ALBERT

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

Description: My thesis covers two general circulation problems that involve the stability of largescale oceanic flows and the importance of non-local effects. The first problem examines the stability of meridional boundary currents, which are found on both sides of most ocean basins because of the presence of continents. A linear stability analysis of a meridional boundary current on the beta-plane is performed using a quasi-geostrophic model in order to determine the existence of radiating instabilities, a type of instability that propagates energy away from its origin region by exciting Rossby waves and can thus act as a source of eddy energy for the ocean interior. It is found that radiating instabilities are commonly found in both eastern and western boundary currents. However, there are some significant differences that make eastern boundary currents more interesting from a radiation point of view. They possess a larger number of radiating modes, characterized by horizontal wavenumbers which would make them appear like zonal jets as they propagate into the ocean interior. The second problem examines the circulation in a nonlinear thermally-forced two-layer quasi-geostrophic ocean. The only driving force for the circulation in the model is a cross-isopycnal flux parameterized as interface relaxation. This forcing is similar to the radiative damping used commonly in atmospheric models, except that it is applied to the ocean circulation in a closed basin and is meant to represent the large-scale thermal forcing acting on the oceans. It is found that in the strongly nonlinear regime a substantial, not directly thermally-driven barotropic circulation is generated. Its variability in the limit of weak bottom drag is dominated by high-frequency barotropic basin modes. It is demonstrated that the excitation of basin normal modes has significant consequences for the mean state of the system and its variability, conclusions that are likely to apply for any other system whose variability is dominated by basin modes, no matter the forcing. A linear stability analysis performed on a wind- and a thermally-forced double-gyre circulation reveals that under certain conditions the basin modes can arise from local instabilities of the flow.

Description: I was supported through a graduate research assistantship from the National Science Foundation Grant OCE-0423975 and the Woods Hole Oceanographic Institution Academic Programs Office.

Description: Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution May 1984

Description: Part one of this thesis discusses the structure of the thermocline and the current pattern within a two-layer model. The corresponding flow field is explored as the amount of water in the upper layer is gradually reduced (or as the wind stress is gradually increased). In the model, when the amount of water in the upper layer is less than a first critical value, the lower layer outcrops near the middle of the western boundary. A dynamically consistent picture includes a whole loop of boundary currents, which surround the outcropping zone completely and have quite different structures. In addition to the boundary currents found in previous models, there is an isolated western boundary current (i.e. bounded on one side by the wall and on the other by a streamline along which the upper layer thickness vanishes), an internal boundary current and possibly isolated northern/southern boundary currents. Within the limitations of the two-layer model, the isolated western boundary current appears to represent the Labrador Current while the internal boundary current may represent the North Atlantic Current. A first baroclinic mode of water mass exchange occurs across the ZWCL (zero-wind-curl-line). When the amount of water in the upper layer is less than a second critical value, the upper layer separates from the eastern wall and becomes a warm water pool in the south-west corner of the basin. Under this warm water pool is the ventilated lower layer. The sea surface density distribution is not specified; it is determined from a consistent dynamical and mass balance. Implicit in this model is the assumption that advection dominates in the mixed layer. The subtropical gyre and the subpolar gyre combine asymmetrically with respect to the ZWCL. Chapter I discusses the case when the lower layer depth is infinite. Chapter II discusses the case when the lower layer depth is finite. In the Addendum the climatological meaning of this two-layer model is discussed. Part two of this thesis concerns the use of a continuously stratified model to represent the thermocline and current structures in subtropical/subpolar basins. The ideal fluid thermocline equation system Is a nonlinear, non-strict hyperbolic system. In an Addendum to Chapter III the mathematical properties of this equation system are studied and a proper way of formulating boundary value problems is discussed. Although the equations are not of standard type, so that no firm conclusions about the existence and uniqueness of solutions have been drawn, some possible approaches to properly posed boundary value problem are suggested. Chapter III presents some simple numerical solutions of the ideal fluid thermocline equation for a subtropical gyre and a subtropical/subpolar basin using one of these approaches. Our model predicts the continuous three dimensional thermocline and current structures in a continuously stratified wind-driven ocean. The upper surface density and Ekman pumping velocity are specified as input data; in addition, the functional form of the potential vorticity is specified. The present model emphasizes the idea that the ideal fluid thermocline model is incomplete. The potential vorticity distribution can not be determined within this idealized model. This suggests that the diffusion and upwelling/downwelling within the western boundary current and the outcropping zone in the north-west corner are important parts of the entire circulation system.

