ALBERT

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

Description: In the present work, we study the statistics of wavefields obtained from non-linear phase-resolved simulations. The numerical model used to generate the waves models wave-wave interactions based on the fully non-linear Zakharov equations. We vary the simulated wavefield's input spectral properties: directional spreading function, Phillips parameter and peak shape parameter. We then investigate the relationships between a wavefield's input spectral properties and its output physical properties via statistical analysis. We investigate surface elevation distribution, wave definition methods in a nonlinear wavefield with a two-dimensional wavenumber, defined waves' distributions, and the occurrence and spacing of large wave events.

Description: This project was supported by the US Office of Naval Research and the Joint Program between MIT and Woods Hole Oceanographic Institute.

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

Description: This thesis develops a new technique for estimating quasi-homogeneous and quasi-stationary sea surface wave frequency-direction spectra using acoustic tomography. The analysis of acoustic (mode and ray) phase and travel time perturbations due to a rough sea surface is presented. Two canonical waveguides (ideal shallow water and linear squared index of refraction) are used as examples for the mode perturbation. The analysis is used to explain high mode coherence measured in the FRAM N experiment. The forward problem of computing the acoustic phase and travel time perturbation spectra given the surface wave spectrum is solved to first order. An application of the technique to ray phase data taken during the MIZEX '84 experiment is shown. The inverse problems for the homogeneous and quasi-homogel1eous frequency-direction spectrum are introduced. The theory is applied to synthetic data which simulate a fetch-dependent sea. The estimates made agree well with the "actual" (synthetic data) spectrum. The effect of noise in the travel time estimates is studied. The sensitivity of the technique. to the number of rays used in the inversion is investigated and the resolution and variance of the inverse method are addressed.

Description: gratefully acknowledge the financial support of the Office of Naval Research, the General Electric Foundation, and the Ford Foundation.

Description: Submitted in partial fulfillment of the requirements for the degree of Ocean Engineer at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution August 1989

Description: When ocean waves in deep water interact with a current, the direction of propagation and characteristics of the waves such as height and length are affected. Swell in the open ocean can undergo significant refraction as it passes through major current systems like the Gulf Stream or Antarctic Circumpolar Current. Remote sensing techniques such as synthetic aperture radars (SAR) have the potential to detect wave systems over a wide geographical area. Combining a model for wave refraction in the presence of currents with SAR measurements, the inverse problem of using the measured wave data can be solved to determine the direction and magnitude of the intervening currents. In this study the behavior of swell measured by SAR on a satellite pass over the Gulf Stream is examined. The refraction predicted by a numerical model under conditions of varying current profiles and velocities is compared to SAR generated wave spectra. By matching the current profile which results in the best correlation of wave refraction to the SAR data, the tomographic problem of measuring the Gulf Stream current is solved. The best correlation between the model and SAR data is obtained when a current is modeled by a top hat velocity profile with a direction of 75° and a current speed of 2 m/s. The direction agrees with that visually observed from the SAR images, and the direction and speeds are close to the Coast Guard estimates for the Gulf Stream at the time of the SEASAT,pass. The current profiles used did not take into account a possible widening of the Gulf Stream at the position of the satellite overpass. There is a great deal of scatter in the SAR data, both before and in the Gulf Stream, so it is difficult to correlate every point with specific current behavior, but the increase in wave length and change in wave angle in the center of the Gulf Stream seem to indicate that there may be a non-uniform feature such as the formation of an eddy or other lateral variability near the current's edge.

Description: I was supported by the U. S. Navy.

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

Description: Acoustic propagation in the ocean can be strongly affected by small random variations in ocean properties, including rough surfaces and volume fluctuations in the ocean or seabed. Such inhomogeneities scatter part of the incident acoustic field, stripping energy from the coherent part of the field. This scattered energy, or reverberation, propagates further in the modes of the ocean waveguide. The distribution of energy among modes is changed and the coherence of the acoustic field is reduced. This thesis introduces several models which describe scattering of low-frequency sound. First, the rough surface scattering theory of Kuperman and Schmidt is reformulated in terms of normal modes. Scattering from rough fluid-fluid interfaces and rough elastic halfspaces is modeled, and statistics of the acoustic field are calculated. Numerical results show the modal formulation agrees well with Kuperman and Schmidt's model, while reducing computation times by several orders of magnitude for the scenarios considered. Next, a perturbation theory describing scattering from sound speed and density fluctuations in acoustic media is developed. The theory is used to find the scattered field generated by volume fluctuations in sediment bottoms. Modal attenuations due to sediment volume scattering are calculated, and agreement is demonstrated with previous work. The surface and volume scattering theories are implemented in a unified modal reverberation code and used to study bottom scattering in shallow water. Numerical examples are used to demonstrate the relationship between volume and surface scattering. Energy distribution among scattered field modes is found to be a complicated function of the scattering mechanism, the scatterer statistics, and the acoustic environment. In particular, the bottom properties strongly influence the coherence of the acoustic field. Examples show that excitation of fluid-elastic interface waves is a potentially important scattering path. Cross-modal coherences are calculated and used to study the loss of signal coherence with range. Finally, earlier work on scattering from the Arctic ice sheet is extended. Simulations of long-range transmissions are compared with data from the April 1994 trans-Arctic propagation test. The results show modal attenuations and group speeds can be predicted reasonably well, indicating that acoustic monitoring of Arctic climate is feasible.

