ALBERT

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: Estimates of natural climate variability during the past millennium provide a frame of reference in which to assess the significance of recent changes. This thesis investigates new methods of reconstructing low-latitude sea surface temperature (SST) and hydrography, and combines these methods with traditional techniques to improve the present understanding of western North Atlantic climate variability. A new strontium/calcium (Sr/Ca) - SST calibration is derived for Atlantic Montastrea corals. This calibration shows that Montastrea Sr/Ca is a promising SST proxy if the effect of coral growth is considered. Further analyses of coral growth using Computed Axial Tomography (CAT) imaging indicate growth in Siderastrea corals varies inversely with SST on interannual timescales. A 440-year reconstruction of low-latitude western North Atlantic SST based on this relationship suggests the largest cooling of the last few centuries occurred from ~1650-1730 A.D., and was ~1ºC cooler than today. Sporadic multidecadal variability in this record is inconsistent with evidence for a persistent 65-80 year North Atlantic SST oscillation. Volcanic and anthropogenic radiative forcing are identified as important sources of externally-forced SST variability, with the latter accounting for most of the 20th century warming trend. An 1800-year reconstruction of SST and hydrography near the Gulf Stream also suggests SSTs remained within about 1ºC of modern values. This cooling is small relative to other regional proxy records and may reflect the influence of internal oceanic and atmospheric circulation. Simulations with an atmospheric general circulation model (AGCM) indicate that the magnitude of cooling estimated by proxy records is consistent with tropical hydrologic proxy records.

Description: Funding for this research was provided by a National Science Foundation Graduate Student Fellowship, National Science Foundation grants OCE-0402728, OCE-0623364, ATM-033746, the WHOI Ocean and Climate Change Institute, the WHOI Ocean Ventures Fund, the WHOI Ocean Life Institute, the MIT Student Assistance Fund, award number USA-0002, made by King Abdullah University of Science and Technology (KAUST), and the Inter-American Institute for Global Change Research.

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 2007.

Description: The water circulation and evolution of water temperature over the inner continental shelf are investigated using observations of water velocity, temperature, density, and bottom pressure; surface gravity waves; wind stress; and heat flux between the ocean and atmosphere during 2001-2007. When waves are small, cross-shelf wind stress is the dominant mechanism driving cross-shelf circulation. The along-shelf wind stress does not drive a substantial cross-shelf circulation. The response to a given wind stress is stronger in summer than winter. The cross-shelf transport in the surface layer during winter agrees with a two-dimensional, unstratified model. During large waves and onshore winds the cross-shelf velocity is nearly vertically uniform, because the wind- and wave-driven shears cancel. During large waves and offshore winds the velocity is strongly vertically sheared because the wind- and wave-driven shears have the same sign. The subtidal, depth-average cross-shelf momentum balance is a combination of geostrophic balance and a coastal set-up and set-down balance driven by the cross-shelf wind stress. The estimated wave radiation stress gradient is also large. The dominant along-shelf momentum balance is between the wind stress and pressure gradient, but the bottom stress, acceleration, Coriolis, Hasselmann wave stress, and nonlinear advection are not negligible. The fluctuating along-shelf pressure gradient is a local sea level response to wind forcing, not a remotely generated pressure gradient. In summer, the water is persistently cooled due to a mean upwelling circulation. The cross-shelf heat flux nearly balances the strong surface heating throughout midsummer, so the water temperature is almost constant. The along-shelf heat flux divergence is apparently small. In winter, the change in water temperature is closer to that expected due to the surface cooling. Heat transport due to surface gravity waves is substantial.

Description: My last three years of thesis work were supported by National Aeronautics and Space Administration Headquarters under the Earth System Science Fellowship Grant NNG04GQ14H, and by WHOI Academic Programs Fellowship Funds. I also benefited from the freedom of a Clare Boothe Luce Fellowship during my first year in the Joint Program, which allowed me more time than is usual to explore different research topics before choosing an advisor. This research was also funded by the National Aeronautics and Space Administration under grant NNG04GL03G and the Ocean Sciences Division of the National Science Foundation under grants OCE-0241292 and OCE-0548961. The Martha's Vineyard Coastal Observatory is partly funded by the Woods Hole Oceanographic Institution and the Jewett/EDUC/Harrison Foundation. The ADCP deployments at CBLAST site F were funded by National Science Foundation Small Grant for Exploratory Research OCE-0337892. Ship time for deployment and recovery of the F ADCP was provided by Robert Weller through Office of Naval Research contracts N00014-01-1-0029 and N00014-05-10090 for the Low-Wind Component of the Coupled Boundary Layers Air-Sea Transfer Experiment.

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 2003

Description: In this thesis I have endeavored to determine the factors and physical processes that controlled SST and thermocline depth at 10°N, 125°W during the Pan American Climate Study (PACS) field program. Analysis based on the PACS data set, TOPEX/Poseidon sea surface height data, European Remote Sensing satellite wind data, and model simulations and experiments reveals that the dominant mechanisms affecting the thermocline depth and SST at the mooring site during the measurement period were local surface fluxes, Ekman pumping, and vertical mixing associated with enhancement of the vertical shear by strong near-inertial waves in the upper ocean superimposed upon intra-seasonal baroclinic Rossby waves and the large scale zonal flow.

Description: This work was funded under NOAA Grant NA17RJ1223 and I also gratefully acknowledge receipt of an MIT Presidential Fellowship in 2000-2001.