Description: This work was supported by NSF Grant 80-19260-0CE.

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

Description: This thesis examines the nature of eddy-mean flow interactions in western boundary current jets and recirculation gyre dynamics from both theoretical and observational perspectives. It includes theoretical studies of eddy-mean flow interactions in idealized configurations relevant to western boundary current jet systems, namely (i) a study of the mechanism by which eddies generated from a localized forcing drive mean recirculation gyres through the process of nonlinear rectification; and (ii) a study of the role of eddies in the downstream evolution of a baroclinic jet subject to mixed instabilities. It also includes an observational analysis to characterize eddy-mean flow interactions in the Kuroshio Extension using data from the downstream location of maximum eddy kinetic energy in the jet. New insights are presented into a rectification mechanism by which eddies drive the recirculation gyres observed in western boundary current systems. Via this mechanism, eddies drive the recirculations by an up-gradient eddy potential vorticity flux inside a localized region of eddy activity. The effectiveness of the process depends on the properties of the energy radiation from the region, which in turn depends on the population of waves excited. In the zonally-evolving western boundary current jet, eddies also act to stabilize the unstable jet through down-gradient potential vorticity fluxes. In this configuration, the role of eddies depends critically on their downstream location relative to where the unstable time-mean jet first becomes stabilized by the eddy activity. The zonal advection of eddy activity from upstream of this location is fundamental to the mechanism permitting the eddies to drive the mean flows. Observational results are presented that provide the first clear evidence of a northern recirculation gyre in the Kuroshio Extension, as well as support for the hypothesis that the recirculations are, at least partially, eddy-driven. Support for the idealized studies’ relevance to the oceanic regime is provided both by indications that various model simplifications are appropriate to the observed system, as well as by demonstrated consistencies between model predictions and observational results in the downstream development of time-mean and eddy properties.

Description: Funding was for this research and my education was provided by the MIT Presidential Fellowship and NSF grants OCE-0220161 and OCE-0825550. The financial assistance of the Houghton Fund, the MIT Student Assistance Fund, and WHOI Academic Programs is also gratefully acknowledged.

Description: Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution February 2008

Description: The subtidal circulation of the southeast Greenland shelf is described using a set of highresolution hydrographic and velocity transects occupied in summer 2004. The main feature present is the East Greenland Coastal Current (EGCC), a low-salinity, highvelocity jet with a wedge-shaped hydrographic structure characteristic of other surface buoyancy-driven currents. The EGCC was observed along the entire Greenland shelf south of Denmark Strait, while the transect north of the strait showed only a weak shelf flow. This observation, combined with evidence from chemical tracer measurements that imply the EGCC contains a significant Pacific Water signal, suggests that the EGCC is an inner branch of the polar-origin East Greenland Current (EGC). A set of idealized laboratory experiments on the interaction of a buoyant current with a submarine canyon also supported this hypothesis, showing that for the observed range of oceanic parameters, a buoyant current such as the EGC could exhibit both flow across the canyon mouth or into the canyon itself, setting the stage for EGCC formation. Repeat sections occupied at Cape Farewell between 1997 and 2004 show that the alongshelf wind stress can also have a strong influence on the structure and strength of the EGCC and EGC on timescales of 2-3 days. Accounting for the wind-induced effects, the volume transport of the combined EGC/EGCC system is found to be roughly constant (~2 Sv) over the study domain, from 68°N to Cape Farewell near 60°N. The corresponding freshwater transport increases by roughly 60% over this distance (59 to 96 mSv, referenced to a salinity of 34.8). This trend is explained by constructing a simple freshwater budget of the EGCC/EGC system that accounts for meltwater runoff, melting sea-ice and icebergs, and net precipitation minus evaporation. Variability on interannual timescales is examined by calculating the Pacific Water content in the EGC/EGCC from 1984-2004 in the vicinity of Denmark Strait. The PW content is found to correlate significantly with the Arctic Oscillation index, lagged by 9 years, suggesting that the Arctic Ocean circulation patterns bring varying amounts of Pacific Water to the North Atlantic via the EGC/EGCC.

