ALBERT

Beschreibung: 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 1998

Beschreibung: This thesis is written in two parts. The first part deals with the problem oflateral dispersion due to mesoscale eddies in the open ocean, and the interaction between the mesoscale strain and horizontal diffusion on spatial scales less than 10 km. The second and major part examines lateral dispersion over the continental shelf on scales of 100 m to 10 km and over time scales of 1- 5 days. PART I: Lateral Dispersion and the North Atlantic Tracer Release Experiment Mixing and stirring of Lagrangian particles and a passive tracer were studied by comparison of float and tracer observations from the North Atlantic Tracer Release Experiment. Statistics computed from the NATRE floats were found to be similar to those estimated by Ledwell et al. (1998) from the tracer dispersion. Mean velocities computed from the floats were (u, v) = ( -1.2±0.3, -0.9±0.2) em s-1 for the (zonal, meridional) components, and large-scale effective eddy diffusivities were (KP. 11 , K:e 22 ) = (1.5±0. 7, 0. 7±0.4) x 103 m2 s-1 . The NATRE observations were used to evaluate theoretical models of tracer and particle dispersal. The tracer dispersion observed by Ledwell et al. (1998) was consistent with an exponential growth phase for about the first 6 months and a linear growth at larger times. A numerical model of mesoscale turbulence that was calibrated with float statistics also showed an exponential growth phase of tracer and a reduced growth for longer times. Numerical results further show that Garrett's (1983) theory, relating the effective small-scale diffusivity to the rms strain rate and tracer streak width, requires a scale factor of 2 when the observed growth rate of streak length is used as a measure of the strain rate. This scale factor will be different for different measures of the strain rate, and may also be affected by temporal and spatial variations in the mesoscale strain field. PART II: Lateral Dispersion over the New England Continental Shelf Lateral dispersion over the continental shelf was examined using dye studies of the Coastal Mixing and Optics (CMO) program. Four experiments performed at intermediate depths and lasting 3 to 5 days were examined. In some cases, the dye patches remained fairly homogeneous both vertically and horizontally throughout an experiment. In other cases, significant patchiness was observed on scales ranging from 2- 10 m vertically and a few hundred meters to a few kilometers horizontally. The observations also showed that the dye distributions were significantly influenced by shearing and straining on scales of 5- 10 m in the vertical and 1- 10 km in the horizontal. Superimposed on these larger-scale distortions were simultaneous increases in the horizontal second moments of the dye patches, with corresponding horizontal diffusivities based on a Fickian diffusion model of 0.3 to 4.9 m2 s-1 . Analysis of the dye data in concert with shear estimates from shipboard ADCP observations showed that the existing paradigms of shear dispersion and dispersion by interleaving water-masses can not account for the observed diffusive spreading of the dye patches. This result suggests that some other mechanisms provided an additional diffusivity of order 0.15 to 4.0 m2 s-1 . An alternative mechanism, dispersion by vortical motions caused by the relaxation of diapycnal mixing events, was proposed which could explain the observed dispersion in some cases. Order-of-magnitude estimates of the effective lateral dispersion due to vortical motions showed that this mechanism could account for effective horizontal diffusivities of order 0.01 to 1.1 m2 s-1 . The upper range of these estimates were within the range required by the observations for two of the four experiments examined.

Beschreibung: The work in Part I relating to the North Atlantic Tracer Release Experiment was supported by the National Science Foundation under grant OCE90-05738. The work in Part II relating to the Coastal Mixing and Optics program was funded by the Office of Naval Research under grant N00014-95-1-0633 (tracer experiments) and grant N00014-95-1-1063 (AASERT fellowship).

Beschreibung: 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 2015

Beschreibung: The spatial and temporal evolution of stratified shear instabilities is quantified in a highly stratified and energetic estuary. The measurements are made using high-resolution acoustic backscatter from an array composed of six calibrated broadband transducers connected to a six-channel high-frequency (120-600 kHz) broadband acoustic backscatter system. The array was mounted on the bottom of the estuary and looking upward. The spatial and temporal evolution of the waves is described in terms of their wavelength, amplitude and turbulent dissipation as a function of space and time. The observed waves reach an arrested growth stage nearly 10 times faster than laboratory and numerical experiments performed at much lower Reynolds number. High turbulent dissipation rates are observed within the braid regions of the waves, consistent with the rapid transition to arrested growth. Further, it appears that the waves do not undergo periodic doubling and do not collapse once their maximum amplitude is reached. Under some conditions long internal waves may provide the perturbation that decreases the gradient Richardson number so as to initiate shear instability. The initial Richardson number for the observed instabilities is likely between 0.1 and 0.2 based on the slope and growth rate of the shear instabilities.

Beschreibung: 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 2014

Beschreibung: Motivated by inspection of complex underwater environments, we have developed a system for multi-sensor SLAM utilizing both structured and unstructured environmental features. We present a system for deriving planar constraints from sonar data, and jointly optimizing the vehicle and plane positions as nodes in a factor graph. We also present a system for outlier rejection and smoothing of 3D sonar data, and for generating loop closure constraints based on the alignment of smoothed submaps. Our factor graph SLAM backend combines loop closure constraints from sonar data with detections of visual fiducial markers from camera imagery, and produces an online estimate of the full vehicle trajectory and landmark positions. We evaluate our technique on an inspection of a decomissioned aircraft carrier, as well as synthetic data and controlled indoor experiments, demonstrating improved trajectory estimates and reduced reprojection error in the final 3D map.

Beschreibung: 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 2014

Beschreibung: Recent work has documented dramatic changes in the West Antarctic Ice Sheet (WAIS) over the past 30 years (e.g., mass loss, glacier acceleration, surface warming) due largely to the influence of the marine environment. WAIS is particularly vulnerable to largescale atmospheric dynamics that remotely influence the transport of marine aerosols to the ice sheet. Understanding seasonal- to decadal-scale changes in the marine influence on WAIS (particularly sea-ice concentration) is vital to our ability to predict future change. In this thesis, I develop tools that enable us to reconstruct the source and transport variability of marine aerosols to West Antarctica in the past. I validate new firn-core sea-ice proxies over the satellite era; results indicate that firn-core glaciochemical records from this dynamic region may provide a proxy for reconstructing Amundsen Sea and Pine Island Bay polynya variability prior to the satellite era. I next investigate the remote influence of tropical Pacific variability on marine aerosol transport to West Antarctica. Results illustrate that both source and transport of marine aerosols to West Antarctica are controlled by remote atmospheric forcing, linking local dynamics (e.g., katabatic winds) with large-scale teleconnections to the tropics (e.g., Rossby waves). Oxygen isotope records allow me to further investigate the relationship between West Antarctic firn-core records and temperature, precipitation origin, sea-ice variability, and large-scale atmospheric circulation. I show that the tropical Pacific remotely influences the source and transport of the isotopic signal to the coastal ice sheet. The regional firn-core array reveals a spatially varying response to remote tropical Pacific forcing. Finally, I investigate longer-term (~200 year) ocean and ice-sheet changes using the methods and results gleaned from the previous work. I utilize sea-ice proxies to reconstruct long-term changes in sea-ice and polynya variability in the Amundsen Sea, and show that the tropics remotely influence West Antarctica over decadal timescales. This thesis utilizes some of the highest-resolution, most coastal records in the region to date, and provides some of the first analyses of the seasonal- to decadal-scale controls on source and transport of marine aerosols to West Antarctica.

Beschreibung: This research was supported by an award from the Department of Energy Office of Science Graduate Fellowship Program (DOE SCGF) to ASC, a James E. and Barbara V. Moltz Research Fellowship, and by grants from the National Science Foundation Office of Polar Programs (NSF-OPP; #ANT-0632031 & #ANT-0631973), the National Science Foundation Major Research Instrumentation Program (NSF-MRI; #EAR-1126217), the NASA Cryosphere Program (#NNX10AP09G), and a WHOI Andrew W. Mellon Foundation Award for Innovative Research.

Beschreibung: 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 2014

Beschreibung: Underwater acoustic communication is an extremely complex field that faces many challenges due to the time-varying nature of the ocean environment. Vector sensors are a proven technology that when utilizing their directional sensing capabilities allows us to minimize the effect of interfering noise sources. A traditional pressure sensor array has been the standard for years but suffers at degraded signal to noise ratios (SNR) and requires maneuvers or a lengthly array aperture to direction find. This thesis explores the effect of utilizing a vector sensor array to steer to the direction of signal arrival and the effect it has on equalization of the signal at degraded SNRs. It was demonstrated that utilizing a single vector sensor we were able steer to the direction of arrival and improve the ability of an equalizer to determine the transmitted signal. This improvement was most prominent when the SNR was degraded to levels of 0 and 10 dB where the performance of the vector sensor outperformed that of the pressure sensor in nearly 100% of cases. Finally, this performance improvement occured with a savings in computational expense.

Beschreibung: ONR Grants #N00014-11-10426 and #N00014-07-10738

Beschreibung: 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 2014

Beschreibung: The ocean is a complex, constantly changing, highly dynamical system. Prediction capabilities are constantly being improved in order to better understand and forecast ocean properties for applications in science, industry, and maritime interests. Our overarching goal is to better predict the ocean environment in regions of complex topography with a continental shelf, shelfbreak, canyons and steep slopes using the MIT Multidisciplinary Simulation, Estimation and Assimilation Systems (MSEAS) primitive-equation ocean model. We did this by focusing on the complex region surrounding Taiwan, and the period of time immediately following the passage of Typhoon Morakot. This area and period were studied extensively as part of the intense observation period during August - September 2009 of the joint U.S. - Taiwan program Quantifying, Predicting, and Exploiting Uncertainty Department Research Initiative (QPE DRI). Typhoon Morakot brought an unprecedented amount of rainfall within a very short time period and in this research, we model and study the effects of this rainfall on Taiwan’s coastal oceans as a result of river discharge. We do this through the use of a river discharge model and a bulk river-ocean mixing model. We complete a sensitivity study of the primitive-equation ocean model simulations to the different parameters of these models. By varying the shape, size, and depth of the bulk mixing model footprint, and examining the resulting impacts on ocean salinity forecasts, we are able to determine an optimal combination of salinity relaxation factors for highest accuracy.

Beschreibung: Office of Naval Research for research support under grants N00014-08-1-0586 (QPE) to the Massachusetts Institute of Technology

Beschreibung: 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 1993

Beschreibung: There is a growing consensus that the sound generated by breaking waves is responsible for much of the ambient noise level in the ocean. While numerous field measurements have shown a strong correlation between the ambient noise spectrum level (N) in the range 100Hz to 25kHz and wind speed in the ocean, very little has been done to establish a comparable correlation between the ambient noise spectrum level and surface wave field parameters. The difficulty in establishing this relationship is remarkable given that the frequency and intensity of wave breaking are dependent on the characteristics of the wave field. In Fall 1991, an experiment was conducted from the research platform Flip 130 kilometers off the coast of Oregon, where the ambient noise between 2.5 and 25 kHz, the wind speed, and the sea surface elevation using wire wave gauges were measured. The correlation between N and the root mean square wave amplitude a was found to be poor but could be improved if the swell was filtered out from the wave elevation time series. The influence of swell on the value of a was disproportionate to the level of ambient noise since its characteristics were not directly due to the local wind-wave conditions. Observations of the dependence of the high frequency wind waves and the directional wave spectrum under turning winds suggested that the high frequency wave components responded more quickly to changes in the wind speed and wind direction than the energy-containing frequencies. The ambient noise level also correlated well with the root mean square wave slopes. This is consistent with previous laboratory measurements which showed that the steepness of a packet of waves correlates with the strength of wave breaking and with characteristics of breaking waves such as loss of momentum flux, dissipation, initial volume of air entrained, mixing, and sound generation. Comparisons of surface wave dissipation estimates using field measurements and models developed by Phillips (1985) and Hasselmann (1974) show that although the two models have very different forms, they give values that are comparable in magnitude. The relationship between the ambient noise level and log of dissipation give correlation coefficients (0.93-0.95) that are comparable to those between ambient noise and wind speed. The mean square acoustic pressure was shown to vary with the dissipation, with p2 ∝ D0.6-0.8. The results suggest that measurements of ambient sound may prove to be useful in inferring surface wave dissipation.

Beschreibung: 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 1997

Beschreibung: Acoustic propagation in the ocean inevitably encounters inhomogeneities of various types, which give rise to scattering. Acoustic scattering from rough water /bottom interfaces comprised of exposed rocks and sea mountains gives way to volumetric scattering in areas with fiat interfaces and thick sediment cover. The data analysis of the ARSRP backscattering experiment revealed that random inhomogeneities in two irregular layers beneath the seafloor were the primary contributors to oblique backscattering in a sediment pond on the western flank of the Mid-Atlantic Ridge. In this thesis, an attempt has been made to model monostatic backscattering from 3-D volume inhomogeneities in the sediment and to compare the results with the ARSRP backscattering data. A scattering process cannot be modeled correctly without a proper account of the incident field . Several approximate propagation models have been evaluated against the exact solution, while the appropriateness of using the equivalent surface scattering strength in volume scattering characterizations is studied. This study concludes that precautions need to be taken in modeling both the propagation effects and the scattering mechanisms associated with the bottom volume scattering process. A volume scattering model based on perturbation theory and the Born approximation is developed incorporating contributions from both sound speed and density fluctuations. With the propagation part handled accurately by OASES and random fluctuations generated effectively by a new scheme modified from the spectral method, the model is capable of simulating the monostatic backscattered field and time series due to 3-D volumetric sediment inhomogeneities. Both the characteristic length scale and power spectrum descriptions of the random inhomogeneities are shown to have great impact on the backscattered field by parameter studies in a free-space scenario. The important roles played by horizontal anisotropy and the vertical correlation of the random field have been demonstrated. Density fluctuations are further confirmed to be the dominant force in backscattering. The model matches the ARSRP backscattering data very well, with the fluctuations of sound speed and density in the two irregular layers described by a power law type of power spectrum.

Beschreibung: 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

Beschreibung: 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.

Beschreibung: 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.

Beschreibung: 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

Beschreibung: TOPEX/POSEIDON altimetry data are employed in the analysis of the global ocean response to atmospheric forcing. We use two different approaches to test the hypothesis that the global sea surface height variability can be adequately described by linear barotropic ocean models: the multichannel regression and the optimal smoothing techniques. We start with the simplest linear vorticity balance and continue by building a hierarchy of more complicated models by including effects of topography and time dependence. We use auto-regressive external (ARX) time-series models to test the hypothesis in all of the Pacific Ocean. We also test whether any significant residual regression on the atmospheric loading is left after the inverted barometer effect is corrected for. We find that no linear barotropic model is consistent with the data. We provide a check on the results of the multichannel regression by using a Kalman filter and optimal smoother. We use sequential estimation in the form of filteringsmoothing algorithm. We run the estimate for an area of 4000 km by 2000 km in the Northeast Pacific. We analyze model and data error structures by simulating the model without data assimilation. The results show that the model forecast on average explains 33% of the data variability. The Kalman filter updates the model very efficiently and produces an estimate which explains 76% of the data variance. The optimal smoother estimate is very similar to the Kalman filter estimate. Running the model in other regions of the Pacific produced worse fits of the model to the data. This supports the conclusion that the linear barotropic dynamics fails to describe the SSH variability.

Beschreibung: This research was partially funded by a NASA Global Change Fellowship.