Description: I am extremely grateful for the financial support of the Office of Naval Research, under contracts N00014-92-J -1282 and N00014-95-1-0307.

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

Description: 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.

Description: Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution December 1980

Description: Data from the COBOLT experiment, which investigated the first 12 km off Long Island's south shore, are analyzed and discussed. Moored current meter records indicate that the nearshore flow field is strongly polarized in the alongshore direction and its fluctuations are well correlated with local meteorological forcing. Complex empirical orthogonal function analysis suggests that subtidal velocity fluctuations are barotropic in nature and are strongly influenced by bottom friction. Wind-related inertial currents were observed within the coastal boundary layer (CBL) under favorable meteorological and hydrographical conditions. The magnitude of these oscillations increases with distance from shore, and they display a very clear 180° phase difference between surface and bottom layers. Nearshore inertial oscillations of both velocity and salinity records appear to lead those further seaward, suggesting local generation and subsequent radiation away from the coast. The response of the coastal zone to impulsive wind forcing is discussed using simple slab and two-layer models, and the behavior of the nearshore current field examined. The major features of the observed inertial motions are in good qualitative agreement with model predictions. It is found that, in a homogeneous domain, the coastal boundary condition effectively prohibits inertial currents over the entire coastal zone. In the presence of stratification the offshore extent of this prohibition is greatly reduced and significant inertial currents may occur within one or two internal deformation radii of the coast. The "coastal effect", in the form of surface and interfacial waves which propagate away from the coast, modifies the "pure" inertial response as it would exist far from shore. The kinematics of this process is such that a 180° phase difference between currents in the two layers is characteristic of the entire coastal zone even before the internal wave has had time to traverse the CBL. It is also suggested that, for positions seaward of several internal deformation radii, interference between the surface and internal components of the coastal response will cause maximum inertial amplitudes to occur for t 〉 x/c2, where c2 is the phase speed of the internal disturbance. The hydrographic structure of the CBL is observed to undergo frequent homogenization. These events are related to both advective and mixing processes. Horizontal and vertical exchange coefficients are estimated from the data, and subsequently used in a diffusive model which accurately reproduces the observed mean density distribution in the nearshore zone. Dynamic balance calculations are performed which indicate that the subtidal cross-shore momentum balance is very nearly geostrophic. The calculations also suggest that the longshore balance may be reasonably represented by a steady, linear equation of motion which includes surface and bottom stresses. Evidence is presented which shows that variations in the longshore wind-stress component are primarily responsible for the energetic fluctuations in the sea surface slope along Long Island. Depth-averaged velocities characteristically show net offshore transport in the study area, and often display dramatic longshore current reversals with distance from shore. These observations are interpreted in terms of a steady circulation model which includes realistic nearshore topography. Model results suggest that longshore current reversals within the CBL may be limited to the eastern end of Long Island, and that this unusual flow pattern is a consequence of flow convergence related to the presence of Long Island Sound.

Description: This work was supported by the Department of Energy through contract no. DE-AC02-EVI0005 entitled Coastal-Shelf Transport and Diffusion.

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

Description: 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.

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

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

Description: The degree to which a baroclinic deep ocean could be responsible for the mean flow on the shallow continental shelf is examined using steady, boundary forced models which incorporate bottom friction. One set of models, for a vertically well mixed shelf, includes the horizontal advection of density. The second set of models comprises a three-layer model without and a two-layer model with interfacial friction. It is found that near bottom flow has a short cross isobath scale due to the steep continental slope and consequently that the deep oceans lower water column could not be responsible for the observed mean flow. The cross isobath scale of flow in the upper deep ocean is predominantly determined by the oceans velocity profile. In a barotropic or near barotropic flow the upper water column follows the near bottom flow and therefore has little influence on the shelf. A surface intensified deep ocean flow is able to cross isobaths until it encounters the bottom. If deep ocean flow is confined to a surface layer thinner than the depth at the shelf break it could be responsible for the observed flow. The depth scale for velocity and density over the slope in the Mid-Atlantic Bight is generally larger than the shelf break depth and consequently it is concluded that the steep continental slope "insulates" this particular shelf from baroclinic deep ocean influence and therefore that the observed shelf flow is not of oceanic origin. Using oxygen isotope data, Chapman et al. (1986) found that the Scotian shelf is the major source of Mid-Atlantic Bight shelf water. Their barotropic modeling results are extended to show that a baroclinic deep ocean also acts to hold shelf water on the shelf.