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 2003

Description: Inertial terms dominate the single-gyre ocean model and prevent western-intensification when the viscosity is small. This occurs long before the oceanically-appropriate parameter range. It is demonstrated here that the circulation is controlled if a mechanism for ultimate removal of vorticity exists, even if it is active only in a narrow region near the boundary. Vorticity removal is modeled here as a viscosity enhanced very near the solid boundaries to roughly parameterize missing boundary physics like topographic interaction and three dimensional turbulence over the shelf. This boundary-enhanced viscosity allows western-intensified mean flows even when the inertial boundary width, is much wider than the frictional region because eddies flux vorticity from within the interior streamlines to the frictional region for removal. Using boundary-enhanced viscosity, western-intensified calculations are possible with lower interior viscosity than in previous studies. Interesting behaviors result: a boundary-layer balance novel to the model, calculations with promise for eddy parameterization, eddy-driven gyres rotating opposite the wind, and temporal complexity including basin resonances. I also demonstrate that multiple-gyre calculations have weaker mean circulation than single-gyres with the same viscosity and subtropical forcing. Despite traditional understanding, almost no inter-gyre flux occurs if no-slip boundary conditions are used. The inter-gyre eddy flux is in control only with exactly symmetric gyres and free slip boundaries. Even without the inter-gyre flux, the multiple-gyre circulation is weak because of sinuous instabilities on the jet which are not present in the single-gyre model. These modes efficiently flux vorticity to the boundary and reduce the circulation without an inter-gyre flux, postponing inertial domination to much smaller viscosities. Then sinuous modes in combination with boundary-enhanced viscosity can control the circulation.

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 2008

Description: The work was motivated by studies of Austin and Lentz (2002) and Pedlosky (2007). The above mentioned works considered two different responses of the stratified flow to a downwelling favorable wind forcing. The first study investigated a time dependent flow with a formation of a constantly expanding relatively well mixed region near the shore and the second considered a steady flow that arises when an offshore varying wind is applied. In my thesis I use ROMS to determine which type of response will take place based on the wind amplitude near the coast. It was demonstrated that if the value of the wind is much smaller than the critical value (determined by the stratification, the rotation rate and the horizontal diffusivity) then the flow is steady (the bbl case) and similar to the one investigated by Pedlosky. If the wind is of the order, or larger than, the critical value then the response is time dependent (the pool case) and similar to the one described by Austin and Lentz. The resulting flow structure of each response was also investigated. I examined the sensitivity of the bbl response to variations in the background vertical diffusivity, the initial stratification and the bottom slope. It was shown that a higher background vertical diffusivity, a higher stratification and a shallower bottom slope correspond to thinner (vertically) and narrower (horizontally) bbl. For the pool case the time dependent structure was also examined, using a number of idealized models. It was shown that the rate of the pool region expansion is a complex function of the local wind stress amplitude and the local depth.

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 2008

Description: Coupled ocean/atmosphere simulations exhibit systematicwarm biases over the SouthWest African (SWA) coastal region. Recent investigations indicate that coastal ocean dynamics may play an important role in determining the SST patterns, but none of them provide a detailed analysis. In this study, I analyze simulations produced both by coupled models and by idealized models. Then results are interpreted on the basis of a theoretical framework. Finally the conclusion is reached that the insufficient resolution of the ocean component in the coupled model is responsible for the warm biases over the SWA coastal region. The coarse resolution used in the ocean model has an artificially stretched coastal side-wall boundary layer, which induces a smaller upwelling velocity in the boundary layer. The vertical heat transport decreases even when the volume transport is unchanged because of its nonlinear relationship with the magnitude of the upwelling velocity. Based on the scaling of the idealized model simulations, a simplified calculation shows that the vertical heat transport is inversely proportional to the zonal resolution over the coastal region. Therefore, increasing the horizontal resolution can considerably improve the coastal SST simulation, and better resolve the coastal dynamics.

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 1980

Description: Observational evidence of seasonal variability below the main thermocline in the eastern North Atlantic is described, and a theoretical model of oceanic response to seasonally varying windstress forcing is constructed to assist in the interpretation of the observations. The observations are historical conductivity-temperature-depth data from the Bay of Biscay region (2° to 20°W, 42° to 52°N), a series of eleven cruises over the three years 1972 through 1974, spaced approximately three months apart. The analysis of the observations utilizes a new technique for identifying the adiabatically leveled density field corresponding to the observed density field. The distribution of salinity anomaly along the leveled surfaces is examined, as are the vertical displacements of observed density surfaces from the leveled reference surfaces, and the available potential energy. Seasonal variations in salinity anomaly and vertical displacement occur as westward propagating disturbances with zonal wavelength 390 (±50) km, phase 71 (±30) days from 1 January, and maximum amplitudes of ±30 ppm and ±20 db respectively. The leveled density field varies seasonally with an amplitude corresponding to a thermocline displacement of ±15 db. The observations are consistent with the predictions of a model in which an ocean of variable stratification with a surface mixed layer and an eastern boundary is forced by seasonal changes in a sinusoidal windstress pattern, when windstress parameters calculated from the observations of Bunker and Worthington (1976) are applied.

Description: This work was supported by the Office of Naval Research under contract N00014~76-C-197, NR 083-400.

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 1982

Description: Four different problems concerning Gulf Stream Rings are considered. The first deals with the particle trajectories of, and advection-diffusion by, a dynamic model of a Ring. It is found that the streaklines computed from the assumptions that the Ring is a steadily propagating and permanent form structure accurately describe its Lagrangian trajectories. The dispersion field of the Ring produces east-west asymmetries in the streaklines, not contained in earlier kinematic studies, which are consistent with observed surface patterns. In the second problem, we compute the core mixed layer evolution of both warm and cold Rings, and compare them to the background SST, in an effort to explain observed SST cycles of Rings. We demonstrate that warm Rings retain their anomalous surface identity, while cold Rings do not, because of differences in both the local atmospheric states of the Sargasso and the Slope and the typical mixed layer structures appropriate to each. The third and fourth problems concern the forced evolution of Gulf Stream Rings as effected by atmospheric interactions. First, we compute the forced spin down of a Gulf Stream Ring. The variations in surface stress across the Ring necessary to spin it down are caused by the variations in relative air-sea velocity, of which the stress is a quadratric function. From numerical simulations, we find the forced decay rates are comparable to those inferred from Ring observations. In the final problem, it is suggested that a substantial fraction of meridional Ring migration is a forced response, caused by Ring SST and the temperature dependence of stress. The warm central waters of anticyclonic Rings are regions of enhanced stress, producing upwelling to the north, and downwelling to the south, which shifts the Ring to the south. A similar, southward shift is computed for cyclonic Rings with cold centers, which tends to reconcile their numerically computed propagation with observations.