Beschreibung: 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 1997

Beschreibung: This work is concerned with coherent communication by means of acoustic signals over underwater communication channels. The estimated scattering functions of real data ranging from the Arctic environment to tropical waters show that underwater communication channels can not be captured by a single, simple channel model. This thesis considers mainly a subset of underwater communication channels where the Doppler spread is more severe than the delay spread. An appropriate representation of the linear time- variant channel is introduced, and the wide sense stationary uncorrelated scattering (WSSUS) channel assumption enables characterization in terms of scattering functions. The concept of Doppler lines, which are frequency domain filters, is used in the derivation of a receiver for Doppler spread channels. The channel is simulated by means of a ray representation for the acoustic field and a time-variant FIR filter. The impact of physical ocean processes on the Doppler spread is demonstrated, and from this modeling explanations for the Doppler spread observed in real data are obtained. A decision feedback equalizer (DFE) adapted with recursive least squares (RLS) is analyzed, and its limit with respect to pure Doppler spread is found. By using the DFE with a phase locked loop (PLL) suboptimal system behavior is found, and this is verified on real data. In the case of a simple Doppler shift the cross-ambiguity function is used to estimate the shift, and the received signal is phase rotated to compensate this before it enters the receiver. A modified RLS called the time updated RLS (TU-RLS) is presented, and it is used in a new receiver. This receiver is initialized by means of the cross-ambiguity function and the performance is characterized by probability of decoding error vs delay spread, Doppler spread and SNR. The receiver uses Doppler lines to compensate both discrete and continuous Doppler spread. The receiver stability depends on the conditioning of the block diagonal correlation matrix propagated by the TU-RLS. The receiver is used to decode both real and simulated data, and some of these data are severely Doppler spread.

Beschreibung: The work in this thesis was funded in part by the Norwegian Research Counsil under a general PhD contract, the education office of Woods Hole Oceanographic Institution and the Office of Naval Research under contracts N00014-95-0153 and N00014-95-l-0322.

Beschreibung: 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 1997

Beschreibung: As part of the Shallow Water Acoustics in a Random Medium (SWARM) experiment [1], a sixteen element WHOI vertical line array (WVLA) was moored in 70 meters of water off the New Jersey coast. This array was sampled at 1395 Hz or higher for the seven days it was deployed. Tomography sources with carrier frequencies of 224 and 400 Hz were moored about 32 km shoreward, such that the acoustic path was anti-parallel to the primary propagation direction for shelf generated internal wave solitons. Two models for the propagation of normal modes through a 2-D waveguide with solitary internal wave (soliton) scattering included are developed to help in understanding the very complicated mode arrivals seen at the WVLA. The simplest model uses the Preisig and Duda [2] sharp interface approximation for solitons, allowing for rapid analysis of the effects of various numbers of solitons on mode arrival statistics. The second model, using SWARM thermistor string data to simulate the actual SWARM waveguides, is more realistic, but much slower. The analysis of the actual WVLA data yields spread, bias, wander, and intensity fluctuation signals that are modulated at tidal frequencies. The signals are consistent with predicted relationships to the internal wave distributions in the waveguides.

Beschreibung: The funds for my education were provided by the Office of Naval Research through an ONR Fellowship (MIT award 002734-001); the funds for SWARM were also provided by the Office of Naval Research through ONR Grant N00014-95-0051.

Beschreibung: 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

Beschreibung: 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.

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

Beschreibung: 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 June 2012

Beschreibung: Adaptive equalization is an important aspect of communication systems in various environments. It is particularly important in underwater acoustic communication systems, as the channel has a long delay spread and is subject to the effects of time- varying multipath fading and Doppler spreading. The design of the adaptation algorithm has a profound influence on the performance of the system. In this thesis, we explore this aspect of the system. The emphasis of the work presented is on applying concepts from inference and decision theory and information theory to provide an approach to deriving and analyzing adaptation algorithms. Limited work has been done so far on rigorously devising adaptation algorithms to suit a particular situation, and the aim of this thesis is to concretize such efforts and possibly to provide a mathematical basis for expanding it to other applications. We derive an algorithm for the adaptation of the coefficients of an equalizer when the receiver has limited or no information about the transmitted symbols, which we term the Soft-Decision Directed Recursive Least Squares algorithm. We will demonstrate connections between the Expectation-Maximization (EM) algorithm and the Recursive Least Squares algorithm, and show how to derive a computationally efficient, purely recursive algorithm from the optimal EM algorithm. Then, we use our understanding of Markov processes to analyze the performance of the RLS algorithm in hard-decision directed mode, as well as of the Soft-Decision Directed RLS algorithm. We demonstrate scenarios in which the adaptation procedures fail catastrophically, and discuss why this happens. The lessons from the analysis guide us on the choice of models for the adaptation procedure. We then demonstrate how to use the algorithm derived in a practical system for underwater communication using turbo equalization. As the algorithm naturally incorporates soft information into the adaptation process, it becomes easy to fit it into a turbo equalization framework. We thus provide an instance of how to use the information of a turbo equalizer in an adaptation procedure, which has not been very well explored in the past. Experimental data is used to prove the value of the algorithm in a practical context.

Beschreibung: Support from the agencies that funded this research- the Academic Programs Office at WHOI and the Office of Naval Research (through ONR Grant #N00014-07-10738 and #N00014-10-10259).

Beschreibung: 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 1998

Beschreibung: Work on the forward problem in zooplankton bioacoustics has resulted in the identification of three categories of acoustic scatterers: elastic-shelled (e.g. pteropods), fluid-like (e.g. euphausiids), and gas-bearing (e.g. siphonophores). The relationship between backscattered energy and animal biomass has been shown to vary by a factor of —19,000 across these categories, so that to make accurate estimates of zooplankton biomass from acoustic backscatter measurements of the ocean, the acoustic characteristics of the species of interest must be well-understood. This thesis describes the development of both feature based and model based classification techniques to invert broadband acoustic echoes from individual zooplankton for scatterer type, as well as for particular parameters such as animal orientation. The feature based Empirical Orthogonal Function Classifier (EOFC) discriminates scatterer types by identifying characteristic modes of variability in the echo spectra, exploiting only the inherent characteristic structure of the acoustic signatures. The model based Model Parameterisation Classifier (MPC) classifies based on correlation of observed echo spectra with simplified parameterisations of theoretical scattering models for the three classes. The Covariance Mean Variance Classifiers (CMVC) are a set of advanced model based techniques which exploit the full complexity of the theoretical models by searching the entire physical model parameter space without employing simplifying parameterisations. Three different CMVC algorithms were developed: the Integrated Score Classifier (ISC), the Pairwise Score Classifier (PSC) and the Bayesian Probability Classifier (BPC); these classifiers assign observations to a class based on similarities in covariance, mean, and variance, while accounting for model ambiguity and validity. These feature based and model based inversion techniques were successfully applied to several thousand echoes acquired from broadband (-350 kHz - 750 kHz) insonifications of live zooplankton collected on Georges Bank and the Gulf of Maine to determine scatterer class. CMVC techniques were also applied to echoes from fluid-like zooplankton (Antarctic krill) to invert for angle of orientation using generic and animal-specific theoretical and empirical models. Application of these inversion techniques in situ will allow correct apportionment of backscattered energy to animal biomass, significantly improving estimates of zooplankton biomass based on acoustic surveys.

Beschreibung: Thanks to the WHOI/MIT Joint Program Education Office for partial funding. Other sources of funding for my thesis work include the Ocean Acoustics, Oceanic Biology and URIP programs of the Office of Naval Research grant numbers N00014-89- J-1729, N00014-95-1-0287 and N00014-924-1527, and the Biological Oceanography program of the National Science Foundation grant number OCE-9201264.

Beschreibung: 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 1990

Beschreibung: An acoustic tomography experiment consisting of a source near Hawaii and seven receivers along the west coast of North America was conducted from November 1987 to May 1988 and from February 1989 to July 1989. In this thesis, the acoustic ray travel times are analyzed in order to investigate inter-annual basin-scale thermal variability. These thermal fluctuations may help detect any greenhouse warming and greater understanding of them will increase knowledge of ocean-atmosphere interactions which affect weather and climate. A discussion of the program for finding the travel times is included along with a comparison of two methods of measuring travel times.

Beschreibung: 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 1990

Beschreibung: Travel-times of acoustic signals were measured between a bottom-mounted source near Oahu and four bottom-mounted receivers located near Washington, Oregon, and California in 1988 and 1989. This paper discusses the observed tidal signals. At three out of four receivers, observed travel times at M2 and S2 periods agree with predictions from barotropic tide models to within ±30° in phase and a factor of 1.6 in amplitude. The discrepancy at the fourth receiver can be removed by including predicted effects of phase-locked baroclinic tides generated by seamounts. Our estimates of barotropic M2 tidal dissipation by seamounts vary between 2 x 1016 and 1 X 1018 erg·s-1. The variation by two orders of magnitude is due to uncertainties in the numbers and sizes of seamounts. The larger dissipation (1 x 1018 erg·s-1) is the same order as previous estimates and amounts to 4% of the total dissipation at M2.

Beschreibung: 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 1990

Beschreibung: A recently developed technique for determining past sea surface temperatures (SST), based on an analysis of the unsaturation ratio of long chain C37 methyl alkenones (Uk37) produced by Prymnesiophyceae phytoplankton, has been applied to late Quaternary sediment cores. Previous studies have shown that the Uk37 ratio of these alkenones is linearly proportional to the sea-water temperature in which the plankton grow, both in culture and water column samples. Furthermore, a reasonable correlation has been found between open ocean paleo-SST estimates based on Uk37 values and those derived from δ180, for the period spanning approximately the last 100,000 years (Brassell, 1986b). These results indicate this technique has potential for determining paleo-SST from analysis of alkenones extracted from marine sediments. In order to apply the Uk37 method quantitatively, it is necessary to calibrate the method for sediment samples, and to assess how well the alkenones maintain their temperature signal under some common conditions of sediment deposition and sample handling. It is also necessary to determine the method's usefulness downcore, that is, back in time, by comparing it to established methods. This study examined the effect on Uk37 of conditions that cause dissolution of carbonates in the sediment, and methods of storage and 'sample handling. These are two problems that must be resolved before the method can be applied rigorously and quantitatively to sediments for paleotemperature estimations. A comparison of duplicate samples collected and stored frozen versus those stored at room temperature for up to four years showed no resolvable differences in Uk37. Laboratory experiments of carbonate dissolution indicated there is no effect on Uk37 values under the acidic conditions that dissolve carbonates. Initial field results support this, but indicate more studies are necessary. The Uk37 "thermometer" was calibrated by analyzing Uk37 in coretops from widely varying open ocean sites. Sediment values of Uk37 reflected overlying SST for the appropriate season of the phytoplankton bloom, which for this study was assumed to be summer in high latitudes. These results fall on the same regression line for culture and water column samples derived by Prahl and Wakeham (1987), indicating that their equation (Uk37 = 0.033 T + 0.043) is suitable for use in converting Uk37 values in sediments to overlying SST for the season of coccolith bloom. Using this calibration for sediments, the Uk37 paleotemperature method can be quantitatively applied down core to open ocean sediments. In the Equatorial Atlantic, Uk37 temperature estimates were compared to those obtained from δ18O of the planktonic foraminifer Globigerinoides sacculifer, and planktonic foraminiferal assemblages for the last glacial cycle. The alkenone method showed ~1.56°C cooling at the last glacial maximum. This is about half the decrease shown by both the isotopic method ( ~3.40°C) and foraminiferal assemblages (~3.75°C), implying that, if Uk37 estimates are correct, SST in the equatorial Atlantic was only reduced slightly in the last glaciation. In the Northeast Atlantic, Uk37 temperature estimates show a profile downcore which is similar to the estimates from foram assemblages but with a constant offset toward warmer values throughout the core. Uk37 SST estimates are substantially warmer than foraminiferal estimates at all times, which may indicate inaccuracy in Uk37 temperatures at this site. Uk37 indicates a SST of 12°C for the late glacial and 18°C for the Recent, whereas assemblages give estimates of 9°C and 13°C, respectively. At 12,700 yrs BP, the Uk37 and foram assemblage methods indicate a 2°C warming. A temperature change of 2°C can account for only 0.44°/oo of the observed 1.2°/oo δ18O signal, indicating that the additional 0.8°/oo change in δ18O must result from changes in surface salinity most likely due to a meltwater lid. Uk37 estimates show the major temperature shift from glacial to interglacial temperatures occured at about 9,000 yrs BP disagreeing with assemblage data which shows the shift to Holocene values at about 12,700 yrs BP. If Uk37 temperature estimates are accurate, this disagreement may reflect differing habitats of flora and fauna under the unusual sea surface conditions in this area during the deglaciation.

Beschreibung: Primary financial support for this research came from the Ocean Ventures Fund (grant 25/ 85.08), without which this project would not have been possible. Funding supporting the labs of John Farrington (OCE 88-11409) and Lloyd Keigwin (OCE 83-08893 and ATM 84-14335) in which I worked also provided funding for part for this research.

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

Beschreibung: In 1991 the Heard Island Feasibility Test demonstrated that it is possible to transmit coded acoustic signals nearly half way around the world. One of the key issues in the feasibility test was to determine the spatial structure of the received transmissions. In this thesis, data from the Canadian Defense Research Establishment Pacific horizontal line array is used to form an estimate of the directional power spectrum. This spectrum determines if any horizontal multipath is detectable. The preliminary signal conditioning, including frequency spectrum estimation and demodulation required before beamforming is described. Conventional and adaptive beamforming methods are examined with synthetic data to demonstrate the limitations on the directional spectrum results. The principle result of this work is that no stable horizontal multipath is evident. The mean arrival angle for the five hours of data analyzed is 212° ± 1.5°.

Beschreibung: The Office of Naval Research provided funding for the author under the ONR fellowship program.

Beschreibung: 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

Beschreibung: The problem of a low-frequency acoustic plane wave incident upon a free surface coupled to a semi-infinite elastic plate surface, is solved using an analytic approach based on the Wiener-Hopf method. By low-frequency it is meant that the elastic properties of the plate are adequately described by the thin plate equation (kH ≲ 1). The diffraction problem relates to issues in long range sound propagation through partially ice-covered Arctic waters, where open leads or polynya on the surface represent features from which acoustic energy can be diffracted or scattered. This work focusses on ice as the material for the elastic plate surface, and, though the solution methods presented here have applicability to general edge diffraction problems, the results and conclusions are directed toward the ice lead diffraction process. The work begins with the derivation of an exact solution to a canonical problem: a plane wave incident upon a free surface (Dirichlet boundary condition) coupled to a perfectly rigid surface (Neumann boundary condition). Important features of the general edge diffraction problem are included here, with the solution serving as a guideline to the more complicated solutions presented later involving material properties of the boundary. The ice material properties are first addressed using the locally reacting approximation for the input impedance of an ice plate, wherein the effects of elasticity are ignored. This is followed by use of the thin plate equation to describe the input impedance, which incorporates elements of elastic wave propagation. An important issue in working with the thin plate equation is the fluid loading pertaining to sea ice and low-frequency acoustics, which cannot be characterized by simplifying heavy or light fluid loading limits. An approximation to the exact kernel of the Wiener-Hopf functional equation is used here, which is valid in this mid-range fluid loading regime. Use of this approximate kernel allows one to proceed to a complete and readily interpretable solution for the far field diffracted pressure, which includes a subsonic flexural wave in the ice plate. By using Green's theorem, in conjunction with the behavior of the diffracted field along the two-part planar boundary, the functional dependence of ∏D (total diffracted power) in terms of k (wavenumber), H (ice thickness), α (grazing angle) and the combined elastic properties of the ice sheet and ambient medium, is determined. A means to convert ∏D into an estimate of dB loss per bounce is developed using ray theoretical methods, in order to demonstrate a mechanism for acoustic propagation loss attributed directly to ice lead diffraction effects. Data from the 1984 MIZEX (Marginal Ice Zone Experiments) narrow-band acoustic transmission experiments are presented and discussed in this context.

Beschreibung: I also gratefully acknowledge financial support provided by the WHOI Education Office and the Office of Naval Research.

Beschreibung: 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

Beschreibung: Variability of the Florida Current has been monitored via acoustic tomography. A reciprocal tomography experiment was conducted in the eastern half of the Florida Straits during mid October and November, 1983. A triangular array of transceivers, with leg separations of approximately 45 kilometers, was deployed at 27°N. The presence of a surface mixed layer in the region allowed for the ducted propagation of acoustic energy in the surface layer. A deeper layer was sampled by an unresolved group of refracted, bottom reflected ray arrivals. Incorporating the complete set of arrivals, we are able to obtain depth dependent estimates of the temperature field, current velocity, and relative vorticity. The oceanography of the region has been shown to be dominated by the lateral shifting of the surface and subsurface core of the Florida Current. The influx of westward flowing water through the Northwest Providence Channel at 26°N also appears as a large scale signal in the eastern Florida Straits. Low frequency fluctuations of temperature, current velocity, and vorticity occur at periods ranging from several days to nearly two weeks, and are intimately related to meandering of the Florida Current system.