Description: This Research was carried out with the support of the National Science Foundation, Grant OCE-85-18487

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

Description: Second-class wave propagation along mid-ocean ridges is investigated in an effort to explain subinertial peaks found in the velocity spectra over the Juan de Fuca Ridge (JdFR, 4 days) and the Iceland-Faeroe Ridge (IFR, 1.8 days). Topographic cross sections of the ridges are fit by a double-exponential depth profile and the linearized shallow water equations are solved with the simplified topography. In the northern hemisphere the western ridge flank supports an infinite set of modes for a topographically trapped northward propagating wave and the eastern flank supports southward propagating modes. The eigenfunctions are calculated and dispersion curves are examined for a variety of ridge profiles. Increasing the slope of a ridge flank increases the frequencies of the modes it supports. In addition, the waves travelling along the flanks 'feel' the topography of the opposite side so t hat increasing the width or steepness of the eastern slope decreases the frequencies of the modes supported by the western side (and vice versa). The dispersion characteristics of the trapped nondivergent oscillations allow a zero group velocity (ZGV) so that energy may accumulate along the ridge as long as the ridge does not approach the isolated shelf profile. Including divergence lowers the frequencies of the longest waves so that a ZGV may be found for all ridge profiles. The nature of the effects of stratification, represented by a two-layer model, are explored by a perturbation procedure for weak stratification. The 0(1) barotropic basic state is accompanied by an 0(E2) baroclinic perturbation. The frequencies of the barotropic modes are increased and the velocities are bottom-trapped. For reasonable values of stratification, however, this effect is small. Plugging the JdFR topography into the models produces an approximate 4-day ZGV wave with wavelengths between 1500 and 4500 km. The IFR oscillation, however, appears to be better modelled by a topographic-Rossby mode model. (Miller et al., 1996) The ridge wave models discussed here also predict the observed anticyclonic velocity ellipses over the ridge and horizontal decay away from the ridge crest.

Description: I have been supported during this research by an ONR fellowship, for which I am grateful. The impetus to this work and the ADCP data were provided by an NSF grant (OCE-9215342).

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

Description: 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.

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

Description: Interest in short waves on the ocean surface has been growing over the last three decades because they play an important role in surface electromagnetic (e.m.) scattering. Currently radars and scatterometers which use e.m. scattering to remotely examine the ocean can produce estimates of the surface wind field, surface currents, and other scientifically important ocean processes. These estimates are based on models which depend on a thorough understanding of electromagnetic scattering mechanisms, and of the three-dimensional surface wave field. Electromagnetic scattering theory is well developed, but the short wavelength portion of the surface wave field has only recently been experimentally explored. A single, consistent, and accurate model of the energy distribution on the ocean surface, also known as the wave height spectrum, has yet to be developed. A new instrument was developed to measure the height of waves with 2-30 cm wavelengths at an array of locations which can be post-processed to generate an estimate of the two-dimensional wave height spectrum. This instrument (a circular wire wave gage buoy) was deployed in an experiment which gathered not only in situ measurements of the two-dimensional wave height spectrum, but also coincident scatterometer measurements, allowing the comparison of current e.m. scattering and surface wave height spectrum models with at sea data. The experiment was conducted at the Buzzards Bay Tower located at the mouth of Buzzards Bay in Massachusetts. A rotating X-band scatterometer, a sonic anemometer, and a capacitive wire wave gage were mounted on the tower. The wave gage buoy was deployed nearby. The resulting data supports a narrowing trend in the two-dimensional spectral width as a function of wavenumber. Two current spectral models support this to some extent, while other models do not. The data also shows a similar azimuthal width for the scatterometer return and the width of the short wavelength portion of the wave height spectrum after it has been averaged and extrapolated out to the appropriate Bragg wavelength. This appears to support current e.m. composite surface (two-scale) theories which suggest that the scattered return from the ocean at intermediate incidence angles is dominated by Bragg scattering which depends principally on the magnitude and shape of the two-dimensional wave height spectrum. However, the mean wind direction (which corresponds well with the peak of the scatterometer energy distribution) and the peak of 20 minute averages of the azimuthal energy distribution were out of alignment in two out of three data sets, once was by nearly 90°. There are a number of tenable explanations for this including instrument physical limitations and the possibility of significant surface currents, but none that would explain such a significant variation. Given that there are so few measurements of short wave directional spectra, however l these results should be considered preliminary in the field and more extensive measurements are required to fully understand the angular distribution of short wave energy and the parameters upon which it depends.