Description: Funding for the cruise and analysis was provided by National Science Foundation grant OCE-0450658, which along with NSF grant OCE- 0095427 provided funds for my tuition and stipend as well.

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 1982

Description: Mean long-isobath drift of the order 5 cm/sec has been observed on several continental shelves, e.g. in the Middle Atlantic Bight and in the Weddell Sea. A theoretical model is developed to explore the driving mechanism of this mean circulation. In the model, the velocity field is decomposed into a depth-independent bottom geostrophic component and a thermohaline component relative to the bottom. The latter can be calculated from the density field, and the former is described by a parabolic equation which expresses the tendency-to balance vorticity between bottom stress curl and vortex stretching. The near-bottom flow field is studied both analytically and numerically under forcing by wind, deep ocean flow, and long-isobath density differences. Model solutions are derived for circulations over a shelf/slope topography driven by wind stress, wind stress curl, and deep ocean currents. The resulting flow patterns show strong dependence on the topography. Over the continental slope, large bottom depth variation suppresses the flow driven by local forcing and insulates the slope region from circulations on the shelf and in the deep-ocean. Geochemical observations on the continental shelf and slope support the argument that the flow on the upper slope below the thermocline is weak. Under the condition of a vertically homogeneous layer below the thermocline, near-bottom density advection is mainly caused by the bottom geostrophic velocity field. Using the parabolic vorticity equation together with a density equation, circulations driven by coastal buoyancy flux and surface cooling are investigated. In the mid-shelf region, away from the coast and the shelf break, the density field is governed by Burgers' equation, which shows longshore self-advection of density perturbations and the formation of front with strong density gradient in the longshore direction. A dense water blob moves in the direction of Kelvin wave propagation. The direction is reversed for the movement of a light water blob. In the near-shore region, the light river water bottom is also self-advected in the direction of Kelvin wave propagation. For a heavy density anomaly at the coast, the initial movement is offshore, and the accumulation of dense water in the mid-shelf region leads to long-isobath propagation of density perturbations, similar to the case of a dense water blob. This theory sheds light on the bottom water movements in the Adriatic Sea, the Antarctic Continent, and the Middle Atlantic Bight. The model solutions are applied to the flow on the western North Atlantic shelf. Southwestward flow is produced near the coast by the self-advection of river water in winter and spring. The southwestward long-isobath propagation of thermal fronts caused by winter cooling contributes significantly to the mean circulation over the mid-shelf. It is suggested that density-driven current is an important component of the near-bottom mean circulation in the Middle Atlantic Bight in spring and summer.

Description: This work was supported by the Department of Energy through contract 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 September 1978

Description: This thesis consists of three papers examining problems related to the crustal structure, isostasy and subsidence history of aseismic ridges and mid-plate island chains. Analysis of gravity and bathymetry data across the Ninetyeast and eastern Walvis Ridges indicates these features are locally compensated by an over thickening of the oceanic crust. Maximum crustal thicknesses are 15-30 km. The western Walvis Ridge is also compensated by crustal thickening; however, the isostasy of this part of the ridge is best explained by a plate model of compensation with elastic plate thicknesses of 5-8 km. These results are consistent with the formation of the Ninetyeast and Walvis Ridges near spreading centers on young lithosphere with flexural rigidities at least an order of magnitude less than those typically determined from flexural studies in older parts of the ocean basins. As the lithosphere cools and thickens, its rigidity increases, explaining the differences in isostasy between aseismic ridges and mid-plate island chains. The long-term subsidence of aseismic ridges and island/ seamount chains can also be explained entirely by lithospheric cooling. Aseismic ridges form near ridge crests and subside at nearly the same rate as normal oceanic crust Mid-plate island chains subside at slower rates because they are built on older crust. However, some island chains have subsided faster than expected based on the age of the surrounding sea floor, probably because of lithospheric thinning over midplate hot spots, like Hawaii. This lithospheric thinning model has major implications both for lithospheric and mantle convection studies as well as the origin of continental rift systems.