Beschreibung: This research was carried out under ONR contract N00014-86-K-0751.

Beschreibung: 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 September 1990

Beschreibung: The propagation of low frequency seismo-acoustic waves in the Arctic Ocean ice canopy is examined through the analysis of hydrophone and geophone data sets collected in 1987 at an ice camp designated PRUDEX in the Beaufort Sea. Study of the geophone time series generated by under-ice explosive detonations reveals not only the expected longitudinal and flexural waves in the ice plate, but also an unexpected horizontally-polarized transverse (SH) wave arriving at a higher amplitude than the other wave types. The travel paths of all three observed wave types are found to be refracted in the horizontal plane along a line coincident with a known ridge separating the ice canopy locally into two distinct half-plates, the first of thin first year ice and the second of thicker multi-year ice. The origin of the SH wave appears to be near the detonation and not associated with the interaction of longitudinal, flexural or waterborne waves with the ridge line. The need to determine the exact location of each detonation from the received time series highlights the dramatic superiority of geophones over hydrophones in this application, as does the ability to detect the anomalous SH waves and the refracted ray paths, neither of which are visible in the hydrophone data. Inversion of the geophone data sets for the low frequency elastic parameters of the ice is conducted initially by treating the ice as a single homogeneous isotropic plate to demonstrate the power of SAFARI numerical modeling in this application. A modified stationary phase approach is then used to extend SAFARI modeling to invert the data sets for the elastic parameters of the two ice half-plates simultaneously. The compressional/shear bulk wave speeds estimated in the half-plates, 3500/1750 m/s in the multi-year ice and 3000/1590 m/ s in the new ice, are comparable to previously obtained values; however, the compressional/shear attenuation values in the two half-plates, 1.0/2.99 d/Bλ. and 1.0/2.67 dB/λ, respectively, are somewhat greater than previously measured values and four times greater than estimates extrapolated from high frequency data.

Beschreibung: 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

Beschreibung: The purpose of this study is to understand the interactions of tropical cyclones with ocean eddies. In particular we examine the influence of a cold-core eddy on the cold wake formed during the passage of Typhoon Fanapi (2010). The three-dimensional version of the numerical Price–Weller–Pinkel (PWP) vertical mixing model has previously been used to simulate and study the cold wakes of Atlantic hurricanes. The model has not been used in comparison with observations of typhoons in the Western Pacific Ocean. In 2010 several typhoons were studied during the Impact of Typhoons on the Ocean in the Pacific (ITOP) field campaign and Fanapi was particularly well observed. We use these observations and the 3DPWP to understand the ocean cold wake generated by Fanapi. The cold wake of Fanapi was advected by a cyclonic eddy that was south of the typhoon track. The 3DPWP model outputs with and without an eddy are compared with observations made during the field campaign. These observations are compared to model outputs with eddies in a series of positions right and left of the storm track in order to study effects of mesoscale eddies on ocean vertical mixing in the cold wake of typhoons.

Beschreibung: 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 September 1990

Beschreibung: Pulse-like acoustic signals are transmitted from an acoustic source near Oahu to seven receivers off the west coast of the United States for a 124-day period in 1988. Acoustic travel-time oscillations are observed in the received signal at periods between 15 and 23 hours, which are caused by barotropic (or first or second mode baroclinic) flu ctuations in the ocean. It is shown that these fluctuations cannot be local processes isolated to either the source or to the receivers. It is further shown that resonant barotropic gravity wave modes (Platzman et al., 1981) are not consistent with the data. The cause of these flu ctuations remains unresolved, but the data and other oceanographic measurements put many constraints on the process causing these fluctuations.

Beschreibung: 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 1996

Beschreibung: The shallow water acoustic channel supports far-field propagation in a discrete set of modes. Ocean experiments have confirmed the modal nature of acoustic propagation, but no experiment has successfully excited only one of the suite of mid-frequency propagating modes propagating in a coastal environment. The ability to excite a single mode would be a powerful tool for investigating shallow water ocean processes. A feedback control algorithm incorporating elements of adaptive estimation, underwater acoustics, array processing and control theory to generate a high-fidelity single mode is presented. This approach also yields a cohesive framework for evaluating the feasibility of generating a single mode with given array geometries, noise characteristics and source power limitations. Simulations and laboratory waveguide experiments indicate the proposed algorithm holds promise for ocean experiments.

Beschreibung: Josko Catipovic funded my research for summer of 1992 on the Office of Naval Research Grant Number N00014-92-J-1661 and from June 1993 through August 1995 on Defense Advanced Research Projects Agency Grant Number MDA972-92-J- 1041. The Office of Naval Research Grant N00014-95-1-0362 to MIT supported the computer facilities used to do much of this work.

Beschreibung: 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

Beschreibung: 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.

Beschreibung: 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.

Beschreibung: 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

Beschreibung: The goal of this thesis is to develop a methodology to interpret sound scattered from the seafloor in terms of seafloor structure and subseafloor geological properties. Specifically, this work has been directed towards the interpretation of matched-filtered, beamformed monostatic acoustic reverberation data acquired on the west flank of the Mid-Atlantic Ridge when the seafloor is insonified by a band-limited, low-grazing-angle acoustic pulse. This research is based on the hypothesis that observed backscatter signals are produced by a combination of seafloor (interface) scattering and subseafloor (volume) scattering from structure having variations at scale lengths similar to the wavelength of the insonifying acoustic field. Analysis of monostatic reverberation data acquired during the Site A experiment (Run 1) of the Acoustic Reverberation Special Research Program 1993 Acoustics Cruise suggests that the scattered signals cannot be accounted for quantitatively in terms of large-scale slope, even though a strong correspondence between high intensity backscatter and seafloor ridges is observed. In order to investigate and quantify the actual sources of seafloor scattering, a numerical modeling study of seafloor models is undertaken using a finite-difference solution to the elastic wave equation. Geological data available at Site A and published reports describing geological properties of similar deep ocean crustal regions are used to develop a realistic seafloor model for the study area with realistic constraints on elastic parameters. Wavelength-scale heterogeneity in each model, in the form of seafloor roughness and subseafloor volume heterogeneity is defined using stochastic distributions with Gaussian autocorrelations. These distributions are quantified by their correlation lengths and standard deviation in amplitude. In order to incorporate all seafloor structure in a single parameterization of seafloor scattering, large-scale slope and wavelength-scale seafloor spatial parameters (rms height and correlation length), are included, along with the acoustic beam grazing-angle relative to a horizontal seafloor, in the definition of an 'effective grazing angle'. The Rayleigh roughness parameter, which depends on grazing angle of the insonification, is then redefined using the effective grazing angle and calculated for a variety of seafloor models. Scattering strengths are shown to vary systematically but nonlinearly with the 'effective Rayleigh roughness parameters' of horizontal rough seafloor models. This leads to an approximate interpretation scheme for backscatter intensity. In general, variation in backscattering is found to be dominated by the scattering from rough seafloor. If the seafloor is smooth or very low velocity (e.g., sediment), then scattering from volume heterogeneity becomes an important factor in the backscattered field. Both wavelength-scale seafloor roughness and volume heterogeneity are shown to be capable of producing the levels of variation in intensity observed in monostatic reverberation experiments. Variations in large-scale seafloor slope and subseafloor average velocity are shown to influence the backscatter response of seafloor models.

Beschreibung: My first year of study at MIT was supported by a Chevron Fellowship. The work in this thesis was funded by the Office of Naval Research under grant numbers N00014-93-1-1352, N00014-90-J-1493, and N00014-95-1-0506.

Beschreibung: 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 May 1995

Beschreibung: The data analysis of a deep-sea bottom backscattering experiment, carried out over a sediment pond on the western flank of the Mid-Atlantic Ridge in July 1993 with a 250- 650 Hz chirp source and a vertical receiving array suspended near the fiat seafloor, is presented in this thesis. Reflected signals in the normal incidence direction as the output of endfire beamforming are used to determine the sediment structure. The sediment is found to be horizontally stratified, except for two irregular regions, each about 20 m t hick, located around 18 m and 60 m beneath the water-sediment interface. Multiple constraints beamforming is shown to be effective in removing coherent reflections from internal stratified layers, which is critical to the analysis of bottom backscattering. With backscattered signals obtained by beamforming, the above-mentioned two inhomogeneous regions are found to be the dominant factors on the bottom backscattered field, both in the normal incidence and oblique directions. The backscattering strength as a function of grazing angle is estimated for each of the two regions.

Beschreibung: 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 1990

Beschreibung: This thesis introduces a new procedure for the enhancement of acoustic images of the bottom of the sea produced by side-scan sonars. Specifically, it addresses the problem of estimating and correcting geometric distortions frequently observed in such images as a consequence of motion instabilities of the sonar array. This procedure estimates the geometric distortions from the image itself, without requiring any navigational or attitude measurements. A mathematical model for the distortions is derived from the geometry of the problem, and is applied to estimates of the local degree of geometric distortion obtained by cross-correlating segments of adjacent lines of the image. The model parameters are then recursively estimated through deterministic least-squares estimation. An alternative approach based on adaptive Kalman filtering is also proposed, providing a natural framework in which a priori information about the array dynamics may be easily incorporated. The estimates of the parameters of the distortion model are used to rectify the image, and may also be used for estimating the attitude parameters of the array. A simulation is employed to evaluate the effectiveness of this technique and examples of its application to high-resolution side-scan sonar images are provided.

Beschreibung: This work was produced under sponsorship of the Conselho Nacional de Desenvolvimento Cient{jico e Tecnol6gico (CNPq), an agency of the Government of the Federative Republic of Brazil, and was supported in part by the Defense Advanced Research Projects Agency monitored by the Office of Naval Research under Grant No. N00014-89-J-1489, in part by the National Science Foundation under Grant No. MIP 87-14969, and in part by Sanders Associates, Incorporated.

Beschreibung: 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 September 1991

Beschreibung: A model is developed for the prediction of the seismo-acoustic noise spectrum in the microseism peak region (0.1 to 0.7 Hz). The model uses a theory developed by Cato [J. Acoust. Soc. Am., 89 , 1096-1112 (1991)] for an infinite depth ocean in which the surface orbital motion caused by gravity waves may produce acoustic waves at twice the gravity wave frequency. Using directional wave spectra as inputs, acoustic source levels are computed and incorporated into a more realistic environment consisting of a horizontally stratified ocean with an elastic bottom. Noise predictions are made using directional wave spectra obtained from the SWADE surface buoys moored off the coast of Virginia and the SAFARI sound propagation code, with a bottom model derived using wave speeds measured in the EDGE deep seismic reflection survey. The predictions are analyzed for noise level variations with frequency, wave height, wind direction, and receiver depth. These predictions are compared to noise measurements made in ECONOMEX using near-bottom receivers located close to the surface buoys. Good agreement is found between the predictions and observations under a variety of environmental conditions.

Beschreibung: 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 1992

Beschreibung: An ultra-short baseline acoustic navigation system has been developed which is capable of determining bearing to a sound source with an error of less than 1° in typical operational conditions. The system has a demonstrated ability to operate in an environment in which multipath interference is significant. A DSP microprocessor is used to process signals received by two hydrophones from a 26 kHz toneburst sound source. This processing power is used to implement features not commonly available with commercial systems. The system has the capability to make an on-line measurement of the signal-tonoise ratio, which can be used to estimate the confidence which should be placed in the data. Estimates of phase difference and signal power are generated many times within each received pulse, so the effects of multi path interference throughout the pulse can be observed. Results of tests at several ranges are presented, and compared to performance models developed in the thesis. System performance is quantified, and an effort is made to understand the effects of multipath arrivals.

Beschreibung: 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

Beschreibung: The dynamical aspects involved in the assimilation of altimeter data in a numerical ocean model have been investigated. The model used for this study is a quasi-geostrophic model of the Gulf Stream region. The data that have been assimilated are maps of sea surface height which have been obtained as the superposition of sea surface height variability deduced from the Geosat altimeter measurements and a mean field constructed from historical hydrographic data. The method used for assimilating the data is the nudging technique. Nudging has been implemented in such a way as to achieve a high degree of convergence of the surface model fields toward the observations. We have analyzed the mechanisms of the model adjustment, and the final statistical equilibrium characteristics of the model simulation when the surface data are assimilated. Since the surface data are the superposition of a mean component and an eddy component, in order to understand the relative role of these two components in determining the characteristics of the final st atistical steady state, we have considered two different experiments: in the first experiment only the climatological mean field is assimilated, while in the second experiment the total surface streamfunction field (mean + eddies) has been used. We have found that the mean component of the surface data determines, to a large extent, the structure of the flow field in the subsurface layers, while the eddy field, as well as the inflow/outflow conditions at the open boundaries, affect its intensity. In particular, if surface eddies are not assimilated only a weak flow develops in the two deeper model layers where no inflow/ outflow is prescribed at the boundaries. Comparisons of the assimilation results with available in situ observations show a considerable improvement in the degree of realism of the climatological model behavior, with respect to the model in which no data are assimilated. In particular, the possibility of building into the model more realistic eddy characteristics, through the assimilation of the surface eddy field, proves very successful in driving components of the mean model circulation that are in good agreement with the available observations.

Beschreibung: This research was carried out with the support of the National Aeronaut ics Space Administration, through a contract to MIT from the Jet Propulsion Laboratory, # 958208, as a part of the TOPEX-Poseidon investigation.

Beschreibung: 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 November 1992

Beschreibung: Cosmogeruc P-32 (14.28 days) and P-33 (25.3 days) are powerful tracers of upper ocean P cycling, when coupled with time-series of the atmospheric sources. A method was developed to determine the low-level beta activities in rainwater and plankton. The wet deposition rates of P-32 and P-33 were determined during 12 months at a marine site, at Bermuda, coinciding with measurements of the activities and activity ratio P-33/P-32 in suspended particles and plankton tows at BATS station. The in situ production rates of radiophosphorus in the upper ocean were estimated by measuring the activities induced in Cl, K and S targets by cosmic rays. Knowledge of all the sources of radiophosphorus to the Sargasso Sea allowed the cycling of P-32 and P-33 in suspended particles and macrozooplankton to be studied. The study was based on the determination of the activity ratio P-33/P-32 in different particulate pools. The activity ratio was higher in particle collections dominated by higher levels in the food web. The increase in the ratio in plankton relative to rain allowed the determination of the turnover times of P in plankton and in situ grazing rates.

Beschreibung: Funding for this research was provided by NSF (grants OCE-8800957. OCE- 8817836 and OCE-902284), DOE (grant DE-FG02-88ER60681), Woods Hole Oceanographic Institution, Ocean Venture Funding of Woods Hole Oceanographic Institution and Scurlock Funds of Mr Arch Scurlock to MIT/WHOI Joint Program.

Beschreibung: 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 1993

Beschreibung: This thesis examines the use of a single, omnidirectional hydrophone as a receiving sensor to passively localize an acoustic beacon. The localization problem is presented as a constrained, nonlinear parameter estimation problem, and Lagrange multipliers are introduced to solve for the maximum likelihood estimate of the acoustic beacon's position. An iterative algorithm is developed using range difference measurements to solve for the maximum likelihood estimate of a stationary acoustic beacon's position. This algorithm is then _extended to include linear, constant velocity motion of the acoustic beacon. Finally, design specifications for a receiver to implement the maximum likelihood estimation algorithms are developed. To test the maximum likelihood estimate algorithms, Monte Carlo simulations are conducted. Results from six representative scenarios are presented. Test results show that as the number of range differences used increases, or the distance that the observer travels between received beacon signals increases, the accuracy of the estimated position improves. Also, tests show that accuracy of the estimated beacon position is directly related to the accuracy in which the observer's position is measured. To test the receiver's design specifications, a prototype receiver is built using commonly available components. It is then shown that the prototype receiver meets or exceeds the design specifications.

