features which are fast to compute and invariant to underwater artifacts. Finally, we implement our framework on real underwater datasets and demonstrate how it can be used to select summary images for the purpose of creating low-bandwidth semantic maps capable of being transmitted acoustically.

Description: Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution June 2013

Description: This thesis explores the role that the circulation in the Gulf of Maine (GOM) plays in determining the distribution of dense aggregations of copepods. These aggregations are an important part of the marine ecosystem, especially for endangered North Atlantic right whales. Certain ocean processes may generate dense copepod aggregations, while others may destroy them; this thesis looks at how different characteristics of the GOM circulation fit into these two categories. The first part of the thesis investigates a hypothetical aggregation mechanism in which frontal circulation interacts with copepod behavior to generate a dense patch of copepods. The first two chapters of this thesis address this mechanism in the context of coastal river plumes and salinity fronts. One chapter describes the characteristics and variability of coastal freshwater and salinity fronts using a historical dataset and a realistic numerical model. The seasonal variability of freshwater is tied in part to seasonality in river discharge, while variability on shorter time scales in the frontal position is related to wind stress. Another chapter applies the hypothetical mechanism to idealized river plumes using a suite of numerical models. The structure of the plume and plume-relative circulation change the resulting copepod aggregation from what is expected from the hypothetical mechanism. The second part of the thesis discusses the GOM circulation and how it may eliminate copepod patches. The summertime mean surface circulation and eddy kinetic energy are computed from a Lagrangian drifter dataset and an adaptive technique that allows for higher spatial resolution while also keeping uncertainty low. Eddy diffusivity is also computed over different regions of the GOM in an attempt to quantify the spreading of a patch of copepods, and is found to be lower near the coast where right whales are often found feeding on copepod patches. In the next chapter, a numerical drifter dataset is used to understand how the results of the previous chapter depend upon the quantity of observations. It is found that the uncertainty in estimating eddy diffusivity is tightly coupled to the number of drifters in the calculation.

Description: This work was supported by the WHOI Coastal Ocean Institute Graduate Student Fellowship and Student Research Award, the WHOI Academic Programs O ce, the NOAA National Marine Fisheries Service Northeast Fisheries Science Center (NOAA Cooperative Agreement NA09OAR4320129), and the O ce of Naval Research Marine Mammals and Biology Program (Grant N00014-12-1-0208).

Description: Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution February 2013

Description: The sinking flux of particles is an important removal mechanism of carbon from the surface ocean as part of the biological pump and can play a role in cycling of other chemical species. This work dealt with improving methods of measuring particle export and measuring export on different scales to assess its spatial variability. First, the assumption of 238U linearity with salinity, used in the 238U–234Th method, was reevaluated using a large sample set over a wide salinity range. Next, neutrally buoyant and surface-tethered sediment traps were compared during a three-year time series in the subtropical Atlantic. This study suggested that previously observed imbalances between carbon stocks and fluxes in this region are not due to undersampling by traps. To assess regional variability of particle export, surface and water-column measurements of 234Th were combined for the first time to measure fluxes on ~20 km scales. Attempts to relate surface properties to particle export were complicated by the temporal decoupling of production and export. Finally, particle export from 234Th was measured on transects of the Atlantic Ocean to evaluate basin-scale export variability. High-resolution sampling through the water-column allowed for the identification of unique 234Th features in the intermediate water column.

Description: I was supported by NASA Headquarters under the NASA Earth and Space Science Fellowship Program (Grant NNX10AO72H). Specific projects were funded by grants from the National Science Foundation, including Carbon Flux Through the Twilight Zone – New Tools to Measure Change (OCE-0628416), WAPflux – New Tools to Study the Fates of Phytoplankton Production in the West Antarctic Peninsula (ANT-0838866), and GEOTRACES Atlantic Section: Trace Element Sources and Sinks Elucidated by Short- Lived Radium and Thorium Isotopes (OCE-0925158).

Description: Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution June 2012

Description: Observations from a three-year field program on the inner shelf south of Martha's Vineyard, MA and a numerical model are used to describe the effect of stratification on inner shelf circulation, transport, and sediment resuspension height. Thermal stratification above the bottom mixed layer is shown to cap the height to which sediment is resuspended. Stratification increases the transport driven by cross-shelf wind stresses, and this effect is larger in the response to offshore winds than onshore winds. However, a one-dimensional view of the dynamics is not sufficient to explain the relationship between circulation and stratification. An idealized, cross-shelf transect in a numerical model (ROMS) is used to isolate the effects of stratification, wind stress magnitude, surface heat flux, cross-shelf density gradient, and wind direction on the inner shelf response to the cross-shelf component of the wind stress. In well mixed and weakly stratified conditions, the cross-shelf density gradient can be used to predict the transport efficiency of the cross-shelf wind stress. In stratified conditions, the presence of an along-shelf wind stress component makes the inner shelf response to cross-shelf wind stress strongly asymmetric.

Description: This work was supported through National Science Foundation grant no. OCE-0548961, the WHOI Academic Programs Office, and the WHOI Coastal Ocean Institute.

Description: Submitted in partial fulfillment of the requirements for the degree of Master of Science at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution August 1993

Description: Control systems for underwater vehicles have reached the level of sophistication where they are limited by the dynamic performance of the thrust actuators. Standard fixed-pitch propellers have been shown to have very poor dynamic characteristics, particularly at low thrust levels The dynamic response of a fixed-pitch propeller is dependent upon highly non-linear transients encountered while the shaft speed approaches its steady-state value. This thesis proposes the use of a controllable pitch propeller system to address this problem. A controllable pitch propeller varies the amount of thrust produced by varying the pitch angle of the blades at a constant shaft speed. The bandwidth of this type of thrust actuator would be dependent primarily on the speed at which the pitch angle of the blades are changed. A variable pitch propeller system suitable for retrofit into an ROV is designed and built. The system is designed for maximal pitch angle bandwidth with low actuator power consumption.

Description: Submitted in partial fulfillment of the requirements for the degree of Master of Science at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution September 2012

Description: Interactions between the ocean circulation in sub-ice shelf cavities and the overlying ice shelf have received considerable attention in the context of observed changes in flow speeds of marine ice sheets around Antarctica. Modeling these interactions requires parameterizing the turbulent boundary layer processes to infer melt rates from the oceanic state at the ice-ocean interface. Here we explore two such parameterizations in the context of the MIT ocean general circulation model coupled to the z-coordinates ice shelf cavity model of Losch (2008). We investigate both idealized ice shelf cavity geometries as well as a realistic cavity under Pine Island Ice Shelf (PIIS), West Antarctica. Our starting point is a three-equation melt rate parameterization implemented by Losch (2008), which is based on the work of Hellmer and Olbers (1989). In this form, the transfer coefficients for calculating heat and freshwater fluxes are independent of frictional turbulence induced by the proximity of the moving ocean to the fixed ice interface. More recently, Holland and Jenkins (1999) have proposed a parameterization in which the transfer coefficients do depend on the ocean-induced turbulence and are directly coupled to the speed of currents in the ocean mixed layer underneath the ice shelf through a quadratic drag formulation and a bulk drag coefficient. The melt rate parameterization in the MITgcm is augmented to account for this velocity dependence. First, the effect of the augmented formulation is investigated in terms of its impact on melt rates as well as on its feedback on the wider sub-ice shelf circulation. We find that, over a wide range of drag coefficients, velocity-dependent melt rates are more strongly constrained by the distribution of mixed layer currents than by the temperature gradient between the shelf base and underlying ocean, as opposed to velocity-independent melt rates. This leads to large differences in melt rate patterns under PIIS when including versus not including the velocity dependence. In a second time, the modulating effects of tidal currents on melting at the base of PIIS are examined. We find that the temporal variability of velocity-dependent melt rates under tidal forcing is greater than that of velocity-independent melt rates. Our experiments suggest that because tidal currents under PIIS are weak and buoyancy fluxes are strong, tidal mixing is negligible and tidal rectification is restricted to very steep bathymetric features, such as the ice shelf front. Nonetheless, strong tidally-rectified currents at the ice shelf front significantly increase ablation rates there when the formulation of the transfer coefficients includes the velocity dependence. The enhanced melting then feedbacks positively on the rectified currents, which are susceptible to insulate the cavity interior from changes in open ocean conditions.

Description: National Science and Engineering Research Council of Canada

Description: Submitted in partial fulfillment of the requirements for the degree of Ocean Engineer at the Massachusetts Institute of Technology and Woods Hole Oceanographic Institution September 1994

Description: Closed loop control of an unmanned underwater vehicle (UUV) in the dynamically difficult environment of shallow water requires explicit consideration of the highly coupled nature of the governing non-linear equations of motion. This coupling between an UUV's six degrees of freedom (6 DOF) is particularly important when attempting complex maneuvers such as coordinated turns (e.g. simultaneous dive and heading change) or vehicle hovering in such an environment. Given the parameter and modelling uncertainties endemic to these equations of motion, then a robust 6 DOF sliding controller employing six-element vector sliding surfaces provides a framework in which satisfactory UUV control can be achieved in shallow water. The vehicle equations of motion are developed and cast in a form that is amenable to non-linear sliding control design. A complete 6 DOF sliding controller with vector sliding surfaces is then formulated via a Lyapunov-like analysis. The sliding controller is then modified via a weighted least-squares approach to work with a particular UUV which has only 4 DOF control authority available. The modified controller is shown to work well for a variety of commanded UUV maneuvers in the presence of significant environmental disturbances and vehicle hydrodynamic parameter uncertainties via numerical simulation. Use of the signals generated by the controller are shown to be of utility in vehicle buoyancy control.

Description: The financial support of the Office of Naval Research under Contract No. N00014-90-J-1912 is gratefully acknowledged.

Description: Submitted in partial fulfillment of the requirements for the degree of Master of Science at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution September 1997

Description: A review of the Odyssey IIB autonomous underwater vehicle shows that energy costs associated with vehicle controls can be reduced and operational flexibility improved with relatively simple, low cost improvements. Because the operating speed that minimizes forward drag is not necessarily the same as that required for optimum sensor performance, a variable speed capability extending to the bottom of the vehicle speed range is sought. Optimizing Odyssey IIB AUV performance for slower speed operations and extended duration missions necessitates a multi-disciplinary review including control system design, hydrodynamic performance and sensor selection and utilization. Reducing the vehicle controls-fixed directional instability by adding vertical fixed fins, implementing an actuation filter, and designing a model based adaptive sliding controller improves the variable speed performance and reduces the control actuation necessary to provide the desired performance level with energy savings.

Description: Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution February 1993

Description: Reciprocal acoustic transmissions made in a region just south of the Gulf Stream are analyzed to determine the structure and variability of temperature, current velocity, and vorticity fields at the northern extent of the southern recirculation gyre. For ten months (November, 1988 through August, 1989), a pentagonal array of tomographic transceivers was situated in a region centered at 38°N, 55°W as part of the eastern array of the SYNOP (SYNoptic Ocean Prediction) Experiment. The region of focus is one rich in mesoscale energy, with the influence of local Gulf Stream meandering and cold-core ring activity strikingly evident. Daily-averaged acoustic transmissions yielded travel times which were inverted to obtain estimates of range-averaged temperature and current velocity fields, and area-averaged relative vorticity fields. The acoustically determined estimates are consistent with nearby current meter measurements and satellite infrared imagery. The signature of cold-core rings is clearly evident in the sections. Spectral estimates of the fields are dominated by motions with periodicities ranging from 32-128 days. Second-order statistics, such as eddy kinetic energies, and heat and momentum fluxes, are also estimated. The integrating nature of the tomographic measurement has been exploited to shed some light on the radiation of eddy energy from the Gulf Stream. The Eliassen-Palm flux diagnostic has been applied to an investigation of wave radiation from the Gulf Stream. Results of the diagnosis suggest that the Gulf Stream itself is the source of wave energy radiating into the far field and found in the interior of the North Atlantic subtropical gyre.

Description: This research was carried out under Office of Naval Research (ONR) University Research Initiative contract N00014-86-K-0751 and ONR contract N00014- 90-J-1481. Construction of the tomographic instruments was supported by grants and contracts with MIT: National Science Foundation grant OCE 85-12430 and by ONR. The field work was supported by ONR under contract N00014-85-G-0241 (Secretary of the Navy Professorship (C. Wunsch)).

Description: Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution March 1989

Description: The general theme of this thesis is the study of systematic mathematical techniques for determining the ocean circulation from classical hydrographic data. Two aspects of this theme are analyzed. The first is finding an efficient representation of hydrographic structure so as to make it most useful and informative. The second is application of inverse methods to the data to determine ocean circulation. Both subjects are examined in the North Atlantic Ocean. The efficient representation is examined in terms of empirical orthogonal functions (EOFs) among the variations in vertical hydrographic profiles. The data used are of a new set of high quality hydrography, all obtained in the early 1980s. Common EOFs are examined among temperature, salinity, oxygen, phosphate, silicate, and nitrate. The EOFs identify a fundamental simplicity in the spatial distributions of t hese properties. Although the volume of numbers involved in the raw data is large, the significant degrees of freedom are only six in space and two among the six properties; temperature and salinity are represented by one mode, while the nutrients by another. The modal structure reflects some underlying simplicity in ocean physics. EOFs form a quantitative basis from which models of the ocean's hydrographic structure can be constructed for various degrees of complexities. As for the second aspect, two applications of inverse methods are explored on small regional scales. The first problem addressed concerns the circulation inside a 12° square located in the eastern basin over the axis of the Mediterranean Water tongue. The study is based on an ocean model constructed by mapping the modes identified in the first half of the thesis over the entire North Atlantic Ocean. A combination of box model inverse and β-spiral method is used to determine the geostrophic reference level velocities. The circulation consists of an anticyclonic circulation near the surface, which is part of the eastern half of the wind-driven subtropical gyre. The flow at depth is weak, and is a cyclonic circulation around the core of the Mediterranean Water tongue. In the second inverse problem, we examine a decaying warm-core ring. Observations of a warm-core ring are used to formulate a model for diagnosing the physics of ring change over a two month period. About 30 hydrographic casts and acoustic doppler current measurements are used to generate estimates of an equivalent radially symmetric ring with radial contrasts of stratification, temperature, salinity, azimuthal velocity, angular momentum, and potential vorticity. A series of related models are inverted for the ring circulation and mixing coefficients. The circulation is insensitive to the model details, is well-resolved, and is a radial outflow and upwelling. Eddy coefficients are only partially resolved; determining the mixing with any degree of confidence appears to require a much more elaborate data set than the one available.

Description: This research was funded in part by the Office of Naval Research (Secretary of the Navy Chair) and the National Science Foundation under grant OCE 85-21685.

Description: Submitted in partial fulfillment of the requirements for the degree of Master of Science at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution July 1996

Description: The continuing development of the autonomous underwater vehicle as an oceanographic research tool has opened up the realm of scientific possibility in the field of deep ocean research. The ability of a vehicle to travel to the ocean floor untethered, collect data for an extended period of time and return to the surface for recovery can make precise oceanographic surveying more economically practical and more efficient. This thesis investigates several scalar parameter searching techniques which have their basis in mathematical optimization algorithms and their applicability for use specifically within the context of autonomous underwater vehicle dynamics. In particular, a modified version of the circular gradient evaluation in the simulated environment of a hydrothermal plume is examined as a test case. Using a priori knowledge of the expected structure of the scalar parameter contour is shown to be advantageous in optimizing the search.

Description: Submitted in partial fulfillment of the requirements for the degree of Master of Science at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution September 1996

Description: Propeller dynamics have typically been ignored in controller design, lumped into the category of 'unmodeled dynamics.' This is acceptable for propellers operating at constant speed in relatively uniform flows. Operational parameters of small remotely operated vehicles and autonomous underwater vehicles require a great deal of transient operation of the propellers. This and the small mass of the vehicles make the dynamics of the propellers a significant factor in vehicle control. Expanding roles of these vehicles require improved control and therefore improved understanding of the dynamics of the thrusters during maneuvering. In this thesis, the dynamics of maneuvering thrusters were explored through numerical simulation and experiments. Vortex lattice propeller code developed for use with nonuniform inflow was adapted to incorporate varying propeller speed and inflow velocity. Test runs were made using a three bladed propeller. Experiments were preformed on a thruster from the ROV Jason using the water tunnel at the Massachusetts Institute of Technology. The thruster incorporated a ducted three bladed propeller. Runs were made using step changes in shaft velocity as well as sinusoidal perturbations on top of steady state velocities. Runs were also made incorporating fully reversing propeller operation. Experiments were done with and without the duct in place. The numerical simulation and experimental results showed that accelerating propeller angular velocity created higher thrust values than steady state propeller operation at the corresponding instantaneous shaft velocity. Decelerating angular velocities created lower thrust values. This is attributed to a lag in the local flow velocity due to the momentum of the fluid. For the case of the accelerating propeller, the angle of attack at the blade is higher, resulting in higher lift force and greater thrust. Errors in the numerical code at low advance coefficients prevented direct comparison of numerical code results to experimental results.

Description: Submitted in partial fulfillment of the requirements for the degree of Master of Science at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution September 1989

Description: Eighteen months of sea surface height data from the GEOSAT altimeter along collinear subtracks were analyzed for information on the circulation pattern in the Bering Sea. Seventy subtracks from both ascending and descending orbits, with as many as 35 repeat cycles along each subtrack, were analyzed. Orbit errors were removed from the height data using a least-squares fit to a cubic polynomial, weighted by the inverse of the height variance. Addition of the weights decreased contamination of residual height profiles by the large geoid signal. Composite maps of variability along each track revealed patterns of increased variability in the regions of the documented Bering slope current (BSC) and the proposed western boundary current (WBC); however, no evidence was found of the expected bifurcation of the BSC near the Siberian coast. Past observations of tides in the Bering Sea were reviewed along with a local tide model to detect tidal contributions to the mesoscale sea surface height variability. The tidal analysis suggested that residual tides contributed primarily to the longer wavelengths which were removed in the collinear processing. Examination of the Schwiderski tidal correction proved it to be a sensible correction, reducing the height variance by approximately 60%. Finally, using a Gaussian model for the BSC velocity profile, synthetic residual heights were generated and fit to the actual data to produce estimates of absolute surface geostrophic velocity and transport. Comparisons of mean flow, height fluctuations and seasonal trends across the BSC, the WBC and Bering Strait support the hypothesis that the BSC turns north at Cape Navarin into the WBC which, in turn, is capable of supplying a major part of the transport through the Bering Strait.