Description: Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy at the Massachusetts Institute and the Woods Hole Oceanographic Institution August 1975

Description: This thesis is made of two separate, but interrelated parts. In Part I the instability of a baroclinic Rossby wave in a two-layer ocean of inviscid fluid without topography, is investigated and its results are applied in the ocean. The velocity field of the basic state (the wave) is characterized by significant horizontal and vertical shears, non-zonal currents, and unsteadiness due to its westward propagation. This configuration is more relevant to the ocean than are the steady, zonal 'meteorological' flows, which dominate the literature of baroclinic instability. Truncated Fourier series are used in perturbation analyses. The wave is found to be unstable for a wide range of the wavelength; growing perturbations draw their energy from kinetic or potential energy of the wave depending upon whether the wavelength, 2πL, is much smaller or larger than 2πLρ, respectively, where Lρ is the internal radius of deformation. When the shears are comparable dynamically, L~Lρ , the balance between the two energy transfer processes is very sensitive to the ratios L/Lρ and U/C as well, where U is a typical current speed, and C a typical phase speed of the wave. For L = Lρ they are augmenting if U 〈 C, yet they detract from each other if U 〉 C. The beta-effect tends to stabilize the flow, but perturbations dominated by a zonal velocity can grow irrespective of the beta-effect. It is necessary that growing perturbations are comprised of both barotropic and baroclinic modes vertically. The scale of the fastest growing perturbation is significantly larger than L for barotropically controlled flows (L 〈 Lρ ), reduces to the wave scale L for a mixed kind (L ~ Lρ ) and is fixed slightly larger than Lρ for baroclinically controlled flows (L 〉 Lρ ). Increasing supply of potential energy causes the normalized growth rate, αL/U, to increase monotonically as L → Lρ from below. As L increases beyond Lρ, the growth rate αLρ /U shows a slight increase, but soon approaches an asymptotic value. In a geophysical eddy field like the ocean this model shows possible pumping of energy into the radius of deformation (~ 40 km rational scale, or 250 km wavelength) from both smaller and larger scales through nonlinear interactions, which occur without interference from the beta-effect. The e-folding time scale is about 24 days if U = 5 cm/sec and L = 90 km. Also it is strongly suggested that, given the observed distribution of energy versus length scale, eddy-eddy interactions are more vigorous than eddy-mean interaction, away from intènse currents like the Gulf Stream. The flux of energy toward the deformation scale, and the interaction of barotropic and baroclinic modes, occur also in fully turbulent 'computer' oceans, and these calculations provide a theoretical basis for source of these experimental cascades. In Part II an available potential energy (APE) is defined in terms appropriate to a limited area synoptic density map (e.g., the 'MODE-I' data) and then in terms appropriate to time-series of hydrographic station at a single geographic location (e. g., the 'Panulirus' data). Instantaneously the APE shows highly variable spatial structure, horizontally as well as vertically, but the vertical profile of the average APE from 19 stations resembles the profile of vertical gradient of the reference stratification. The eddy APE takes values very similar to those of the average kinetic energy density at 500 m, 1500 m and 3000 m depth in the MODE area. In and above the thermocline the APE has roughly the same level in the MODE area (centered at 28°N, 69° 40'W) as at the Panulirus station (32° 10'N, 64° 30'W), yet in the deep water there is significantly more APE at the Panulirus station. This may in part indicate an island effect near Bermuda.

Description: This research has been supported by the National Science Foundation grant IDO 73-09737, formerly GX-36342.