Beschreibung: 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

Beschreibung: This thesis presents the design and simulation of an acoustic listen-only navigation system for use in a time-varying high-multipath environment. The system uses a ray tracing model of sound propagation in a horizontally stratified ocean to associate each multipath arrival with a particular ray path through the ocean. This ray path identification allows an inversion for both vehicle position and sound speed profile parameters. Tracking changes in the sound speed profile allows the vehicle to maintain navigation accuracy in a time-varying acoustic environment. Simulation results for typical Arctic conditions indicate the potential for accurate navigation from acoustic beacons at ranges up to tens of kilometers.

Beschreibung: I also wish to acknowledge the support received from a National Science Foundation Graduate Fellowship under Grant #RCD-9154652 and from the Office of Naval Research Arctic Programs Office, which made possible this work.

Beschreibung: 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

Beschreibung: Ambient noise in the sea has been observed for over 100 years. Previous studies conclude that the primary source of microseisms is nonlinear interaction of surface gravity waves at the sea surface. Though this source relationship is generally accepted, the actual processes by which the wave generated acoustic noise in the water column couples and propagates to and along the sea floor are not well understood. In this thesis, the sources and propagation of sea floor and sub-sea floor microseismic noise between 0.2 and 10 Hz are investigated. This thesis involves a combination of theoretical, observational and numerical analysis to probe the nature of the microseismic field in the Blake Bahama Basin. Surface waves are the primary mechanism for noise propagation in the crust and fall into two separate groups depending on the relative wavelength/water depth ratio. Asymptotic analysis of the Sommerfeld integral in the complex ray parameter plane shows results that agree with previous findings by Strick (1959) and reveal two fundamental interface wave modes for short wavelength noise propagation in the crust: the Stoneley and pseudo-Rayleigh wave. For ocean sediments, where the shear wave velocity is less than the acoustic wave velocity of water, only the Stoneley interface wave can exist. For well consolidated sediments and basalt, the shear velocity exceeds the acoustic wave velocity of water and the pseudo-Rayleigh wave can also exist. Both interface waves propagate with retrograde elliptic motion at the sea floor and attenuate with depth into the crust, however the pseudo-Rayleigh wave travels along the interface with dispersion and attenuation and "leaks" energy into the water column for a half-space ocean over elastic crust model. For finite depth ocean models, the pseudo-Rayleigh wave is no longer leaky and approaches the Rayleigh wave velocity of the crust. The analysis shows that longer wavelength noise propagates as Rayleigh and Stoneley modes of the ocean+crust waveguide. These long wavelength modes are the fundamental mechanism for long range noise propagation. During the Low Frequency Acoustic Seismic Experiment (LFASE) a four-node, 12- channel borehole array (SEABASS) was deployed in the Blake Bahama Basin off the coast of eastern Florida (DSDP Hole 534B). This experiment is unique and is the first use of a borehole array to measure microseismic noise below the sea floor. Ambient background noise from a one week period is compared between an Ocean Bottom Seismometer (OBS) and SEABASS at sub-bottom depths of 10, 40, 70 and 100 meters below the sea floor. The 0.3 H z microseism peak is found to be nearly invariant with depth and has a power level of 65 and 75 dB rel 1 (nm/ s2)2)/ H z for the vertical and horizontal components respectively. At 100 m depth, the mean microseismic noise levels above 0.7 Hz are 10 dB and 15-20 dB quieter for the vertical and horizontal components respectively. Most of this attenuation occurs in the upper 10 m above 1.0 Hz, however higher modes in the spectra show narrow bandwidth variability in the noise field that is not monotonic with depth. Dispersion calculations show normal mode Stoneley waves below 0.7 Hz and evidence of higher modes above 0.8 Hz. A strong correlation between noise levels in the borehole and local sea state conditions is observed along with clear observation of the nonlinear frequency doubling effect between ocean surface waves and microseisms. Particle motion analysis further verifies that noise propagates through the array as Rayleigh/Stoneley waves. Polarization direction indicates at least two sources; distant westerly swell during quiescent times and local surface waves due to a passing storm. Above 1.0 Hz the LFASE data shows little coherence and displays random polarization. Because of this, we believe scattered energy is a significant component of the noise field in the Blake Bahama Basin. A fully 3-D finite difference algorithm is used to model both surface and volume heterogeneities in the ocean crust. Numerical modeling of wave propagation for hard and soft bottom environments shows that heterogeneities on the order of a seismic wavelength radiate energy into the water column and convert acoustic waves in the water into small wavelength Stoneley waves observed at the borehole. Sea floor roughness is the most important elastic scattering feature of the ocean crust. Comparisons of 2D and 3D rough sea floor models show that out-of-plane effects necessitate the use of 3D methods. The out-of-plane energy that is present in the LFASE data comes from either heterogeneities in the source field (i.e. mixed gravity wave directions) or, equally likely, scattering of the source field from surface or volume heterogeneities in the sea floor.

Beschreibung: This research was supported by Office of Naval Research grants N00014-89-C-0018, N00014-89-J-1012, N00014-90-C-0098, N00014-90-J-1493 and N00014-93-1-1352.

Beschreibung: 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

Beschreibung: 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.

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

Beschreibung: 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

Beschreibung: The unusual twice-yearly cycle of mixed layer deepening and cooling driven by the monsoon is analyzed using a recently collected (1994-95) dataset of concurrent local air-sea fluxes and upper ocean dynamics from the Arabian Sea. The winter northeast monsoon has moderate wind forcing and a strongly destabilizing surface buoyancy flux, driven by large radiative and latent heat losses at the sea surface. Convective entrainment is the primary local mechanism driving the observed mixed layer cooling and deepening, although horizontal advection of thermocline depth variations affect the depth which the mixed layer attains. Modifications of a one-dimensional mixed layer model and heat balance show that the primary nonlocal forcing of the upper ocean is the horizontal advection of temperature gradients below the mixed layer base. The summer southwest monsoon has strong wind stresses and a neutral to stabilizing surface buoyancy flux, limited by the extreme humidity of the atmosphere, which suppresses both the radiative and latent heat losses at the surface. Wind-driven shear instabilities at the base of the mixed layer, which entrain cooler and fresher water primarily produces the observed mixed layer cooling and deepening. Horizontal advection of cooler water within the mixed layer influences the local heat balance at the mooring site. Ekman pumping velocities play only a small role in the upper ocean evolution during both monsoon seasons.

Beschreibung: This research was funded by ONR grant N00014-94- 1-0161, and I was supported during this time by a generous NDSEG fellowship.

Beschreibung: 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 1992

Beschreibung: A basin-scale acoustic tomography experiment was conducted in the northeast Pacific from May 1987 to September 1987. In this thesis, the stability of the forward model is analyzed. There are large non-linearities in the changes in travel time between ray paths for the four seasons . I constructed a model in which the change in warming in the upper 100 m of the ocean was due only to changes in surface solar irradiance. The value of the surface solar irradiance anomalies necessary to cause the tomography results for warming (Spiesberger and Metzger, 1991) was computed. This value was larger than the actual value of surface solar irradiance anomaly which was computed using inputs measured by satellite (Chertock, 1989).

Beschreibung: 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

Beschreibung: 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.

Beschreibung: 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.

Beschreibung: 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 1991

Beschreibung: This thesis reviews observational and theoretical work on the equatorial deep jets and work related to the study of the role of the horizontal Coriolis parameter. Most existing analytical models interpret the equatorial deep jets as either low frequency, long Rossby waves or stationary,long Kelvin waves generated at or near the ocean surface. These models are unable to answer the question of how wind generated energy propagates down through the equatorial undercurrent and thermocline into the deep ocean. Existing numerical models do not display deep jet features due mainly to their in low vertical resolution and the high eddy viscosity associated with these models. These numerical models also suggest that very tittle energy is able to get into the deep ocean. A natural question is raised: can the equatorial deep jets possibly be interpreted as free, steady inertial motion below the thermocline? We develop a simple model for the deep jets as a free, stationary inertial motion. After scaling the fluid dynamical equations in the appropriate regime, it is found that neither the advective nonlinearity nor the horizontal Coriolis parameter can be neglected. An important conservation equation, the so called potential zonal vorticity conservation equation which governs the equatorial steady and zonal independent equatorial flow is derived. From this conservation principle, an inertial equatorial deep jets model is developed which captures some important features of the deep jets. The horizontal Coriolis parameter is important in this inertial model. The role of the horizontal Coriolis parameter has long been controversial in the literature. We discuss this role for several equatorial flow systems. It is found that the horizontal Coriolis parameter is not significant for inviscid linear equatorial waves due to the presence of stratification in the real ocean. However, when the ratio of momentum eddy viscosity to the density dissipation coefficient becomes small enough, the effect of the horizontal Coriolis parameter becomes more important in a simple viscous model. Some general aspects of this parameter have also been discussed in terms of angular momentum conservation and energy conservation principles. It is suggested that for the ocean circulation of large vertical excursion of the fluid particle, the horizontal Coriolis parameter effect may not be small and should be included in future numerical models.

Beschreibung: 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 1991

Beschreibung: In this thesis, I study the time-varying behavior of a ventila ted thermocline with basin scales at annual and decadal time scales. The variability is forced by three external forcings: the wind stress (chapter 3), the surface heat flux (chapter 4) and the upwelling along the eastern boundary (chapter 5). It is found that the thermocline variability is forced mainly by wind in a shadow zone while m~inly by surface buoyancy flux in a ventilated zone. A two-layer planetary geostrophic model is developed (chapter 2) to simulate a thermocline. The model includes some novel physical mechanisms. Most importantly, it captures the essential feature of subduction; it also is able to account for a time-varying surface temperature. The equation for the interface is a quasi-linear equation, which can be solved analytically by the method of characteristics. The effect of a varying Ekman pumping is investigated. In a shadow zone, it is found that the driving due to the Ekman pumping is mainly balanced by the propagation of planetary waves. However, in a ventilated zone, the cold advection of subducted water plays the essential role in opposing the Ekman pumping. The different dynamics also results in different thermocline variability between the two zones. After a change of Ekman pumping, in the shadow zone, since the baroclinic Ross by wave responds to a changing Ekman pumping slowly (in years to decades), an imbalance arises between the Rossby wave and the Ekman pumping, which then excites thermocline variability. However, in the ventilated zone, both the advection and the Ekman pumping vary rapidly after a barotropic process (about one week) to reach a new steady balance, leaving little thermocline variability. In addition, the evolution of the thermocline and circulation are also discussed. Furthermore, with a periodic Ekman pumping, it is found that linear solutions are approximate the fully nonlinear solution well, particularly for annual forcings. However, the linear disturbance is strongly affected by the basic thermocline structure and circulation. The divergent group velocity field, which is mainly caused by the divergent Sverdrup flow field, produces a decay effect on disturbances. The mean thermocline structure also strongly affects the relative importance of the local Ekman pumping and remote Rossby waves. As a result, in a shadow zone, local response dominates for a shallow interface while the remote Rossby wave dominates for a deep interface. With a strong decadal forcing, the nonlinearity becomes important in the shadow zone, particularly in the western part. The time-mean thermocline which results, becomes shallower than the steady thermocline under the mean Ekman pumping. Then, we investigate the effect on the permanent thermocline by a moving outcrop line, which simulates the effect of a varying surface heat flux. The two layer model is modified by adding an (essentially passive) mixed layer atop. The outcrop line and the mixed layer depth are specified. It is found that, opposite to a surface wind stress, a surface buoyancy flux causes strong variability in the ventilated zone through subducted water while it affects the shadow zone very little. Furthermore, two regimes of buoyancy-forced solution are found. When the outcrop line moves slowly, the solutions are non-entrainment solutions. For these solutions, the surface heat flux is mainly balanced by the horizontal advection. The mixed layer is never entrained. The time-mean thermocline is close to the steady thermocline with the time-mean outcrop line. When the outcrop line moves southward rapidly during the cooling season, the solutions become entrainment solutions. Now, deep vertical convection must occur, because the horizontal advection in the permanent thermocline is no longer strong enough to balance the surface cooling. The mixed layer penetrates rapidly such that water mass is entrained into the mixed layer through the bottom. The time-mean thermocline resembles the steady thermocline with the early spring mixed layer, as suggested by Stommel (1979). The local variability in the permanent thermocline is most efficiently produced by decadal forcings. Finally, two issues about the waves radiating from the eastern boundary are discussed. The first is the penetration of planetary waves across the southern boundary of a subtropical gyre. We find that the wave penetration across the southern boundary is substantially changed by the zonal variation of the thermocline structure. The zonal variation alters both the effective β and the wave front orientation. As a result, the wave penetration differs for interfaces at different depths. For an interface near the surface, part of the waves penetrate into the equatorial region. For middle depths, most waves will be trapped within the subtropical gyre. In contrast, for deep depths, all waves penetrate southward. The second issue of the eastern boundary waves mainly concerns with the breaking of planetary waves in the presence of an Ekman pumping and the associated two-dimensional mean flow. It is found that the breaking is affected significantly by an Ekman pumping and the associated mean flow. With an Ekman pumping, downwelling breaking is suppressed and the breaking time is delayed; upwelling breaking is enhanced and their times are shortened. The breaking times and positions are mainly determined by the maximum vertical perturbation speed while the intensity of the breaking front mainly depends on the amplitude of the perturbation. The intensity of a breaking front increases with the amplitude of the forcing, but decreases with the distance from the eastern boundary. The orientation of a breaking front is overall in northeast-southwest (x ~ -1/f2).

Beschreibung: This thesis is supported by National Science Foundation, Division of Atmospheric Sciences.

Beschreibung: 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 1998

Beschreibung: This thesis presents an investigation of the influence of surface waves on momentum exchange. A quantitative comparison of direct covariance friction velocity measurements to bulk aerodynamic and inertial dissipation estimates indicates that both indirect methods systematically underestimate the momentum flux into developing seas. To account for wave-induced processes and yield improved flux estimates, modifications to the traditional flux parameterizations are explored. Modification to the bulk aerodynamic method involves incorporating sea state dependence into the roughness length calculation. For the inertial dissipation method, a new parameterization for the dimensionless dissipation rate is proposed. The modifications lead to improved momentum flux estimates for both methods.

Beschreibung: This project was funded by the Oceanographer of the Navy.

Beschreibung: 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

Beschreibung: Signal detection and synchronization in the time varying ocean environment is a difficult endeavor. The current common methods include using a linear frequency modulated chirped pulse or maximal length sequence as a detection pulse, then match filtering to that signal. In higher signal to noise ratio (SNR) environments (~0 dB and higher) this has been a suitable solution. As the SNR drops lower however, this solution no longer provides an acceptable probability of detection for a given tolerable probability of false alarm. The issue derives from the inherent coherence issues in the ocean environment which limit the useful matched filter length. This thesis proposes an alternative method of detection based on a recursive least squares linearly adaptive equalizer which we term the Adaptive Linear Equalizer Detector (ALED). This detectors performance has demonstrated reliable probability of detection with minimal interfering false alarms with SNR as low as -20 dB. Additionally this thesis puts forth a computationally feasible method for implementing the detector.

Beschreibung: Support from the Office of Naval Research (through ONR grant #N00014-07-10738 and #N00014-11-10426).

Beschreibung: 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 September 1990

Beschreibung: This paper develops a beam pattern design procedure for general multidimensional irregular sonar arrays that incorporates the not well understood effects of array geometry into the design process. The procedure is implemented by generating a "penalty function" in a spectral covariance function form. Processing the penalty function causes beam pattern high sidelobes to be penalized and the main lobe to be emphasized. This is accomplished by forming the penalty function in terms of an isotropic noise field of specified strength modified with a finite sector of low coherent energy and stabilized with incoherent sensor noise. By inputting the penalty function into a minimum variance beamformer, the beam pattern and aperture weights are calculated based on the given array geometry. The beamformer used is Capon's Maximum Likelihood Method. The array used to test the procedure is located on a sixty degree sector of a cylindrical surface. The procedure is implemented by two different methods, each with some desirable characteristics. One method suppresses sidelobes directly by the placement of nulls. The other method suppresses sidelobes indirectly by the enhancement of the main lobe with anti-nulls. Both methods are evaluated in terms of a sensitivity factor which constrains the maximum white noise array gain. Results show that both methods result in sidelobe levels that range from 20 to 35 dB lower compared to a conventional beam pattern with uniform aperture weighting and that the design procedure is applicable to beam patterns steered to both true broadside and to off-broadside directions.