Description: Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution January 1993

Description: Although the association between soft-sediment invertebrates and a specific sediment type has been documented for many habitats, most studies have been correlative and have failed to convincingly demonstrate any single mechanism to explain this association. Sediment type has generally been characterized by grain size, however, many other potential causal factors correlate with grain size, including organic content, microbial content, stability, food supply, and larval supply. One hypothesis for animal-sediment associations is that settling larvae are transported as passive particles and are sorted into different sedimentary habitats much like sediment grains. To test the hypothesis that near-bed hydrodynamics may modify larval settlement, field and flume experiments were conducted where larval settlement was compared between microdepositional environments (small depressions) and non-trapping environments (flush treatments). Depressions have been observed to trap passive particles, and these experiments were therefore designed to test whether settling larvae would be trapped in depressions like passive particles. Flume flow simulations were carried out with the polychaete Capitella sp. I and the bivalve Mulinia latera/is. Experiments with flush and depression sediment treatments were conducted in the absence of the potentially confounding effects of suspended sediment and organic matter and therefore offered a highly controlled, explicit test of passive hydrodynamic deposition of larvae in depressions. Although larvae of both species were generally able to actively select a high-organic sediment over a low-organic alternative with a comparable grain size, elevated densities of both species were observed in depressions for a given sediment treatment. Thus, both species appeared to be vulnerable to hydrodynamic trapping. M. latera/is larvae, however, often made a "poor choice" by settling in high numbers in depressions containing the low-organic sediment while Capitella sp. I larvae were generally able to "escape" from depressions if the sediment was unsuitable. In field experiments carried out at Station R in Buzzards Bay, Massachusetts, significantly higher densities of Mediomastus ambiseta juveniles, spionid polychaete juveniles, bivalves, gastropod larvae, and nemerteans were observed in depressions compared with flush treatments over 5 relatively short experimental periods (3 or 4 days each) during the summer of 1990. Of the abundant taxa, only Capitella spp. was not significantly more abundant in depressions compared with flush treatments, although numbers tended to be higher in depressions. Experiments were conducted over a short time period to minimize potential biological interactions between taxa and reduce the likelihood that organic material would accumulate in depressions and provide a cue for settling larvae. Thus, higher numbers in depressions suggest that larvae were passively entrained. These flume and field experiments suggest that near-bed hydrodynamics may modify settlement at some scales, and that both active and passive processes may operate in determining larval distributions in shallow-water, muddy habitats. In deep-sea ecosystems, the role of near-bed hydrodynamics is also of interest because of the potential role that larval settlement in organic patches may play in maintaining the immense species diversity characteristic of many deep-sea ecosystems. To try to understand the role of organic patches in deep-sea communities, several investigators have used colonization trays containing sediments that have been treated in different ways. These experiments have been criticized in the past because the sediment surface in the trays was elevated above the bottom and may therefore have interfered with natural boundary layer flow. Flume simulations of flow over these colonization trays revealed serious flow artifacts generated by the trays, and that flow across the sediment surface of the trays was characterized by turbulent eddies, accelerated velocities and boundary layer thickening. These sorts of flow characteristics would not be expected over natural sediments, and an alternative colonization tray was designed to eliminate these artifacts. To test the hypothesis that different types of food patches would result in different types of larval response, and determine how near-bed hydrodynamics may influence larval settlement, flush colonization trays filled with prefrozen sediment were deployed in tandem with artificial depressions south of St. Croix, U.S.V.I at 900 m depth. Colonization trays and artificial depressions were either unenriched or enriched with Thalassiosira sp. and Sargassum sp. two types of algae chosen to mimic natural food patches on the sea floor. Unexpectedly high densities of organisms colonized trays after only 23 days. The Thalassiosira trays were colonized by high densities of a relatively low diversity, opportunistic fauna, Sargassum trays were colonized by lower densities of a higher diversity fauna, and unenriched trays were colonized by very low numbers of a very diverse fauna. All tray faunas were markedly different in composition from the natural, ambient fauna. These fmdings suggest that different patch types did, indeed, result in a specialized faunal response to each of the "patch" types. Depressions on the sea floor provide a natural mechanism for food patch formation because passive particles such as detritus and algae tend to be entrained in the depressions. To determine whether dominant colonizers would be entrained in depressions like passive particles or could differentiate between depression "patch" types in a flow environment that might be expected to make active selection more difficult, artificial depressions were unenriched or enriched with Sargassum sp. or Thalassiosira sp. Total densities of organisms and densities of the most abundant species were substantially lower in artificial depressions than in trays. Densities in Thalassiosira depressions were lower than in Sargassum depressions and densities in unenriched depressions were extremely low, suggesting that dominant colonizers were not passively entrained in depressions and that colonization was specialized and highly active for these taxa. A different fauna was also observed in natural depressions compared with flush sediments, suggesting that natural depressions do contribute to species coexistence. Long-term tray deployments designed to test whether different faunas would be present in "patches" of different ages indicated that time may also play an important part in a deep-sea patch mosaic.

Description: This was funded by NSF and ONR, NOAA, NSERC (Canada), WHOI Ocean Ventures Fund and the WHOI Ditty Bag Fund.

Description: Submitted in partial fulfillment of the requirements for the degree of Ocean Engineer at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution August 1993

Description: Travel time perturbations of adiabatic normal modes due to an internal tide and internal mode field in the Barents Sea are examined. A formalism for the travel time perturbation due to a change in sound speed is presented. Internal tide and internal wave amplitude spectra are calculated from Brancker temperature loggers which were deployed on moorings in the Barents Sea during the August 1992 Barents Sea Polar Front Experiment. In particular, the first three internal wave mode amplitudes are estimated from the four Brancker temperature loggers on the southwest mooring of the array. Modal perturbations in acoustic pulse travel time and the travel time covariance are calculated and compared for consistency to a simple ray model. These perturbations are small for the modal arrivals that the vertical acoustic array which was deployed is expected to resolve. The third internal wave mode has the largest impact on the acoustic arrivals, per unit amplitude, but the first internal wave mode dominates the scattering due to having a much larger amplitude overall.

Description: Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution September 2012

Description: This thesis focuses on ocean circulation and atmospheric forcing in the Atlantic Ocean at the Last Glacial Maximum (LGM, 18-21 thousand years before present). Relative to the pre-industrial climate, LGM atmospheric CO2 concentrations were about 90 ppm lower, ice sheets were much more extensive, and many regions experienced significantly colder temperatures. In this thesis a novel approach to dynamical reconstruction is applied to make estimates of LGM Atlantic Ocean state that are consistent with these proxy records and with known ocean dynamics. Ocean dynamics are described with the MIT General Circulation Model in an Atlantic configuration extending from 35°S to 75°N at 1° resolution. Six LGM proxy types are used to constrain the model: four compilations of near sea surface temperatures from the MARGO project, as well as benthic isotope records of δ18O and δ13C compiled byMarchal and Curry; 629 individual proxy records are used. To improve the fit of the model to the data, a least-squares fit is computed using an algorithm based on the model adjoint (the Lagrange multiplier methodology). The adjoint is used to compute improvements to uncertain initial and boundary conditions (the control variables). As compared to previous model-data syntheses of LGM ocean state, this thesis uses a significantly more realistic model of oceanic physics, and is the first to incorporate such a large number and diversity of proxy records. A major finding is that it is possible to find an ocean state that is consistent with all six LGM proxy compilations and with known ocean dynamics, given reasonable uncertainty estimates. Only relatively modest shifts from modern atmospheric forcing are required to fit the LGM data. The estimates presented herein successfully reproduce regional shifts in conditions at the LGM that have been inferred from proxy records, but which have not been captured in the best available LGM coupled model simulations. In addition, LGM benthic δ18O and δ13C records are shown to be consistent with a shallow but robust Atlantic meridional overturning cell, although other circulations cannot be excluded.

Description: Primary support was provided by a National Defense Science and Engineering Graduate Fellowship and two National Science Foundation awards: Award #OCE-0645936: “Beyond the Instrumental Record: the Case of Circulation at the Last Glacial Maximum” and Award #OCE-1060735: “Collaborative Research: Beyond the Instrumental Record - the Ocean Circulation at the Last Glacial Maximum and the de-Glacial Sequence”. Important secondary support came from the National Ocean Partnership Program and the National Aeronautics and Space Administration via the ECCO effort at MIT.

Description: Submitted in partial fulfillment of the requirements for the degree of Master of Science at the Massachusetts Institute of Technology and Woods Hole Oceanographic Institution August 1994

Description: Ocean modellers seek to understand the circulation of the oceans, or portions thereof, by developing models of the ocean they can solve. This tractability constraint forces ocean modellers to make choices. Naturally, they hope to make intelligent choices, but whenever a new model is being developed or an existing one extended, the issue of tractability lurks. The large-scale, basin-wide, circulation of the oceans can be divided into two components, classified by their driving force. The wind-driven circulation, whose flow occurs mainly above the thermocline, was first explained qualitatively by Stommel (1948) with a simple, elegant analytical model. The other component of the oceans' circulation, the density-driven, or thermohaline circulation, flows below the thermocline. Again, the first simple analytical model for the deep thermohaline flow was proposed by Stommel (1958) and developed by Stommel and Aarons (1959) whose basic ideas underlie even the most recent conceptual models of the large-scale circulation. The details of the thermohaline circulation and its interaction with the wind-driven circulation in a realistic ocean basin is a problem which is not tractable analytically. This has driven ocean modellers interested in this aspect of the oceans' circulation to numerical models: ocean circulation models.

Description: Submitted in partial fulfillment of the requirements for the degree of Master of Science at the Massachusetts Institute of Technology and Woods Hole Oceanographic Institution May 1994

Description: This thesis compares classical nonlinear control theoretic techniques with recently developed neural network control methods based on the simulation and experimental results on a simple electromechanical system. The system has a configuration-dependent inertia, which contributes a substantial nonlinearity. The controllers being studied include PID, sliding control, adaptive sliding control, and two different controllers based on neural networks: one uses feedback error learning approach while the other uses a Gaussian network control method. The Gaussian network controller is tested only in simulation due to lack of time. These controllers are evaluated based on the amount of a priori knowledge required, tracking performance, stability guarantees, and computational requirements. Suggestions for choosing appropriate control techniques to one's specific control applications are provided based on these partial comparison results.

Description: Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution September 1994

Description: This work investigates whether large-scale coherent vortex structures driven by wave-current interaction (Langmuir circulation) are responsible for maintaining the oceanic mixed layer. Langmuir circulations dominate the near-surface vertical transport of momentum and density when the characteristic scale for forcing (defined as the Craik-Leibovich instability parameter γCLS) is stronger than the characteristic scale for diffusive decay γdiff. Since the wave-current forcing is concentrated near the surface both terms depend on the cell geometry. Cells with long wavelengths penetrate more deeply into the water column. These cells grow more slowly than the fastest growing mode for most cases, but always dominate the solution in the absence of Coriolis forces. In the presence of Coriolis forces, the horizontal wavelength and thus the depth of penetration are limited. When a cell geometry is found such that γCLS » γdiff, the current profile produced by small-scale diffusion is unstable to Langmuir cells and the cells replace small-scale diffusion as the dominant vertical transport mechanism for momentum and density. The perturbation crosscell shear is predicted to scale as γCLS. Such a scaling is observed during two field experiments. The observed velocity profile during these experiments is more sheared than predicted by a model which implicitly assumes instantaneous mixing by large eddies, but less sheared than predicted by a model which assumes small-scale mixing by near-isotropic turbulence. The latter profile is unstable to Langmuir cells when waves are present. The inclusion of cells driven by wave-current interaction explains the failure of the mixed layer to restratify on two days with high waves and low wind. Wave-current interaction introduces a small but efficient source of energy for transporting density which goes as the surface stress times the Stokes drift.

Description: The Office of Naval Research supported me throughout graduate school, first as an ONR Graduate Fellow. and later as a research assistant under the Surface Waves Processes Program (ONR Grant N00014-90-J-1495).

Description: Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy at the Massachusetts Institute of Technology and Woods Hole Oceanographic Institution, February 2012

Description: Trace metal cycling is one of many processes that influence ocean ecosystem dynamics. Cobalt, iron, and manganese are redox active trace metal micronutrients with oceanic distributions that are influenced by both biological and abiotic sources and sinks. Their open ocean concentrations range from picomolar to nanomolar, and their bioavailabilities can impact primary production. Understanding the biogeochemical cycling of these hybrid-type metals with an emphasis on cobalt was the focus of this thesis. This was accomplished by determining the dissolved distributions of these metals in oceanic regions that were characterized by different dominant biogeochemistries. A large subsurface plume of dissolved cobalt, iron, and manganese was found in the Eastern South Atlantic. The cause of this plume is a combination of reductive dissolution in coastal sediments, wind-driven upwelling, advection, biological uptake, and remineralization. Additional processes that are discussed as sources of metals to the regions studied during this thesis include isopycnal uplift within cold-core eddies (Hawaii), ice melt (McMurdo Sound, Antarctica), riverine input (Arctic Ocean), and winter mixing (McMurdo Sound). The biological influence on surface ocean distributions of cobalt was apparent by the observation of linear relationships between cobalt and phosphate in mid to low latitudes. The cobalt:phosphate ratios derived from these correlations changed over orders of magnitude, revealing dynamic variability in the utilization, demand, and sources of this micronutrient. Speciation studies suggest that there may be two classes of cobalt binding ligands, and that organic complexation plays an important role in preventing scavenging of cobalt in the ocean. These datasets provided a basis for comparing the biogeochemical cycles of cobalt, iron, and manganese in three oceanic regimes (Hawaii, South Atlantic, McMurdo Sound). The relative rates of scavenging for these metals show environmental variability: in the South Atlantic, cobalt, iron, and manganese were scavenged at very different rates, but in the Ross Sea, mixing and circulation over the shallow sea was fast, scavenging played a minor role, and the cycles of all three metals were coupled. Studying the distributions of these metals in biogeochemically distinct regions is a step toward a better understanding of their oceanic cycles.

Description: Funding for this research was provided by the the National Science Foundation Chemical Oceanography (Division of Ocean Sciences OCE-0452883, OCE-0752291, OCE-0928414, OCE-0732665, OCE-0440840, OCE-0327225), the Center for Microbial Research and Education, the WHOI Coastal Ocean Institute, and the WHOI Ocean Life Institute, WHOI Academic Programs Office, and a Fye Teaching Fellowship.

Description: Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution May 1989

Description: The relationship between depth-averaged velocity and bottom stress for wind-driven flow in unstratified coastal waters is examined here. The adequacy of traditional linear and quadratic drag laws is addressed by comparison with a 2 1/2-D model. A 2 1/2-D model is one in which a simplified 1-D depth-resolving model (DRM) is used to provide an estimate of the relationship between the flow and bottom stress at each grid point of a depth-averaged model (DAM). Bottom stress information is passed from the DRM to the DAM in the form of drag tensor with two components: one which scales the flow and one which rotates it. This eliminates the problem of traditional drag laws requiring the flow and bottom stress to be collinear. In addition , the drag tensor field is updated periodically so that the relationship between the velocity and bottom stress can be time-dependent. However, simplifications in the 2 1/2-D model that render it computationally efficient also impose restrictions on the time-scale of resolvable processes. Basically, they must be much longer than the vertical diffusion time scale. Four progressively more complicated scenarios are investigated. The important factors governing the importance of bottom friction in each are found to be 1) non-dimensional surface Ekman depth, u.5/fh where u.s is the surface shear velocity, f is the Coriolis parameter and h is the water depth 2) the non-dimensional bottom roughness, zo/h where zo is the roughness length and 3) the angle between the wind stress and the shoreline. Each has significant influence on the drag law. The drag tensor magnitude, r, and the drag sensor angle, θ are functions of all three, while a drag tensor which scales with the square of the depth-averaged velocity has a magnitude, Cd, that only depends on zo/h. The choice of drag Jaw is found to significantly affect the response of a domain. Spin up times and phase relationships vary between models. In general, the 2 1/2-D model responds more quickly than either a constant r or constant Cd model. Steady-state responses are also affected. The two most significant results are that failure to account for θ in the drag law sometimes leads to substantial errors in estimating the sea surface height and to extremely poor resolution of cross-shore bottom stress. The latter implies that cross-shore near-bottom transport is essentially neglected by traditional DAMs.

Description: Financial support during my time in graduate school came from the Woods Hole Oceanographic Institution and grants from the National Science Foundation (OCE84-03249) and the Office of Naval Research (N00014-86-K-0061).

Description: Submitted in partial fulfillment of the requirements for the degree of Master of Science at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution September 1989

Description: This thesis studies mixing and convection in a rectangular basin driven by a specified heat flux at the surface. A numerical model is constructed for this purpose. The main focus of the study is on the density and circulation structure resulting from the thermal forcing. In chapter two, a simple vertical one-dimensional model is developed to examine the mixing processes under a given surface heat flux. In order to simulate strong vertical mixing in the region where stratification is unstable, turbulent processes are modeled by a convective overturning parameterization of eddy viscosity and diffusivity. The results show that the density structure is strongly affected by the convective overturning adjustment as surface cooling prevails, and the resulting density field is nearly depth independent. In chapter three, a more complicated two-dimensional model is constructed to simulate mixing and circulation in a vertical rectangular basin with rigid boundaries. The aspect ratio of the basin ranges from 1 to 0.001 and Rayleigh number from 104 to 2 x 1012. It is found that the circulation pattern is dominated by these two important numbers. The roles of density overturning and density-momentum overturning mixing are further investigated. The results show that the convective overturning not only homogenizes the density field in the unstably stratified region but also contributes to increase the circulation. A crude scale analysis of the system shows that the characteristics of the density and momentum fields from the analysis agree well with the numerical results.

Description: Submitted in partial fulfillment of the requirements for the degree of Master of Science at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution September 1989

Description: In order to provide an accurate and efficient method of docking, station keeping, and navigation for the JASON remotely operated vehicle, an optical tracking method is investigated. The method needs to be accurate enough for underwater control and fast enough to enable timely control. The method presented solves the camera location problem in a closed form and is accurate for accurate measurements of image plane coordinates. Target design criteria are discussed and a prepared, passive target is selected. Testing and error analysis reveal that the approach gives good results for camera lenses with focal lengths greater than 4.8mm. The effects of underwater use are discussed and a non-numerical method of compensating for the underwater effects is presented. The compensation method shifts the image plane coordinates toward the image plane center and gives results that are within acceptable error margins.