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

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

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

Description: Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution June 2002

Description: A numerical model of the tropical Atlantic ocean is used to investigate the upper layer pathways of the Meridional Overturning Circulation (MOC) in the tropical Atlantic. The main focus of this thesis is on those parts of the tropical circulation that are thought to be important for the MOC return flow, but whose dynamics have not been understood yet. It is shown how the particular structure of the tropical gyre and the MOC act to inhibit the flow of North Atlantic water into the equatorial thermocline. As a result, the upper layers of the tropical Atlantic are mainly fed by water from the South Atlantic. The processes that carry the South Atlantic water across the tropical Atlantic into the North Atlantic as part of the MOC are described here, and three processes that were hitherto not understood are explained as follows: The North Brazil Current rings are created as the result of the reflection of Rossby waves at the South American coast. These Rossby waves are generated by the barotropically unstable North Equatorial Countercurrent. The deep structure of the rings can be explained by merger of the wave's anticyclones with the deeper intermediate eddies that are generated as the intermediate western boundary current crosses the equator. The bands of strong zonal velocity in intermediate depths along the equator have hitherto been explained as intermediate currents. Here, an alternative interpretation of the observations is offered: The Eulerian mean flow along the equator is negligible and the observations are the signature of strong seasonal Rossby waves. The previous interpretation of the observations can then be explained as aliasing of the tropical wave field. The Tsuchyia Jets are driven by the Eliassen-Palm flux of the tropical instability waves. The equatorial current system with its strong shears is unstable and generates tropical instability waves. These waves cause a poleward temperature flux which steepens the isotherms which in turn generates are geostrophically balanced zonal flow. In the eastern part of the basin this zonal flow feeds the southeastward flow of the equatorial gyre.

Description: NASA and ONR ~ho generously funded me with their respective grants NAG5- 7194 and N00014-98-10881.

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 1972

Description: Free-air and simple Bouguer anomaly maps of the Venezuelan continental margin (from 60°W to 72°W and from 7°N to 13°N) are presented. The major features of the free-air map are: the large lows associated with the deep sedimentary basins, -200 mgal in the Eastern Venezuela basin and -164 mgal in the Maracaibo basin; the high of greater than 300 mgal over the Venezuelan Andes; and a belt of highs associated with the offshore islands extending from Blanquilla to Curacao and then over the Guajira peninsula, where they terminate. The Bouguer anomaly map shows a large low (-196 mgal) over the Eastern Venezuela basin and relative minimums over the coastal mountains. A minimum associated with the Venezuelan Andes is shifted to the northwest of the topographic axis and lies over the flank of the Andes and part of the Maracaibo basin. Using the gravity data, structural sections were constructed for a series of profiles across the Venezuelan Andes and Caribbean mountains. They show that there is no light crustal root under the Andes, the relative mass excess is as much as 600 kg/cm2, and that there is an excess of low density material under the Maracaibo basin. This appears to be caused by a combination of a southeastward dipping shear zone in the lithosphere under the basin-mountain boundary and a component of compressive stress perpendicular to this zone, both of which have resulted in the uplift of the crust under the Andes, and downwarp under the basin. The apparent flexural rigidity of the lithosphere under the Maracaibo basin is 0.6 x 1023 newton-m, a normal value for lithosphere deformations of Miocene age. The Caribbean mountains have a light crustal root which has been formed by the sliding of blocks of crustal material from the north over the rocks to the south, and perhaps by the underthrusting of oceanic crust under the continental crust. This underthrusting may have been a result of the formation of a downgoing slab of lithosphere along the Venezuelan continental margin during the late Cretaceous. The downgoing slab may have existed until mid-Eocene time. The gravity minimum over the Eastern Venezuela basin is due to the downwarping of lighter crustal material into the higher density mantle. This may be a result of compression from the north along a north-south direction causing plastic downbuckling of the lithosphere. The present deformation along the northern boundary appears to be due to differences in relative motion between the North and South American plates. Because the Caribbean mountains are partially isostatically compensated, while the Venezuelan Andes are above isostatic equilibrium, this suggests that the relative motion of the Caribbean plate with respect to the South American plate is eastward. The compressive stress across the boundary in the region of the Venezuelan Andes is probably greater than the compressive stress across the Caribbean mountains.

Description: This investigation was supported by a grant from the National Science Foundation GA-12204, and by contract N00014-66-C-024l with the Office of Naval Research.