Beschreibung: 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 1997

Beschreibung: In order to constrain the processes controlling the cycles of biogeochemically important gases such as 02 and C02, and thereby infer rates of biological activity in the upper ocean or the uptake of radiatively important "greenhouse" gases, the noble gases are used to characterize and quantify the physical processes affecting the dissolved gases in aquatic environments. The processes of vertical mixing, gas exchange, air injection, and radiative heating are investigated using a 2 year time-series of the noble gases, temperature, and meteorological data from Station S near Bermuda, coupled with a 1- dimensional upper ocean mixing model to simulate the physical processes in the upper ocean. The rate of vertical mixing that best simulates the thermal cycle is 1.1±0.1 x104 m The gas exchange rate required to simulate the data is consistent with the formulation of Wanninkhof (1992) to ± 40%, while the formulation of Liss and Merlivat 1986 must be increased by a factor of 1.7± 0.6. The air injection rate is consistent with the formulation of Monahan and Torgersen (1991) using an air entrainment velocity of 3±1 cm s1. Gas flux from bubbles is dominated on yearly time-scales by larger bubbles that do not dissolve completely, while the bubble flux is dominated by complete dissolution of bubbles in the winter at Bermuda. In order to obtain a high-frequency time-series of the noble gases to better parameterize the gas flux from bubbles, a moorable, sequential noble gas sampler was developed. Preliminary results indicate that the sampler is capable of obtaining the necessary data. Dissolved gas concentrations can be significantly modified by ice formation and melting, and due to the solubility of He and Ne in ice, the noble gases are shown to be unique tracers of these interactions. A three-phase equilibrium partitioning model was constructed to quantify these interactions in perennially ice-covered Lake Fryxell, and this work was extended to oceanic environments. Preliminary surveys indicate that the noble gases may provide useful and unique information about interactions between water and ice.

Beschreibung: This work has been supported by the National Science Foundation - OCE 9302812 and DPP 9118363, the Ocean Ventures Fund Ditty Bag Fund and Westcott Fund and the WHOI Education Office.

Beschreibung: 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

Beschreibung: During July and August of 1996, a large acoustics/physical oceanography experiment was fielded in the Mid-Atlantic Bight, south of Nantucket Island, MA. Known as the Shelfbreak Front PRIMER Experiment, the study combined acoustic data from a moored array of sources and receivers with very high resolution physical oceanographic measurements. This thesis addresses two of the primary goals of the experiment, explaining the properties of acoustic propagation in the region, and tomographic inversion of the acoustic data. In addition, this thesis develops a new method for predicting acoustic coherence in such regions. Receptions from two 400 Hz tomography sources, transmitting from the continental slope onto the shelf, are analyzed. This data, along with forward propagation modeling utilizing SeaSoar thermohaline measurements, reveal that both the shelfbreak front and tidally-generated soliton packets produce stronger coupling between the acoustic waveguide modes than expected. Arrival time wander and signal spread show variability attributable to the presence of a shelf water meander, changes in frontal configuration, and variability in the soliton field. The highly-coupled nature of the acoustic mode propagation prevents detailed tomographic inversion. Instead, methods based on only the wander of the mode arrivals are used to estimate path-averaged temperatures and internal tide "strength". The modal phase structure function is introduced as a useful proxy for acoustic coherence, and is related via an integral transform to the environmental sound speed correlation function. Advantages of the method are its flexibility and division of the problem into independent contributions, such as from the water column and seabed.

Beschreibung: Office of Naval Research for providing funding for this thesis through AASERT Grant N00014-96-1-0918, and through ONR Grant N00014-98-1-0059.

Beschreibung: 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

Beschreibung: In this thesis the analysis of natural ice events is carried out based on direct measurements of ice-borne seismo-acoustic waves generated by ice fracturing processes. A major reason for studying this phenomenon is that this acoustic emission is a significant contributor to Arctic ocean ambient noise. Also the Arctic contains rich mineral and oil resources and in order to design mining facilities able to withstand the harsh environmental conditions, we need to have a better understanding of the processes of sea ice mechanics. The data analyzed in this thesis were collected during the Sea Ice Mechanics Initiative SIMI’94 experiment which was carried out in the spring of 1994 in the Central Arctic. One of the contributions of this thesis was the determination of the polarization characteristics of elastic waves using multicomponent geophone data. Polarization methods are well known in seismology, but they have never been used for ice event data processing. In this work one of the polarization methods so called Motion Product Detector method has been successfully applied for localization of ice events and determination of polarization characteristics of elastic waves generated by fracturing events. This application demonstrates the feasibility of the polarization method for ice event data processing because it allows one to identify areas of high stress concentration and "hot spots" in ridge building process. The identification of source mechanisms is based on the radiation patterns of the events. This identification was carried out through the analysis of the seismo-acoustic emission of natural ice events in the ice sheet. Previous work on natural ice event identification was done indirectly by analyzing the acoustic energy radiated into the water through coupling from elastic energy in the ice sheet. After identification of the events, the estimation of the parameters of fault processes in Arctic ice is carried out. Stress drop, seismic moment and the type of ice fracture are determined using direct near-field measurements of seismo-acoustic signals generated by ice events. Estimated values of fracture parameters were in good agreement with previous work for marginal ice zone. During data processing the new phenomenon was discovered: "edge waves", which are waves propagating back and forth along a newly opened ice lead. These waves exhibit a quasi-periodic behavior suggesting some kind of stick-slip generation mechanism somewhere along the length of the lead. The propagation characteristics of these waves were determined using seismic wavenumber estimation techniques. In the low frequency limit the dispersion can be modeled approximately by an interaction at the lead edges of the lowest order, antisymmetric modes of the infinite plate.

Beschreibung: Support for this thesis was provided by Office of Naval Research.

Beschreibung: 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

Beschreibung: Remote sensing and in situ observations are used to investigate the ocean response to the Tokar Wind Jet in the Red Sea. The wind jet blows down the pressure gradient through the Tokar Gap on the Sudanese coast, at about 18°N, during the summer monsoon season. It disturbs the prevailing along-sea (southeastward) winds with strong cross-sea (northeastward) winds that can last from days to weeks and reach amplitudes of 20-25 m/s. By comparing scatterometer winds with along-track and gridded sea level anomaly observations, it is shown that an intense dipolar eddy spins up in less than seven days in response to the wind jet. The eddy pair has a horizontal scale of 140 km. Maximum ocean surface velocities can reach 1 m/s and eddy currents extend at least 200 m into the water column. The eddy currents appear to cover the width of the sea, providing a pathway for rapid transport of marine organisms and other drifting material from one coast to the other. Interannual variability in the strength of the dipole is closely matched with variability in the strength of the wind jet. The dipole is observed to be quasi-stationary, although there is some evidence for slow eastward propagation—simulation of the dipole in an idealized high-resolution numerical model suggests that this is the result of self-advection. These and other recent in situ observations in the Red Sea show that the upper ocean currents are dominated by mesoscale eddies rather than by a slow overturning circulation.

Beschreibung: 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).

Beschreibung: 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

Beschreibung: Analysis of vertical profiles of absolute horizontal velocity collected in January 1981, February 1982 and April 1982 in the central equatorial Pacific as part of the Pacific Equatorial Ocean Dynamics (PEQUOD) program, revealed two significant narrow band spectral peaks in the zonal velocity records, centered at vertical wavelengths of 560 and 350 stretched meters (sm). Both signals were present in all three cruises, but the 350 sm peak showed a more steady character in amplitude and a higher signal-to-noise ratio. In addition, its vertical scales corresponded to the scales of the conspicuous alternating flows generically called the equatorial deep jets in the past (the same terminology will be used here). Meridional velocity and vertical displacement spectra did not show any such energetic features. Energy in the 560 sm band roughly doubled between January 1981 and April 1982. Time lagged coherence results suggested upward phase propagation at time scales of about 4 years. East-west phase lines computed from zonally lagged coherences, tilted downward towards the west, implying westward phase propagation. Estimates of zonal wavelength (on the order of 10000 km) and period based on these coherence calculations, and the observed energy meridional structure at this vertical wavenumber band, seem consistent, within experimental errors, with the presence of a first meridional mode long Rossby wave packet, weakly modulated in the zonal direction. The equatorial deep jets, identified with the peak centered at 350 sm, are best defined as a finite narrow band process in vertical wavenumber (311-400 sm), accounting for only 20% of the total variance present in the broad band energetic background. At the jets wavenumber band, latitudinal energy scaling compared well with Kelvin wave theoretical values and a general tilt of phase lines downward towards the east yielded estimates of 10000-16000 km for the zonal wavelengths. Time-lagged coherence calculations revealed evidence for vertical shifting of the jets on interannual time scales. Interpretation of results in terms of single frequency linear wave processes led to inconsistencies, but finite bandwidth (in frequency and wavenumber) Kelvin wave processes of periods on the order of three to five years could account for the observations. Thus, the records do not preclude equatorial waves as a reasonable kinematic description of the jets.

Beschreibung: This research was supported by grant OCE-8600052 from the National Science Foundation, through the Woods Hole Oceanographic Institution.

Beschreibung: 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

Beschreibung: An algorithm is developed for underwater acoustic signal processing with an array of hydrophones. With various acoustic signals coming from different directions, the maximum likelihood approach is used to estimate the source bearings and time series. Simulated annealing is used to implement the resulting time-domain beamformer. Broadband signals in spatially correlated noise are treated. Previous time-domain beamformers did not consider the correlation between random noise, and they did not use the concept of maximum likelihood, which is asymptotically optimal. We show that improved resolution can be achieved using this new method.

Beschreibung: 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 June 1988

Beschreibung: Two approaches to determining the ocean sound speed profile using measured acoustic modal eigenvalues are examined. Both methods use measured eigenvalues and mode dependent assumed values of the WKB phase integral as input data and use the WKB phase integral as a starting point for relating the index of refraction to depth. Inversion method one is restricted to monotonic or symmetric sound speed profiles and requires a measurement of the sound speed at one depth to convert the index of refraction profile to a sound speed profile. Inversion method two assumes that the sound speed at the surface and the minimum sound speed in the profile are known and is applicable to monotonic profiles and to general single duct sound speed profiles. For asymmetric profiles, inversion method two gives the depth difference between two points of equal sound speed in the portion of the profile having two turning points, and in the remainder of the profile it gives sound speed versus depth directly. A numerical implementation of the methods is demonstrated using idealized ocean sound speed profiles numerical experiments used to test the performance of the inversions using noisy data. The two methods are used to determine the sediment sound speed profiles in two shallow water waveguide models, and inversion method one is used to find the sediment sound speed profile using data from an experiment performed in the Gulf of Mexico.

Beschreibung: Funding through the Office of Naval Research Fellowship Program.

Beschreibung: 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 2011

Beschreibung: Current underwater acoustic channel estimation techniques generally apply linear MMSE estimation. This approach is optimal in a mean square error sense under the assumption that the impulse response fluctuations are well characterized by Gaussian statistics, leading to a Rayleigh distributed envelope. However, the envelope statistics of the underwater acoustic communication channel are often better modeled by the K-distribution. In this thesis, by presenting and analyzing field data to support this claim, I demonstrate the need to investigate channel estimation algorithms that exploit K-distributed fading statistics. The impact that environmental conditions and system parameters have on the resulting distribution are analyzed. In doing so, the shape parameter of the K-distribution is found to be correlated with the source-to-receiver distance, bandwidth, and wave height. Next, simulations of the scattering behavior are carried out in order to gain insight into the physical mechanism that cause these statistics to arise. Finally, MAP and MMSE based algorithms are derived assuming K-distributed fading models. The implementation of these estimation algorithms on simulated data demonstrates an improvement in performance over linear MMSE estimation.

Beschreibung: This work was supported by the Office of Naval Research grant #N00014-05-10085 and the National Science Foundation grant #OCE-0519903.

Beschreibung: 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 8, 1988

Beschreibung: Several experiments are presented in this thesis which examine methods to measure and monitor fluid flow from hydrothermal vent fields. Simultaneous velocity, temperature, and conductivity data were collected in the convective flow emanating from a hydrothermal vent field located at 10°S6'N, 103° 41'W on the East Pacific rise. The horizontal profiles obtained indicate that the flow field approaches an ideal plume in the temperature and velocity distribution. Such parameters as total heat flow and maximum plume height can be estimated using either the velocity or the temperature information. The results of these independent calculations are in close agreement, yielding a total heat flow from this vent site of 3.7 ± 0.8 MW and a maximum height of 150±10 m. The nonlinear effects of large temperature variations on heat capacity and volume changes slightly alter the calculations applied to obtain these values. In Guaymas Basin, a twelve day time series of temperature data was collected from a point three centimeters above a diffuse hydrothermal flow area. Using concurrent tidal gauge data from the town of Guaymas it is shown that the effects of tidar currents can be strong enough to dominate the time variability of a temperature signal at a fixed point in hydrothermal flow and are a plausible explanation for the variations seen in the Guaymas Basin temperature data. Theoretical examination of hot, turbulent, buoyant jets exiting from hydrothermal chimneys revealed acoustic source mechanisms capable of producing sound at levels higher than ambient ocean noise. Pressure levels and frequency generated by hydrothermal jets are dependent on chimney dimensions, fluid velocity and temperature and therefore can be used to monitor changes in these parameters over time. A laboratory study of low Mach number jet noise and amplification by flow inhomogeneities confirmed theoretical predictions for homogeneous jet noise power and frequency. The increase in power due to convected flow inhomogeneities, however, was lower in the near field than expected. Indirect evidence of hydrothermal sound fields (Reidesel et al., 1982; Bibee and Jacobson, 1986) showing anomalous high power and low frequency noise associated with vents is due to processes other than jet noise. On Axial Seamount, Juan de Fuca Ridge, high quality acoustic noise measurements were obtained by two hydrophones located 3 m and 40 m from an active hydrothermal vent, in an effort to determine the feasibility of monitoring hydrothermal vent activity through floW noise generation. Most of the noise field could be attributed to ambient ocean noise sources of microseisms, distant shipping and weather, punctuated by local ships and biological sources. Water/rock interface waves of local origin, were detected which showed high pressure amplitudes near the seafloor and, decaying with vertical distance, produced low pressures at 40 m above the bottom. Detection of vent signals was hampered by unexpected spatial non- stationarity due to shadowing effects of the caldera wall. No continuous vent signals were deemed significant based on a criterion of 90% probability of detection and 5% probability of false alarm. However, a small signal near 40 Hz, with a power level of 1x10-4 Pa2/Hz was noticed on two records taken near the Inferno black smoker. The frequency of this signal is consistent with predictions and the power level suggests the occurrence of jet noise amplification due to convected density inhomogeneities. Ambient noise from the TAG (Trans-Atlantic Geotraverse) hydrothermal area on the Mid-Atlantic Ridge near 26°N, in the frequency band 1-30 Hz at a range of 0.75-14 km from the site of an extremely active high temperature hydrothermal vent field (Rona, 1986) was examined. The ambient noise field exhibits great temporal and spatial variations attributed in part to typical ocean noise sources such as distant shipping and microseisms. Power spectral levels as measured at each of six ocean bottom hydrophones (OBH) were used to estimate the location of point sources of sound in the area, if any. The hydrothermal vent did not produce enough sound to be located as a point source using data from the OBH array. The only consistently identifiable point source found with the data set was generating sound in a 0.8-3.5 Hz bandwidth and located outside the median valley. It appears to be harmonic tremor associated with the tip of a ridge on the western side of the spreading axis and may be volcanic in origin.

Beschreibung: This work was supported by the WHOI/MIT Education Office, the Center for Analysis of Marine Systems, the National Science Foundation (grant OCE83-l0l75), NOAA National Sea Grant College Program Office, Dept. of Commerce under grant #NA86-AA-D-SG090, WHOI Sea Grant (R/6-l4), the Office of Naval Research grant #N0014-87-K-0007, and the NOAA Vents Program.