Description: Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution September 1993

Description: An extensive set of new high-quality hydrographic data is assembled in order to determine the mean circulation in the equatorial Pacific, and thus the pathways for cross-equatorial and cross-gyre exchange. Making up the core of the data set are two onetime transpacific zonal sections nominally at 10°N and 14°S. Supplementing these are repeat surveys of the equatorial currents along the 165°E meridian with direct shear measurements, and repeat surveys of the western boundary current at 8°N including direct velocity measurements. The repeat survey data are crucial for obtaining a good estimate of the mean conditions in the face of strong annual and interannual variability of the near-equatorial flow field. A comparison with historical XBT and hydrographic data shows that the interior thermocline transports in the one-time sections are fortuitously representative of the mean conditions. A detailed study of the water mass distribution along the sections is the basis for choosing reference levels for the thermal wind shear in an initial guess circulation field. Using an inverse model, the initial guess circulation is adjusted such that volume, heat and salt arc conserved in a set of subthermocline layers (δΘ 〉 26.7). Cross-isopycnal diffusion and advection are explicitly accounted for in the inverse model, and the diapycnal diffusivity is constrained to be positive, though its value is allowed to vary with depth and location. Net mass conservation constraints are applied to the enclosed volumes of the North Pacific and eastern Pacific, and essentially require that the Ekman divergence be equal to the geostrophic convergence. The Ekman fluxes as estimated from wind-stress climatologies are an important element of the mass budget, and yet are subject to large uncertainties. The model is therefore given the freedom to determine the Ekman fluxes within the range of error of the wind-stresses. The circulation of the coldest waters (Θ 〈 1.2°C) is dominated by the northward flow of Lower Circumpolar Water (LCPW) in a system of narrow western boundary currents. A net transport of 12.1 Sv of LCPW flows across 14°S, 9.6 Sv of which flows into the North Pacific across 10°N. The bulk of the LCPW flux across the equator appears to occur in the denser part of the western boundary current which follows topography directly across the equator. Dissipation in the boundary layer can thus modify the potential vorticity of the fluid and allow it to cross the equator. The circulation of the upper part of the LCPW is dominated by a strong westward jet at the equator which is supplied both by upwelling from below and the recirculation of modified LCPW from the North Pacific. At mid-depth (4.0 〉 Θ 〉 1.2°C) high silica and low oxygen concentrations mark the North Pacific Deep Water (NPDW) which is present in both the North and South Pacific Oceans. Across both 10°N and 14°S, a net of 11 Sv of NPDW flows southward, returning the northward mass flux associated with the LCPW. In contrast to the LCPW, narrow western boundary currents are not present in this layer, and it is not clear how the deep water flows across the equator. Strong zonal jets on and about the equator may be important in allowing mass to cross the equator by increasing the time available for the cross-equatorial diffusion of potential vorticity to act on a fluid parcel. At intermediate depths equatorward advection is suggested by the presence of intermediate water salinity minima formed in the subpolar latitudes: Antarctic Intermediate Water dominates the 4 to 8°C classes south of the equator, while North Pacific Intermediate Water occupies this range north of the equator. Determination of the mean circulation of the intermediate waters is, however, confounded by the large eddies that dominate the geostrophic transport stream function along the onetime zonal sections. The equatorial thermocline is occupied by waters of subtropical origin: the shallow salinity minimum waters and saline Central Water from both the North and South Pacific Ocean. The equator marks the location of a front between northern and southern subtropical gyre waters, except in the lower thermocline where water from the South Pacific subtropical gyre penetrates to about 4°N to feed the Northern Subsurface Countercurrent at 165°E. All of the equatorward flowing thermocline waters are entrained in the eastward equatorial currents which in turn feed the upwelling system in the eastern Pacific. The upwelled waters largely supply the South Equatorial Current in the eastern Pacific, accounting for its large transport compared to that predicted by Sverdrup dynamics. Northward flow across the equator of the upwelled waters in the thermocline or surface layer in the western Pacific is necessary to supply the Ekman flux into the North Pacific. The analysis indicates that the Pacific Ocean does not convert a large amount of abyssal water to thermocline water, as required by several theories of the global thermohaline circulation. In contrast to the Atlantic Ocean, the thermocline circulation in the Pacific appears decoupled from the abyssal overturning, with little upwelling of abyssal waters occurring in either the North Pacific or the equatorial Pacific. The leakage of Pacific water into the Indian Ocean is deduced to be essentially zero, though an error analysis allows a range of 0-8 x 106m3s-1.

Description: I was supported by the 1986 Caltex Graduate Women Scholarship, and a NASA Scholarship in Global Change Research.

Description: Submitted in partial fulfillment of the requirements for the degree of Master of Science at the Massachusetts Institute of Technology and Woods Hole Oceanographic Institution February 1994

Description: Planktonic foraminiferal flux was collected at four sediment trap locations spanning a 34° latitude range in the North Atlantic during 1988-1990. Satellite-derived sea surface temperature (SST) and CTD data were integrated with time-corrected flux data to determine the effects of seasonal hydrographic changes on foraminiferal production and species succession in surface waters. The thermal structure of the upper water column controls foraminiferal production by regulating levels of phytoplankton production and by directly influencing the preferred temperature habitats of individual species in the community. Sediment traps deployed at the two southern sites (34°N and 48°N) were part of the U.S. Joint Global Ocean Flux Study (JGOFS) and were located in regions influenced by upwelling events induced by mesoscale eddy activity. The timing of maximum foraminiferal production at each trap location coincides with the northward progression of the spring bloom in the North Atlantic. The magnitude of total flux during bloom periods varies considerably with location and is positively correlated with the amount of primary productivity in surface waters. Foraminiferal production is highest at JGOFS 48 and probably results from the greater influence of mesoscale variability in this region on local hydrographic conditions. The upwelling associated with cyclonic cold-core eddies appears to be an effective mechanism for increasing local foraminiferal production in the North Atlantic by enhancing food availability. The preferred production of individual species during upwelling periods may depend on the vertical distribution of chlorophyll in the water column. Seasonal variation in SST is also an important factor controlling the relative abundance of species with preferred thermal habitats. A distinct seasonal species succession occurs at both JGOFS sites but is absent at higher latitudes due to decreases in both faunal diversity and seasonal temperature variations with increasing latitude in the North Atlantic. Foraminiferal :flux data, expressed in terms of relative abundance, is the best direct method of comparing species distributions of living populations with fossil assemblages in the sedimentary record. Preferred temperature ranges for G. ruber, G. bulloides, and N. pachyderma are estimated in this study. The tolerance limits for these species and other ecological inferences derived from these flux data may prove useful for paleoceanographic reconstruction in deep-sea cores. Future studies will be necessary, however, to establish the consistency of these results among different ocean basins.

Description: This research was funded by National Science Foundation Grant No. ATM-9115619.

Description: Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution September 1994

Description: Paleo-tracers such as carbon 13 and cadmium show that the deep Atlantic was enriched in nutrients during the Last Ice Age. The conventionally accepted interpretation of these higher nutrient levels is that a reduction of the rate of formation of nutrient-depleted Lower North Atlantic Deep Water (Lower NADW) allowed nutrient-rich Antarctic Bottom Water (AABW) to push further north during the Last Glacial Maximum (LGM) (Boyle and Keigwin, 1982; 1987; Duplessy et al., 1988). The evidence for this interpretation is re-examined in this work, with an emphasis on the quantitative analysis of the paleo-data. An end-member analysis of the δ13C data indicates a larger volume of AABW and a smaller volume of Lower NADW during the LGM. It is not yet possible, however, to quantify the extent of the volume differences between the modern and the glacial distributions, because the LGM δ13C end-members are poorly known. The second issue examined in this thesis deals with the interpretation of the water mass distribution, inferred from paleo-tracers, in terms of the oceanic circulation. Using a dynamical inverse model of the North Atlantic and a kinematic inverse model of the South Atlantic, it is shown that a tracer distribution corresponding to a significantly reduced volume of Lower NADW does not necessarily correspond to a reduced flux of NADW. Indeed, a circulation almost identical to a modem ocean reference circulation is consistent with the available LGM δ13C and δ18O data A flux of Lower NADW reduced by 50%, though not needed to explain the LGM tracer distribution, is also consistent with the data Thus, the paleo-tracers δ13C and δ18O do not suffice to quantify the flux of NADW in the glacial ocean. The modem ocean circulation is one of many possible circulations consistent with the available δ13C and δ18O data.

Description: This research was funded by the National Science Foundation under grant OCE-9205942.

Description: Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution October 1993

Description: In this thesis, production of dense water that feeds the dense overflows across the Greenland-Scotland Ridge has been considered. A new circulation scheme is developed which is consistent with the water masses, currents and air-sea fluxes in the region, and with the important observation that the dense overflows show little or no seasonal or interannual variability. An inverse box model has been constructed that shows that the new circulation scheme is consistent with conservation statements for mass, heat and salt as well. According to the new circulation scheme the major buoyancy is lost in the North Atlantic Current, which enters the Norwegian Sea between Iceland and Scotland, and flows northward towards the Arctic Ocean and the Barents Sea. The transformation is due to a large net annual heat loss over the North Atlantic Current, combined with a long residence time (2-3 years) and a large surface area. After subduction, one branch of the North Atlantic Current enters the Arctic Ocean, is modified in hydrographic properties into those associated with the Denmark Strait Overflow Waters in the western North Atlantic, exits the Arctic Ocean in the western Fram Strait and flows with the East Greenland Current towards the Denmark Strait Another branch of the North Atlantic Current recirculates directly in the Fram Strait and flows towards the Denmark Strait with the East Greenland Current This branch will not sink to the bottom of the North Atlantic as it is less compressible than the Arctic branch. The third branch of the North Atlantic Current enters the Barents Sea, continues to lose buoyancy, and enters the Arctic Ocean at intermediate depth. This branch exits the Arctic Ocean in the western Fram Strait, circulates around the Greenland Sea, enters the Norwegian Sea, and flows towards the Frer¢-Shetland Channel. The traditional view holds that the major sources of the dense overflows are the Iceland and Greenland gyres, west of the North Atlantic Current. Aside from the finding that the new circulation scheme is more likely in terms of water mass properties, currents etc., one fundamental problem with the old scheme lies with supplying a substantial overflow. There are indications that the production of dense water in the gyres is sensitive to the highly variable surface conditions and that indeed the production tends to shut on and off. The reservoirs in the gyres are so small that they would be drained within a few years if they were to supply the overflows during a shutdown period. Production of dense water within the North Atlantic Current is less sensitive to surface conditions. The density in the gyres is gained at a temperature around freezing, whereas in the North Atlantic Current the density is gained well above freezing. Therefore a freshwater anomaly in the two domains will have different consequences for vertical · overturning: within the North Atlantic Current the freshening can be overcome by further cooling, whereas in the gyres freezing will occur and the vertical overturning will cease. The observed lack of a significant seasonal signal associated with the dense overflows is consistent with the new circulations scheme. The net annual cooling dominates the seasonal oscillation in the atmospheric heat loss for time scales comparable with the residence time of the Atlantic Water within the domain. Thus winter formation of dense water within the North Atlantic Current does not induce a seasonal signal in the transport field of the dense water.

Description: Funding for this work was partly provided by a NASA Global Change Fellowship.

Description: Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution July 1993

Description: In this thesis, the dynamic role of bottom topography in a β-plane channel is systematically studied in both linear homogeneous and stratified layer models in the presence of either wind stress (Chapters 2, 3, 4, and 6) or buoyancy forcing (Chapter 5). In these studies, the structure of the geostrophic contour plays a fundamental role, and the role of bottom topography is looked at from two different angles. It is shown that blocking all the geostrophic contours leads to two different physical processes in which bottom topographic form drag is generated (Chapters 2, 3 and 4) and enables geostrophic flow in a β-plane channel to support a net cross-channel volume transport (Chapters 5 and 6). It is demonstrated that by blocking all the geostrophic contours in the presence of a sufficiently high ridge, the dynamics of both source-sink and wind driven circulations in a β-plane is similar to that in a dosed basin. First, wind-driven circulation in the inviscid limit is discussed in a linear barotropic channel model in the presence of a bottom ridge. There is a critical height of the ridge, above which all geostrophic contours in the channel are blocked. In the subcritical case, the Sverdrupian balance does not apply and there is no solution in the inviscid limit. In the supercritical case, however, the Sverdrupian balance applies. The form drag is generated through two different physical processes: the through-channel recirculating flow and the Sverdrupian gyre flow. These processes are fundamentally different from the nonlinear Rossby wave drag generation. In this linear model, the presence of a supercritical high ridge is essential in the inviscid limit. With this form drag generation determined, an explicit form for the zonal transport in the channel is obtained, which shows what model parameters determine the through-channel transport. In addition, the model demonstrates that most of the potential vorticity dissipation occurs at the northern boundary where the ridge intersects. The result from the homogeneous channel model in Chapter 2 is then extended to a model whose geometry consists of a zonal channel and two partial meridional barriers along each boundary at the same longitude. Both the model transport and especially the model circulation are significantly affected by the presence of the two meridional barriers. The presence of the northern barrier always leads to a decrease in the transport. The presence of the southern barrier, however, increases the transport for a narrow ridge. The northern barrier only has a localized influence on the circulation pattern, while the southern barrier has a global influence in the channel. Then a multi-layer Q-G model is constructed by assuming that potential vorticity in all subsurface layers is homogenized. The circulation is made up of baroclinic and the barotropic part. The barotropic part is same as that in a corresponding barotropic model, and is solely determined by the wind stress, while the baroclinic part is not directly related to the wind stress. It is determined by the potential vorticity homogenization and lateral boundary conditions. The presence of the stratification does not affect the bottom topographic form drag generation. The interfacial form drag is generated by the stationary eddies. Corresponding to the circulation structure, the zonal through-channel transport associated with the barotropic circulation is determined by the wind stress and bottom topography. The other part associated with the baroclinic circulation, however, is not directly related to the wind stress and it is determined by the background stratification. Based upon the discussion on the geostrophic contour, a simple barotropic model of abyssal circulation in a circumpolar ocean basin is constructed. The presence of a supercritically high ridge is both necessary and sufficient for geostrophic flow in a β-plane channel to support a net cross-channel volume flux. In the presence of a sufficiently high ridge, the classical Stommel & Arons theory applies here, but with significant modifications. The major novelty is that a throughchannel recirculation is generated. Both its strength and direction depend critically upon the model parameters. Then, a schematic picture of the abyssal circulation in a rather idealized Southern Ocean is obtained. The most significant feature is the narrow current along the northern boundary of the circumpolar basin, which feeds the deep western boundary currents of the Indian Ocean and Pacific Ocean and connects all the oceanic basins in the Southern Ocean. Finally, the question of how the northward surface Ekman transport out of the circumpolar ocean is returned is discussed in a two-layer model with an infinitesimally thin surface Ekman layer on top of a homogeneous layer of water in a rather idealized Southern Ocean basin. First, the case with a single subtropical ocean basin is discussed. In the case with a sufficiently high ridge connecting the Antarctic and the meridional barrier, an explicit solution is found. The surface Ekman layer sucks water from the lower layer in the circumpolar basin. This same amount of water flows northward as the surface Ekman drift. It downwells in the subtropical gyre, and is carried to the western boundary layer. From there, the same amount of water flows southward as a western boundary current across the inter-gyre boundary between the circumpolar ocean and the subtropical gyre along the west coast to the southern boundary of the meridional barrier. Then, the same amount of water is carried southward and feeds the water loss to the surface Ekman layer due to the Ekman sucking in the interior circumpolar ocean. The case with multiple subtropical ocean basins such as the Southern Ocean is also discussed. It is demonstrated that the surface Ekman drift drives a strong inter-basin water mass exchange.

Description: This thesis was supported by National Science Foundation through grant OCE OCE90-17158. Part of the numerical simulation was performed at NCAR's supercomputer, and was supported by SCD/NCAR.

Description: Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy at the Massachusetts Institute of Technology and Woods Hole Oceanographic Institution February 1994

Description: In this work we study motion of a baroclinic upper-ocean eddy over a large-scale topography which simulates a continental slope. We use a quasigeostrophic f-plane approximation with continuous stratification. To study this problem we develop a new numerical technique which we call "semi-lagrangian contour dynamics". This technique resembles the traditional 2-D contour dynamics method but differs significantly from it in the numerical algorithm. In addition to "Lagrangian" moving contours it includes an underlying "Eulerian" regular grid to which vorticity or density fields are interpolated. To study topographic interactions in a continuously stratified model we use density contours at the bottom in a similar manner as vorticity contours are used in the standard contour dynamics. For the case of a localized upper-ocean vortex moving over a sloping bottom the problem becomes computationally 2-dimensional (we need to follow only bottom density contours and the position of the vortex itself) although the physical domain is still 3-dimensional. Results of the numerical model lndicate importance of baroclinic effects in the vortex-topography interaction. After the initial surge of topographic Rossby waves a vortex moves almost steadily due to the interaction with a bottom density anomaly which is created and supported by a vortex itself. This anomaly is equivalent to a region of opposite-signed vorticity with a total circulation exactly compensating that of a vortex. This results in a vertically aligned dipolar structure with the total barotropic component equal to zero. Analytical considerations explaining this effect are presented and formulated in a more general statement which resembles but does not coincide with the "zero angular momentum theorem" of Flierl, Stern and Whitehead, 1983. In such steady translation the centroid of a bottom density anomaly is displaced horizon tally from the center of an upper-ocean vortex so the whole system moves due to this misalignment, which is known as a "he tonic mechanism". Cyclonic vortices go generally upslope, and anticyclones - in a downslope direction. The along-slope component of their motion depends upon the strength of a vortex, curvature of the bottom slope and background flows. When surrounded by a bowl-shaped topography anticyclonic vortices tend to stay near the deepest center of a basin, even resisting ambient flows which advect them outward. Application of this results to various oceanic examples (particularly to the "Shikmona eddy" in the Eastern Meditenanian) is discussed. Our results show that the behavior of a vortex over a sloping bottom differs significantly from its motion on the planetary beta-plane (but with a flat bottom). To explain this difference we introduce the concept of a "wave-breaking regime" relevant for the case of a planetary beta-effect, and a "wave-gliding regime" which characterizes the interaction of an eddy with a topographic slope.

Description: This work was supported by the NSF grant #OCE 90-12821.

Description: Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution February 2013

Description: Efforts to monitor the ocean for signs of climate change are hampered by ever-present noise, in the form of stochastic ocean variability, and detailed knowledge of the character of this noise is necessary for estimating the significance of apparent trends. Typically, uncertainty estimates are made by a variety of ad hoc methods, often based on numerical model results or the variability of the data set being analyzed. We provide a systematic approach based on the four-dimensional frequency-wavenumber spectrum of low-frequency ocean variability. This thesis presents an empirical model of the spectrum of ocean variability for periods between about 20 days and 15 years and wavelengths of about 200{10,000 km, and describes applications to ocean circulation trend detection, observing system design, and satellite data processing. The horizontal wavenumber-frequency part of the model spectrum is based on satellite altimetry, current meter data, moored temperature records, and shipboard ADCP data. The spectrum is dominated by motions along a "nondispersive line". The observations considered are consistent with a universal ω-2 power law at the high end of the frequency range, but inconsistent with a universal wavenumber power law. The model spectrum is globally varying and accounts for changes in dominant phase speed, period, and wavelength with location. The vertical structure of the model spectrum is based on numerical model results, current meter data, and theoretical considerations. We find that the vertical structure of kinetic energy is surface intensified relative to the simplest theoretical predictions. We present a theory for the interaction of linear Rossby waves with rough topography; rough topography can explain both the observed phase speeds and vertical structure of variability. The improved description of low-frequency ocean variability presented here will serve as a useful tool for future oceanographic studies.

Description: This research was supported by NASA under grants NNG06GC28G and NNX08AR33G.