Description: The present research has been conducted under NOAA contract # NA80AA-D-0057 and NSF contract II OCE-8240455

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 2007

Description: Observations of current velocity, temperature, salinity and pressure from a 2-year moored array deployment and four hydrographic cruises conducted by the United States Southern Ocean GLOBEC program on the western Antarctic Peninsula continental shelf are used to characterize the ocean circulation and its connection to fresh water and heat fluxes on the shelf. Mean velocities on the shelf are of the order of 5 cm/s or less. Tidal motions are dominated by the M2 and S2 semi-diurnal tides and the O1 and K1 diurnal tides, although the tidal velocities are typically less than 2 cm/s. Near-inertial motions are relatively large, with current velocities as high as 26 cm/s. It is shown that Marguerite Trough, a large bathymetric feature connecting the shelf-break to Marguerite Bay, plays a critical role in determining the circulation. The mean flow is strongly steered in the along-slope direction, and the tidal currents also show increasing current polarization at depth in Marguerite Trough. At timescales of 5 to 20 days, the observations show bottom-intensified motion in Marguerite Trough consistent with bottom-trapped topographic Rossby waves. The subtidal circulation in the trough has a significant wind-driven component in Marguerite Trough, with downwelling-favorable winds forcing cross-shelf flow on the northern side of the trough and along the shore on the outer shelf. Upwelling-favorable winds force roughly the opposite circulation. The cyclonic circulation on the trough helps advect blobs of salty, warm and nutrient-rich water across the shelf. These intrusions are small (≈4 km) and frequent (4 events/month). Also, the Antarctic Peninsula Coastal Current (APCC), a coastal buoyant current which is described for the first time here. The APCC is a seasonal current which is only present during the ice-free season and is forced by freshwater fluxes associated with large glacier melt and precipitation rates in the region.

Description: Thanks goes to the agencies who made this thesis possible: the National Science Foundation Office of Polar programs through U.S. Southern Ocean GLOBEC grants OPP 99-10092 and 06-23223, the Chilean government through its Presidential Fellowship program and the Coastal Ocean Institute and the Cooperative Institute for Climate and Ocean Research.

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 2008

Description: This study presents observations of turbulence dynamics made during the low winds portion of the Coupled Boundary Layers and Air-Sea Transfer experiment (CBLAST-Low). Observations were made of turbulent fluxes, turbulent kinetic energy, and the length scales of flux-carrying and energy-containing eddies in the ocean surface boundary layer. A new technique was developed to separate wave and turbulent motions spectrally, using ideas for turbulence spectra that were developed in the study of the bottom boundary layer of the atmosphere. The observations of turbulent fluxes allowed the closing of heat and momentum budgets across the air-sea interface. The observations also show that flux-carrying eddies are similar in size to those expected in rigid-boundary turbulence, but that energy-containing eddies are smaller than those in rigid-boundary turbulence. This suggests that the relationship between turbulent kinetic energy, depth, and turbulent diffusivity are different in the ocean surface boundary layer than in rigid-boundary turbulence. The observations confirm previous speculation that surface wave breaking provides a surface source of turbulent kinetic energy that is transported to depth where it dissipates. A model that includes the effects of shear production, wave breaking and dissipation is able to reproduce the enhancement of turbulent kinetic energy near the wavy ocean surface. However, because of the different length scale relations in the ocean surface boundary layer, the empirical constants in the energy model are different from the values that are used to model rigid-boundary turbulence. The ocean surface boundary layer is observed to have small but finite temperature gradients that are related to the boundary fluxes of heat and momentum, as assumed by closure models. However, the turbulent diffusivity of heat in the surface boundary layer is larger than predicted by rigid-boundary closure models. Including the combined effects of wave breaking, stress, and buoyancy forcing allows a closure model to predict the turbulent diffusivity for heat in the ocean surface boundary layer.

Description: This work was supported by Office of Naval Research grants N00014-00-1-0409, N00014-01-1-0029, and N00014-03-1-0681, the Woods Hole Oceanographic Institution Academic Programs Office, and National Aeronautics and Space Administration grant NAG5-11933.

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 2004

Description: The uncertainty in the determination of the momentum and scalar fluxes remains one of the main obstacles to accurate numerical forecasts in low to moderate wind conditions. For example, latent heat fluxes computed from data using direct covariance and bulk aerodynamic methods show that there is good agreement in unstable conditions when the latent heat flux values are generally positive. However, the agreement is relatively poor in stable conditions, particularly when the moisture flux is directed downward. If the direct covariance measurements are indeed accurate, then they clearly indicate that the bulk aerodynamic formula overestimate the downward moisture flux in stable conditions. As a result, comparisons of the Dalton number for unstable and stable conditions indicate a marked difference in value between the two stability regimes. Investigations done for this thesis used data taken primarily at the Air-Sea Interaction Tower (ASIT) during the Coupled Boundary Layers and Air-Sea Transfer (CBLAST) Experiment 2003 from the 20-27 August 2003. Other data from the shore based Martha's Vineyard Coastal Observatory (MVCO) and moored buoys in the vicinity of the ASIT were also incorporated. During this eight day period, the boundary layer was often characterized by light winds, a stably stratified surface layer and a swell dominated wave field. Additionally, the advection of warm moist air over cooler water resulted in fog formation and a downward flux of moisture on at least three occasions. Therefore, a primary objective of this thesis is to present a case study to investigate the cause of this shortcoming in the bulk formula under these conditions by examining the physical processes that are unique to these boundary layers. Particular attention will be paid to the behavior of the Dalton number in a stable marine atmospheric boundary layer under foggy conditions using insights derived from the study of fog formation and current flux parameterization methods.