Beschreibung: 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 June 2011

Beschreibung: Throughout this thesis we will discuss the theoretical background and empirical observation of a swell band shore normal flux divergence reversal. Specifically, we will demonstrate the existence and persistence of the energy flux divergence reversal in the nearshore region of Atchafalaya Bay, Gulf of Mexico, across storms during the March through April 2010 deployment. We will show that the swell band offshore component of energy flux is rather insignificant during the periods of interest, and as such we will neglect it during the ensuing analysis. The data presented will verify that the greatest flux divergence reversal is seen with winds from the East to Southeast, which is consistent with theories which suggest shoreward energy flux as well as estuarine sediment transport and resuspension prior to passage of a cold front. Employing the results of theoretical calculations and numerical modeling we will confirm that a plausible explanation for this phenomena can be found in situations where temporally varying wind input may locally balance or overpower bottom induced dissipation, which may also contravene the hypothesis that dissipation need increase shoreward due to nonlinear wave-wave interactions and maturation of the spectrum. Lastly, we will verify that the data presented is consistent with other measures collected during the same deployment in the Atchafalaya Bay during March - April 2010.

Beschreibung: 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

Beschreibung: 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.

Beschreibung: 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.

Beschreibung: 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 1998

Beschreibung: In practical applications with bathymetric sidescan sonars, the multipath reflections and other directional interferences are the key limiting factors for a better performance. This thesis proposes a new scheme to deal with the interferences using a multiple-row bathymetric sidescan sonar. Instead of smoothing the measurements over some time or angle intervals, which was previously widely investigated, we resolve the multipath interferences from the direct signal. Two approaches on signal direction-of-arrival DOA and amplitude estimation are developed, the correlated signal direction estimate CSDE for three-row systems and the ESPRIT-based method. These approaches are compared using different sonar data models, including a stochastic model from the statistical analysis on bottom scattering and a coherent model from the analysis on interference field; the simulations show the ESPRIT-based approach is quite robust at the angular separation of 100 between two sources and at the signal-to-noise ratio above 10dB except for highly coherent or temporally correlated signals, for which CSDE works very well. The computer simulation results and the discussions on practical algorithm implementation indicate the proposed scheme can be applied to a real multiple-row bathymetric sidescan sonar. With the capability to simultaneously resolve two or more directional signals, the new sonar model should work better for a wider variety of practical situations in shallow water with out significant increase of the system cost.

Beschreibung: Funding supporting my thesis research project was provided by the Office of Naval Research ONR.

Beschreibung: 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

Beschreibung: 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.

Beschreibung: 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.

Beschreibung: 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

Beschreibung: In order to determine the efficacy of tomographic reconstructions of the ocean sound speed structure in improving acoustic field predictions for source localization, a 150 km by 350 km volume of ocean 3000 meters deep was synthetically modeled to be similar to the Gulf Stream system, including an eddy and a front. The features were Gaussian, with the eddy's maximum sound speed perturbation being 10ms-1 and the front's maximum perturbation 15ms-1. Two vertical slices through this system were inverted in a synthetic tomography experiment using linear optimal estimation theory. Inversions were also performed using XSV and satellite sea surface temperature data. Gaussian fits to the reconstructed features were constructed for use with a three dimensional raytrace program (HARPO). Three dimensional rays were propagated both through the reconstructions and the original model. Travel time versus intensity (transmission loss) for the eigenrays was used as a basis for intercomparison. Tomographic results showed good reconstruction for a first iteration of the inversion, but inadequate vertical resolution. Iterations and the use of more refractive eigenrays are needed for improvement of the reconstruction, especially for the front. Reconstructed results for the acoustic field should improve conventional beamforming, but are probably inadequate for matched field processing.

Beschreibung: 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 1999

Beschreibung: Climate simulation with numerical oceanic models requires a proper parameterization scheme in order to represent the effects of unresolved mesoscale eddies. Even though a munber of schemes have been proposed and some have led to improvements in the simulation of the bulk climatological properties, the success of the parameterizations in representing the mesoscale eddies has not been investigated in detail. This thesis examines the role of eddies in a 105-years long basin scale eddy resolving simulation with the MIT General Circulation Model (GCM) forced by idealized wind stress and relaxation to prescribed meridional temperature; this thesis also evaluates the Fickian diffusive, the diabatic Green-Stone (GS) and the quasi-adiabatic Gent-McWilliams (GM) parameterizations in a diagnostic study and a series of coarse resolution experiments with the same model in the same configuration. The mesoscale eddies in the reference experiment provide a significant contribution to the thermal balance in limited areas of the domain associated with the upper 1000M of the boundary regions. Specifically designed diagnostic tests of the schemes show that the horizontal and vertical components of the parameterized flux are not simultaneously downgradient to the eddy heat flux. The transfer vectors are more closely aligned with the isopycnal surfaces for deeper layers, thus demonstrating the adiabatic nature of the eddy heat flux for deeper layers. The magnitude of the coefficients is estimated to be consistent with traditionally used values. However, the transfer of heat associated with timedependent motions is identified as a complicated process that cannot be fully explained with any of the local parameterization schemes considered. The eddy parameterization schemes are implemented in the coarse resolution configuration with the same model. A series of experiments exploring the schemes' parameter space demonstrate that Fickian diffusion has the least skill in the climatological simulations because it overestimates the temperature of the deep ocean and underestimates the total heat transport. The GS and GM schemes perform better in the simulation of the bulk climatological properties of the reference solution, although the GM scheme in particular produces an ocean that is consistently colder than the reference state. Comparison of the eddy heat flux divergence with the parameterized divergences for typical parameter values demonstrates that the success of the schemes in the climatological simulation is not related to the representation of the eddy heat flux but to the representation of the overall internal mixing processes.

Beschreibung: The financial support for this research was provided by ONR grant number NOOOl4- 98-1-0881, Alliance for Global Sustainability and American Automobile Manufactures Association.

Beschreibung: 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

Beschreibung: 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.

Beschreibung: 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

Beschreibung: Observational and modeling techniques are employed to investigate the thermal and inertial upper ocean response to wind and buoyancy forcing in the North Atlantic Ocean. First, the seasonal kinetic energy variability of near-inertial motions observed with a moored profiler is described. Observed wintertime enhancement and surface intensification of near-inertial kinetic energy support previous work suggesting that near-inertial motions are predominantly driven by surface forcing. The wind energy input into surface ocean near-inertial motions is estimated using the Price-Weller- Pinkel (PWP) one-dimensional mixed layer model. A localized depth-integrated model consisting of a wind forcing term and a dissipation parameterization is developed and shown to have skill capturing the seasonal cycle and order of magnitude of the near-inertial kinetic energy. Focusing in on wintertime storm passage, velocity and density records from drifting profiling floats (EM-APEX) and a meteorological spar buoy/tethered profiler system (ASIS/FILIS) deployed in the Gulf Stream in February 2007 as part of the CLIvar MOde water Dynamics Experiment (CLIMODE) were analyzed. Despite large surface heat loss during cold air outbreaks and the drifting nature of the instruments, changes in the upper ocean heat content were found in a mixed layer heat balance to be controlled primarily by the relative advection of temperature associated with the strong vertical shear of the Gulf Stream. Velocity records from the Gulf Stream exhibited energetic near-inertial oscillations with frequency that was shifted below the local resting inertial frequency. This depression of frequency was linked to the presence of the negative vorticity of the background horizontal current shear, implying the potential for near-inertial wave trapping in the Gulf Stream region through the mechanism described by Kunze and Sanford (1984). Three-dimensional PWP model simulations show evidence of near-inertial wave trapping in the Gulf Stream jet, and are used to quantify the resulting mixing and the effect on the stratification in the Eighteen Degree Water formation region.

Beschreibung: This work was supported by National Science Foundation grants OCE-0241354 and OCE-0424865, as well as the Woods Hole Oceanographic Institution's Ocean and Cli- mate Change Institute. Funding to initiate the McLane Moored Pro ler observations at Line W were provided by grants from the G. Unger Vetlesen Foundation and the Comer Charitable Fund to the Woods Hole Oceanographic Institutions Ocean and Climate Change Institute.

Beschreibung: 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 1985

Beschreibung: The acoustic properties of marine sediments have a direct effect on the propagation of sound in the ocean. In the frequency range of interest (50 - 500 Hz) the sediment can be modelled as a fluid. Assuming horizontal stratification of the ocean bottom, the acoustic parameters of interest are the compressional wave speed, the compressional wave attenuation and density as a function of depth. An inverse method based on a perturbation technique is presented in this thesis for the determination of these parameters. A monochromatic source experiment is proposed because of the desirability of such an experiment for determining the acoustic properties of an anelastic medium. The input information is the plane wave reflection coerricent as a function of the angle of incidence at a fixed frequency. A nonlinear integral equation relating the variations of these acoustic properties from a known reference value to the plane wave reflection coefficient is derived. This is then linearised using the Born approximation. The region of validity of the Born approximation is derived and based on this the optimum angular aperture for the input data is obtained. The linearised integral equation is a Fredholm integral equation of the first kind. An acceptable stable solution of the integral equation is obtained by imposing a priori constraints on the solution. The inversion method is tested using synthetic data and inversions are carried out for various examples of the attenuation coefficient profile and the sound speed profile. The results obtained with noise free data show good agreement between the true profiles and the reconstructed profiles. The resolution obtainable with the data set is studied using the resolving power theory of Backus and Gilbert and the inversion method is shown to provide adequate resolution. The effect of additive noise in data is examined and inversions performed with noisy data yielded stable acceptable results.

Beschreibung: I acknowledge the financial support provided by the education office in the Woods Hole Oceanographic Institution and the Office of Naval Research.

Beschreibung: 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 May 1986

Beschreibung: The dual issues of modal decomposition for tonal sound fields and the temporal coherence of the modal amplitudes are investigated for the case of the central Arctic sound channel at very low frequencies (15-80 Hz). A detailed study of the Arctic modal structure for these frequencies reveals the central role played by the strong Arctic surface duct. The performance of each of four different modal beamforming algorithms when applied to the vertical array deployed during the FRAM IV Arctic Acoustic Experiment is analyzed. A multiple beam (or decoupled beam) least squares processor produces the most acceptable results for Arctic conditions. The modal decomposition is sensitive to vertical array tilt caused by hydrodynamic drag; a technique for its estimation from the acoustie data is developed. Tonal data taken from both the horizontal and vertical arrays deployed during FRAM IV is analyzed. Horizontal array results confirm the modal amplitudes generated from vertical array data. The rough surface scattering from the ice canopy places an upper limit of 40 Hz on efficient surface duct propagation. Attenuation measurements for the first mode show excellent agreement with predictions made for ice scattering using the method of small perturbations and experimental ice statistics. The high levels of coherence observed (O.95 to 0.99) show that tonal signal propagation in the Arctic channel is essentially deterministic for time periods well in excess of one hour. The various modes may then be considered to maintain a constant phase relationship over time.

Beschreibung: 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 1991

Beschreibung: Breaking waves dissipate energy, transfer momentum from the wind to surface currents and breaking enhances the transfer of gas and mass across the air-sea interface. Breaking waves are believed to be the dominant source of sea surface sound at frequencies greater than 500 Hz and the presence of breaking waves on the ocean surface has been shown to enhance the scattering of microwave radiation. Previous studies have shown that breaking waves can be detected by measuring the microwave backscatter and acoustic radiation from breaking waves. However, these techniques have not yet proven effective for studying the dynamics of breaking. The primary motivation for the research presented in this thesis was to determine whether measurements of the sound generated by breaking waves could be used to quantitatively study the dynamics of the breaking process. Laboratory measurements of the microwave backscatter and acoustic radiation from two-dimensional breaking waves are described in Chapter 2. The major findings of this Chapter are: 1) the mean square acoustic pressure and backscattered microwave power correlate with the wave slope and dissipation for waves of moderate slope, 2) the mean square acoustic pressure and backscattered microwave power correlate strongly with each other, and 3) the amount of acoustic energy radiated by an individual breaking event scaled with the amount of mechanical energy dissipated by breaking. The observed correlations with the mean square acoustic pressure are only relevant for frequencies greater than 2200 Hz because lower frequencies were below the first acoustic cut-off frequency of the wave channel. In order to study the lower frequency sound generated by breaking waves another series of two-dimensional breaking experiments was conducted. Sound at frequencies as low as 20 Hz was observed and the mean square acoustic pressure in the frequency band from 20 Hz-l kHz correlated strongly with the wave slope and dissipation. A characteristic low frequency signal was observed immediately following the impact of the plunging wave crest. The origin of this low frequency signal was found to be the pulsating cylinders of air which are entrained by the plunging waves. The pulsation frequency correlated with both the wave slope and dissipation. Following the characteristic constant frequency signal, approximately 0.25 s after the initial impact of the plunging crest, another low frequency signal was typically observed. These signals were generally lower in frequency initially and then increased in frequency as time progressed. To determine if three-dimensional effects were important in the sound generation process and to measure the sound beneath larger breaking waves a series of experiments was conducted in a large multi-paddle wave basin. Three-dimensional breaking waves were generated and the sound produced by breaking was measured in the frequency range from 10 Hz to 20 kHz. The observed sound spectra showed significant increases in level across the entire bandwidth from 10 Hz to 20 kHz and the spectra sloped at -5 to-6 dB per octave at frequencies greater than 1 kHz. The mean square acoustic pressure in the frequency band from 10 Hz to 150 Hz correlated with the wave amplitude similar to the results obtained in the two-dimensional breaking experiments. Large amplitude low frequency spectral peaks were observed approximately 0.75 s after the initial impact of the plunging crests. It was postulated that the low frequency signals observed some time after the initial impact of the plunging crests for both the two and three-dimensional breakers were caused by the collective oscillation of bubble clouds. Void fraction measurements taken by Eric Lamarre were available for five breaking events and therefore the average sound speed inside the bubble clouds and their radii were known. Using this information the resonant frequencies of a two-dimensional cylindrical bubble cloud of equal radius and sound speed were calculated. The frequencies of the observed signals matched closely with the calculated resonant frequencies of the first and second mode of the two-dimensional cylindrical bubble cloud. The close agreement supports the hypothesis that the low frequency signals were produced by the collective oscillation of bubble clouds. In Chapter 4 a model of the sound produced by breaking waves is presented which uses the sound radiated by a single bubble oscillating at its linear resonant frequency and the bubble size distribution to estimate the sound spectrum. The model generates a damped sinusiodal pulse for every bubble formed, as calculated from the bubble size distribution. If the range to the receiver is known then the only unknown parameters are ε, the initial fractional amplitude of the bubble oscillation and L, the dipole moment arm or twice the depth of the bubble below the free surface. It was found that if the product εxL is independent of the bubble radius the model reproduces the shape and magnitude of the observed sound spectrum accurately. The success of the model implies that it may be possible to calculate the bubble size distribution from the sound spectrum. The model was validated using data from experiments where the breaking events were small scale gently spilling waves (Medwin and Daniel, 1990).

Beschreibung: 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 2000

Beschreibung: Estimation of the upper ocean heat budget from one year of observations at a moored array in the north central Arabian Sea shows a rough balance between the horizontal advection and time change in heat when the one-dimensional balance between the surface heat flux and oceanic heat content breaks down. The two major episodes of horizontal advection, during the early northeast (NE) and late southwest (SW) monsoon seasons, are both associated with the propagation of mesoscale eddies. During the NE monsoon, the heat fluxes within the mixed layer are not significantly different from zero, and the large heat flux comes from advected changes in the thermocline depth. During the SW monsoon a coastal filament exports recently upwelled water from the Omani coast to the site of the array, 600 km offshore. Altimetry shows mildly elevated levels of surface eddy kinetic energy along the Arabian coast during the SW monsoon, suggesting that such offshore transport may be an important component of the Arabian Sea heat budget. The sea surface temperature (SST) and mixed layer depth are observed to respond to high frequency (HF, diurnal to atmospheric synoptic time scales) variability in the surface heat flux and wind stress. The rectified effect of this HF forcing is investigated in a three-dimensional reduced gravity thermodynamic model of the Arabian Sea and Indian Ocean. Both the HF heat and wind forcing act locally to increase vertical mixing in the model, reducing the SST. Interactions between the local response to the surface forcing, Ekman divergences, and remotely propagated signals in the model can reverse this, generating greater SSTs under HF forcing, particularly at low latitudes. The annual mean SST, however, is lowered under HF forcing, changing the balance between the net surface heat flux (which is dependent on the SST) and the meridional heat flux in the model. A suite of experiments with one-dimensional upper ocean models with different representations of vertical mixing processes suggests that the rectified effect of the diurnal heating cycle is dependent on the model, and overstated in the formulation used in the three-dimensional model.