Description: Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution June 1995

Description: Moored time series from the Coastal Ocean Dynamics Experiment (CODE), Shelf Mixed Layer Experiment (SMILE), Sediment Transport Events over the Shelf and Slope (STRESS) study, and Northern California Coastal Circulation Study (NCCCS) are used to study subtidal cross-shelf circulation over the northern California shelf. The northern California shelf, like much of the United States Pacific coast, is subject to strong wind forcing which exhibits characteristic seasonality. In winter and early spring, it is distinguished by poleward and equatorward fluctuations on time scales of days and by weak monthly means. In summer, it is distinguished by periods of equatorward stress lasting several weeks and by relatively strong monthly means. The intensive winter and spring SMILE and STRESS and summer CODE-2 field programs permit the examination of cross-shelf circulation under both types of wind forcing conditions at a mid-shelf site (~90 m) 6 km from the northern California coast. The primary thesis goal is to examine the applicability of a two-dimensional conceptual model of wind-forced cross-shelf circulation. In this conceptual model, surface and bottom cross-shelf flows are forced by along-shelf wind stress and bottom stress, and interior cross-shelf flow compensates such that the depth-averaged flow is zero. A secondary thesis goal is to use the seasonal coverage of available field programs to gain insight into seasonal variability of cross-shelf circulation on the northern California shelf. To accomplish these goals, the observed subtidal cross-shelf circulation is examined in the context of the winter and spring heat and salt balances, an analytic model of wind-forced cross-shelf circulation, and the spatial scales of subtidal velocity. Mean and fluctuating heat and salt balances estimated between December, 1988 and May, 1989 demonstrate the importance of cross-shelf fluxes and their general consistency with the simple conceptual model. Mean fluxes are consistent with the weak mean equatorward wind stress observed during SMILE. The dominant terms in the fluctuating balances are the cross-shelf fluxes and local changes in heat and salt content. These are well correlated with each other and with the local along-shelf wind stress. The along-shelf heat flux divergence is of secondary importance to the fluctuating heat balance. It is uncorrelated with the along-shelf wind stress, and occurrences when it is strong are interpreted as effects of mesoscale features. To examine the applicability of the wind-forced conceptual model in more detail, a simple analytic model incorporating the assumptions of the conceptual model and observed local wind forcing is compared quantitatively to estimates of surface mixed layer, interior, and bottom mixed layer cross-shelf transport for winter SMILE and STRESS and summer CODE-2 observations. This comparison suggests the model is more suited to the transient wind forcing observed during SMILE and STRESS than to the steady wind forcing observed during CODE-2. For 2-3 day wind events between December, 1988 and February, 1989, the model is well correlated with observed depthdependent (total minus depth-averaged) transports throughout the water column and with total surface mixed layer transports. For 2-3 week wind events between April and July, 1982, the model does not work nearly as well below the surface mixed layer. In the absence of other processes, the locally wind-forced model implies that the wind stress sets the horizontal scales of subtidal velocity. Correlation scales estimated for subtidal along-shelf velocity over the northern California shelf are for all field programs longer than the maximum mooring separation (60 km) and are similar to those of the wind stress. However, along-shelf correlation scales of cross-shelf velocity are shorter than minimum mooring separations for CODE. SMILE and NCCCS time series do resolve along-shelf correlation scales for near surface cross-shelf velocity. During this time, along-shelf correlation scales for near surface cross-shelf velocity vary on a monthly time scale. They are generally long (30 km or more) when correlation with wind stress is high and short (15 km or less) when correlation with wind stress is low. On at least one occasion, short along-shelf correlation scales coincide with the intrusion of an offshore mesoscale feature onto the shelf. Results of the three studies show the two-dimensional model offers some insight into the observed subtidal cross-shelf circulation, particularly in winter. During this time, the heat balance, analytical transport model, and correlation scales all provide evidence that the winter wind-forced circulation is quasi-two-dimensional. Threedimensional variability on the shelf, though important on occasion, does not appear to be wind-driven and may result from the influence of offshore mesoscale features. A quite different story emerges for summer when the simple conceptual model of crossshelf circulation fails to describe adequately subsurface cross-shelf flow. Two useful areas of further investigation may be the non-linear response of cross-shelf velocity to wind forcing and its response to other processes such as remotely generated mesoscale features.

Description: In my first year at MIT, Carl Wunsch supported me through NSF grant OCE 88- 23043. At WHOI, I first started work on SMILE with Bob Beardsley under NSF grant OCE 87-16937 and continued working on it with Steve Lentz under NSF grant OCE 91-15713.

Description: Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution August 1995

Description: Data from fifteen globally distributed, modern, high resolution, hydrographic oceanic transects are combined in an inverse calculation using large scale box models. The models provide estimates of the global meridional heat and freshwater budgets and are used to examine the sensitivity of the global circulation, both inter and intra-basin exchange rates, to a variety of external constraints provided by estimates of Ekman, boundary current and throughflow transports. A solution is found which is consistent with both the model physics and the global data set, despite a twenty five year time span and a lack of seasonal consistency among the data. The overall pattern of the global circulation suggested by the models is similar to that proposed in previously published local studies and regional reviews. However, significant qualitative and quantitative differences exist. These differences are due both to the model definition and to the global nature of the data set. The picture of the global circulation which emerges from the models IS a complex, turbulent flow. When integrated across ocean basins not one, but two major cells emerge. The first connects an Atlantic overturning cell (estimated at 18± 4x 109 kg s- 1) to the Southern Ocean where the Antarctic Circumpolar Current carries lower deep waters to the Indian and Pacific basins where they are converted to upper deep and intermediate waters before returning to the Atlantic. The second cell connects the Pacific and Indian Basins to the north and south of Australia. In t his cell deep waters pass into the Pacific and return within the Indian Basin as intermediate waters after passing through the Indonesian Passages. The two cells are found to be independent of one another, i.e. within the models, the Indonesian Passages do not represent a significant element in a net global circulation. While there is ample evidence of westward flow around the southern tip of South Africa which would support a "warm" water path scenario, the variability of flow in this region, rich with eddies makes hydrography a poor estimator of the relative strengths of the controversial "warm" and "cold" water paths. All existing estimates of Indonesian Passage throughflow, including the smallest (O x 106 m3 s-1) and the largest (20 x 106 m3 s-1), are consistent with the model constraints. When the Pacific- Indian throughflow is not constrained, the model produces an estimate of 11 ± 14x 109 kg s-1. The model heat flux estimates are both significantly different from zero and quite robust to changes in initial assumptions, with the exception of the choice of wind field. Although in this work it was not possible to compute freshwater fluxes which were significantly different from zero, future inclusion of salinity anomaly constraints along with terms describing vertical diffusion may yet make it possible to compute significant freshwater :flux estimates from hydrography.

Description: This research was partially funded by a NASA Global Change Fellowship and was also supported by NASA under contract NAGW-1048 and NSF under contract OCE-9205942.

Description: Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution February 1995

Description: Inverse modeling activities in oceanography have recently been intensified, aided by the oncoming observational data stream of WOCE and the advance of computer power. However, interpretations of inverse model results from climatological hydrographic data are far from simple. This thesis examines the behavior of an inverse model in the WOCE CME (Community Modeling Effort) results where the physics and the parameter values are known. The ultimate hypotheses to be tested are whether the inferred circulations from a climatological hydrographic data set (where limited time means and spatial smoothing are usually used) represent the climatological ocean general circulations, and what the inferred "diffusion" coefficients really are. The inverse model is first tested in a non-eddy resolving numerical GCM ocean. Numerical/scale analyses are used to test whether the inverse model properly represents the GCM ocean. Experiments show how biased answers could result from an incorrect model, and how a correct model must produce the right answers. When the inverse model is applied to the time-mean hydrographic data of an eddy-resolving GCM ocean in the fine grid resolution of the GCM, the estimated horizontal circulation is statistically consistent with the EGCM time means in both patterns and values. Although the flow patterns are similar, the uncertainties for the GCM time means and the inverse model estimates are different. The former are very large, such that the GCM time-mean circulation has no significance in the deep ocean. The latter are much smaller, and with them the estimated circulations are well defined. This is consistent with the concept that ocean motions are very energetic, while variations of tracers (temperature, salinity) are low frequency. The inverse model succeeded in extracting the ocean general circulation from the "climatological" hydrographic data. The estimated vertical velocities are also statistically indistinguishable from the GCM time means. However, significant differences between the estimated "diffusion" coefficients and the EGCM eddy diffusion coefficients are found at certain locations. These discrepancies are attributed to the differences in physics of the inverse model and the EGCM ocean. The "diffusion" coefficients from the inversion parameterize not only the eddy fluxes, but also (part of) the temporal variation and biharmonic terms which are not explicitly included in the inverse model. Given the essentially red spectrum of the ocean, it makes sense to look for smooth solutions. Aliasing due to subsampling on a coarse grid and the effects of spatial smoothing are addressed in the last part of this thesis. It is shown that this aliasing could be greatly reduced by spatial smoothing. The estimated horizontal circulation from the spatially smoothed time-mean EGCM hydrographic data with a coarse grid resolution (2.4° longitude by 2.0° latitude) is generally consistent with the spatially smoothed EGCM time means. Significant differences only occur at some grid points at great depths, where the GCM circulations are very weak. The conclusions of this study are different from some previous studies. These discrepancies are explained in the concluding chapter. Finally, it should be pointed out that the issue of properly representing a GCM ocean by an inverse model is not identical to the issue of represent ing the real ocean by the same inverse model, since the GCM ocean is not identical to the real ocean. Numerical calculations show that both the non-eddy resolving and the eddy-resolving GCM oceans used in this work are evolving towards a statistical equilibrium. In the real ocean, the importance of temporal variation terms in the property conservation equations should also be analyzed when a steady mverse model is applied to a limited time-mean (the climatological) data set.

Description: This research was carried out under National Science Foundation grant OCE- 90-04396.

Description: Submitted in partial fulfillment of the requirements for the degree of Master of Science at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution September 1991

Description: Based on the Levitus atlas, we find that the application of the Montgomery streamfunction to the isopycnal surfaces induces an error which can not be ignored in some regions in the ocean. The error arises from the sloping effect of the specific volume anomaly along isopycnal surfaces. By including the major part of this effect, new streamfunctions, namely the pressure anomaly and main pressure streamfunctions, are suggested for the use in potential density coordinates. By using the newly proposed streamfunction and by including the variations of specific volume anomaly along isopycnal surfaces, the inverse model proposed by Hogg (1987) is modified for increasing accuracy and applied to the Brazil Basin to study the circulation, diffusion and water mass balances. The equations in the model, i.e. the dynamic equation, continuity equation, integrated vorticity equation, and conservation equations for heat, salt and oxygen (in which a consumption sink term is allowed), are written in centered finite difference form with lateral steps of 2 degree latitude and longitude and 8 levels in the vertical. This system of equations with constraints of positive diffusivities and oxygen consumption rates is solved by the inverse method. The results indicate that the circulation in the upper oceans is consistent with previous works, but that in the deep ocean is quite different. In the NADW region, we find a coincidence of the flows with the tongues of water properties. The diffusivities and diapycnal velocities seem stronger in the region near the equator than in the south, with reasonable values. Diffusion plays an important role in the water mass balance. Examples show that similar property fields may results from different processes.

Description: Submitted in partial fulfillment of the requirements for the degree of Master of Science at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution July 1991

Description: Six hydrographic basinwide sections, two in each of the three major ocean basins, are employed in a set of inverse calculations to determine the extent of exchange between the Pacific and Indian Oceans through the Indonesian Archipelago and the net global oceanic heat flux at 30°S. Using a model which combines the data for the South Pacific and South Indian Oceans, it is found that even the largest existing estimates of Indonesian Passage through flow (20 Sv) are consistent with the data. However, the available information cannot limit the extent of the exchange, i.e. both smaller and larger through flows produce physically reasonable circulation patterns. The seasonal and interannual variations which have been found by other investigators and which we are incapable of resolving, lead us to conclude that in the long term mean an estimate of ~10 Sv for the through flow is most reasonable. Globally, at 30°S, we find a net oceanic heat flux of -1.1 ± 1.7 PW, which is not significantly different from zero. It is dominated by a large (〉1 PW) southward heat flux in the Indian Ocean. Large equatorward (~0.8 PW) heat flux values in the South Atlantic Basin are not consistent with our data. We therefore conclude that although our data are consistent with some water following the warm water return path for NADW (Gordon 1986), the cold water path must play the dominant role in the maintenance of the global thermohaline cell associated with the formation process of NADW.

Description: Submitted in partial fulfillment of the requirements for the degree of Master of Science at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution August 1991

Description: Undersea technology is on the verge of equipping remotely operated vehicle (ROV) pilots with a three-dimensional (3-D), real-time display incorporating data from a wide variety of sensors including sonar (sound navigation and ranging), cameras, and lasers. Effective collection, computation, and presentation of this data to the pilot in a single display presents hardware, software and human factors problems. This thesis focuses on human factors issues associated with the display of information which could enhance the pilot's efficiency of performance. Background information on human factors engineering, 3-D computer graphics displays, and application of the 3-D perspective display precede the details of the experiment. Five specific display enhancements tested include altering the displayed field of view, providing a screen grid, displaying the current range to the target of interest, using a vertical color scheme, and controlling the display update rate. Seven tests measure the effects of these display enhancements on the simulated piloting of an ROV. The effects of the ROV simulation and operator learning curves are removed to compare performance changes due to the various enhancements directly. Operator comments during and after testing as well as test monitor/author observations provide insight into the experiment Test result implications for system design trade-offs are discussed in detail. Recommendations for future research and the proposed construction of a fully equipped ROV simulator complete the work.

Description: Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution May 1998

Description: Observations of bedforms, suspended sediment and water velocities were used to examine sediment transport processes at the sandy LE0-15 site located off the New Jersey coast. The bedforms were observed during storms using a rotary sidescan sonar and were found to be wave orbital scale ripples. The onshore migration of these ripples was forced by non-linear wave velocities, and could be related to a simple bedload model. Observations of suspended sand flux were calculated from acoustic backscattering profiles and water velocity profiles. Suspended sand transport forced by wave velocities was found to occur primarily during the weaker offshore phase of wave motion, as part of a vortex ejection mechanism. This net offshore suspended sediment flux was an order of magnitude less than the flux associated with onshore ripple migration. Thus it is hypothesized that ripple migration was forced by unobserved bedload or near bottom suspended flux. The net suspended sediment flux due to mean currents was a factor of five less than the waveforced offshore suspended flux. These wave dominated events at LE0-15 represent a contradiction of the conceptual idea that waves are primarily responsible for suspending sediment and mean currents provide the transport mechanism.

Description: Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution August 1991

Description: The oceanic distributions of tritium 3H), 3He, and the chlorofluorocarbons (CFCs) can be used to constrain the time-scales of the major ventilation pathways for an ocean basin such as the North Atlantic. I present a new global model function, developed from a factor analysis of the WMO/IAEA data set, for predicting the spatial and temporal variability of bomb-tritium in precipitation. Model estimates for the atmospheric 3H delivery to the North Atlantic are recomputed and combined with advective 3H input estimates in a budget for the North Atlantic Basin. Key features of the model budget include refined estimates of the 3H vapor flux and southward advection of 3H in the low salinity, surface flow from the Arctic. Arctic tritium sources contribute about half of the observed increase (40%) in the decay corrected tritium inventory from the 1972 GEOSECS program and the 1981 TTO/NAS program. The 3H concentration in the intermediate and deep waters for the sub-polar North Atlantic increased substantially between 1972 and 1981. A time dependent model for the 3H and 3He inflow to the abyssal Atlantic from the Nordic Seas is developed. The 3H and 3He distributions in the abyssal North Atlantic and Deep Western Boundary Current (DWBC) are also presented. A simple model of abyssal circulation is constructed using the model Nordic Seas overflow curves, the observed tracer gradients in the DWBC, and the GEOSECS and TTO tracer inventories for the deep basins. Although the tracer concentrations in the boundary current are rather insensitive to the velocity of the boundary current, they do place bounds on the magnitude of recirculation between the boundary current and the interior. On average, a volume equal to the boundary current transport is entrained/detrained over a length scale of about 5000 km. About half of the overflow water entering the western basin of North Atlantic since the mid-1960's has been mixed into the deep Labrador Sea and subpolar gyre. The effects of tracer surface boundary conditions on thermocline ventilation and oxygen utilization rate estimates are discussed. Tracers that equilibrate rapidly with the atmosphere, such as 3He and the CFCs, have lower apparent ventilation time scales than t racers, such as tritium and radiocarbon, t hat are reset slowly in the surface layer. The results of a simple box-mixing model are compared with tritium and 3He data from a 1979 survey of the eastern subtropical North Atlantic. On shallow density surfaces, the computed tritium ventilation rates are two to three times slower than those for 3He; deeper in the thermocline, the two tracer ventilation rates converge. This trend may be related to the decreasing effectiveness of 3He gas exchange in equilibrating the deeper winter mixed layers of the more northerly isopycnal outcrops. Box models using limited surface exchange tracers (e.g. tritium and 14C) can under predict oxygen utilization rates (OUR) by up to 3 times due to differences between tracer and oxygen boundary conditions while 3He may overestimate OUR by 10- 20%. I present and discuss the distributions of two chlorofluorocarbons (CFCs) in the eastern North Atlantic measured on a 1988 hydrographic cruise between Iceland and the equator. CFC tagged seawater fills the entire sub-polar water column and subtropical thermocline. Measurable CFC levels are found at the ocean bottom as far south as 35°N; the CFC penetration depth shoals to about 750 meters in the tropics. The CFC data are used to illustrate the ventilation time-scales for the water masses in the eas tern basin and to calculate OUR values in the subtropical thermocline. The CFC data in the tropical oxygen minimum off of Africa are significantly lower than the values on similar density surfaces in the subtropics, providing support for the idea that the tropical oxygen minima are controlled primarily by physical rather than biological mechanisms. The evolution of the tropical and subtropical CFC distributions between the 1972-73 TTO/TAS program and the 1988 cruise are also examined. Other features of the CFC data include a clear signal of Labrador Sea Water mid-depth ventilation, a CFC-enriched overflow water boundary current along the Iceland slope, a northward flowing deep boundary current along the eastern margin of the basin, and a mid-depth equatorial plume of upper North Atlantic Deep Water.

Description: The work carried out in my thesis has been supported in part by a National Science Foundation Graduate Fellowship and by grants OCE-8615289 and OCE-8800957 from the National Science Foundation.

Description: Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution March 1988

Description: Inverse methods are applied to historical hydrographic data to address two aspects of the general circulation of the Atlantic Ocean. The method allows conservation statements for mass and other properties, along with a variety of other constraints, to be combined in a dynamically consistent way to estimate the absolute velocity field and associated property transports. The method is first used to examine the exchange of mass and heat between the South Atlantic and the neighboring ocean basins. The Antarctic Circumpolar Current (ACC) carries a surplus of intermediate water into the South Atlantic through Drake Passage which is compensated by a surplus of deep and bottom water leaving the basin south of Africa. As a result, the ACC loses .25±.18x1015 W of heat in crossing the Atlantic. At 32°S the meridional flux of heat is .25±.19x1015 W equatorward, consistent in sign but smaller in magnitude than other recent estimates. This heat flux is carried primarily by a meridional overturning cell in which the export of 17 Sv of North Atlantic Deep Water (NADW) is balanced by an equatorward return flow equally split between the surface layers, and the intermediate and bottom water. No "leak" of warm Indian Ocean thermocline water is necessary to account for the equatorward heat flux across 32°S; in fact, a large transfer of warm water from the Indian Ocean to the Atlantic is found to be inconsistent with the present data set. Together these results demonstrate that the Atlantic as a whole acts to convert intermediate water to deep and bottom water, and thus that the global thermohaline cell associated with the formation and export of NADW is closed primarily by a "cold water path," in which deep water leaving the Atlantic ultimately returns as intermediate water entering the basin through Drake Passage. The second problem addressed concerns the circulation and property fluxes across 24°and 36°N in the subtropical North Atlantic. Conservation statements are considered for the nutrients as well as mass, and the nutrients are found to contribute significant information independent of temperature and salinity. Silicate is particularly effective in reducing the indeterminacy of circulation estimates based on mass conservation alone. In turn, the results demonstrate that accurate estimates of the chemical fluxes depend on relatively detailed knowledge of the circulation. The zonal-integral of the circulation consists of an overturning cell at both latitudes, with a net export of 19 Sv of NADW. This cell results in a poleward heat flux of 1.3±.2x1015 Wand an equatorward oxygen flux of 2900±180 kmol S-l across each latitude. The net flux of silicate is also equatorward: 138±38 kmol s-1 and 152±56 kmol s -1 across 36°and 24° N, respectively. However, in contrast to heat and oxygen, the overturning cell is not the only important mechanism responsible for the net silicate transport. A horizontal recirculation consisting of northward flow of silica-rich deep water in the eastern basin balanced by southward flow of low silica water in the western basin results in a significant silicate flux to the north. The net equatorward flux is thus smaller than indicated by the overturning cell alone. The net flux of nitrate across 36°N is n9±35 kmol 8- 1 to the north and is indistinguishable from zero at 24°N (-8±39 kmol 8-1 ), leading to a net divergence of nitrate between these two latitudes. Forcing the system to conserve nitrate leads to an unreasonable circulation. The dominant contribution to the nitrate flux at 36°N results from the correlation of strong northward flow and relatively high nitrate concentrations in the sub-surface waters of the Gulf Stream. The observed nitrate divergence between 24°and 36°N, and convergence north of 36°N, can be accounted for by a shallow cell in which the northward flow of inorganic nitrogen (nitrate) in the Gulf Stream is balanced by a southward flux of dissolved organic nitrogen in the recirculation gyre. Oxidation of the dissolved organic matter during its transit of the subtropical gyre supplies the required source of regenerated nitrate to the Gulf Stream and consumes oxygen, consistent with recent observations of oxygen utilization in the Sargasso Sea.