Description: Funding: the MIT Ocean Engineering Department, the WHOI Rienhart Coastal Research Center, the WHOI Education Office, the National Defense Science and Engineering Graduate Fellowship Program, and grant N0001493-1-0726 from the Office of Naval Research.

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

Description: Planktonic protozoan grazers have the potential to significantly affect the chemistry of particle-associated trace metals. This is due both to the importance of protists as consumers of bacterial-sized particles, and to the unique low-pH, enzyme-rich microenvironment of the grazer food vacuole. This thesis examines the role of protozoan grazers in the marine geochemistry of strongly hydrolyzed, particle-reactive trace metals, in particular Th and Fe. A series of tracer experiments was carried out in model systems in order to determine the effect of grazer-mediated transformations on the chemical speciation and partitioning of radioisotopes C9Fe, 234Th, 51Cr) associated with prey cells. Results indicate that protozoan grazers are equally able to mobilize intracellular and extracellular trace metals. In some cases, protozoan regeneration of trace metals appears to lead to the formation of metal-organic complexes. Protozoan grazing may generate colloidal material that can scavenge trace metals and, via aggregation, lead to an increase in the metal/organic carbon ratio of aggregated particles. Model system experiments were also conducted in order to determine the effect of grazers on mineral phases, specifically colloidal iron oxide (ferrihydrite). Several independent techniques were employed, including size fractionation ors9Fe-labeled colloids, competitive ligand exchange, and iron-limited diatoms as "probes" for bioavailable Fe. Experimental evidence strongly suggests that protozoan grazing can affect the surface chemistry and increase the dissolution rate of iron oxide phases through phagotrophic ingestion. In further work on protozoan-mediated dissolution of colloidal Fe oxides, a novel tracer technique was developed based on the synthesis of colloidal ferrihydrite impregnated with 133Ba as an inert tracer. This technique was shown to be a sensitive, quantitative indicator for the extent of ferrihydrite dissolution/alteration by a variety of mechanisms, including photochemical reduction and ligand-mediated dissolution. In field experiments using this technique, grazing by naturally occuring protistan assemblages was shown to significantly enhance the dissolution rate of colloidal ferrihydrite over that in non-grazing controls. Laboratory and field results indicate that, when integrated temporally over the entire euphotic zone, protozoan grazing may equal or exceed photoreduction as a pathway for the dissolution of iron oxides.

Description: This work was financially supported by a Department of Defense ONR-NDSEG Graduate Fellowship, Office ofNaval Research AASERT Award (N00014-94-1-0711), and the National Science Foundation EGB Program (OCE-9523910).

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

Description: This thesis is not subject to U.S. copyright.

Description: A demonstrated need exists for better wind field information over the open ocean, especially as a forcing function for ocean circulation models. Microwave scatterometry, as a means of remotely sensing surface wind information, developed in response to this requirement for a surface wind field with global coverage and improved spatial and temporal resolution. This development led to the 1978 deployment of the SEASAT Satellite Scatterometer (SASS). Evaluations of the three months of SEASAT data have established the consistency of SASS winds with high quality surface wind data from field experiments over limited areas and time periods. The directional ambiguity of the original SASS vectors has been removed by Atlas et al. (1987) for the entire data set, and the resulting SASS winds provide a unique set of scatterometer wind information for a global comparison with winds from conventional sources. A one-month (12 August to 9 September 1978) subset of these dealiased winds, in the western North Atlantic, is compared here with a conventional, pressure-derived wind field from the 6-hourly surface wind analyses of the Fleet Numerical Oceanographic Center (FNOC), Monterey, CA. Through an objective mapping procedure, the irregularly spaced SASS winds are regridded to a latitude-longitude grid, facilitating statistical comparisons with the regularly spaced FNOC wind vectors and wind stress curl calculations. The study includes qualitative comparisons to synoptic weather maps; calculations of field statistics and boxed mean differences; scatter plots of wind speed, direction, and standard deviation; statistical descriptions of the SASS-FNOC difference field, and wind stress curl calculations. The SASS and FNOC fields are consistent with each other in a broad statistical sense, with wide scatter of individual values about a pattern of general agreement. The FNOC wind variances are slightly smaller than the SASS values, reflecting smoothing on larger spatial scales than the SASS winds, and the SASS mean values tend to be slightly higher than the FNOC means, though the increase is frequently lost in the large scatter. Exceptions to the pattern of relatively small consistent variations between the two fields are the pronounced differences associated with extremely strong winds, especially during Hurricane Ella, which traveled up the East Coast of the United States during the latter part of the study period. These large differences are attributed mainly to differences in the inferred positions of the pressure centers and in the response at the highest wind speeds (〉 20m/s). The large statistical differences between the SASS and FNOC fields, present under high wind conditions, may yield significantly different ocean forcing, especially when the strong winds persist over longer periods of time. Under less intense wind conditions, usually prevailing over the ocean, the two fields correspond well statistically and the ocean responses forced by each should be similar.