Beschreibung: Funded by the Office of Naval Research (ONR), grant NOOO14-94-1-0161 and National Defense Science and Engineering Graduate Fellowship and by ONR grant NOOO14-99-1-0090.

Beschreibung: 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 2010

Beschreibung: Wind waves in the ocean are a product of complex interaction of turbulent air flow with gravity driven water surface. The coupling is strong and the waves are non-stationary, irregular and highly nonlinear, which restricts the ability of traditional phase averaged models to simulate their complex dynamics. We develop a novel phase resolving model for direct simulation of nonlinear broadband wind waves based on the High Order Spectral (HOS) method (Dommermuth and Yue 1987). The original HOS method, which is a nonlinear pseudo-spectral numerical technique for phase resolving simulation of free regular waves, is extended to simulation of wind forced irregular broadband wave fields. Wind forcing is modeled phenomenologically in a linearized framework of weakly interacting spectral components of the wave field. The mechanism of wind forcing is assumed to be primarily form drag acting on the surface through wave-induced distribution of normal stress. The mechanism is parameterized in terms of wave age and its magnitude is adjusted by the observed growth rates. Linear formulation of the forcing is adopted and applied directly to the nonlinear evolution equations. Development of realistic nonlinear wind wave simulation with HOS method required its extension to broadband irregular wave fields. Another challenge was application of the conservative HOS technique to the intermittent non-conservative dynamics of wind waves. These challenges encountered the fundamental limitations of the original method. Apparent deterioration of wind forced simulations and their inevitable crash raised concerns regarding the validity of the proposed modeling approach. The major question involved application of the original HOS low-pass filtering technique to account for the effect of wave breaking. It was found that growing wind waves break more frequently and violently than free waves. Stronger filtering was required for stabilization of wind wave simulations for duration on the time scale of observed ocean evolution. Successful simulations were produced only after significant sacrifice of resolution bandwidth. Despite the difficulties our modeling approach appears to suffice for reproduction of the essential physics of nonlinear wind waves. Phase resolving simulations are shown to capture both - the characteristic irregularity and the observed similarity that emerges from the chaotic motions. Energy growth and frequency downshift satisfy duration limited evolution parameterizations and asymptote Toba similarity law. Our simulations resolve the detailed kinematics and the nonlinear energetics of swell, windsea and their fast transition under wind forcing. We explain the difference between measurements of initial growth driven by a linear instability mechanism and the balanced nonlinear growth. The simulations validate Toba hypothesis of wind-wave nonlinear quasi-equilibrium and confirm its function as a universal bound on combined windsea and swell evolution under steady wind.

Beschreibung: 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 2001

Beschreibung: Matched-field methods concern estimation of source location and/or ocean environmental parameters by exploiting full wave modeling of acoustic waveguide propagation. Typical estimation performance demonstrates two fundamental limitations. First, sidelobe ambiguities dominate the estimation at low signal-to-noise ratio (SNR), leading to a threshold performance behavior. Second, most matched-field algorithms show a strong sensitivity to environmental/system mismatch, introducing some biased estimates at high SNR. In this thesis, a quantitative approach for ambiguity analysis is developed so that different mainlobe and sidelobe error contributions can be compared at different SNR levels. Two large-error performance bounds, the Weiss-Weinstein bound (WWB) and Ziv-Zakai bound (ZZB), are derived for the attainable accuracy of matched-field methods. To include mismatch effects, a modified version of the ZZB is proposed. Performance analyses are implemented for source localization under a typical shallow water environment chosen from the Shallow Water Evaluation Cell Experiments (SWellEX). The performance predictions describe the simulations of the maximum likelihood estimator (MLE) well, including the mean square error in all SNR regions as well as the bias at high SNR. The threshold SNR and bias predictions are also verified by the SWellEX experimental data processing. These developments provide tools to better understand some fundamental behaviors in matched-field performance and provide benchmarks to which various ad hoc algorithms can be compared.

Beschreibung: Financial support for my research was provided by the Office of Naval Research and the WHOI Education Office.

Beschreibung: 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 2000

Beschreibung: The plausibility of local baroclinic instability as a generation mechanism for midocean mesoscale eddies is examined with a two-layer, quasi-geostrophic (QG) model forced by an imposed, horizontally homogeneous, vertically sheared mean flow and dissipated through bottom Ekman friction, Explanations are sought for two observed features of mid-ocean eddies: 1) substantial energy is retained in the baroclinic mode and in the associated deformation radius (Rd) scale, and 2) the ratio of eddy to mean kinetic energy is much larger than one, The tendency of QG to cascade energy into the barotropic mode and into scales larger than Rd can be counteracted when stratification is surface-trapped, for then the baroclinic mode is weakly damped, and hence enhanced, Numerical experiments are performed with both surface-trapped and uniform stratification to quantify this, Experiments with equal Ekman frictions in the two layers are also performed for purposes of contrast, Interpretation is aided with an inequality derived from the energy and enstrophy equations, The inequality forbids the simultaneous retention of substantial energy in the baroclinic mode and in scales near Rd when Ekman friction is symmetric, but points towards surface-trapped stratification and bottomtrapped friction as an environment in which both of these can be achieved, The dissertation also contains a systematic study of geostrophic turbulence forced by nonzonal flows, Narrow zonal jets emerge when shear-induced mean potential vorticity (PV) gradients are small compared to the planetary gradient (β), and energy is a strong function of the angle shear presents to the east-west direction, When shear-induced PV gradients are comparable to β, and the mean shear has a westward component, fields of monopolar vortices form and persist, Energy is asymmetric between fields of cyclones and anticyclones, Such asymmetry was commonly thought not to occur in QG, but is shown here to be introduced by the nonzonal basic state, In both jet and vortex regimes, eddy energy can be much larger than mean kinetic energy, contrary to the expectation that β stabilizes weak shear flows,

Beschreibung: My first three years here were funded by an Office of Naval Research/National Defense Science and Engineering Graduate Fellowship administered by Jeff Jarocz at the American Society for Engineering Education. During the last three years, most of my support has come the National Science Foundation via grant OCE-9617848, with some additional support coming from the Office of Naval Research via grant N00014-95-1-0824.

Beschreibung: 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 2000

Beschreibung: A channel characterization experiment for the underwater acoustic communication channel was carried out at Scripps Pier in May 1999. The experiment investigated acoustic transmission in very shallow water and breaking waves. In analyzing the data, several questions arose. The majority of the acoustic channel probe data was corrupted by crosstalk in the receiver array cable. This thesis investigates methods to correct for the effects of the crosstalk, to attempt to recover the channel probe data. In selected regions, the crosstalk could be removed quite effectively using a linear least-squares method to estimate the crosstalk coefficients. The bulk of the data could not be corrected, however, primarily due to crosstalk from a receiver channel which was not recorded, and hence could not be well estimated. A second question addressed by this thesis is concerned with acoustic propagation in shallow water under bubble clouds. The breaking waves injected air deep into the water column. The resulting bubble clouds heavily attenuated acoustic signals, effectively causing total dropouts of the acoustic communication channel. Due to buoyancy, the bubbles gradually rise, and the communication channel clears. The channel clearing was significantly slower than predicted by geometric ray acoustic propagation models, however. Proposed explanations included secondary, unobserved, breaking events causing additional bubble injection; delayed rising of bubbles due to turbulent currents; or failure of the geometric ray model due to suppression by bubble clouds of acoustic signals which are not along the geometric ray paths. This thesis investigated the final hypothesis, modeling the acoustic propagation in Scripps Pier environment, using the full wave equation modeling package OASES. It was determined that the attenuation of the propagating acoustic signal is not accurately predicted by the bubble-induced attenuation along the geometric ray path.

Beschreibung: For financial support, thanks to the National Science Foundation for funding me on a Graduate Research Fellowship, and thanks to the WHOI Education Office for supplementing that fellowship.

Beschreibung: 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 January 1987

Beschreibung: This thesis presents the theoretical and computational underpinnings of a novel approach to the determination of the acoustic parameters of the ocean bottom using a monochromatic source. The problem is shown to be equivalent to that of the reconstruction of the potential in a Schrodinger equation from the knowledge of the plane-wave reflection coefficient as a function of vertical wavenumber, r(kz) for all real positive k z. First, the reflection coefficient is shown to decay asymptotically at least as fast as (1/kz2) for large kz and is therefore inteqrable. The Gelfand-Levitan inversion procedure is extended to include the case of basement velocity higher than the velocity of sound in water. The neglect of bound states is shown to be justified in both clayey silt and silty clay at the 220 Hz frequency of operation. Three methods for the numerical solution of the integral equation are investigated. The first one is an "Improved Born approximation" wherein the solution is given as a series expansion the first term of which is the Born approximation while the second term represents a substantial and yet easy to implement improvement over Born. The two other methods are based on a discretization of the Gelfand-Levitan integral equation, and both avoid a matrix inversion: one by employing a recursive procedure, and the other by coupling the Gelfand-Levitan equation with a partial differential equation. Bounds are obtained on errors in the solution due either to discretization or to data inaccuracy. These methods are tested on synthetic data obtained from known geoacoustic models of the ocean bottom. Results are found to be very accurate particularly at the top of the sediment layer with resolution of less than the wavelength of the acoustic source in the water. Several effects are investigated, such as sampling, attenuation, and noise. Also examined is the gradual restriction of the reflection coefficient to a finite range of vertical wave numbers and the consequent progressive deterioration of the reconstruction. The analysis shows how to reconstruct velocity profiles in the presence of density variation when the experiment is conducted at two frequencies. Our results provide a good understanding of the issues involved in conducting a monochromatic deep ocean bottom experiment and constitute a promising technique for processing the experimental data when it becomes available.

Beschreibung: Support for this thesis was provided in part by the education office at WHOI and by the Office of Naval Research.

Beschreibung: 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 2002

Beschreibung: One of the major problems in wireless communications is compensating for the time-varying intersymbol interference (ISI) due to multipath. Underwater acoustic communications is one such type of wireless communications in which the channel is highly dynamic and the amount of ISI due to multipath is relatively large. In the underwater acoustic channel, associated with each of the deterministic propagation paths are macro-multipath fluctuations which depend on large scale environmental features and geometry, and micro-multipath fluctuations which are dependent on small scale environmental inhomogeneities. For arrivals which are unsaturated or partially saturated, the fluctuations in ISI are dominated by the macro-multipath fluctuations resulting in correlated fluctuations between different taps of the sampled channel impulse response. Traditional recursive least squares (RLS) algorithms used for adapting channel equalizers do not exploit this structure. A channel subspace post-filtering algorithm that treats the least squares channel estimate as a noisy time series and exploits the channel correlation structure to reduce the channel estimation error is presented. The improvement in performance of the algorithm with respect to traditional least squares algorithms is predicted theoretically, and demonstrated using both simulation and experimental data. An adaptive equalizer structure that explicitly uses this improved estimate of the channel impulse response is discussed. The improvement in performance of such an equalizer due to the use of the post-filtered estimate is also predicted theoretically, and demonstrated using both simulation and experimental data.

Beschreibung: This research was supported by an ONR Graduate Traineeship Award Grant #N00014-00-10049.

Beschreibung: 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 2009

Beschreibung: A confluence of several coastal oceanographic features creates an acoustically interesting region with high variability along the New England Shelfbreak. Determining the effect of the variability on acoustic propagation is critical for sonar systems. In the Nantucket Shoals area of the Middle Atlantic Bight, two experiments, the New England Shelfbreak Tests (NEST), were conducted in May and June, 2007 and 2008, to study this variability. A comprehensive climatology of the region along with the experimental data provided detailed information about the variability of the water column, particularly the temperature and sound speed fields. Empirical orthogonal function (EOF) analysis of the ocean sound speed field defined a set of perturbations to the background sound speed field for each of the NEST Scanfish surveys. Attenuation due to bottom sediments is the major contributor of transmission loss in the ocean. In shallow water, available propagation paths most often include bottom interaction. Perturbations in the ocean sound speed field can cause changes in the angle of incidence of sound rays with the bottom, which can result in changes to the amount of sound energy lost to the bottom. In lieu of complex transmission loss models, the loss/bounce model provides a simpler way to predict transmission loss changes due to perturbations in the background sound speed field in the ocean. Using an acoustic wavenumber perturbation method, sound speed perturbations, defined by the ocean EOF modes, are translated into a change in the horizontal wavenumber, which in turn changes the modal angle of incidence. The loss/bounce model calculates the loss of sound energy (dB) per bottom bounce over a given distance based on the change in angle of incidence. Evaluated using experimental data from NEST, the loss/bounce model provided accurate predictions of changes to transmission loss due to perturbations of the background sound speed field.

Beschreibung: 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

Beschreibung: 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.

Beschreibung: 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.

Beschreibung: 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 2009

Beschreibung: Mode filtering is most commonly implemented using the sampled mode shape or pseudoinverse algorithms. Buck et al placed these techniques in the context of a broader maximum a posteriori (MAP) framework. However, the MAP algorithm requires that the signal and noise statistics be known a priori. Adaptive array processing algorithms are candidates for improving performance without the need for a priori signal and noise statistics. A variant of the physically constrained, maximum likelihood (PCML) algorithm is developed for mode filtering that achieves the same performance as the MAP mode filter yet does not need a priori knowledge of the signal and noise statistics. The central innovation of this adaptive mode filter is that the received signal's sample covariance matrix, as estimated by the algorithm, is constrained to be that which can be physically realized given a modal propagation model and an appropriate noise model. The first simulation presented in this thesis models the acoustic pressure field as a complex Gaussian random vector and compares the performance of the pseudoinverse, reduced rank pseudoinverse, sampled mode shape, PCML minimum power distortionless response (MPDR), PCML-MAP, and MAP mode filters. The PCML-MAP filter performs as well as the MAP filter without the need for a priori data statistics. The PCML-MPDR filter performs nearly as well as the MAP filter as well, and avoids a sawtooth pattern that occurs with the reduced rank pseudoinverse filter. The second simulation presented models the underwater environment and broadband communication setup of the Shallow Water 2006 (SW06) experiment. Data processing results are presented from the Shallow Water 2006 experiment, showing the reduced sensitivity of the PCML-MPDR filter to white noise compared with the reduced rank pseudoinverse filter. Lastly, a linear, decision-directed, RLS equalizer is used to combine the response of several modes and its performance is compared with an equalizer applied directly to the data received on each hydrophone.

Beschreibung: 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 2009

Beschreibung: Marine mammal whistle calls present an attractive medium for covert underwater communications. High quality models of the whistle calls are needed in order to synthesize natural-sounding whistles with embedded information. Since the whistle calls are composed of frequency modulated harmonic tones, they are best modeled as a weighted superposition of harmonically related sinusoids. Previous research with bottlenose dolphin whistle calls has produced synthetic whistles that sound too “clean” for use in a covert communications system. Due to the sensitivity of the human auditory system, watermarking schemes that slightly modify the fundamental frequency contour have good potential for producing natural-sounding whistles embedded with retrievable watermarks. Structured total least squares is used with linear prediction analysis to track the time-varying fundamental frequency and harmonic amplitude contours throughout a whistle call. Simulation and experimental results demonstrate the capability to accurately model bottlenose dolphin whistle calls and retrieve embedded information from watermarked synthetic whistle calls. Different fundamental frequency watermarking schemes are proposed based on their ability to produce natural sounding synthetic whistles and yield suitable watermark detection and retrieval.