Description: This research was supported by NASA under contract NAG5-534 and NSF under contract OCE-8521685.

Description: Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution June 1998

Description: A freshwater plume often forms when a river or an estuary discharges water onto the continental shelf. Freshwater plumes are ubiquitous features of the coastal ocean and usually leave a striking signature in the coastal hydrography. The present study combines both hydrographic data and idealized numerical simulations to examine how ambient currents and winds influence the transport and mixing of plume waters. The first portion of the thesis considers the alongshore transport of freshwater using idealized numerical simulations. In the absence of any ambient current, the downstream coastal current only carries a fraction of the discharged fresh water; the remaining fraction recirculates in a continually growing "bulge" of fresh water in the vicinity of the river mouth. The fraction of fresh water transported in the coastal current is dependent on the source conditions at the river mouth. The presence of an ambient current augments the transport in the plume so that its freshwater transport matches the freshwater source. For any ambient current in the same direction as the geostrophic coastal current, the plume will evolve to a steady-state width. A key result is that an external forcing agent is required in order for the entire freshwater volume discharged by a river to be transported as a coastal current. The next section of the thesis addresses the wind-induced advection of a river plume, using hydrographic data collected in the western Gulf of Maine. The observations suggest that the plume's cross-shore structure varies markedly as a function of fluctuations in alongshore wind forcing. Consistent with Ekman dynamics, upwelling favorable winds spread the plume offshore, at times widening it to over 50 km in offshore extent, while downwelling favorable winds narrow the plume width to a few Rossby radii. Near-surface current meters show significant correlations between cross-shore currents and alongshore wind stress, consistent with Ekman theory. Estimates of the terms in the alongshore momentum equation calculated from moored current meter arrays also indicate an approximate Ekman balance within the plume. A significant correlation between alongshore currents and alongshore wind stress suggests that interfacial drag may be important. The final section of the thesis is an investigation of the advection and mixing of a surface-trapped river plume in the presence of an upwelling favorable wind stress, using a three-dimensional model in a simple, rectangular domain. Model simulations demonstrate that the plume thins and is advected offshore by the crossshore Ekman transport. The thinned plume is susceptible to significant mixing due to the vertically sheared horizontal currents. The first order plume response is explained by Ekman dynamics and a Richardson number mixing criterion. Under a sustained wind event, the plume evolves to a quasi-steady, uniform thickness. The rate of mixing slowly decreases for longer times as the stratification in the plume weakens, but mixing persists under a sustained upwelling wind until the plume is destroyed. Mixing is most intense at the seaward plume front due to an Ekman straining mechanism in which the advection of cross-shore salinity gradients balances vertical mixing. The mean mixing rate observed in the plume is consistent with the mixing power law suggested by previous studies of I-D mixing, in spite of the two-dimensional dynamics driving the mixing in the plume.

Description: This research was funded by a National Science Foundation graduate fellowship, and Gulf of Maine Regional Marine Research Program grants UM-S227 and UM-S276.

Description: Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution June 2011

Description: Eastern oceanic boundary currents are subject to hydrodynamic instability, generate small scale features that are visible in satellite images and may radiate westward into the interior, where they can be modified by the large-scale circulations. This thesis studies the stability of an eastern boundary current with and without the large-scale flow influence in an idealized framework represented by barotropic quasi-geostrophic dynamics. The linear stability analysis of a meridional current with a continuous velocity profile shows that meridional eastern and western boundary currents support a limited number of radiating modes with long meridional and zonal wavelengths and small growth rates. However, the linearly stable, long radiating modes of an eastern boundary current can become nonlinearly unstable by resonating with short trapped unstable modes. This phenomenon is clearly demonstrated in the weakly nonlinear simulations. Results suggest that linearly stable longwave modes deserve more attention when the radiating instability of a meridional boundary current is considered. A large-scale flow affects the short trapped unstable mode and long radiating mode through different mechanisms. The large-scale flow modifies the structure of the boundary current to stabilize or destabilize the unstable modes, leading to a meridionally localized maximum in the perturbation kinetic energy field. The shortwave mode is accelerated or decelerated by the meridional velocity adjustment of the large-scale flow to have an elongated or a squeezed meridional structure, which is confirmed both in a linear WKB analysis and in nonlinear simulations. The squeezed or elongated unstable mode detunes the nonlinear resonance with the longwave modes, which then become less energetic. These two modes show different meridional structures in kinetic energy field because of the different mechanisms. In spite of the model simplicity, these results can potentially explain the formation of the zonal jets observed in altimeter data, and indicate the influence of the large-scale wind-driven circulation on eastern boundary upwelling systems in the real ocean. Studies with more realistic configurations remain future challenges.

Description: Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution February 1999

Description: This thesis studies the problems of generation and maintenance of recirculations by Gulf Stream instabilities. Observations show that the horizontal structure of the jet and its recirculations suffer significant changes in time. Here, the role of internal dynamics of the jet is isolated as one of the possible sources of such variability, and the differences between barotropic and baroclinic instabilities are investigated. The problem of recirculation development is considered in a framework of a free spin down of the 2-layer and the 1-layer, zonally symmetric, quasi-geostrophic jets. Linear stability analysis shows that in strongly baroclinic basic flows, eddies are capable of driving recirculations in the lower layer through the residual meridional circulation. In strongly barotropic jets, the linearly most unstable wave simply diffuses the jet. Nonlinear stability analysis indicates that recirculations are robust features of the 2-layer model. The strength of recirculations is a function of the model’s parameters. It increases with a decrease in the value of the nondimensional /3 due to potential vorticity homogenization constrained by enstrophy conservation. The recirculation strength is a non-monotonic function of the baroclinic velocity parameter; it is the strongest for strongly baroclinic basic flows, weakest for flows with intermediate baroclinic structure and of medium strength for strongly barotropic basic flows. Such non-monotonic behavior is the result of two different processes responsible for the recirculation development: linear eddy-mean flow interactions for strongly baroclinic basic flows and strongly nonlinear eddy-eddy and eddy-mean flow interaction for strongly barotropic flows. In the case of the reduced-gravity model, recirculations develop only for infinite deformation raduis. Basic flows with finite deformation radius are only weakly supercritical and therefore produced negligible recirculations after equilibration. The problem of maintenance of the recirculations is coupled to the questions of existence of low frequency variability and of multiple dynamical regimes of a system consisting of a quasi-geostrophic jet and its recirculations. The problem is studied in a framework of a 2-layer or a reduced-gravity colliding jets model which has no windforcing. Instead, it is forced by inflows and outflows through the open boundaries. Oniy the western boundary of the domain is closed, and a free slip boundary condition is used there. The results of the numerical experiments show that when oniy the mechanism of barotropic instability is present, the model has two energy states for a wide range of interfacial friction coefficients. The high energy state is characterized by well-developed recirculations and displays strong variability associated with either large recirculating gyres and a weak eddy field or small recirculations and a strong eddy field. The iow energy state is characterized by large meridional excursions in the separation point and large amplitude, westward propagating meanders that produce strong rings after interacting with the western wall. For physically relevant bottom friction values, the presence of baroclinic in stability in the recirculation regions of the 2-layer model allows for a unique dynamical regime characterized by well-developed recirculations in both layers. The low-frequency variability associated with the regime is weak and is related to meridional shifts in the position of the jet, to wrapping of the recirculations around each other, and to pulsations in their zonal extent. For strong bottom friction, the 2-layer model has only the mechanism of barotropic instability which reduces it to a 1 1/2-layer configuration; the model displays two dynamical regimes and strong low frequency variability in the upper layer, while the lower layer is strongly frictional.

Description: Financial support for this research was provided by NSF grant number OCE 9617848.

Description: Submitted in partial fulfillment of the requirements for the degree of Master of Science at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution January 1999

Description: Today, deep waters produced in the North Atlantic are exported through the western South Atlantic. Antarctic intermediate water AAJW also enters the Atlantic in this region. Circumpolar deep water (CDW) fills the depths below AAIW and above and below northern source waters. A depth transect of cores from 1567-3909 m water depth in the western South Atlantic are ideally located to monitor inter-ocean exchange of deep water, and variations in the relative strength of northern versus southern source water production. Last glacial maximum (LGM) Cd/Ca and δ13C data indicate a nutrient-depleted intermediate-depth water mass. In the mid-depth western South Atlantic, a simple conversion of LGM δ13C data suggests significantly less nutrient enrichment than LGM Cd/Ca ratios, but Cd/Ca and δ13C data can be reconciled when plotted in CdW/δ13C space. Paired LGM Cd/Ca and δ13C data from mid-depth cores suggest increasingly nutrient rich waters below 2000 m, but do not require an increase in Southern Ocean water contribution relative to today. Cd/Ca data suggest no glacial-interglacial change in the hydrography of the deepest waters ofthe region. To maintain relatively low Cd/Ca ratios low nutrients in the deepest western South Atlantic waters, and in CDW in general, during the LGM requires an increased supply ofnutrient-depleted glacial North Atlantic intermediate water (GNA1W) and/or nutrient-depleted glacial Subantarctic surface waters to CDW to balance reduced NADW contribution to CDW. LGM Cd/Ca and δ13C data suggest strong GNA1W influence in the western South Atlantic which in turn implies export of GNAIW from the Atlantic, and entrainment of GNA1W into the Antarctic Circumpolar current.

Description: Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution May 1999

Description: This thesis focuses on improving the productivity of autonomous and telemanipulation systems consisting of a manipulator arm mounted to a free flying underwater vehicle. Part I minimizes system sensitivity to misalignment by developing a gripper and a suite of handles that passively self align when grasped. After presenting a gripper guaranteed to passively align cylinders we present several other self aligning handles. The mix of handle alignment and load resisting properties enables handles to be matched to the needs of each task. Part I concludes with a discussion of successful field use of the system on the Jason Remotely Operated Undersea Vehicle operated by the Woods Hole Oceanographic Institution. To enable the exploitation of contact with the environment to help stabilize the vehicle, Part II develops a technique which identifies the contact state of a planar vehicle interacting with a fixed environment. Knowing the vehicle geometry and velocity we identify kinematically feasible contact points, from which we construct the set of feasible contact models. The measured vehicle data violates each model’s constraints; we use the associated violation power and work to select the best overall model. Part II concludes with experimental confirmation of the contact identification techniques efficacy.

Description: Submitted in partial fulfillment of the requirements for the degree of Master of Engineering at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution June 1999

Description: A review of six methods for deriving sediment quality criteria SQC was conducted. Each approach was analyzed according to cost, difficulty, applicability and reliability. Results of this analysis led to the recommendation that the state of Massachusetts should use a combination of the Equilibrium Partitioning modeling approach and the Threshold Effects Level/Probable Effects Level correlative approach to SQC derivation. Criteria should then be applied as screening values for evaluation of sediment toxicity. One significant component of sediment quality that all criteria approaches lack is the bioaccumulation/biomagnification of contaminants. Nor are there accurate, easily implemented models for benthically-coupled organisms such as Mya arenaria. In this document a bioaccumulation model for PAHs and PCBs was developed. The model considers contaminant partitioning into organism lipid and protein and incorporates organism exposure to contaminants through sediment and ingestion of contaminated food particles. Results show the model predicts PCB accumulation in M. arenaria with only a slight variation from observed data. Partitioning into sedimentary soot fraction was added to the model for PARs. Results showed an increase in model accuracy, but predicted concentrations still remain greater than observed concentrations. A combination of sediment quality criteria used as screening criteria and the bioaccumulation model for M.arenaria will improve the accuracy of site assessment of PAH and PCB compounds.

Description: Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution June 1998

Description: The evolution of a coastal ocean undergoing uniform surface heat loss is examined. The dynamics of this ocean are initially modulated by the intense vertical mixing driven by surface cooling. The strong vertical mixing prevents the formation of geostrophic flows and inhibits the cross-shelf flux of heat. The vertical mixing is eventually suppressed by the advective transport of cold, dense water offshore. Once this happens, alongshore geostrophic flows form, and become baroclinically unstable. The surface heat flux is then balanced by a cross-shelf eddy heat flux. Scales are found for the cross-shelf density gradient which results from this balance. Solutions for linear internal waves are found for a wedge-shaped bathymetry with bottom friction. Bottom friction is capable of entirely dissipating the waves before they reach the coast, and waves traveling obliquely offshore are reflected back to the coast from a caustic. The internal wave climate near two moorings of the Coastal Ocean Dynamics Experiment observation program is analyzed. The high frequency internal wave energy levels were elevated above the Garrett and Munk spectrum, and the spectrum becomes less red as one moves to the shore. The wave field is dominated by vertical-mode one waves, and internal wave energy propagates shoreward.

Description: This work was funded by an Office of Naval Research fellowship and and Office of Naval Research AASERT fellowship, N00014-95:-1-0746.

Description: Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution January 1988

Description: This study focuses on the zonal weakening, eastern termination and seasonal variations of the Atlantic equatorial undercurrent (EUC). The main and most original contribution of the dissertation is a detailed analysis of the Atlantic EUC simulated by Philander and Pacanowski's (1986) general circulation model (GCM), which provides a novel description of the dynamical regimes governing various regions of a nonlinear stratified undercurrent. Only in a narrow deep western region of the simulation does one find an approximately inertial regime corresponding to zonal acceleration. Elsewhere frictional processes cannot be ignored. The bulk of the mid-basin model EUC terminates in the overlying westward surface flow while only a small fraction (the deeper more inertial layers) terminates at the eastern coast. In agreement with observations, a robust feature of the GCM not present in simpler models is the apparent migration of the EUC core from above the thermocline in the west to below it in the east. In the GCM, this happens because the eastward flow is eroded more efficiently by vertical friction above the base of the thermocline than by lateral friction at greater depths. This mechanism is a plausible one for the observed EUC. A scale analysis using a depth scale which decreases with distance eastwards predicts the model zonal transition between western inertial and eastern inertio-frictional regimes. Historical and recent observations and simple models of the equatorial and coastal eastern undercurrents are reviewed, and a new analysis of current measurements in the eastern equatorial Atlantic is presented. Although the measurements are inadequate for definitive conclusions, they suggest that Lukas' (1981) claim of a spring surge of the Pacific EUC to the eastern coast and a seasonal branching of the EUC into a coastal southeastward undercurrent may also hold for the Atlantic Ocean. To improve the agreement between observed and modelled strength of the eastern undercurrent, it is suggested that the eddy coefficient of horizontal mixing should be reduced in future GCM simulations.

Description: This work was supported by NSF grants OCE82-14771, OCE82-08744 and OCE85-14885.

Description: Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution June 1998

Description: The steady states of two models of the double-gyre wind-driven ocean circulation are studied. The link between the steady state solutions of the models and their time-mean and low-frequency variability is explored to test the hypothesis that both stable and unstable fixed points influence shape the model's attractor in phase space. The steady state solutions of a barotropic double-gyre ocean model in which the wind-stress curl input of vorticity is balanced primarily by bottom friction are studied. The bifurcations away from a unique and stable steady state are mapped as a function of two nondimensional parameters, (δI,δS), which can be thought of as measuring respectively the relative importance of the nonlinear advection and bottom damping of relative vorticity to the advection of planetary vorticity. A highly inertial branch characterized by a circulation with transports far in excess of those predicted by Sverdrup balance is present over a wide range of parameters including regions of parameter space where other solutions give more realistic flows. For the range of parameters investigated, in the limit of large Reynolds number, δI,δS → ∞, the inertial branch is stable and appears to be unique. This branch is anti-symmetric with respect to the mid-basin latitude like the prescribed wind-stress curl. For intermediate values of δI,δS, additional pairs of mirror image non-symmetric equilibria come into existence. These additional equilibria have currents which redistribute relative vorticity across the line of zero wind-stress curl. This internal redist~ibution of vorticity prevents the solution from developing the large transports that are necessary for the anti-symmetric solution to achieve a global vorticity balance. Beyond some critical Reynolds number, the nonsymmetric solutions are unstable to time-dependent perturbations. Time-averaged solutions in' this parameter regime have transports comparable in magnitude to those of the non-symmetric steady state branch. Beyond a turning point, where the non-symmetric steady state solutions cease to exist, all the computed time-dependent model trajectories converge to the anti-symmetric inertial runaway solution. The internal compensation mechanism which acts through explicitly simulated eddies is itself dependent explicit dissipation parameter. Using the reduced-gravity quasigeostrophic model an investigation of the link between the steady state solutions and the model's low-frequency variability is conducted. If the wind-stress curl is kept anti-symmetric, successive pairs of non-symmetric equilibria come into existence via symmetry-breaking pitchfork bifurcations as the model's biharmonic viscosity is reduced. Succesive pairs of mirror image equilibria have an additional half meander in the jet. The distinct energy levels of the steady state solutiOris can be understood in part by there different inter-gyre fluxes of vorticity. Those solutions with weak inter-gyre fluxes of vorticity have large and energetic recirculation cells which remove excess vorticity through bottom friction. Those solutions with strong inter-gyre fluxes of vorticity have much smaller and ·less energetic recirculation cells. A significant fraction of the variance (30%) of the interface height anomaly can be accounted by four coherent structures which point away from the time-mean state and towards four steady state solutions in phase space. After removing the variance which projects onto the four modes, the remaining variance is reduced predominantly at low-frequencies, showing that these modes are linked to the low-frequency variability of the model. Furthermore, the time-averaged flow fields within distinct energy ranges show distinct patterns which are in turn similar to the distinct steady state solutions.

Description: Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution August 1998

Description: Double-diffusive processes are studied and parameterized, and their impacts on the oceanic thermohaline circulation are investigated by single-hemisphere numerical models and scaling analysis. Scaling analysis on the thermohaline circulation has been done under three types of surface boundary conditions. (a) Under "relaxation" conditions, there is a two-thirds power law dependence of the meridional overturning rate (and the poleward heat transport) on the diapycnal diffusivity. For any given external forcing, there is only one equilibrium state for the thermohaline circulation. (b) Under "flux" boundary conditions, there is a half power law dependence of the meridional overturning rate on the diapycnal diffusivity. Only one mode is possible for given external forcing. (c) Under "mixed" boundary conditions, multiple equilibria become possible. For given thermal forcing, the existence of multiple equilibria depends on the relative contributions of diapycnal diffusivity and the hydrologic forcing. Numerical experiments are implemented to test the above scaling arguments. Consistent results have been obtained under the above three types of boundary conditions. These provide a basis for understanding how the thermohaline circulation depends on the diapycnal diffusivity, which we know is influenced by the double-diffusive processes of "salt fingering" and "diffusive layering" in some parts of the ocean. In order to examine this issue, the double-diffusive processes are parameterized by diapycnal eddy diffusivities for heat and salt that are different and depend on the local density ratio, Rp= αTz/βSz. A background diffusivity is applied to represent turbulent mixing in the stratified environment. The implementation of this double-diffusive - parameterization in numerical models has significant impacts on the thermohaline circulation. (a) Under "relaxation" boundary conditions, the meridional overturning rate and the poleward heat transport are reduced, and water mass properties are also changed. Similar results are obtained under "flux" boundary conditions. (b) Under "mixed" boundary conditions, the critical freshwater flux for the existence of the thermal mode becomes smaller with the double-diffusive parameterization. The extent to which the thermohaline circulation is affected by double-diffusive processes depends on the magnitude of the freshwater forcing.