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

Description: Using the maximum-likelihood estimation method and minimization techniques, quasi-geostrophic wave solutions were fitted to the observations of the 1981 Ocean Acoustic Tomography Experiment. The experiment occupied a 300 km square area centered at 26°N, 70°W, and had a duration of ~80 days. The data set consisted of acoustic travel-time records, temperature records and CTD profiles, obtained from the acoustic tomographic array, moored temperature sensors and recorders, and ship surveys, respectively. While the latter two were conventional spot measurements, the former corresponds to integral measurements of the temperature (or sound-speed) field. The optimal fit to the data corresponded to 3 waves in the first baroclinic mode, evolving under the presence of a westward mean flow with vertical shear. The flow was estimated to be weak (~2 cm/s), but it changed the wave periods significantly by producing large Doppler shifts. The waves were dynamically stable to the mean flow, had weak nonlinear interactions with each other and did not form a resonant traid; thus they constituted a fully linear solution. Evidence for the existence of the waves was strongly supported by the high correlation (~0.9) between the data and the fit, the large amount of signal energy resolved (~80 percent), the excellent quality of the wave-parameter estimate (only about 10 percent in error), and the general agreement between the observations and quasi-geostrophic linear dynamics.

Description: My financial support for the first two years came from the Education Office at W.H.O.I. My dissertation research was supported by ONR Grant NOOOl4-82-C0019.

Description: Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution May 1990

Description: The importance of wave breaking in both microwave remote sensing and air-sea interaction has led to this investigation of the utility of a Ku-Band CW Doppler scatterometer to detect and characterize wave breaking in the open ocean. Field and laboratory measurements by previous authors of microwave backscatter from sharp-crested and breaking waves have shown that these events can exhibit characteristic signatures in moderate incidence angle measurements of the radar cross-section (RCS) and Doppler spectrum. Specifically, breaking events have been associated with polarization independent sea spikes in the RCS accompanied by increased mean frequency and bandwidth of the Doppler spectrum. Simultaneous microwave, video, and environmental measurements were made during the SAXON experiment off Chesapeake Bay in the fall of 1988. The scatterometer was pointed upwind with an incidence angle of 45 degrees and an illumination area small compared to the wavelength of the dominant surface waves. An autocovariance estimation technique was used to produced time series of the RCS, mean Doppler frequency, and Doppler spectral bandwidth in real-time. The joint statistics of the microwave quantities indicative of breaking are used to investigate detection schemes for breaking events identified from the video recordings. The most successful scheme is based on thresholds in both the RCS and the Doppler bandwidth determined from joint distributions for breaking and non-breaking waves. Microwave events consisting of a sea spike in the RCS accompanied by a large bandwidth are associated with the steep forward face of waves in the early stages of breaking. The location of the illumination area with respect to the phase of the breaking wave, the stage of breaking development, and the orientation of an individual crest with respect to the antenna look-direction all influence the detect ability of a breaking event occurring in the vicinity of the radar spot. Since sea spikes tend to occur on the forward face of waves in the process of breaking, the whitecap associated with a given sea spike may occur after the crest of the wave responsible for the sea spike has passed the center of the illumination area. Approximately 70% of the waves which produce whitecaps within a distance of 5m of the bore sight location are successfully identified by a threshold-based detection scheme utilizing both RCS and bandwidth information. The sea spike statistics are investigated as functions of wave field parameters and friction velocity u*. For VV and HH polarization, the frequency of sea spike occurrence and the sea spike contribution to the mean RCS show an approximately cubic dependence on u*, which is consistent with theoretical modelling and various measures of whitecap coverage. The data also suggest that the average RCS of an individual sea spike is not dependent on u*. At high friction velocities (u*≈40-50cms-l), the contribution of sea spikes to the mean RCS is in the range of 5-10% for VV and 10-20% for HH. The wind speed dependence of the percentage of crests producing sea spikes is comparable to that of the fraction of breaking crests reported by previous authors. The percentage of wave crests producing sea spikes is found to vary approximately as (Re*)1.5, where Re* is a Reynolds number based on u* and the dominant surface wavelength. This result agrees with measurements of the degree of wave breaking by. previous authors and is shown to be consistent with a cubic dependence on u *. Models for the probability of wave breaking as a function of moments of the wave height spectrum are compared to our results. The Doppler frequency and bandwidth measurements are also used to inquire into the kinematics of the breaking process.