Description: Submitted to the Department of Mechanical Engineering in partial fulfillment of the requirements for the degree of Doctor of Philosophy in Applied Ocean Sciences at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution February 2007

Description: This thesis introduces an algorithm for inverting for the geoacoustic properties of the seafloor in shallow water. The input data required by the algorithm are estimates of the amplitudes of the normal modes excited by a low-frequency pure-tone sound source, and estimates of the water column sound speed profiles at the source and receiver positions. The algorithm makes use of perturbation results, and computes the small correction to an estimated background profile that is necessary to reproduce the measured mode amplitudes. Range-dependent waveguide properties can be inverted for so long as they vary slowly enough in range that the adiabatic approximation is valid. The thesis also presents an estimator which can be used to obtain the input data for the inversion algorithm from pressure measurements made on a vertical line array (VLA). The estimator is an Extended Kalman Filter (EKF), which treats the mode amplitudes and eigenvalues as state variables. Numerous synthetic and real-data examples of both the inversion algorithm and the EKF estimator are provided. The inversion algorithm is similar to eigenvalue perturbation methods, and the thesis also presents a combination mode amplitude/eigenvalue inversion algorithm, which combines the advantages of the two techniques.

Description: The funding that made this research possible came from the Office of Naval Research, and 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 September, 2005

Description: Minor and trace element records from planktic and benthic foraminifera from Atlantic sediment cores, as well as outputfrom a coupled OA·GCM, were used to investigate the magnitude and distribution of the oceanic response to abrupt Climate events.of the past 20,000 years. The study addressed three major questions: 1) What is the magnitude of high-latitude sea surface temperature and salinity variability during abrupt climate events? 2) Does intermediate depth ventilation change in conjunction with high-latitude climate variability? 3) Are the paleoclimate data consistent with the response of a coupled OAGCM to a freshwater perturbation? To address these questions, analytical methods were implemented for the simultaneous measurement of Mg/Ca, Zn/Ca, Cd/Ca, Mn/Ca and All Ca in foraminiferal samples using inductively-coupled plasma mass spectrometry. Paired records of planktic foraminiferal ()IRO and Mg/Ca from the subpolar North Atlantic reveal trends of increasing temperatures (-3°C) and salinities over the course of the Holocene. The records provide the first evidence of open':'ocean cooling (nearly 2°C) and freshening during the 8.2 kyr event, and suggest similar conditions at 9.3 ka. Benthic foraminiferal Cd/Ca results from an intermediate depth, western South Atlantic core (l,268 ni) are consistent with reduced export into the South Atlantic of North Atlantic Intermediate Water during the Younger Dryas. Paired records. of benthic foraminiferal Mg/Ca and bIRO from two intermediate depth low latitude western Atlantic sites - one from the Florida Current (751 m) and one from the Little Bahama Bank (l,057 m) - provicie insights into the spatial distribution of intermediate depth temperature and sii!.inity variability during" the Younger Dryas. The intermediate depth paleoceanographic temperature and salinity data are consistent with the results of a GFDL R30 freshwater forced model simulation, suggesting that freshwater forcing is a possible driver or amplifier for B011ing-Aller0d to Younger Dryas climate variability. Benthic foraminiferal Cd/Ca results from an intermediate depth Florida Current core (751 m) are consistent with a decrease in the northward penetration of southern source waters within the return flow of the Atlantic meridional overturning circulation (MOC) and an increase in the influence of intermediate depth northern source waters during the Younger Dryas.

Description: This work was funded by a John Lyons Fellowship and a WHOI Ocean and Climate Change Institute Fellowship. Analyses were funded by the Ocean and Climate Change Institute and the following grants from the National Science Foundation: OCE98-86748, OCE02- 20776, OCE96-33499,ATM05-01391, and OCE04-02565.

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 March, 1980

Description: The Southern Ocean as defined here is the body of water between the Antarctic Continent and the Antarctic Polar Front, (APF). This ocean is considered important in the global thermodynamic balance of the ocean-atmosphere system because large planetary heat losses are believed to occur at high latitudes. The ocean and atmosphere must transport heat poleward to balance these losses. In the Southern Hemisphere, the oceanic contribution to this flux involves a southward transport of heat across the APF into the Southern Ocean where it is given up to the atmosphere through air-sea interactions. In Part I, the air-sea interactions and structure of the near surface waters of the Southern Ocean are investigated with a three dimensional time dependent numerical model. The surface waters in this region in summer are characterized by a relatively warm surface mixed layer with low salinity. Below this layer, a cold temperature extremum is usually observed in vertical profiles which is believed to be the remnant of a deep surface mixed layer produced in winter. The characteristics of this layer, the surface mixed layer and the observed distribution of wintertime sea ice are reproduced well by this model. Unlike some other sea-ice models the air-sea heat exchange is a free variable. Model estimates of the annual heat loss by the Southern Ocean exhibit the observed meridional variation of heat gained by the ocean along the APF with heat lost further south. The model's area average heat loss is much smaller than that estimated with direct observations. While several model parameterizations were made which could be in error, the model results suggest that the Southern Ocean does give up vast amounts of heat to the atmosphere away from the continental margins. The model results and direct calculations of air-sea exchanges suggest a southward heat flux must occur across the APF. The lateral water mass transition across the front is not discontinuous but occurs over a finite sized zone of fluid which is dominated by intrusive finestructure. The characteristics and dynamics of these features are investigated in Part II to try and assess their importance in the meridional heat budget. Observations made on two cruises to the APF are presented and the space-time scales of the features and thermohaline characteristics are discussed. It is suggested that double diffusive processes dominated by salt fingering are active within the intrusions. An extension of Stern's (1967) model of the stability of a thermohaline front to intrusive finestructure driven by saltfingering where small scale viscous processes are included, is presented to explain why intrusions are observed in frontal zones. The model successfully predicts vertical scales of intrusions observed in the ocean and the observed dependence of the intrusions' slopes across density surfaces on the vertical scale. Since the fastest growing intrusion is not strongly determined by the model, though, it is likely that finite amplitude effects determine the dominant scale of interleaving in the ocean. The analysis predicts that intrusions transport heat, salt and density down the mean gradients of the front. For the APF, this heat flux is poleward which is the direction required by the global heat budget. This model does not describe intrusions at finite amplitude or in steady state and so cannot be used to estimate the magnitude of the poleward heat flux due to intrusions in the APF.