Description: This thesis is supported by a grant from the Ocean Sciences Division of the National Science Foundation, OCE94-155S9.

Description: Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution February 2011

Description: The sinking flux of particulate matter into the ocean interior is an oceanographic phenomenon that fuels much of the metabolic demand of the subsurface ocean and affects the distribution of carbon and other elements throughout the biosphere. In this thesis, I use a new suite of observations to study the dynamics of marine particulate matter at the contrasting sites of the subtropical Sargasso Sea near Bermuda and the waters above the continental shelf of the Western Antarctic Peninsula (WAP). An underwater digital camera system was employed to capture images of particles in the water column. The subsequent analysis of these images allowed for the determination of the particle concentration size distribution at high spatial, depth, and temporal resolutions. Drifting sediment traps were also deployed to assess both the bulk particle flux and determine the size distribution of the particle flux via image analysis of particles collected in polyacrylamide gel traps. The size distribution of the particle concentration and flux were then compared to calculate the average sinking velocity as a function of particle size. I found that the average sinking velocities of particles ranged from about 10-200 m d-1 and exhibited large variability with respect to location, depth, and date. Particles in the Sargasso Sea, which consisted primarily of small heterogeneous marine snow aggregates, sank more slowly than the rapidly sinking krill fecal pellets and diatom aggregates of the WAP. Moreover, the average sinking velocity did not follow a pattern of increasing velocities for the larger particles, a result contrary to what would be predicted from a simple formulation of Stokes’ Law. At each location, I derived a best-fit fractal correlation between the flux size distribution and the total carbon flux. The use of this relationship and the computed average sinking velocities enabled the estimation of particle flux from measurements of the particle concentration size distribution. This approach offers greatly improved spatial and temporal resolution when compared to traditional sediment trap methods for measuring the downward flux of particulate matter. Finally, I deployed specialized in situ incubation chambers to assess the respiration rates of microbes attached to sinking particles. I found that at Bermuda, the carbon specific remineralization rate of sinking particulate matter ranged from 0.2 to 1.1 d-1, while along the WAP, these rates were very slow and below the detection limit of the instruments. The high microbial respiration rates and slow sinking velocities in the Sargasso Sea resulted in the strong attenuation of the flux with respect to depth, whereas the rapid sinking velocities and slow microbial degradation rates of the WAP resulted in nearly constant fluxes with respect to depth.

Description: The Scurlock Bermuda Biological Station for Research Fund provided travel support to and from Bermuda. A grant from the National Science Foundation (NSF) Carbon and Water Program (06028416) enabled all the Sargasso Sea research as well as the opportunity to develop and test much of the methodology presented in this thesis. Internal awards from the WHOI Rinehart Access to the Sea Program and the WHOI Coastal Oceans Institute provided early funding that supported my first season of research in Antarctica and were instrumental in securing the larger external NSF Office of Polar Programs (OPP) Western Antarctic Peninsula Flux Project (OPP 0838866) grant for a second year of science in the region. The NSF OPP Palmer Long-Term Ecological Research Project and the Food for Benthos on the Antarctic Continental Shelf Project provided logistical support in the region. Phoebe Lam and Scott Doney’s grant from the WHOI Ocean Carbon and Climate Institute supported a semester of my time. The Henry G. Houghton Fund and the MIT Student Assistance Fund subsidized educational costs, textbooks, equipment, and travel expenses to conferences. In my first year I was supported by funding from Scott Doney’s NSF grant (OCCE-0312710).

Description: Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution June 1998

Description: Efforts to understand the Arctic system have recently focused on the role in local and global circulation of waters from the Arctic shelf seas. In this study, steady-state exchanges between the Arctic shelves and the central basins are estimated using an inverse box model. The model accounts for data uncertainty in the estimates, and quantifies the solution uncertainty. Other features include resolution of the two-basin Arctic hydrographic structure two-way shelf-basin exchange in the surface mixed layer, the capacity for shelfbreak upwelling, and recognition that most inflows enter the Arctic via the shelves. Aggregate estimates of all fluxes across the Arctic boundary, with their uncertainties, are generated from flux estimates published between 1975 and 1997. From the aggregate estimates, mass-, heat-, and salt-conserving boundary flux estimates are derived, which imply a net flux of water from the shelves to the basins of 1.2±0.4 Sv. Due primarily to boundary flux data uncertainty, constraints of mass, heat, and salt conservation alone cannot determine how much shelf-basin exchange occurs via dense overflows, and how much via the surface mixed layer. Adding δ180 constraints, however, greatly reduces the uncertainty. Dense water flux from the shelves to the basins is necessary for maintaining steady state, but shelfbreak upwelling is not required. Proper representation of external sources feeding the shelves, rather than the basins, is important to obtain the full range of plausible steady solutions. Implications of the results for the study of Arctic change are discussed.

Description: This work was supported by National Science Foundation grant OPP-9422292 as part of the Arctic System Science ARCSS program, administered by the Office of Polar Programs.

Description: Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution August 1999

Description: The thesis investigates the circulation at a 76-m deep study site on the southern flank of Georges Bank, a shallow submarine bank located between the deeper Gulf of Maine and the continental slope. Emphasis is placed on the vertical structure of the bottom boundary layer driven by the semi diurnal tides and the flow field's response to wind forcing. The observational analysis presented here is based on a combination of moored array and bottom tripod-mounted current, temperature, conductivity, and meteorological data taken between February and August 1995. Results from the bottom boundary layer analysis are compared to numerical model predictions for tidal flow over rough bottom topography. The flow response to wind forcing is examined and brought into context with the existing understanding of the wind-induced circulation in the Georges Bank region. Particular attention is given to the vertical distribution of the wind-driven currents, whose variation with background stratification is discussed and compared to observations from open ocean studies.

Description: The research presented in this thesis was generously supported by the National Science Foundation under grants OCE 93-13671 and OCE 96-32357 as part of the U.S. GLOBEC/Georges Bank Program.

Description: Submitted in partial fulfillment of the requirements for the degree of Master of Science at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution September 2011

Description: Coastal barriers are particularly susceptible to the predicted effects of accelerated of sea-level rise and the potential for increased impacts of intense storms. Over centennial scales, barriers are maintained via overtopping during storms, causing deposition of washover fans on their landward sides. This study examines three washover fans on the south shore of Martha’s Vineyard using a suite of data including vibracores, ground penetrating radar, high resolution dGPS, and LiDAR data. From these data, the volumes of the deposits were determined and range from 2.1—2.4 x 104 m3. Two overwashes occurred during Hurricane Bob in 1991. The water levels produced by this storm have a return interval of ~28 years, resulting in an onshore sediment flux of 2.4—3.4 m3/m/yr. The third washover was deposited by a nor’easter in January 1997, which has a water level return interval of ~6 years, resulting in a flux of 8.5 m3/m/yr. These fluxes are smaller than the flux of sediment needed to maintain a geometrically stable barrier estimated from shoreline retreat rates, suggesting that the barrier is not in long-term equilibrium, a result supported by the thinning of the barrier over this time interval.

Description: Funding for this research was provided by an Emery Fellowship through the Woods Hole Oceanographic Institution Academic Programs Office, the National Science Foundation (grants NSF-GEO-0815875 and NSF-OCE-0840894), and the Department of Defense Strategic Environmental Research and Development Program (contract W912HQ-09-C-0043).

Description: Submitted in partial fulfillment of the requirements for the degree of Doctor of Science at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution February 1999

Description: A state-of-the-art, high-resolution ocean general circulation model is used to estimate the time-dependent global ocean heat transport and investigate its dynamics. The north-south heat transport is the prime manifestation of the ocean’s role in global climate, but understanding of its variability has been fragmentary owing to uncertainties in observational analyses, limitations in models, and the lack of a convincing mechanism. These issues are addressed in this thesis. Technical problems associated with the forcing and sampling of the model, and the impact of high-frequency motions are discussed. Numerical schemes are suggested to remove the inertial energy to prevent aliasing when the model fields are stored for later analysis. Globally, the cross-equatorial, seasonal heat transport fluctuations are close to +4.5 x 1015 watts, the same amplitude as the seasonal, cross-equatorial atmospheric energy transport. The variability is concentrated within 200 of the equator and dominated by the annual cycle. The majority of it is due to wind-induced current fluctuations in which the time-varying wind drives Ekman layer mass transports that are compensated by depth-independent return flows. The temperature difference between the mass transports gives rise to the time-dependent heat transport. The rectified eddy heat transport is calculated from the model. It is weak in the central gyres, and strong in the western boundary currents, the Antarctic Circumpolar Current, and the equatorial region. It is largely confined to the upper 1000 meters of the ocean. The rotational component of the eddy heat transport is strong in the oceanic jets, while the divergent component is strongest in the equatorial region and Antarctic Circumpolar Current. The method of estimating the eddy heat transport from an eddy diffusivity derived from mixing length arguments and altimetry data, and the climatological temperature field, is tested and shown not to reproduce the model’s directly evaluated eddy heat transport. Possible reasons for the discrepancy are explored.

Description: Funding for this research came from the Department of Defense under a National Defense Science and Engineering Graduate Fellowship. Financial support was also contributed by the National Science Foundation through grants #OCE-9617570 and #OCE-9730071, and the Tokyo Electric Power Company through the TEPCO/MIT Environmental Research Program. The author received partial support from an MIT Climate Modeling Fellowship, made possible by a gift from the American Automobile Manufacturers Association.

Description: Submitted in partial fulfillment of the requirements for the degree of Master of Science at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution August 1988

Description: A tomographic array is placed in a 2-layer, flat bottom, steady-wind driven quasi-geostrophic circulation model to investigate whether the analysis of acoustic travel time changes can detect large-scale barotropic oscillations. Time series of sea surface elevation and upper and lower layer meridional currents are generated for comparison against a series of acoustic travel times. The spectra of these time series exhibit a broad mesoscale peak near a period of 40 days. The spectrum of the acoustic travel time contains a significant peak due to a resonant barotropic oscillation with a period of 28.6 days which is not present in the spectra of the point measurements. In this numerical model, basin-scale tomographic measurements are a better method of sensing the large-scale resonant barotropic oscillations than are conventional point measurements because the tomographic system attenuates the "noise" from the mesoscale.

Description: Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution February 2000

Description: Benthic foraminiferal δ13C, Cd/Ca, and Ba/Ca are important tools for reconstructing nutrient distributions, and thus ocean circulation, on glacial-interglacial timescales. However, each tracer has its own "artifacts" that can complicate paleoceanographic interpretations. It is therefore advantageous to measure multiple nutrient proxies with the aim of separating the various complicating effects. Zn/Ca is introduced as an important aid toward this goal. Benthic (Hoeglundina elegans) Cd/Ca ratios from the Bahama Banks indicate that the North Atlantic subtropical gyre was greatly depleted in nutrients during the last glacial maximum (LGM). A high-resolution Cd/Ca record from 965 m water depth suggests that Glacial North Atlantic Intermediate Water formation was strong during the LGM, weakened during the deglaciation, and strengthened again during the Younger Dryas cold period. Comparison of Cd/Ca and δ13C data reveals apparent short-term changes in carbon isotopic air-sea signatures. Benthic foraminiferal Zn/Ca could be a sensitive paleoceanographic tracer because deep water masses have characteristic Zn concentrations that increase about ten-fold from the deep North Atlantic to the deep North Pacific. A "core top calibration" shows that Zn/Ca is controlled by bottom water dissolved Zn concentration and, like Cd/Ca and BalCa, by bottom water saturation state with respect to calcite Since Zn/Ca responds to a different range of saturation states than Cd/Ca, the two may be used together to evaluate changes in deep water carbonate ion (CO32-) concentration. Zn/Ca and Cd/Ca ratios in the benthic foraminifer Cibicidoides wuellerstorfi exhibit large fluctuations over the past 100,000 years in a deep (3851 m) eastern equatorial Pacific sediment core. The data imply that bottom water CO32- concentrations were lowest during glacial Marine Isotope Stage 4 and highest during the last deglaciation. LGM CO32- concentrations appear to have been within a few μmol kg-1 of modern values. Deep North Atlantic Cd/Ca ratios imply much higher nutrient concentrations during the LGM. Although such data have usually been explained by a northward penetration of Southern Ocean Water (SOW), it has been suggested that they could result from increased preformed nutrient levels in the high-latitude North Atlantic or by increased aging of lower North Atlantic Deep Water (NADW). Glacial Zn/Ca data, however, require a substantially increased mixing with SOW and thus a reduction in NADW formation. Large changes in carbon isotopic air-sea exchange are invoked to reconcile benthic δ13C and trace metal data.

Description: This work was supported by a JOIlUSSAC Ocean Drilling Fellowship (subgrant JSG-CY 12-4), the R. H. Cole Ocean Ventures Fund, the Joint Program Education Office, and the National Science Foundation (grants OCE-9402804 and OCE-9503135 to W. Curry, and grant OCE-9633499 to D. Oppo).

Description: Submitted in partial fulfillment of the requirements for the degree of Master of Science at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution April 2000

Description: A simple quasi-two dimensional dynamical model of Thermohaline circulation (THC) is developed, assuming that the mixing only occurs near western and eastern boundary layers. When the surface density is prescribed, the climatically important quantities, such as the strength of overturning and meridional heat transport, are related to the zonal integral over the vigorously mixing regions and scaled as (KvΔx )2/3. The numerical results suggest that the density difference between eastern and western boundaries play an important role in the meridional overturning. The eastern boundary is characterized by the upwelling on top of downwelling. The western boundary layer is featured by the universal upwelling. The inefficiency of diffusion heat transport accounts for the narrowness of sinking region and shallowness of overturning cell in one-hemisphere. The experiments with other surface boundary conditions are also explored. The circulation patterns obtained are similar under various surface temperature distributions, suggesting these are very robust features of THC. The role of boundary mixing is further explored in global ocean. The 2 1/2 dynamical model is extended to two-hemisphere ocean. Additional dynamics such as Rayleigh friction and abyssal water properties are taken into account. A set of complicated governing equations are derived and numerically solved to obtain steady state solution. The basic circulation features are revealed in our dynamical model. An equtorially asymmetric meridional circulation is observed due to small perturbation at the surface temperature in the high latitude. The density differences between eastern and western boundaries are distinct in both hemispheres. This is achieved during the spin-up process. Although the dynamical model results agree well with OGCM results in one-hemisphere, several important dynamics are lacking and exposed in two-hemisphere experiments. We need to consider horizontal advection terms which will effectively advect positive density anomalies across the equator and form the deep water for the entire system.

Description: Submitted in partial fulfillment of the requirements for the degree of Master of Science at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution September 1999

Description: This study uses geophysical and sedimentological data collected from the Lower Hudson River estuary to identify the depositional response of the estuary to high river discharge events. Erosional and depositional environments in the estuary are identified through the use of side-scan sonar, bottom penetrating sonar and surficial sediment sampling. Sediment cores are used to document deposit thicknesses and to obtain the spatial distribution of estuarine deposits. Results show a high degree of spatial and temporal variability in sedimentation within the estuary. Two primary deposits are identified underneath the turbidity maximum for the estuary. Approximately 300,000 metric tons of sediment were deposited within these two deposits during May and June of 1998. This short-term accumulation underneath the turbidity maximum of the estuary can account for 30 to 98 percent of the estimated, river-borne sediment load supplied to the estuary during the 1997-1998 water year. Both the tidally produced stratigraphy observed in sediment cores and the spatial distribution of identified deposits, support the theory that sedimentation underneath the turbidity maximum of the estuary is primarily the results of a convergence in bottom water flow, caused by the formation of a salinity front during ebb tide.

Description: This research was funded by the Hudson River Foundation and a National Science Foundation Coastal Trainee Fellowship.

Description: Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution June 1999

Description: Data assimilation methods, such as the Kalman filter, are routinely used in oceanography. The statistics of the model and measurement errors need to be specified a priori. In this study we address the problem of estimating model and measurement error statistics from observations. We start by testing the Myers and Tapley (1976, MT) method of adaptive error estimation with low-dimensional models. We then apply the MT method in the North Pacific (5°-60° N, 132°-252° E) to TOPEX/POSEIDON sea level anomaly data, acoustic tomography data from the ATOC project, and the MIT General Circulation Model (GCM). A reduced state linear model that describes large scale internal (baroclinic) error dynamics is used. The MT method, closely related to the maximum likelihood methods of Belanger (1974) and Dee (1995), is shown to be sensitive to the initial guess for the error statistics and the type of observations. It does not provide information about the uncertainty of the estimates nor does it provide information about which structures of the error statistics can be estimated and which cannot. A new off-line approach is developed, the covariance matching approach (CMA), where covariance matrices of model-data residuals are "matched" to their theoretical expectations using familiar least squares methods. This method uses observations directly instead of the innovations sequence and is shown to be related to the MT method and the method of Fu et al. (1993). The CMA is both a powerful diagnostic tool for addressing theoretical questions and an efficient estimator for real data assimilation studies. It can be extended to estimate other statistics of the errors, trends, annual cycles, etc. Twin experiments using the same linearized MIT GCM suggest that altimetric data are ill-suited to the estimation of internal GCM errors, but that such estimates can in theory be obtained using acoustic data. After removal of trends and annual cycles, the low frequency /wavenumber (periods〉 2 months, wavelengths〉 16°) TOPEX/POSEIDON sea level anomaly is of the order 6 cm2. The GCM explains about 40% of that variance. By covariance matching, it is estimated that 60% of the GCM-TOPEX/POSEIDON residual variance is consistent with the reduced state linear model. The CMA is then applied to TOPEX/POSEIDON sea level anomaly data and a linearization of a global GFDL GCM. The linearization, done in Fukumori et al.(1999), uses two vertical mode, the barotropic and the first baroclinic modes. We show that the CMA method can be used with a global model and a global data set, and that the estimates of the error statistics are robust. We show that the fraction of the GCMTOPEX/ POSEIDON residual variance explained by the model error is larger than that derived in Fukumori et al.(1999) with the method of Fu et al.(1993). Most of the model error is explained by the barotropic mode. However, we find that impact of the change in the error statistics on the data assimilation estimates is very small. This is explained by the large representation error, i.e. the dominance of the mesoscale eddies in the TIP signal, which are not part of the 20 by 10 GCM. Therefore, the impact of the observations on the assimilation is very small even after the adjustment of the error statistics. This work demonstrates that simultaneous estimation of the model and measurement error statistics for data assimilation with global ocean data sets and linearized GCMs is possible. However, the error covariance estimation problem is in general highly underdetermined, much more so than the state estimation problem. In other words there exist a very large number of statistical models that can be made consistent with the available data. Therefore, methods for obtaining quantitative error estimates, powerful though they may be, cannot replace physical insight. Used in the right context, as a tool for guiding the choice of a small number of model error parameters, covariance matching can be a useful addition to the repertory of tools available to oceanographers.

Description: This research was supported by NASA through Earth Science Fellowship under contract number NGTS-30084, Global Change Science Fellowship under contract number NGT-30309, contract NAG 5-3274, and NASA-JPL under contract number 958125.