Description: This work was funded by grants from the MIT Sloan Basic Research Fund, the National Science Foundation (Physical Oceanography), and the Office of Naval Research (Physical Oceanography). Additional funding was provided by the National Aeronautics and Space Administration through the Graduate Student Researchers' Fellowship Program.

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

Description: Garrett and Munk use linear dynamics to synthesize frequency-wavenumber energy spectra for internal waves (GM72, GM75, GM79). The GM internal wave models are horizontally isotropic, vertically symmetric, purely propagating, and universal in both time and space. This set of properties effectively eliminates all the interesting physics, since such models do not allow localized sources and sinks of energy. Thus an important step in understanding internal wave dynamics is to make measurements of deviations from the simple GM models. This thesis continues the search for deviations from the GM models. It has three advantages over earlier work: extensive data from an equatorial region, long time series (2 years), and relatively sophisticated linear internal wave models. Since the GM models are based on mid-latitude data, having data from an equatorial region which has a strong mean current system offers an opportunity to examine a region with a distinctly different basic state. The longer time series mean there is a larger statistical ensemble of realizations, making it possible to detect smaller internal wave signals. The internal wave models include several important extensions to the GM models: horizontal anisotropy and vertical asymmetry, resolution between standing modes and propagating waves, general vertical structure, and kinematic effects of mean shear flow. Also investigated are the effects of scattering on internal waves, effects that are especially strong on the equator because the buoyancy frequency variability is a factor of ten higher than at mid-latitudes. In the high frequency internal wave field considered (frequencies between .125 cph and .458 cph), several features are found that are not included in the GM models. Both the kinematic effects of a mean shear flow and the phase-locking that distinguishes standing modes from propagating waves are observed. There is a seasonal dependence in energy level of roughly 10% of the mean level. At times the wave field is zonally and vertically asymmetric, with resulting energy fluxes that are a small (4% to 10%) fraction of the maximum energy flux the internal wave field could support. The fluxes are, however, as big as many of the postulated sources of energy for the internal wave field.

Description: This work has been supported under grants from the National Science Foundation and the Office of Naval Research, grants numbered NSF-89076, ONR-88914, NSF-9l002, NSF-94971, and NSF-93661.

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

Description: A finite difference computer model is developed to simulate the exposure statistics of a radio frequency buoyant antenna as it is towed in a three-dimensional random seaway. The model allows the user to prescribe antenna properties (length, diameter, density, etc.), sea conditions (significant wave height, development of sea), tow angle, and tow speed. The model then simulates the antenna-sea interaction for the desired duration to collect statistics relating to antenna performance. The model provides design engineers with a tool to predict antenna performance trends, and to conduct design tradeoff studies. The floating antenna envisioned is for use by a submarine operating at modest speed and depth.

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

Description: A simple dynamic model is used with various observations to provide an approximate spectral description of low frequency oceanic variability. Such a spectrum has wide application in oceanography, including the optimal design of observational strategy for the deployment of floats, the study of Lagrangian statistics and the estimate of uncertainty for heat content and mass flux. Analytic formulas for the frequency and wavenumber spectra of any physical variable, and for the cross spectra between any two different variables for each vertical mode of the simple dynamic model are derived. No heat transport exists in the model. No momentum flux exists either if the energy distribution is isotropic. It is found that all model spectra are related to each other through the frequency and wavenumber spectrum of the stream-function for each mode, Φ(k, I, w, n, φ, λ), where (k, I) represent horizontal wavenumbers, W stands for frequency, n is vertical mode number, and (φ,λ) are latitude and longitude, respectively. Given Φ(k, I, w, n, φ, λ), any model spectrum can be estimated. In this study, an inverse problem is faced: Φ(k, I, w, n, φ, λ) is unknown; however, some observational spectra are available. I want to estimate Φ(k, I, w, n, φ, λ) if it exists. Estimated spectra of the low frequency variability are derived from various measurements: (i) The vertical structure of and kinetic energy and potential energy is inferred from current meter and temperature mooring measurements, respectively. (ii) Satellite altimetry measurements produce the geographic distributions of surface kinetic energy magnitude and the frequency and wavenumber spectra of sea surface height. (iii) XBT measurements yield the temperature wavenumber spectra and their depth dependence. (v) Current meter and temperature mooring measurements provide the frequency spectra of horizontal velocities and temperature. It is found that a simple form for Φ(k, I, w, n, φ, λ) does exist and an analytical formula for a geographically varying Φ(k, I, w, n, φ, λ) is constructed. Only the energy magnitude depends on location. The wavenumber spectral shape, frequency spectral shape and vertical mode structure are universal. This study shows that motion within the large-scale low-frequency spectral band is primarily governed by quasigeostrophic dynamics and all observations can be simplified as a certain function of Φ(k, I, w, n, φ, λ). The low frequency variability is a broad-band process and Rossby waves are particular parts of it. Although they are an incomplete description of oceanic variability in the North Pacific, real oceanic motions with energy levels varying from about 10-40% of the total in each frequency band are indistinguishable from the simplest theoretical Rossby wave description. At higher latitudes, as the linear waves slow, they disappear altogether. Non-equatorial latitudes display some energy with frequencies too high for consistency with linear theory; this energy produces a positive bias if a lumped average westward phase speed is computed for all the motions present.