Description: The research reported on here, and my support as a graduate student was provided by the National Science Foundation through grants OCE 75 14056. OCE 76 82036 and OCE 77 28355.

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, 2006

Description: Benthic and planktonic foraminiferal δ18O (δ18Oc) from a suite of well-dated, high-resolution cores spanning the depth and width of the Straits of Florida reveal significant changes in Gulf Stream cross-current density gradient during the last millennium. These data imply that Gulf Stream transport during the Little Ice Age (LIA: 1200-1850 A.D.) was 2-3 Sv lower than today. The timing of reduced flow is consistent with cold conditions in Northern Hemisphere paleoclimate archives, implicating Gulf Stream heat transport in centennial-scale climate variability of the last 1,000 years. The pattern of flow anomalies with depth suggests reduced LIA transport was due to weaker subtropical gyre wind stress curl. The oxygen isotopic composition of Florida Current surface water (δ18O w) near Dry Tortugas increased 0.4% during the course of the Little Ice Age (LIA: ~1200-1850 A.D.), equivalent to a salinity increase of 0.8-1.5 psu. On the Great Bahama Bank, where smface waters are influenced by the North Atlantic subtropical gyre, δ18Ow increased by 0.3% during the last 200 years. Although a portion (~0.1%) of this shift may be an artifact of anthropogenically-driven changes in surface water ΣCO2, the remaining δl8Ow signal implies a 0.4 to 1 psu increase in salinity after 200 yr BP. The simplest explanation of the δ18Ow data is southward migration of the Atlantic Hadley circulation during the LIA. Scaling of the δ18Ow records to salinity using the modern low-latitude δ18Ow-S slope produces an unrealistic reversal in the salinity gradient between the two sites. Only if C180 w is scaled to salinity using a high-latitude δ18Ow-S slope can the records be reconciled. Changes in atmospheric 14C paralleled shifts in Dry Tortugas δ18Ow , suggesting that variable solar irradiance paced centennial-scale Hadley cell migration and changes in Florida Current salinity during the last millennium.

Description: I would also like to thank the National Science Foundation (NSF grant OCE-0096469) and WHOI Academic Programs for supporting this 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 February 2007

Description: Oceanic spreading centers are sites of magmatic, tectonic, and hydrothermal processes. In this thesis I present experimental and seismological constraints on the evolution of these complex regions of focused crustal accretion and extension. Experimental results from drained, triaxial deformation experiments on partially molten olivine reveal that melt extraction rates are linearly dependent on effective mean stress when the effective mean stress is low and non-linearly dependent on effective mean stress when it is high. Microearthquakes recorded above an inferred magma reservoir along the TAG segment of the Mid-Atlantic Ridge delineate for the first time the arcuate, subsurface structure of a long-lived, active detachment fault. This fault penetrates the entire oceanic crust and forms the high-permeability pathway necessary to sustain long-lived, high-temperature hydrothermal venting in this region. Long-lived detachment faulting exhumes lower crustal and mantle rocks. Residual stresses generated by thermal expansion anisotropy and mismatch in the uplifting, cooling rock trigger grain boundary microfractures if stress intensities at the tips of naturally occurring flaws exceed a critical stress intensity factor. Experimental results coupled with geomechanical models indicate that pervasive grain boundary cracking occurs in mantle peridotite when it is uplifted to within 4 km of the seafloor. Whereas faults provide the high-permeability pathways necessary to sustain high-temperature fluid circulation, grain boundary cracks form the interconnected network required for pervasive alteration of the oceanic lithosphere. This thesis provides fundamental constraints on the rheology, evolution, and alteration of the lithosphere at oceanic spreading centers.

Description: Research was funded by a MIT Presidential Fellowship and NSF grants OCE-0095936, OCE-9907224, OCE-0137329, OCE-6892222, and OCE-6897400.

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, 1978

Description: A two-layer linear analytic model is used to study the response of the mid-latitude ocean to the seasonal variation of the windstress. The most important component of the response is a barotropic quasi-steady Sverdrup balance. A meridional ridge such as the Antilles Arc is modeled as an infinitely thin meridional barrier that blocks the lower layer but does not protrude into the upper layer. It is found that such a barrier has little effect on the upper layer flow across the barrier. This result is obtained provided the frequency of the motion is low enough so that free short Rossby waves are essentially nondivergent. In this case there is little coupling between the layers for energy propagating to the east away from the barrier. A study of the dynamics of flow over a sloping bottom is made and the results are used to determine the effect on seasonal oscillations of eastern boundary slopes and triangular ridges. It is found that the presence of a slope at the eastern boundary has little effect. A meridional ridge that does not reach the interface may cause substantial scattering of free Rossby waves, but unless the ridge is steep its effect on the quasi-steady Sverdrup balance is minimal. However, if the ridge height is a substantial fraction of the lower layer depth and the width is comparable to the scale of free short Rossby waves, the ridge will tend to block flow in the lower layer, acting like the infinitely thin barrier. The theory suggests that the Antilles Arc should have the effect of a thin barrier, while the Mid-Atlantic Ridge should have little effect on the response of the ocean to seasonal wind variations.

Description: For three and a half years of generous financial support I am grateful to the John and Fannie Hertz Foundation, from which I received a Graduate Fellowship. Research money and other support were provided by the National Science Foundation under contract OCE 77 15600.