Description: Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution December 1985

Description: Samples from time-series sediment traps deployed in three distinct oceanographic settings (North Pacific, Panama Basin, and Black Sea) provide strong evidence for rapid settling of marine particles by aggregates. Particle water column residence times were determined by measuring the time lag between the interception of a flux event in a shallow trap and the interception of the same event in a deeper trap at the same site. Effective sinking speeds were determined by dividing the vertical offset of the traps (meters) by the interception lag time (days). At station Papa in the North Pacific, all particles settle at 175 m day-1, regardless of their composition, indicating that all types of material may be settling in common packages. Evidence from the other two sites (Panama Basin and Black Sea) shows that particle transport may be vertical, lateral, or a combination of directions, with much of the Black Sea flux signal being dominated by lateral input. In order to ascertain whether marine snow aggregates represent viable transport packages, surveys were conducted of the abundance of these aggregates at several stations in the eastern North Atlantic and Panama Basin using a photographic technique. Marine snow aggregates were found in concentrations ranging from ~1 mm3 liter-1 to more than 500 mm3 liter-1. In open ocean environments, abundances are higher near the surface (production) and decline with depth (decomposition). However, in areas near sources of deep input of resuspended material, concentrations reach mid-water maxima, reflecting lateral transport. A model is proposed to relate the observed aggregate abundances, time series sediment flux and inferred circulation. In this model, depthwise variations in sediment flux and aggregate abundance result from suspension from the sea floor and lateral transport of suspended aggregates which were produced or modified on the sea floor. Temporal changes in sediment flux result from variations in the input of fast-sinking material which falls from the surface, intercepts the suspended aggregates, and transports them to the sea floor. A new combination sediment trap and camera system was built and deployed in the Panama Basin with the intent of measuring the flux of marine snow aggregates. This device consists of a cylindrical tube which is open at the top and sealed at the bottom by a clear plate. Material lying on the bottom plate is illuminated by strobe lights mounted in the wall of the cylinder and photographed by a camera which is positioned below the bottom plate. Flux is determined as the number of aggregates arriving during the time interval between photographic frames (# area-1 time-1). Results show that essentially all material arrives in the form of aggregates with minor contributions of fecal pellets and solitary particles. Sinking speeds (m day-1), calculated by dividing the flux of aggregates (# m-2 day-1) by their abundance (# m-3), indicate that the larger (4-5mm) aggregates are flocculent and sink slowly (~1m day-1) while the smaller aggregates (1-2.5mm) are more compact and sink more quickly (~36m day-1). These large, slow-sinking aggregates may have been re-suspended from the sediment water interface at nearby basin margins.

Description: This research was supported by ONR contract numbers N00014-82-C-0019 and N00014-85-C-0001, NSF grant numbers OCE-83-09024, OCE-84-17106, and DPP-85-01152 and the WHO1 education office.

Description: Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution September 2009

Description: Interaction between the Antarctic Circumpolar Current and the continental slope/shelf in the Marguerite Bay and west Antarctic Peninsula is examined as interaction between a wind-driven channel flow and a zonally uniform slope with a bay-shaped shelf to the south. Two control mechanisms, eddy advection and propagation of topographic waves, are identified in barotropic vortex-escarpment interactions. The two mechanisms advect the potential vorticity (PV) perturbations in opposite directions in anticyclone-induced interactions but in the same direction in cyclone-induced interactions, resulting in dramatic differences in the two kinds of interactions. The topographic waves become more nonlinear near the western(eastern if in the Northern Hemisphere) boundary of the bay, where strong cross-escarpment motion occurs. In the interaction between a surface anticyclone and a slope penetrating into the upper layer in a two-layer isopycnal model, the eddy advection decays on length scales on the order of the internal deformation radius, so shoreward over a slope that is wider than the deformation radius, the wave mechanism becomes noticeably significant. It acts to spread the cross-isobath transport in a much wider range while the transport directly driven by the anticyclone is concentrated in space. A two-layer wind-driven channel flow is constructed to the north of the slope in the Southern Hemisphere, spontaneously generating eddies through baroclinic instability. A PV front forms in the first layer shoreward of the base of the topography due to the lower-layer eddy-slope interactions. Perturbed by the jet in the center of the channel, the front interacts with the slope/shelf persistently yet episodically, driving a clockwise mean circulation within the bay as well as crossisobath transport. Both the transports across the slope edge and out of the bay are comparable with the maximum Ekman transport in the channel, indicative of the significance of the examined mechanism. The wave-boundary interaction identified in the barotropic model is found essential for the out-of-bay transport and responsible for the heterogeneity of the transport within the bay. Much more water is transported out of the bay from the west than from the east, and the southeastern area is the most isolated region. These results suggest that strong out-of-bay transport may be found near the western boundary of the Marguerite Bay while the southeastern region is a retention area where high population of Antarctic krill may be found.

Description: Submitted in partial fulfillment of the requirements for the degree of Doctor of Science at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution August 1987

Description: Several problems connected by the theme of thermal forcing are addressed herein. The main topic is the stratification and flow field resulting from imposing a specified heat flux on a fluid that is otherwise confined to a rigid insulating basin. In addition to the traditional eddy viscosity and diffusivity, turbulent processes are also included by a convective overturning adjustment at locations where the local density field is unstable. Two classes of problems are treated. The first is the large scale meridional pattern of a fluid in an annulus. The detailed treatment is carried out in two steps. In the beginning (chapter 2) it is assumed that the fluid is very diffusive, hence, to first approximation no flow field is present. It is found that the convective overturning adjustment changes the character of the stratification in all the regions that are cooled from the top, resulting in a temperature field that is nearly depth independent in the northernmost latitudes. The response to a seasonal cycle in the forcing, and the differences between averaging the results from the end of each season compared to driving the fluid by a mean forcing are analyzed. In particular, the resulting sea surface temperature is warmer in the former procedure. This observation is important in models where the heat flux is sensitive to the gradient of air to sea surface temperatures. The analysis of the problem continues in chapter 5 where the contribution of the flow field is included in the same configuration. The dimensionless parameter controlling the circulation is now the Rayleigh number, which is a measure of the relative importance of gravitational and viscous forces. The effects of the convective overturning adjustment is investigated at different Rayleigh numbers. It is shown that not only is the stratification now always stable, but also that the vigorous vertical mixing reduces the effective Rayleigh number; thereby the flow field is more moderate, the thermocline deepens, and the horizontal surface temperature gradients are weaker. The interior of the fluid is colder compared to cases without convective overturning, and, because the amount of heat in the system is assumed to be fixed, the surface temperature is warmer. The fluid is not only forced by a mean heat flux, or a seasonally varying one, but its behavior under permanent winter and summer conditions is also investigated. A steady state for the experiments where the net heat flux does not vanish is defined as that state where the flow field and temperature structure are not changing with time except for an almost uniform temperature decrease or increase everywhere. It is found that when winter conditions prevail the circulation is very strong, while it is rather weak for continuous summer forcing. In contrast to those results, if a yearly cycle is imposed, the circulation tends to reach a minimum in the winter time and a maximum in the summer. This suggests that, depending on the Rayleigh number, there is a phase leg of several months between the response of the ocean and the imposed forcing. Differences between the two averaging procedures mentioned before are also observed when the flow field is present, especially for large Rayleigh numbers. The circulation is found to be weaker and the sea surface temperature colder in the mean of the seasonal realizations compared to the steady state derived by the mean forcing. As an extension to the numerical results, an analytic model is presented in chapter 4 for a similar annular configuration. The assumed dynamics is a bit different, with a mixed layer on top of a potential vorticity conserving interior. It is demonstrated that the addition of the thermal wind balance to the conservation of potential vorticity in the axially symmetric problem leads to the result that typical fluid trajectories in the interior are straight lines pointing downward going north to south. The passage of information in the system is surprisingly in the opposite sense to the clockwise direction of the flow. A model for water mass formation by buoyancy loss in the absence of a flow field is introduced in chapter 3. The idea behind it is to use the turbulent mixing parameterization to generate chimney-like structures in open water, followed by along-isopycnal advection and diffusion. This model can be applied to many observations of mode water. In particular, in this work it is related to the chimneys observed by the MEDOC Group (1970), and the Levantine Intermediate Water in the Eastern Mediterranean Basin. An analytic prediction of the depth of the water mass is derived and depends on the forcing and initial stratification. It suggests that the depth of shallow mode water like the 18°C water or the Levantine Intermediate Water would not be very sensitive to reasonable changes in atmospheric forcing. Similar conclusions were also reached by Warren (1972) by assuming that the temperature in the thermocline decreases linearly with depth, and by approximating the energy balance in a water column by a Newtonian cooling law.

Description: Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution June 2000

Description: The thesis develops and demonstrates methods of classifying ocean processes using an underwater moving platform such as an Autonomous Underwater Vehicle (AUV). The "mingled spectrum principle" is established which concisely relates observations from a moving platform to the frequency-wavenumber spectrum of the ocean process. It clearly reveals the role of the AUV speed in mingling temporal and spatial information. For classifying different processes, an AUV is not only able to jointly utilize the time-space information, but also at a tunable proportion by adjusting its cruise speed. In this respect, AUVs are advantageous compared with traditional oceanographic platforms. Based on the mingled spectrum principle, a parametric tool for designing an AUVbased spectral classifier is developed. An AUV's controllable speed tunes the separability between the mingled spectra of different processes. This property is the key to optimizing the classifier's performance. As a case study, AUV-based classification is applied to distinguish ocean convection from internal waves. The mingled spectrum templates are derived from the MIT Ocean Convection Model and the Garrett-Munk internal wave spectrum model. To allow for mismatch between modeled templates and real measurements, the AUVbased classifier is designed to be robust to parameter uncertainties. By simulation tests on the classifier, it is demonstrated that at a higher AUV speed, convection's distinct spatial feature is highlighted to the advantage of classification. Experimental data are used to test the AUV-based classifier. An AUV-borne flow measurement system is designed and built, using an Acoustic Doppler Velocimeter (ADV). The system is calibrated in a high-precision tow tank. In February 1998, the AUV acquired field data of flow velocity in the Labrador Sea Convection Experiment. The Earth-referenced vertical flow velocity is extracted from the raw measurements. The classification test result detects convection's occurrence, a finding supported by more traditional oceanographic analyses and observations. The thesis work provides an important foundation for future work in autonomous detection and sampling of oceanographic processes.

Description: This thesis research has been funded by the Office of Naval Research (ONR) under Grants NOOOl4-95-1-1316, NOO0l4-97-1-0470, and by the MIT Sea Grant College Program under Grant NA46RG0434.

Description: Submitted in partial fulfillment of the requirements for the degree of Doctor of Science at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution December 1999

Description: A new, global inversion is used to estimate the large scale oceanic circulation based on the World Ocean Circulation Experiment and Java Australia Dynamic Experiment hydrographic data. A linear inverse "box" model is used to combine consistently the transoceanic sections. The circulation is geostrophic with an Ekman layer at the surface and oceanic layers defined by neutral surfaces. Near-conservation of mass, salt and top-to-bottom silica is required and, in addition, heat and the phosphate-oxygen combination (170[P04]+[02]) are conserved in layers that are not in contact with the surface. A globally-consistent solution is obtained for a depth-independent adjustment to the thermal wind field, freshwater flux divergenees, the Ekman transport, and the advective and diffusive dianeutral fluxes between layers. A detailed error budget permits calculation of statistical uncertainties, taking into account both the non-resolved part of the solution and the systematic errors due to the temporal oceanic variability. The estimated water mass transports during the WOCE period (1985-1996) are generally similar to previous published estimates. However, important differences are found. In particular, the inflow of bottom waters into the Pacific Ocean is smaller than in most previous estimates. Utilization of property anomaly conservation constraints allows the estimation of significant dianeutral diffusivities in deep layers, with a global average of 3 ± lcm2s- 1 north of 30°S. Dianeutral transfers indicate that about 20 Sv of bottom water is formed in the Southern Ocean. Significant ocean-atmosphere heat fluxes are found, with a global heating of 2.3 ± 0.4PW in the tropical band and a corresponding cooling at high latitudes. The signature of a large-scale average export production is found for nutrients in several temperate regions. Despite the large uncertainties, the production magnitudes are consistent with independent measurements from sediment traps and isotopic data. Net nutrient sources or sinks are found in several regions, suggesting either transport of dissolved organic matter or a seasonal alias. Oxygen indicates large exchanges with the atmosphere, with intake at high latitudes and outgassing/remineralization at low latitudes.

Description: This work was supported in part by the Jet Propulsion Laboratory/CALTECH (contract #958125), and by gifts from Ford, General Motors, and Daimler-Chrysler to MIT's Climate Modelling Initiative.

Description: Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution February 1981

Description: The equatorial Pacific heat flow low, a major oceanic geothermal anomaly centered on the equatorial sediment bulge, was investigated using deeply penetrating heat flow probes (6-11 meters penetration) within three detailed surveys (400 km2) and along over 10,000 km of continuous seismic profiles (CSP). Previous heat flow measurements in this region defined a broad region characterized by a heat flux well below 1 HFU. We report 98 new measurements collected during cruises PLEIADES 3 and KNORR 73-4 that verify the anomalous nature of the heat flux and also define non-linear temperature gradients (concave down). Temperature field disturbances due to perturbations of a purely conductive heat transport regime are incapable of suitably explaining either of these observations . A simple model incorporating heat transport by both conduction and fluid convection through the sediments fits the observations. A volume flux of (hydrothermal) fluid in the range of 10-6 to 10-5 cm3/sec/cm2 (0.1 liter/yr/cm2) is required. The sense of the flow for all measurements exhibiting non-linear gradients is upward out of the sediment column; no evidence for the recharging of the system was observed. Investigation of a well-defined boundary of the low zone at 4°N and 114°W showed a transition from low and variable heat flow to values compatible with thermal models that correlated with a change in the nature of the basement from rough to smooth. A few outcrops occur in the area of rough basement, but otherwise the region is well-sedimented (greater than 200 meters). Measurements within a detailed survey centered at this transition showed a dramatic increase in heat flow from 1.21 HFU to values greater than 3 HFU over a horizontal distance of 10km. A similar transition from non-linear to linear temperature gradients was not observed as nearly every measurement was non-linear. Heat flow measurements located in well-sedimented, outcrop-free areas (A environments) were associated with linear gradients and a heat flux greater than 1 HFU, however, several of these values were well below the theoretical heat flow for the appropriate age crust. Values measured in environments other than A exhibited variable heat flow and non-linear gradients. The average value of measurements located in A environments within the equatorial Pacific heat flow low was 1.37±0.27 HFU. The previously reported average was 0.92±0.48 HFU based on several measurements from L-DGO cruise VEMA 24-3. The average heat flow measured at a survey located outside the low heat flow zone on crust of 55 ±5 m.a. was 1.76 ±0.30 HFU which is in good agreement with the theoretical value of 1.60. The measurements in this survey were not located in A environments suggesting that crustal convection has ceased or is greatly attenuated within crust of this age. Error analysis of the geothermal data reduction using the convective/conductive heat transport model suggests that the volume flux parameter is sensitive to temperature measurement errors greater than a few millidegrees. Volume fluxes less than 10-7 cm/sec are difficult to distinguish from the purely conductive case assuming instrumental accuracies of 0.001°C. Resolution of the volume flux deteriorates as heat flow decreases and is poor for values less than 0.5 HFU. A detailed survey located within the low zone confirmed previous measurements of low heat flow, however, due to the low value of heat flow (about 0.5 HFU) the small-scale variability could not be clearly defined.

Description: Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution September 1987

Description: This thesis presents an application of the Grant-Madsen-Glenn bottom boundary layer model (Grant and Madsen, 1979; Glenn and Grant, 1987) to predictions of sediment transport on the continental shelf. The analysis is a two-stage process. Via numerical experiment, we explore the sensitivity of sediment transport to variations in model parameters and assumptions. A notable result is the enhancement of suspended sediment stratification due to wave boundary layer effects. When sediment stratification is neglected under conditions of large wave bottom velocities, concentration predictions can be more than an order of magnitude higher than any observed during storm conditions on the continental shelf. A number of limitations to application emerged from the analysis. Solutions to the stratified model are not uniquely determined under a number of cases of interest, potentially leading to gross inaccuracies in the prediction of sediment load and transport. Load and sediment transport in the outer Ekman Layer, beyond the region of emphasis for the model, can be as large or larger than the near-bottom estimates in some cases; such results suggest directions for improvements in the theoretical model. In the second step of the analysis, we test the ability of the model to make predictions of net sediment transport that are consistent with observed sediment depositional patterns. Data from the Mid-Atlantic Bight and the Northern California coast are used to define reasonable model input to represent conditions on two different types of shelves. In these examples, the results show how the intensification of wave bottom velocities with decreasing depth can introduce net transport over a region. The patterns of erosion/deposition are shown to be strongly influenced by sediment stratification and moveable bed roughness. Also predicted by the applications is a rapid winnowing out of fine grain size components when there is even a small variation of bed grain size texture in the along-flow direction.

Description: Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution September 2009

Description: Internal tides are internal waves of tidal period generated by tidal currents flowing over submarine topography. Tall ridges that are nominally two-dimensional (2-D) are sites of particularly strong generation. The subsequent dissipation of internal tides contributes to ocean mixing, thereby playing an important role in the circulation of the ocean. Strong internal tides can also evolve into internal wave solitons, which affect acoustic communication, offshore structures and submarine navigation. This thesis addresses the generation of internal tides by tall submarine ridges using a combined analytical and experimental approach. The first part of the thesis is an experimental investigation of a pre-existing Green function formulation for internal tide generation by a tall symmetric ridge in a uniform density stratification. A modal decomposition technique was developed to characterize the structure of the experimental wave fields generated by 2D model topographies in a specially configured wave tank. The theory accurately predicts the low mode structure of internal tides, and reasonably predicts the conversion rate of internal tides infinite tidal excursion regimes, for which the emergence of non-linearities was notable in the laboratory. In the second part of the thesis, the Green function method is advanced for asymmetric and multiple ridges in weakly non-uniform stratifications akin to realistic ocean situations. A preliminary investigation in uniform stratification with canonical asymmetric and double ridges reveals asymmetry in the internal tide that can be very sensitive to the geometric configuration. This approach is then used with realistic topography and stratification data to predict the internal tide generated by the ridges at Hawaii and at the Luzon Strait. Despite the assumption of two-dimensionality, there is remarkably good agreement between field data, simulations and the new theory for the magnitude, asymmetry and modal content of the internal tide at these sites. The final part of the thesis investigates the possibility of internal wave attractors in the valley of double-ridge configurations. A one-dimensional map is developed to identify the existence and stability of attractors as a function of the ridge geometry. The Green function method is further advanced to include a viscous correction to balance energy focusing and dissipation along an attracting orbit of internal wave rays, and very good agreement is obtained between experiment and theory, even in the presence of an attractor.

Description: My Ph. D. and the work in this thesis have been generously funded by the National Science Foundation under grants OCE 0645529 and OCE 04-25283 and the Office of Naval Research under grants N00014-08-0390, N00014-05-1-0573 and N00014-09-0282.