Beschreibung: 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 2002

Beschreibung: Sound propagation in shallow water is highly dependent on the interaction of the sound field with the bottom. In order to fully understand this problem, it is necessary to obtain reliable estimates of bottom geoacoustic properties that can be used in acoustic propagation codes. In this thesis, perturbative inversion methods and exact inverse methods are discussed as a means for inferring geoacoustic properties of the bottom. For each of these methods, the input data to the inversion is the horizontal wavenumber spectrum of a point-source acoustic field. The main thrust of the thesis work concerns extracting horizontal wavenumber content for fully three-dimensionally varying waveguide environments. In this context, a high-resolution autoregressive (AR) spectral estimator was applied to determine wavenumber content for short aperture data. As part of this work, the AR estimator was examined for its ability to detect discrete wavenumbers in the presence of noise and also to resolve closely spaced wavenumbers for short aperture data. As part of a geoacoustic inversion workshop, the estimator was applied to extract horizontal wavenumber content for synthetic pressure field data with range-varying geoacoustic properties in the sediment. The resulting wavenumber content was used as input data to a perturbative inverse algorithm to determine the sound speed profile in the sediment. It was shown using the high-resolution wavenumber estimator that both the shape and location of the range-variability in the sediment could be determined. The estimator was also applied to determine wavenumbers for synthetic data where the water column sound speed contained temporal variations due to the presence of internal waves. It was shown that reliable estimates of horizontal wavenumbers could be obtained that are consistent with the boundary conditions of the waveguide. The Modal Mapping Experiment (MOMAX), an experimental method for measuring the full spatial variability of a propagating sound field and its corresponding modal content in two-dimensions, is also discussed. The AR estimator is applied to extract modal content from the real data and interpreted with respect to source/receiver motion and geometry. For a moving source, it is shown that the wavenumber content is Doppler shifted. A method is then described that allows the direct measure of modal group velocities from Doppler shifted wavenumber spectra. Finally, numerical studies are presented addressing the practical issues associated with using MOMAX type data in the exact inversion method of Gelfand-Levitan.

Beschreibung: I am especially grateful to ONR for providing the funding for me to do this work.

Beschreibung: 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.

Beschreibung: 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.

Beschreibung: 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.

Beschreibung: 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

Beschreibung: This thesis covers a comprehensive analysis of long-range, deep-ocean, low-frequency, sound propagation experimental results obtained from the North Pacific Ocean. The statistics of acoustic fields after propagation through internal-wave-induced sound-speed fluctuations are explored experimentally and theoretically. The thesis starts with the investigation of the North Pacific Acoustic Laboratory 98-99 data by exploring the space-time scales of ocean sound speed variability and the contributions from different frequency bands. The validity of the Garret & Munk internal-wave model is checked in the upper ocean of the eastern North Pacific. All these results impose hard bounds on the strength and characteristic scales of sound speed fluctuations one might expect in this region of the North Pacific for both internal-wave band fluctuations and mesoscale band fluctuations. The thesis then presents a detailed analysis of the low frequency, broadband sound arrivals obtained in the North Pacific Ocean. The observed acoustic variability is compared with acoustic predictions based on the weak fluctuation theory of Rytov, and direct parabolic equation Monte Carlo simulations. The comparisons show that a resonance condition exists between the local acoustic ray and the internal wave field such that only the internal-waves whose crests are parallel to the local ray path will contribute to acoustic scattering: This effect leads to an important filtering of the acoustic spectra relative to the internal-wave spectra. We believe that this is the first observational evidence for the acoustic ray and internal wave resonance. Finally, the thesis examined the evolution with distance, of the acoustic arrival pattern of the off-axis sound source transmissions in the Long-range Ocean Acoustic Propagation EXperiment. The observations of mean intensity time-fronts are compared to the deterministic ray, parabolic equation (with/without internal waves) and (one-way coupled) normal mode calculations. It is found the diffraction effect is dominant in the shorter-range transmission. In the longer range, the (internal wave) scattering effect smears the energy in both the spatial and temporal scales and thus has a dominant role in the finale region.

Beschreibung: The funding that made this research possible came from the Office of Naval Research, and the WHOI Academic Programs Office.

Beschreibung: 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

Beschreibung: 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.

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

Beschreibung: 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

Beschreibung: 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.

Beschreibung: 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

Beschreibung: A novel distributed underwater acoustic networking (UAN) protocol suitable for ad-hoc deployments of both stationary and mobile nodes dispersed across a relatively wide coverage area is presented. Nodes are dynamically clustered in a distributed manner based on the estimated position of one-hop neighbor nodes within a shallow water environment. The spatial dynamic cellular clustering scheme allows scalable communication resource allocation and channel reuse similar in design to land-based cellular architectures, except devoid of the need for a centralized controlling infrastructure. Simulation results demonstrate that relatively high degrees of interference immunity, network connectivity, and network stability can be achieved despite the severe limitations of the underwater acoustic channel.

Beschreibung: 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

Beschreibung: 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.

Beschreibung: 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

Beschreibung: 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.

Beschreibung: 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 2002

Beschreibung: This thesis investigates the complexities of acoustic scattering by finite bodies in general and by fish in particular through the development of an advanced acoustic scattering model and detailed laboratory acoustic measurements. A general acoustic scattering model is developed that is accurate and numerically effcient for a wide range of frequencies, angles of orientation, irregular axisymmetric shapes and boundary conditions. The model presented is an extension of a two-dimensional conformal mapping approach to scattering by irregular, finite-length bodies of revolution. An extensive series of broadband acoustic backscattering measurements has been conducted involving alewife fish (Alosa pseudoharengus), which are morphologically similar to the Atlantic herring (Clupea harengus). A greater-than-octave bandwidth (40-95 kHz), shaped, linearly swept, frequency modulated signal was used to insonify live, adult alewife that were tethered while being rotated in 1-degree increments over all angles of orientation in two planes of rotation (lateral and dorsal/ventral). Spectral analysis correlates frequency dependencies to morphology and orientation. Pulse compression processing temporally resolves multiple returns from each individual which show good correlation with size and orientation, and demonstrate that there exists more than one significant scattering feature in the animaL. Imaging technologies used to exactly measure the morphology of the scattering features of fish include very highresolution Phase Contrast X-rays (PCX) and Computerized Tomography (CT) scans, which are used for morphological evaluation and incorporation into the scattering modeL. Studies such as this one, which combine scattering models with high-resolution morphological information and high-quality laboratory data, are crucial to the quantitative use of acoustics in the ocean.

Beschreibung: 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 1983

Beschreibung: Ocean acoustic tomography was proposed in 1978 by Munk and Wunsch as a possible technique for monitoring the evolution of temperature, density, and current fields over large regions. In 1981, the Ocean Tomography Group deployed four 224 Hz acoustic sources and five receivers in an array which fit within a box 300 km. on a side centered on 26°N, 70°W (southwest of Bermuda). The experiment was intended both to demonstrate the practicality of tomography as an observation tool and to extend the understanding of mesoscale evolution in the low-energy region far from the strong Gulf Stream recirculation. The propagation of 224 Hz sound energy in the ocean can be described as a set of rays traveling from source to receiver, with each ray taking a different path through the ocean in a vertical plane connecting the source and receiver. The sources transmitted a phase-coded signal which was processed at the receiver to produce a pulse at the time of arrival of the signal. Rays can be distinguished by their different pulse travel times, and these travel times change in response to variations in sound speed and current in the ocean through which the rays passed. In order to reconstruct the ocean variations from the observed travel time changes, it is necessary to specify models for both the variations and their effect on the travel times. The dependence of travel time on the oceanic sound speed and current fields can be calculated using ray paths traced by computer. The vertical structure of the sound speed and current fields in the ocean were modelled as a combination of Empirical Orthogonal Functions (EOFs) from MODE. The horizontal structure was continuous, but was constrained to have a gaussian covariance with a 100 km. e- folding scale. The resulting estimator closely resembles objective mapping as used in meteorology and physical oceanography. The tomographic system has at present only been used to estimate sound speed structure for comparison with the traditional measurements, especially the first two NOAA CTD surveys, but the method provides means for estimating density, temperature or velocity fields, and these will be produced in the future. The sound speed estimates made using the tomographic system match the traditional measurements to within the associated error bars, and there are several possibilities for improving the signal to noise ratio of the data. Given high-precision data, tomographic systems can resolve ocean structures at small scales, such as in the Gulf Stream, or at large scales, over entire ocean basins. Work is in progress to evaluate the usefulness of tomography as an observation tool in these applications.

Beschreibung: My support for the first 3 years came from an NSF graduate fellowship, and I was then supported as a research assistant by NSF Grant OCE-8017791.

Beschreibung: 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

Beschreibung: 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.

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

Beschreibung: 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

Beschreibung: 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.

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

Beschreibung: 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

Beschreibung: 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.

Beschreibung: 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.

Beschreibung: Submitted in partial fulfillment of the requirements for the degree of Electrical Engineer at the Woods Hole Oceanographic Institution and the Massachusetts Institute of Technology June 1979

Beschreibung: For a spherical acoustic wave incident on a horizontally stratified ocean bottom, the reflected pressure field and the plane-wave reflection coefficient are related through a two-dimensional spatial-wavenumber Fourier transform. An algorithm is proposed to evaluate the plane-wave reflection coefficient from the bottom reflected field as a function of angle of incidencè. The algorithm is based on the "Projection-Slice" theorem associated with the two-dimensional Fourier transform. This technique is implemented to evaluate the plane-wave reflection coefficient for a perfectly reflecting ocean bottom and for an isovelocity-low speed ocean bottom model.

Beschreibung: 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

Beschreibung: 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.

Beschreibung: 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.

Beschreibung: 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 2008.

Beschreibung: In this thesis, I provide quantitative descriptions of toothed whale echolocation and foraging behavior, including assessment of the effects of noise on foraging behavior and the potential influence of ocean acoustic propagation conditions on biosonar detection ranges and whale noise exposure. In addition to presenting some novel basic science findings, the case studies presented in this thesis have implications for future work and for management. In Chapter 2, I describe the application of a modified version of the Dtag to studies of harbor porpoise echolocation behavior. The study results indicate how porpoises vary the rate and level of their echolocation clicks during prey capture events; detail the differences in echolocation behavior between different animals and in response to differences in prey fish; and show that, unlike bats, porpoises continue their echolocation buzz after the moment of prey capture. Chapters 3-4 provide case studies that emphasize the importance of applying realistic models of ocean acoustic propagation in marine mammal studies. These chapters illustrate that, although using geometric spreading approximations to predict communication/target detection ranges or noise exposure levels is appropriate in some cases, it can result in large errors in other cases, particularly in situations where refraction in the water column or multi-path acoustic propagation are significant. Finally, in Chapter 5, I describe two methods for statistical analysis of whale behavior data, the rotation test and a semi-Markov chain model. I apply those methods to test for changes in sperm whale foraging behavior in response to airgun noise exposure. Test results indicate that, despite the low-level exposures experienced by the whales in the study, some (but not all) of them reduced their buzz production rates and altered other foraging behavior parameters in response to the airgun exposure.

Beschreibung: Work presented in this thesis was supported by a National Science Foundation Graduate Research Fellowship, the WHOI Ocean Life Institute (Grant Numbers 32031300 and 25051351), the Office of Naval Research, the U.S. Department of the Interior Minerals Management Service (Cooperative Agreement Numbers 1435-01-02-CA-85186 and NA87RJ0445; WHOI Grant Number 15205601), the Industry Research Funding Coalition, and the WHOI/MIT Joint Program in Oceanography/Applied Ocean Science & Engineering (including a Fye Teaching Fellowship).

Beschreibung: 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

Beschreibung: 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.

Beschreibung: 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

Beschreibung: Oceanographic applications of robotics are as varied as the undersea environment itself. As underwater robotics moves toward the study of dynamic processes with multiple vehicles, there is an increasing need to distill large volumes of data from underwater vehicles and deliver it quickly to human operators. While tethered robots are able to communicate data to surface observers instantly, communicating discoveries is more difficult for untethered vehicles. The ocean imposes severe limitations on wireless communications; light is quickly absorbed by seawater, and tradeoffs between frequency, bitrate and environmental effects result in data rates for acoustic modems that are routinely as low as tens of bits per second. These data rates usually limit telemetry to state and health information, to the exclusion of mission-specific science data. In this thesis, I present a system designed for communicating and presenting science telemetry from untethered underwater vehicles to surface observers. The system's goals are threefold: to aid human operators in understanding oceanographic processes, to enable human operators to play a role in adaptively responding to mission-specific data, and to accelerate mission planning from one vehicle dive to the next. The system uses standard lossy compression techniques to lower required data rates to those supported by commercially available acoustic modems (O(10)-O(100) bits per second). As part of the system, a method for compressing time-series science data based upon the Discrete Wavelet Transform (DWT) is explained, a number of low-bitrate image compression techniques are compared, and a novel user interface for reviewing transmitted telemetry is presented. Each component is motivated by science data from a variety of actual Autonomous Underwater Vehicle (AUV) missions performed in the last year.

Beschreibung: National Science Foundation Center for Subsurface Sensing and Imaging (CenSSIS ERC)

Beschreibung: 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

Beschreibung: This thesis develops methods for estimating wideband shallow-water acoustic communication channels. The very shallow water wideband channel has three distinct features: large dimension caused by extensive delay spread; limited number of degrees of freedom (DOF) due to resolvable paths and inter-path correlations; and rapid fluctuations induced by scattering from the moving sea surface. Traditional LS estimation techniques often fail to reconcile the rapid fluctuations with the large dimensionality. Subspace based approaches with DOF reduction are confronted with unstable subspace structure subject to significant changes over a short period of time. Based on state-space channel modeling, the first part of this thesis develops algorithms that jointly estimate the channel as well as its dynamics. Algorithms based on the Extended Kalman Filter (EKF) and the Expectation Maximization (EM) approach respectively are developed. Analysis shows conceptual parallels, including an identical second-order innovation form shared by the EKF modification and the suboptimal EM, and the shared issue of parameter identifiability due to channel structure, reflected as parameter unobservability in EKF and insufficient excitation in EM. Modifications of both algorithms, including a two-model based EKF and a subspace EM algorithm which selectively track dominant taps and reduce prediction error, are proposed to overcome the identifiability issue. The second part of the thesis develops algorithms that explicitly find the sparse estimate of the delay-Doppler spread function. The study contributes to a better understanding of the channel physical constraints on algorithm design and potential performance improvement. It may also be generalized to other applications where dimensionality and variability collide.

Beschreibung: Financial support for this thesis research was provided by the Office of Naval Research and the WHOI Academic Program Office.

Beschreibung: 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

Beschreibung: 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.

Beschreibung: The funding that made this research possible came from the Office of Naval Research, and the WHOI Academic Programs Office.

Beschreibung: 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

Beschreibung: 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.

Beschreibung: 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.

Beschreibung: 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

Beschreibung: 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.

Beschreibung: 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.

Beschreibung: 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

Beschreibung: 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.

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

Beschreibung: Submitted in partial fulfillment of the requirements for Master of Science at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution September 2005

Beschreibung: This thesis develops and utilizes a method for analyzing data from the North Pacific Acoustic Laboratory's (NPAL) Basin Acoustic Seamount Scattering Experiment (BASSEX). BASSEX was designed to provide data to support the development of analytical techniques and methods which improve the understanding of sound propagation around underwater seamounts. The depth-dependent sound velocity profile of typical ocean waveguides force sound to travel in convergence zones about a minimum sound speed depth. This ducted nature of the ocean makes modeling the acoustic field around seamounts particularly challenging, compared to an isovelocity medium. The conical shape of seamounts also adds to the complexity of the scatter field. It is important to the U.S. Navy to understand how sound is diffracted around this type of topographic feature. Underwater seamounts can be used to conceal submarines by absorbing and scattering the sound they emit. BASSEX measurements have characterized the size and shape of the forward scatter field around the Kermit-Roosevelt Seamount in the Pacific Ocean. Kermit- Roosevelt is a large, conical seamount which shoals close to the minimum sound speed depth, making it ideal for study. Acoustic sources, including M-sequence and linear frequency-modulated sources, were stationed around the seamount at megameter ranges. A hydrophone array was towed around the seamount to locations which allowed measurement of the perturbation zone. Results from the method developed in this thesis show that the size and shape of the perturbation zone measured coincides with theoretical and experimental results derived in previous work.