Description: This work is supported financially by National Science Foundation through grants OCE-9529545, Jet Propulsion Laboratory, California Institute of Technology through contract 958125, and University of Texas-Austin through contract UTA-98-0222.

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

Description: A study is conducted of the scattering of freely-propagating subinertial frequency coastal-trapped waves (CTWS) by large variations in coastline and topography using analytical and numerical techniques. Particular attention is paid to the role of stratification because, as shown, the introduction of even modest stratification can eliminate backscattered free-waves with large wavenumbers which occur, theoretically, in a barotropic ocean. An analytical solution is presented for the scattering of barotropic waves incident upon a discontinuity in shelf width. Discussion of solutions relying on backscattered free-waves is avoided by considering only the range of parameters over which energy transmission is nearly 100%. The solution shows there is a substantial transfer of energy to modes other than that of the incident wave. The transmitted mode most readily excited is that which has the across-shelf structure most closely coinciding with that of the incident wave. For a widening shelf, energy is therefore readily transferred to higher modes. The resultant presence of multiple modes produces a strong modulation in flow intensity and phase progression downstream of the scattering region which may affect the interpretation of shelf wave observations. A non-dispersive shelf wave 'pulse' of limited a10ngshelf extent scatters into a train of similarly shaped waves of all allowable modes, each propagating at its own free-wave speed. To overcome limitations of the analytical study a numerical model which accomodates arbitrary density stratification, bathymetry, and coastline, is employed. Numerical simulations are conducted of the scattering of CTWs by a set of topographic and coastline variations which are representative of many continental shelves. The strength of the scattering observed is found to be proportional to a topographic warp factor which estimates the severity of the topographic irregularities. The scattering is amplified by density stratification. A comparison of the effects of widening and narrowing topographies shows that the gross scattering effects of 'reciprocal' topographies are qnite similar. Within the scattering region itself, the strengths of the scattered-wave-induced currents exhibit substantial variation over short spatial scales. On both widening and narrowiag shelves, there is generally a marked intensification of the flow within the scattering region, and significant differences in the directions of the currents at points separated by a few tens of kilometers indicate the occurence of rapid variations ia phase. On narrowing shelves, the influence of the scattering can extend upstream into the region of uniform topography even when no freely-propagating backscattered waves exist. A simulation is condncted of CTW scattering at a site on the East Coast of Australia where observations suggest the presence of scattered freely-propagating CTWs. The success of the model simulation in reproducing features of observations confirms that realistic shelf geometries can scatter significant levels of CTW energy, and that the scattered waves can have an appreciable signal in current-meter observations made on the continental shelf. This demonstrates that along irregular coastlines it is necessary to account for the possibility that CTW scattering processes filay be in effect if oceanographic observations are to be interpreted correctly.

Description: This research was supported by the National Science Foundation under grants OCE8417769 and OCE85-21837, and by the WHOI Education Program.

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

Description: The problem of estimating boundary and initial conditions for a regional open-ocean model is addressed here. With the objective of mimicking the SYNOP experiment in the Gulf Stream system, a meandering jet is modeled by the fully nonlinear barotropic vorticity equation. Simulated observations of current velocity are taken using current meters and acoustic tomography. Twin experiments are performed in which the adjoint method is used to reconstruct the flow field. The estimated flow is forced to resemble the true flow by minimizing a cost function with respect to some control variables. At the minimum, the error covariance matrix of the estimated control variables can be evaluated from the inverse Hessian of the cost function. The first major scientific objective of this work is the estimation of initial and boundary conditions for the model from sparse interior data. First the vorticity initial conditions are used as control variables and the boundary conditions are kept fixed. The jet-like flow is found to induce strong dependence of the model/data misfit upon the specified boundary conditions. Successively, the boundary values of streamfunction and vorticity are included among the control variables and estimated together with the initial conditions. Experiments are performed with various choices of scaling and first guess for the control variables, and various observational strategies. The first major new result obtained is the successful estimation of the complete set of initial and boundary conditions, necessary to integrate the vorticity equation forward in time. From a time-invariant first guess for the boundary conditions, the assimilation is able to create temporal variations at the boundaries that make the interior flow match well the velocity observations, even when noise is added. It is found that information from the observations is communicated to the boundaries by advection of vorticity and by the instantaneous domain-wide connections in the streamfunction field due to the elliptic character of the Poisson equation. The second major scientific objective is the estimation of error covariances in the presence of strongly nonlinear dynamics. The evaluation of the full error covariance matrix for the estimated control variables from the inverse Hessian matrix is investigated along with its dependence upon the degree of nonlinearity in the dynam1cs. Further major new results here obtained are the availability of off-diagonal covariances, the successful calculation of error covariances for all boundary and initial conditions, and the estimation of errors for the interior fields of streamfunction and vorticity. The role of the Hessian matrix is assessed in gauging the sensitivity of the estimated boundary and initial conditions to the data. Also, the importance is stressed of retaining off-diagonal structure of the Hessian to obtain more accurate error estimates.