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 2006

Description: The impact of the Greenland tip jet on the wintertime mixed-layer of the southwest Irminger Sea is investigated using in-situ moored profiler data and a variety of atmospheric data sets. The mixed-layer was observed to reach 400 m in the spring of 2003, and 300 m in the spring of 2004. Both of these winters were mild and characterized by a low North Atlantic Oscillation (NAO) index. All of the storms that were advected through the region were tracked, and the tip jet events that occurred throughout the two winters were identified. Composite images of the tip jets elucidated the conditions during which tip jets were likely to take place, which led to an objective method of determining tip jet occurrences by taking into account the large-scale pressure gradients. Output from a trajectory model indicates that the air parcels entering a tip jet accelerate and descend as they are deflected around southern Greenland. A heat flux timeseries for the mooring site was constructed that includes the enhancing influence of the tip jet events. This was used to drive a one-dimensional mixed-layer model, which was able to reproduce the observed mixed-layer deepening in both winters. All of the highest heat flux events took place during tip jets, and removal of the tip jets from the heat flux timeseries demonstrated their importance in driving convection east of Greenland. The deeper mixed-layer of the first winter was in large part due to a higher number of robust tip jet events, which in turn was caused by a greater number of storms passing northeast of southern Greenland. This interannual change in storm tracks was attributable to a difference in upper level steering currents. Application of the mixed-layer model to the winter of 1994-l995, during a period characterized by a high NAO index, resulted in convection reaching 1600 m. This predict ion is consistent with concurrent hydrographic data, supporting the notion that deep convection can occur in the Irminger Sea during strong winters.

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 Doctor of Philosophy at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution February 2007

Description: The role of ocean dynamics in driving air-sea interaction is examined at two contrasting sites on 125°W in the eastern tropical Pacific Ocean using data from the Pan American Climate Study (PACS) field program. Analysis based on the PACS data set and satellite observations of sea surface temperature (SST) reveals marked differences in the role of ocean dynamics in modulating SST. At a near-equatorial site (3°S), the 1997-1998 El Nino event dominated the evolution of SST and surface heat fluxes, and it is found that wind-driven southward Ekman transport was important in the local transition from El Nino to La Nina conditions. At a 10°N site near the summertime position of the Inter-tropical Convergence Zone, oceanic mesoscale motions played an important role in modulating SST at intraseasonal (50- to 100-day) timescales, and the buoy observations suggest that there are variations in surface solar radiation coupled to these mesoscale SST variations. This suggests that the mesoscale oceanic variability may influence the occurrence of clouds. The intraseasonal variability in currents, sea surface height, and SST at the northern site is examined within the broader spatial and temporal context afforded by satellite data. The oscillations have zonal wavelengths of 550-1650 km and propagate westward in a manner consistent with the dispersion relation for first baroclinic mode, free Rossby waves in the presence of a mean westward flow. The hypothesis that the intraseasonal variability and its annual cycle are associated with baroclinic instability of the North Equatorial Current is supported by a spatio-temporal correlation between the amplitude of intraseasonal variability and the occurrence of westward zonal flows meeting an approximate necessary condition for baroclinic instability. Focusing on 10°N in the eastern tropical Pacific, the hypothesis that mesoscale oceanic SST variability can systematically influence cloud properties is investigated using several satellite data products. A statistically significant relationship between SST and columnar cloud liquid water (CLW), cloud reflectivity, and surface solar radiation is identified within the wavenumber-frequency band corresponding to oceanic Rossby waves. Analysis of seven years of CLW data and 20 years surface solar radiation data indicates that 10-20% of the variance of these cloud-related properties at intraseasonal periods and wavelengths on the order of 10° longitude can be ascribed to SST signals driven by oceanic Rossby waves.

Description: I gratefully acknowledge support from the following sources: NOAA Grants NA87RJ0445 (2002-2003) and NA17RJ1223 (2005-2006), and an MIT Presidential Fellowship (2000-2001). I also received support from The Cooperative Institute for Climate and Ocean Research, a NOAA-WHOI joint institute (NOAA Grant NA17RJ1223).

Description: Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy at the Woods Hole Oceanographic Institution February 1980

Description: The existence of resonant, baroclinic, equatorially-trapped inertia-gravity waves (discovered by Wunsch and Gill (1976)) is confirmed in the mid-Pacific by spectral analysis of long sea level records. The energy of the low-mode inertia-gravity waves is found to decrease toward the meridional boundaries. A simple spectral model, acknowledging the dispersive characteristics of the equatorial waves, adequately reproduces the observed mid-Pacific sea level spectra in the 1-6 day band. Model spectra computed at latitudes outside the equatorial waveguide of the gravest meridional modes suggest the presence of "inertial" peaks in several observed sea level spectra. Resonant, low-mode inertia-gravity waves may also exist in the Indian Ocean. Sea level fluctuations along the Pacific equator are found to have Kelvin wave characteristics in the 35-80 day band, and, in particular, propagation from the western Pacific to the coast of South America is observed. The Kelvin waves are atmospherically-forced in the central- western Pacific and have a computed equivalent depth corresponding to the first-baroc1inic mode. Outside of the equatorial mid-Pacific, a non-static ocean response to air pressure in the 4-6 day band is dominated by a basin-wide, barotropic, planetary mode. The low Q of this mode suggests that the ocean is viscous with respect to large-scale barotropic oscillations. The dynamical components of the observed long-period tides have been isolated for the first time using the "self-consistent" equilibrium tide of Agnew and Farrell (1978). The tides are slightly non-equilibrium with large horizontal scales. The relatively short-scale Rossby modes predicted by Wunsch (1967) are not observed, perhaps because of the poor spatial coverage of the dataset. Considering the low Q of the 4-6 day planetary basin mode, it is suggested that the long-period tides are frictionally-controlled. The 4- and 5-day equatorial inertia-gravity waves, the 35-80 day Kelvin waves and the 4-6 day planetary basin mode are clearly atmospherically forced, and, perhaps surprisingly, they are forced by atmospheric waves that have similar horizontal structures, i.e., 4-5 day Rossby-gravity waves, 40-50 day Kelvin waves and a 5-day global barotropic mode. The surface expressions of these atmospheric waves are determined in order to understand the nature of the oceanic response, e.g., resonant or forced. Much of the information about the surface atmospheric fields that has been collected, including frequency-wavenumber descriptions, awaits an accurate model of the coupling between wind stress and internal ocean waves.