Description: Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution August 1987

Description: This thesis consists of two loosely related problems, both of which analyze some consequences of the failure of Sverdrup relation. In the first part, Chapters 2 and 3, the Sverdrup relation is invalidated because substantial flow is obtained at the bottom where topography exists. The eddies play an essential role in transfering momentum vertically from the surface, where the forcing is applied, to the bottom, which is otherwise unforced. If the topography has a structure in the longitudinal direction, then the inviscid theory predicts the occurence of strong jets in the interior of the model ocean. According to the structure of the topography these internal jets can occur in both vertically homeogenous and baroclinic oceans. If the topographic slope changes sign, then one kind of jets is observed both in stratified and in homogeneous oceans. This phenomenon is robust to moderate amounts of dissipation and is not disturbed by the occurrence of recirculating gyres within the basin. If the topographic slope is constant, then another kind of internal jets is observed, and it occurs in stratified models only. I was unable to observe this kind of jets in the presence of weak dissipation. The reason for this failure is twofold: on one hand friction, especially interfacial friction, tends to make the flow more barotropic (and we believe that indeed this is one of the processes that the eddies accomplish in a stratified fluid) and therefore the phenomena that rely strongly on baroclinicity are discouraged. On the other hand, reduction of the dissipation leads to the onset of a strong recirculating, inertial gyre which, although confined in space, affects the global properties of the flow. In the second part of the thesis (Chapters 4 and 5) I developed a simple model of the recirculating, inertial gyre. Again the dynamics of this feature are far from being in Sverdrup balance. In this case inertia is responsible for the failure of Sverdrup relation, together with the eddy field which provides a mean for transfering momentum vertically and laterally into regions away from where the forcing is applied. In this model there is no direct forcing in the recirculation region, and the input of momentum is confined to the boundary currents surrounding the gyre, for example the separated Gulf Stream. One of the results of the recirculation model is the prediction of its transport. It is shown that most of the transport is depth independent, i.e. it can be calculated without detailed knowledge of the density structure of the ocean. It is also shown that the "barotropic" part of the transport increases as the cube of the meridional extent of the gyre.

Description: The thesis work has been supported by a National Foundation grant from the Office of Atmospheric Sciences.

Description: Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution February 2010

Description: Aspects of the circulation and convection in the Irminger Sea are investigated using a variety of in-situ, satellite, and atmospheric reanalysis products. Westerly Greenland tip jet events are intense, small-scale wind phenomena located east of Cape Farewell, and are important to circulation and convection in the Irminger Sea. A climatology of such events was used to investigate their evolution and mechanism of generation. The air parcels constituting the tip jet are shown to have a continental origin, and to exhibit a characteristic deflection and acceleration around southern Greenland. The events are almost invariably accompanied both by a notable coherence of the lower-level tip jet with an overlying upper-level jet stream, and by a surface cyclone located in the lee (east) of Greenland. It is argued that the tip jet arises from the interplay of the synopticscale flow evolution and the perturbing effects of Greenland’s topography upon the flow. The IrmingerGyre is a narrow, cyclonic recirculation confined to the southwest Irminger Sea. While the gyre’s existence has been previously documented, relatively little is known about its specific features or variability. The mean strength of the gyre’s circulation between 1991 and 2007 was 6.8 ± 1.8 Sv. It intensified at a rate of 4.3 Sv per decade over the observed period despite declining atmospheric forcing. Examination of the temporal evolution of the LSW layer thickness across the Irminger Basin suggests that local convection formed LSW during the early 1990s within the Irminger Gyre. In contrast, LSW appeared outside of the gyre in the eastern part of the Irminger Sea with a time lag of 2-3 years, consistent with transit from a remote source in the Labrador Sea. In the winter of 2007-08 deep convection returned to both the Labrador and Irminger seas following years of shallow overturning. The transition to a convective state took place abruptly, without going through a preconditioning phase, which is contrary to general expectations. Changes in the hemispheric air temperature, tracks of storms, flux of freshwater to the Labrador Sea, and distribution of pack ice all conspired to enhance the air-sea heat flux, resulting in the deep overturning.

Description: Financial support for this work was provided by National Science Foundation grant OCE-0450658.

Description: Submitted in partial fulfillment of the requirements for the degree of Master of Science at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution September 2001

Description: As part of Phase III of the U.S. GLOBEC Georges Bank program, drogued drifters and dye tracer were released into the pycnocline on the southern flank of Georges Bank to measure advective and diffusive transport relative to the tidal mixing front in May 1999. Potential density measurements placed the tidal mixing front around the 50-55 m isobath on the southern flank. Drogued drifter movement relative to the front was on the order of the drifter's slip velocity and therefore did not support the existence of a mean, advective flow. No movement relative to the front of the dye patch center of mass also indicated a lack of advective flow. Diffusive transport did occur as the dye patch spread laterally both toward and away from the front much as would be predicted by the diffusion relationship of Okubo (1971), who summarized diffusion experiments in the surface ocean. The dye did not spread symmetrically, but was rather elongated along the isobaths. This can be attributed to vertical shear in the along-isobath current that was measured by the shipboard ADCP.

Description: The National Science Foundation (Grant #OCE9806498) sponsored the data collection described within. The United States Navy funded the author's participation in this thesis project.

Description: Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution June 2010

Description: In a stratified rotating fluid, frictionally driven circulations couple with the buoyancy field over sloping topography. Analytical and numerical methods are used to quantify the impact of this coupling on the vertical circulation, spindown of geostrophic flows, and the formation of a shelfbreak jet. Over a stratified slope, linear spindown of a geostrophic along-isobath flow induces cross-isobath Ekman flows. Ekman advection of buoyancy weakens the vertical circulation and slows spindown. Upslope (downslope) Ekman flows tend to inject (remove) potential vorticity into (from) the ocean. Momentum advection and nonlinear buoyancy advection are examined in setting asymmetries in the vertical circulation and the vertical relative vorticity field. During nonlinear homogeneous spindown over a flat bottom, momentum advection weakens Ekman pumping and strengthens Ekman suction, while cyclonic vorticity decays faster than anticyclonic vorticity. During nonlinear stratified spindown over a slope, nonlinear advection of buoyancy enhances the asymmetry in Ekman pumping and suction, whereas anticyclonic vorticity can decay faster than cyclonic vorticity outside of the boundary layers. During the adjustment of a spatially uniform geostrophic current over a shelfbreak, coupling between the Ekman flow and the buoyancy field generates Ekman pumping near the shelfbreak, which leads to the formation of a jet. Scalings are presented for the upwelling strength, the length scale over which it occurs, and the timescale for jet formation. The results are applied to the Middle Atlantic Bight shelfbreak.

Description: Funding for my research and education was provided by MIT EAPS, the WHOI Academic Programs O ce and the MIT Presidential Fellowship. Financial assistance from the Houghton Fund is also acknowledged.

Description: Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution February 1987

Description: This thesis studies the role of cross-isopycnal mixing in general circulation dynamics, from both the theoretical and observational points of view. The first two chapters discuss some theoretical aspects of cross-isopycnal mixing in the oceans. In chapter one, an integral constraint relating the interior stratification and air-sea heat fluxes is derived, based on the condition that the total mass of water of given density is constant in a steady state ocean. Two simple models are then used to examine the way the numerically small mixing, together with air-sea fluxes, determines the average vertical density stratification of the oceans, and the deep buoyancy driven circulation. In chapter two, a more complete model of a deep flow driven by cross isopycnal diffusion is presented, motivated by the Mediterranean outflow into the North Atlantic. Mixing in this model is responsible for the determination of the detailed structure of the flow and density field, while in the models of the first chapter it was allowed to determine only the average vertical density stratification. In chapter three, a hydrographic data set from the Mediterranean sea is analyzed by inverse methods. The purpose is to examine the importance of mixing when trying to explain tracer distributions in the ocean. The time-mean circulation and the appropriate mixing coefficients are calculated from the hydrographic data. We conclude that the numerically small cross isopycnal mixing processes are crucial to the dynamics, yet difficult to parameterize and measure using available hydrographic data.

Description: NSF grants OCE-8521685 and OCE-8017791 supported me during my studies in the joint program.

Description: Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution January 1987

Description: The various distributions of tracer associated with the Northern Recirculation Gyre of the Gulf Stream (NRG) are studied to try to obtain information about the flow. An advective-diffusive numerical model is implemented to aid in the investigation. The model is composed of a gyre adjacent to a boundary current in which a source of tracer is specified at the upstream edge of the current. This set up attempts to simulate the lateral transfer of properties from the Deep Western Boundary Current (DWBC) to the NRG in the region where the two flows are in close contact west of the Grand Banks. The results of the model are analyzed in some detail. Tracer is entrained into the gyre as a plume which extends from the boundary current and spirals across streamlines toward the gyre center. The maintenance of the spiral during spin-up and its relationship to the occurrence of homogenization at steady state is examined. An asymmetry in the spiral exists due to the ellipticity of the gyre, which also effects homogenization. The anomalous properties that are fluxed into the NRG include salt, oxygen, and freon. These particular tracers are independent from each other, the former two because they are characterized by different vertical profiles in the deep layer. This results in a decay of oxygen but not salt, due to the presence of vertical mixing as discussed by Hogg et al. (1986, Deep-Sea Research, 33, 1139-1165). Their analysis is expanded upon here. The effect of vertical mixing on the gyre/boundary current system is examined within the context of the numerical model. Results are applied to recently collected water sample data from the region which leads to an estimate of the lateral and vertical eddy diffusion coefficients and an estimate of the amount of oxygen in the NRG that has diffused from the DWBC. The accumulation of freon within the NRG is considered in addition to salt and oxygen. Appreciable levels of freon have been present in the ocean only since 1950, and the atmospheric source functions have been increasing steadily since then. A simple overflow model is presented of the manner in which freon may be stirred in the Norwegian-Greenland basin prior to overflowing and entering the DWBC. Once in the boundary current the concentrations are diluted by way of mixing with surrounding water. Two different schemes are considered in which the immediate surrounding water accumulates a substantial amount of freon as time progresses. These models suggest that the freon-11:freon-12 ratio may not be a conserved quantity for the water in the core of the DWBC. It is found that the level of freon in the NRG is barely above the existing background level.

Description: This work was supported by the office of Naval Research through contracts N00014-76-C-0197 and N00014-84-C-0134, NR 083-400; and N00014-82-C-0019 and N00014-85-C-0001, NR 083-004, and the National Science Foundation through grant 0CE82-14925.

Description: Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution April 1985

Description: The focus of this thesis is on rates of transport of metals both across the sediment/water interface and within the sediment column of nearshore sediments. The early diagenesis of several first-row transition metals exhibiting a variety of behaviors in the ocean -- Mn, Fe, Co, Ni, and Cu -- has been studied intensively at a site in Buzzards Bay, Mass. By limiting the study to a single site, independent measurements over the seasonal cycle of the concentrations of the metals in pore water, of the pore water constituents important to metal cycling, and of particle and solute transport rates could be made at the same site. In addition, a direct, in situ study of the interaction of chemical and transport processes was undertaken using radiotracer techniques. Thus, the study emphasizes the mechanisms of metal cycling near the interface of nearshore sediments. Transport rates were estimated using excess 234Th distributions for particle transport, and pore water 222Rn deficit distributions for solute transport. Particle transport rates, modeled by analogy to Fickian diffusion, ranged from 7-80x10-8 cmz/sec, with excess 234Th reaching to 2-2.5 cm below the interface. There was a significant seasonal variation in rates, with a warm-season average of 40x10-8 cm2/sec and a cold-season average of 20x10-8 cm 2/sec. 234Th-derived mixing rates were applied to Mn distributions through a mass balance model of Mn cycling. It was found that a particulate flux due to bioturbation, from the net dissolved Mn removal layer to a net dissolved Mn production layer adjacent to the interface, was as large as 38% of net dissolved Mn production. Mixing of particulate Fe sulfides may have a similar importance for Fe cycling. Solute transport was estimated using measured 222Rn/ 226 Ra disequilibrium. The pore water 222Rn deficit could be explained using a model including vertical molecular diffusion and exchange with overlying seawater via exchange of pore water with bottom water in rapidly flushed burrows. Cores taken in all seasons could be split into three groups: (1) December through March: the 222Rn deficit was explained by vertical molecular diffusion alone; (2) early summer (June): irrigation affected the 222Rn profile to a depth of at least 20cm; (3) late summer/fall: irrigation was still important near the interface, affecting 222Rn profiles to depths of 10-12 cm. 222Rn deficits were adequately explained by an exchange parameter (a) which decreased exponentially with depth below the interface, but not by a constant-α model. Previous studies have explained irrigation using a constant exchange parameter throughout the irrigated layer. For comparative purposes, an α averaged over the upper 20 cm of the sediment column was calculated at the Buzzards Bay site: the range of depth-averaged α values found, 4-12x10-7 sec-1, is in agreement with values reported previously for a variety of nearshore sediments, using pore water Si02 as a tracer, of 1-20x10-7 sec-1. 222Rn-derived irrigation rates were applied to pore water Mn and Fe distributions. It was estimated that irrigation may contribute 20-40% of the dissolved Mn flux across the interface and about 20% of the dissolved Fe flux. Study of pore water metal chemistry at the Buzzards Bay site included measurements of pore water Mn and Fe during all seasons, and measurements of Co, Cu, and Ni in two cores: one under late winter conditions when the interface is most oxidizing; one when sulfate reduction was very important in the upper centimeter of the sediments. Fe regeneration sufficient to produce enrichments on water column particles was observed only during periods of summer and fall when the interface was reducing; otherwise, oxidation of Fe to insoluble Fe(III) limited Fe fluxes. Mn, Co, Cu, and Ni fluxes varied inversely to Fe fluxes; the primary control on fluxes of these elements was their limited solubility in reducing marine systems. The control was least important for Mn and Co; fluxes of Ni and Cu were significantly greater than zero only when sulfate reduction was unimportant in the upper centimeter of the sediment column. Fluxes of Mn were sufficient to affect the water column Mn distribution, with enrichments on water column particulates of up to 10,000 ppm inferred from calculated fluxes. Tentative estimates of the turnover time of dissolved Co, Cu, and Ni in the water column relative to the benthic flux indicated that the flux may be a significant contributor to the coastal Co cycle (turnover time = 1 yr), but is less likely to be important to Cu and Ni cycles (turnover times greater than 2 yrs). In situ radiotracer migration experiments were carried out at the Buzzards Bay site. 54Mn, 59Fe, 60Co, and 63Ni were released into the sediments at depths ranging from 2.5 to 7 cm below the interface. The order of mobilities was Mn»Fe〉Co,Ni, which is similar to the solubility trend for these metals in reducing marine systems. 63Ni and 60Co were essentially particle-bound in these experiments; apparent diffusion coefficients calculated from their dispersion rates agreed with particle mixing rates from excess 234Th distributions. Solid:solution distribution coefficients were calculated from 54Mn dispersion and found to agree with directly measured values. The coefficient was approximately 15 (dpm/gm solid ÷ dpm/gm pore water) in the upper 0.5 cm and below 5 cm, and 5-10 from 0.5 to 5 cm. Distribution coefficients for 59Fe were approximately 120 below 0.5 cm. Although the trend of the distribution coefficients is clear, the quantitative results from these experiments are preliminary, in that the model used to explain metal ion dispersion, when applied to the nonreactive tracer, 36Cl, could only explain a portion of the 36Cl distribution. The agreement between calculated and directly measured s4Mn distribution coefficients, as well as the way the distributions of tracers varied as a function of apparent diffusion coefficient and time, provides evidence in favor of the adequacy of the model used.

Description: Financial support for parts of this work have come from: National Science Foundation grant #OCE-82-16425; Department of Commerce, NOAA, National Sea Grant College Program, under grant #NA80-AA-D-00077 (R/P/7); from the Coastal Research Center of the Woods Hole Oceanographic Institution; and from the WHOI Education Office.

Description: Submitted in partial fulfillment of the requirements for the degree of Ocean Engineer at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution August 1987

Description: In 1983, continuous acoustic transmissions centered at 133 Hz and with a resolution of 60 ms were transmitted for five days from Oahu to the coast of Northern California (4000 km range). A maximum likelihood estimate of the change in acoustic travel time (based on phase) between received pulses is used to estimate barotropic fluctilations. Analysis of the resulting time series reveals resonant oscillations at nontidal frequencies in the Northeast Pacific. Some of the periods of the resonant oscillations are consistent with theory (Platzman, Curtis, Hansen, and Slater, 1981). A Wiener filter is formulated for estimating the barotropic tides from a basin scale tomographic array. Error analyses indicate an ability to estimate barotropic currents and surface displacements with errors less than 0.01 cm/s and 1 cm, respectively, over a large portion of the Northeast Pacific.

Description: Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution February 2009

Description: Self-localization of an underwater vehicle is particularly challenging due to the absence of Global Positioning System (GPS) reception or features at known positions that could otherwise have been used for position computation. Thus Autonomous Underwater Vehicle (AUV) applications typically require the pre-deployment of a set of beacons. This thesis examines the scenario in which the members of a group of AUVs exchange navigation information with one another so as to improve their individual position estimates. We describe how the underwater environment poses unique challenges to vehicle navigation not encountered in other environments in which robots operate and how cooperation can improve the performance of self-localization. As intra-vehicle communication is crucial to cooperation, we also address the constraints of the communication channel and the effect that these constraints have on the design of cooperation strategies. The classical approaches to underwater self-localization of a single vehicle, as well as more recently developed techniques are presented. We then examine how methods used for cooperating land-vehicles can be transferred to the underwater domain. An algorithm for distributed self-localization, which is designed to take the specific characteristics of the environment into account, is proposed. We also address how correlated position estimates of cooperating vehicles can lead to overconfidence in individual position estimates. Finally, key to any successful cooperative navigation strategy is the incorporation of the relative positioning between vehicles. The performance of localization algorithms with different geometries is analyzed and a distributed algorithm for the dynamic positioning of vehicles, which serve as dedicated navigation beacons for a fleet of AUVs, is proposed.

Description: This work was funded by Office of Naval Research grants N00014-97-1-0202, N00014-05-1-0255, N00014-02-C-0210, N00014-07-1-1102 and the ASAP MURI program led by Naomi Leonard of Princeton University.

Description: Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution June 2009

Description: This research presents a parallel hypothesis method for autonomous underwater vehicle navigation that enables a vehicle to expand the operating envelope of existing long baseline acoustic navigation systems by incorporating information that is not normally used. The parallel hypothesis method allows the in-situ identification of acoustic multipath time-of-flight measurements between a vehicle and an external transponder and uses them in real-time to augment the navigation algorithm during periods when direct-path time-of-flight measurements are not available. A proof of concept was conducted using real-world data obtained by the Woods Hole Oceanographic Institution Deep Submergence Lab's Autonomous Benthic Explorer (ABE) and Sentry autonomous underwater vehicles during operations on the Juan de Fuca Ridge. This algorithm uses a nested architecture to break the navigation solution down into basic building blocks for each type of available external information. The algorithm classifies external information as either line of position or gridded observations. For any line of position observation, the algorithm generates a multi-modal block of parallel position estimate hypotheses. The multimodal hypotheses are input into an arbiter which produces a single unimodal output. If a priori maps of gridded information are available, they are used within the arbiter structure to aid in the elimination of false hypotheses. For the proof of concept, this research uses ranges from a single external acoustic transponder in the hypothesis generation process and grids of low-resolution bathymetric data from a ship-based multibeam sonar in the arbitration process. The major contributions of this research include the in-situ identification of acoustic multipath time-of-flight measurements, the multiscale utilization of a priori low-resolution bathymetric data in a high-resolution navigation algorithm, and the design of a navigation algorithm with a exible architecture. This flexible architecture allows the incorporation of multimodal beliefs without requiring a complex mechanism for real-time hypothesis generation and culling, and it allows the real-time incorporation of multiple types of external information as they become available in situ into the overall navigation solution.