Description: Funding for this research was provided by Office of Naval Research (ONR) Grant N00014-90-J-1481 and, since 1/1/95, ONR Grant N00014-95-1-0226.

Description: Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution September 1990

Description: The contribution of tropical instability waves to the momentum and energy balances of the Pacific Equatorial Undercurrent is investigated using velocity and temperature time series from the three-dimensional Equatorial Pacific Ocean Climate Study mooring array at 110°W. Tropical instability waves are an energetic band of variability typically with periods between 14 and 36 days which are thought to be generated by instability of the equatorial currents. They are frequently observed as meanders of the equatorial front in satellite sea surface temperature maps. Here, they are observed as large oscillations in the meridional velocity records at l10°W with an energy peak at 21 days. Westward phase propagation is observed in this band with a phase speed of -0.9 (±0.3) m s-1 and a wavelength of 1660 km. Upward phase propagation is observed which is consistent with downward energy propagation. The observed propagation characteristics are compared with those of the mixed Rossby-gravity wave. The variability in this band produces large northward fluxes of eastward momentum and southward fluxes of temperature which affect the dynamics of the mean Undercurrent through the Reynolds stress divergence, and the Eliassen-Palm flux divergence. The waves produce a northward flux of eastward momentum, uv, which is largest at the northern mooring in the upper part of the array. The meridional divergence of eastward momentum, -δ(uv)/δy, decelerates the Undercurrent core down to 150 m. This implies a coupling between the Undercurrent and the South Equatorial Current with the eastward momentum of the Undercurrent transferred to the westward flowing South Equatorial Current. To estimate the vertical momentum flux divergence, the vertical eddy flux of eastward momentum, uw, is inferred using the eddy temperature equation. The vertical eddy momentum flux is positive and largest at the core of the Undercurrent, implying an acceleration of the eastward flow above the core and a deceleration below. The Eliassen-Palm flux divergence is small above the core of the Undercurrent at 75 m, but below the core, is sufficient to balance the deeply penetrating eastward pressure gradient force. The instability waves are important to the energetics of the mean Undercurrent. An exchange of kinetic energy from the mean Undercurrent to the waves through shear production is estimated. A local exchange is suggested since the rate at which the mean Undercurrent loses kinetic energy through instability is comparable to the rate at which the waves gain energy through shear production. The conversion from mean to eddy potential energy is an order of magnitude smaller with the waves gaining potential energy through conversion of mean available potential energy. The observations of upward phase propagation and downward Eliassen-Palm flux suggest that the waves propagate energy downward into the deep ocean. The energetics and momentum balance of the mean Undercurrent is investigated further by analyzing the downstream change in the Bernoulli function on the equator along isentropes or potential density surfaces using mean hydrographic sections at 150°W and 110°W. A downstream decrease in the Bernoulli function is observed which is due to a decrease in the Acceleration Potential since the mean kinetic energy of the Undercurrent changes little from 150°W to 110°W. The lateral divergence of eddy momentum fluxes calculated on isotherms is sufficient to balance the observed decrease in the Acceleration Potential. The downstream decrease in the Acceleration potential has further implications for the mean energetics since this "downhill" flow releases mean available potential energy stored in the east-west sloping thermocline. The rate at which the Undercurrent releases available potential energy, is shown to be comparable to the rate at which the mean flow loses kinetic energy by interaction with the waves, with the waves gaining kinetic energy in the process. Thus, it is hypothesized that in the eastern Pacific this downstream release of available potential energy is ultimately converted into a downstream increase in the kinetic energy of the waves rather than the kinetic energy of the mean flow as occurs in the western Pacific. To maintain an equilibrium, the waves radiate energy into the deep ocean as is suggested by the upward phase propagation and the downward Eliassen-Palm flux.

Description: Financial support of the National Science Foundation under contracts OCE 82-14955 and OCE 85-19551, for participation in Tropic Heat, and OCE 85-04125.