Description: Monetary support for this research was provided by the National Science Foundation through contract OCE73-0l384. At various times, tuition and living expenses were paid by funds from the NSF contract above, the Office of Naval Research (contract N00014-C-75-029l), the Cecil and Ida Green Professorship in Earth Sciences, the J. P. Luther Educational Fund and by an M.I.T. Educational Tuition Award.

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 1997

Description: A new tomographic technique is employed to investigate the structure and dynamics of the Pacific upper mantle. We invert band-center travel times of ScS reverberations and frequency-dependent travel times of direct S phases, upper-mantle guided waves such as SS and SSS, and the R1 and G1 surface waves for the 2D composite structure in the plane of two Pacific corridors. The frequency-dependent travel times of the turning and surface waves are measured from all three components of ground motion as phase delays relative to a radially-anisotropic, spherically-symmetric oceanic mantle model, and their 2D Fréchet kernels are constructed by a coupled-mode algorithm. The travel times of the primary ScSn and sScSn phases and their first-order reverberations from the 410 and 660 discontinuities are measured as individual phases and the 2D Fréchet kernels for these band-limited signals are calculated using the paraxial ray approximation. The model parameters include shear-speed variations throughout the mantle, perturbations to radial shear-wave anisotropy in the uppermost mantle, and the topography of the 410 and 660 discontinuities. We construct vertical tomograms through two mantle corridors: one between the Tonga subduction zone and Oahu, Hawaii, which traverses the central Pacific Ocean; and the other between the Ryukyu subduction zone and Oahu, which samples the northern Philippine Sea, the western Pacific, and the entire Hawaiian swell. Tests demonstrate that the data sets for the two corridors resolve the lateral structure in the upper mantle with a scale length of a few hundreds kilometers and greater but that the resolving power decreases rapidly in the lower mantle. The model for the Tonga-Hawaii corridor reveals several interesting features, the most significant being a regular pattern of high and low shear velocities in the upper mantle between Tonga and Hawaii. These variations, which are well resolved by the data set, have a horizontal wavelength of 1500 km, a vertical dimension of 700 km, and an amplitude of about 3%, and they show a strong positive correlation with seafloor topography and geoid-height variations along this corridor. The geoid highs correspond to a series of northwest-trending swells associated with the major hotspots of the Society, Marquesas, and Hawaiian Islands. Where these swells cross the corridor, they are underlain by high shear velocities throughout the uppermost mantle, so it is unlikely that their topography is supported by thermal buoyancy. This result is substantiated by the model from the Ryukyu-Hawaii corridor, which exhibits a prominent, fast region that extends beneath the entire Hawaiian swell. This anomaly, which resides in the uppermost 200-300 km of the mantle, is also positively correlated with the undulations of the Hawaiian-swell height. The other dominant features in the Ryukyu-Hawaii model include the high-velocity subducting slabs beneath the Ryukyu and Izu-Bonin seismic zones, which extend throughout the entire upper mantle; a very low-velocity in the uppermost 160 km of the mantle beneath the northern Philippine Sea, which is ascribed to the presence of extra water in this region; and a pronounced minimum in the amount of radial anisotropy near Hawaii, which is also seen along the Tonga-Hawaii corridor. A joint inversion of the data from the two corridors reveals the same anomaly pattern and clearly demonstrates that the swells in the Central Pacific are underlain by fast velocities. It is therefore implied that the topography of the swells in the central Pacific is supported by a chemical buoyancy mechanism which is generated by basaltic volcanism and the formation of its low-density peridotitic residuum. While the basaltic depletion mechanism can produce high shear velocities in the uppermost 200 km, it cannot explain the depth extent of the fast anomalies beneath the swells which, along Tonga-Hawaii corridor, extend well into the transition zone. It is therefore hypothesized that the central Pacific is underlain by a system of convective rolls that are confined above the 660-km discontinuity. It is likely that these rolls are predominantly oriented in the direction of plate motion (like "Richter rolls ") but the limited depth of the fast anomaly beneath the Hawaiian swell (200-300 km) suggests that their pattern is probably more complicated. Nevertheless, this convection pattern appears to be strongly correlated with the locations of the Tahitian, Marquesan, and Hawaiian hotspots, which raises interesting questions for Morgan's hypothesis that these hotspots are the surface manifestations of deep-mantle plumes.

Description: This research was supported by the National Science Foundation under grant EAR- 9628351 and by the Defense Special Weapons Agency under grant DSW A-F49620-95-1- 0051.

Description: Submitted in partial fulfillment of the requirements of the degree of Master of Science at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution June 2004

Description: Laterally extensive, well-developed clinoforms have been mapped in Early Cretaceous deposits located in the northeastern 27,000 km2 of the Colvile Basin, North Slope of Alaska. Using public domain 2-D seismic data, well logs, core photographs, and grain size data, depositional geometries within the Nanushuk and Torok formations were interpreted in order to constrain the transport conditions associated with progradation of the shoreline and construction of the continental margin out of detritus shed from the ancestral Brooks Range. Using STRATA, a synthetic stratigraphic modeling package, constructional clinoform geometries similar to those preserved in the North Slope clinoform volume (32,400 km3) were simulated. Sediment flux, marine and nonmarine diffusivities, and basin subsidence were systematically varied until a match was found for the foreset and topset slopes, as well as progradation rates over a 6 milion year period. The ability of STRATA to match the seismically interpreted geometries allows us to constrain measures of possible water and sediment discharges consistent with the observed development of the Early Cretaceous c1inoform suite. Simulations indicate that, in order to reproduce observed geometries and trends using constant input parameters, the subsidence rate must be very small, only a fraction of the most likely rate calculated from the seismic data. Constant sediment transport parameters can successfully describe the evolution of the prograding margin only in the absence of tectonic subsidence. However, further work is needed to constrain the absolute magnitude of these values and determine a unique solution for the NPR-A clinoforms.

Description: Seychelles is composed of over 100 islands with a land area of approximately 455 km², centred close to 4°30'S and 55°30'E. The combined coastline is approximately 600 km long, the oceanic shelf totals about 50 000 km² and the Exclusive Economic Zone (EEZ) is over 1 370 000 km². The total population (1994 census) stands at just under 74 000. in 1994, the population registered a growth rate of 2.2%. The GDP (1994) was SR 2373.8 million, fisheries representing 4.8% of this sum. Licensing agreements for foreign fishing activities provided a yearly revenue of SR8 million. Port Victoria is seen as a prime centre for tuna fishing operations in the Indian Ocean. In the artisanal fishery just under 900 persons are working. The largest contributor to catch by vessel type are the traditional whaler vessels representing 47.8% of the total catch. Over 66.3% of the catch is by the handline method. Carangidae representing 24% and Lutjanidae 19% of total landings. There are six specific objectives to the fisheries sector policy, which aims as resource development and maximisation of potential benefits. Nearshore fishery resources are considered to be heavily exploited, however opportunities exist around the distant islands and in deeper waters off the Mahe plateau shelf. Aquaculture of molluscs and prawns is being developed and carried out. The main constraints to development are seen as the lack of skilled manpower and foreign exchange.

Description: Published

Description: Published

Description: Industrial tuna fishing

Description: Submitted in partial fulfillment of the requirements of the degree of Doctor of Philosophy at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution September 2006

Description: Two years of moored oceanographic and automatic weather station data which span the winter ice seasons of 2001-2003 within Marguerite Bay on the western Antarctic Peninsula (wAP) shelf were collected as part of the Southern Ocean Global Ocean Ecosystems Dynamics program. In order to characterize the ice environment in the region, a novel methodology is developed for determining ice coverage, draft and velocity from moored upward-looking acoustic Doppler current profiler data. A linear momentum balance shows the importance of internal ice stresses in the observed motion of the ice pack. Strong inertial, not tidal, motions were observed in both the sea ice and upper ocean. Estimates of upward diapycnal fluxes of heat and salt from the Upper Circumpolar Deep Water to the surface mixed layer indicate almost no contribution from double diffusive convection. A one-dimensional vertical mixed layer model adapted for investigation of mixing beneath an ice-covered ocean indicates that the initial wind event, rather than subsequent inertial shear, causes the majority of the mixing. This work points towards episodic wind-forced shear at the base of the mixed layer coupled with static instability from brine rejection due to ice production as a major factor in mixing on the wAP shelf.

Description: The work described in this thesis was funded by the National Science Foundation Office of Polar Programs grant # 99-100092. Additional funding came from the MIT Houghton Fund, the WHOI Ocean Venture Fund and the WHOI Education Office.

Description: The series of observations described in this report were planned with the double purpose of measuring the evaporation and transport of water vapor from the ocean into an unstable atmosphere, and of studying the diffusion processes operating in air of this stability class. Measured values of the evaporation from ocean surfaces were conspicuously absent from the meteorological literature until Craig and Montgomery (1949) published values for hydrostatically stable air. The present set of measurements extends our knowledge to include evaporation into a hydrostatically unstable air mass. In addition to evaporation values at the surface, net transports of water vapor at many levels up to 2000 meters have been measured.

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, 1971

Description: The objective of this study was to describe the mechanics of wind wave generation and spectral development. Intermittency, high frequency microstructure in wind and wave fields, and strong nonlinear coupling involving a wide range of scales are shown to be crucial elements in the transfer of momentum to, from, and within the wave field. None of these elements are included in available theories. Measurements of wave height and of the turbulent atmospheric and subsurface boundary layers were made, from a small surface following platform and from a stable 38.5m spar buoy. The structure of moving gust patterns (cat's paws) is described and related to the generation of surface waves. Results from this and other background studies are then applied to a discussion of spectral growth during a two day period of active wave generation. Cat's paws contain 'bursts' of intense turbulent stress and buoyancy fluctuations separated by quiescent 'intervals'. There is a difference of over three orders of magnitude in fluctuation strength between these features. Rapid growth rate generation of high frequency surface waves and atmospheric turbulence occurs during the bursts. The resultant microscale components aid the growth of lower frequency instabilities by strong nonlinear coupling between scales of motion and by acting as drag or roughness elements. Evidence of strong coupling between frequency bands and of weakly resonant capillary-gravity wave interactions is presented. Thermal stratification has a strong influence on fluctuation magnitude and can delay the onset of surface wave generation. Major spectral growth is highly unsteady. Much of the momentum flux from air to sea occurs during intermittent events that are similar in nature to cat's paws, and goes directly into high frequency waves. The bursts occur predominantly over large groups of surface waves and involve strong nonlinear interactions between media and frequency bands. The long-term equilibrium balance between wind and water is disrupted by variations in surface currents. There are 'critical' wind speeds characterized by anomalous relationships between parameters of the predominantly logarithmic velocity profile.

Description: The purpose of the present paper is to analyze, in a reasonably comprehensive fashion, the principal factors controlling the mean state of turbulence and hence the mean velocity distribution in wind and ocean currents near the surface. The plan of the investigation is theoretical but efforts have been made to check each major step or result through an analysis of available measurements. The comparison of theory and observations is made diffcult by the fact that in most cases measurements have been arranged without the aid of a working hypothesis concerning the dynamics of the effect studied; thus information is often lacking concerning parameters essential to the interpretation of the data.