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 January 1981

Description: The results of a detailed geophysical survey are used in conjunction with all available information in a study of the tectonic development of the Cayman Trough and the Greater Antilles Ridge. This development is connected with the relative motions of the North and South Americas and the eastern Pacific plates. Thus, the pre-Tertiary history of the region is one of simple convergence. This contrasts with the complex tectonism of primary translation, with secondary convergence and divergence during the Tertiary. The ancestral Greater Antillean Arc suffered fracturing during collision with the Bahamas stable platform in the Late Cretaceous. Oblique convergence re-established itself across the remnant fragments of the ancestral arc in the Tertiary, producing a sheared welt partially decoupled from both the North American and Caribbean plates. Pronounced temporal and structural heterogeneity occurs within this Plate Boundary Zone. Along its northern margin secondary convergence with the North American plate formed the massive subduction complex of the Cuchillas Uplift and the Sierra Septentrional. Convergence between the Plate Boundary Zone and the Caribbean plate resulted in the triple virgation of the fold belts extending westward from the Los Muertos Trough to Oriente Province (Cuba), the Cayman Trough and the Nicaraguan Rise. Tectonism along these fold belts youngs southwestward preserving the stratigraphy of the Caribbean Basin at the time of their formation during the early, middle, and late Tertiary. The Caribbean/North American Plate boundary occurred along the zones of major strain accomodation within the Plate Boundary Zone. The Cayman Trough was produced during a period of divergence between the Nicaraguan Rise and the North American plates during the Miocene. Since the Pliocene, the shear boundary within the Cayman Trough occurs along the Oriente Deep proceeding via the Windward Passage Deep and the Valle del Cibao to the Puerto Rico Trench. Convergence and shear predominate the present tectonic framework of the Plate Boundary Zone.

Description: Cruise #97 of the R. V. ATLANTIS II was sponsored by the National Science Foundation (OCE78-20/ 11336.00). Further support was received from the Ocean Industry Program and the Educational Program of 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 August 1981

Description: This thesis consists of three loosely related theoretical studies. In chapters 1 - 3 the physical mechanisms which determine the three dimensional structure of the currents in the Sverdrup interior of a wind-driven gyre are discussed. A variety of simple analytic models suggest that the subsurface geostrophic contours in a wind gyre are closed and so the flow in these regions is not determined by lateral boundary conditions. Instead a turbulent, quasigeostrophic extension of the Batchelor-Prandtl theorem suggests that the potential vorticity is uniform inside these laterally isolated regions. The requirement that the potential vorticity be uniform leads simply and directly to predictions of the shape and extent of the wind gyre and the vertical structure of the currents within it. In chapter 4 the propogation of Rossby wave trains through slowly varying forced mean flows is examined by solving the linearized potential vorticity equation using the WKB method. If the mean flow is forced the action defined by Bretherton and Garrett (1968) is not conserved. Surprisingly, there is another quadratic wave property which is conserved, the wave enstrophy. In chapter 5 shear dispersion in an oscillatory velocity field, similar to that of an inertial oscillation, is discussed. The goal of this section is to develop intuition about the role of internal waves in horizontal ocean mixing. The problem is examined using a variety of models and techniques. The most important result is (23.2) which is an expression for the effective horizontal diffusivity produced by the interaction of vertical diffusivity and oscillatory vertical shear. Given an empirical velocity shear spectrum and an estimate of the vertical diffusivity this result could be used to calculate a horizontal eddy diffusivity which parameterizes the horizontal mixing due to the internal wave field.

Description: NSF Grant OCE-78-25692 has supported me throughout my stay in the Joint Program.

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

Description: A general discussion of possible techniques for observation of near-surface currents indicates that the surface-following frame of reference will provide several advantages over the Eulerian or Lagrangian frames. One problem with surface-following measurements is the biasing effects of the waves. A technique for making unbiased measurements is developed. This technique requires that both the sensor velocity and the fluid velocity be measured. A sensor platform, the Surface Acoustic Shear Sensor (SASS), which makes the required measurements is described. The processing scheme for interpreting the measurements from the SASS is described at length. The data that SASS has obtained from two deployments in the Shelf Mixed Layer Experiment (SMILE) is presented. This data shows clearly that the biasing effects of waves can not, in general, be ignored. In the summary of the data we find surprisingly little shear in the downwind direction in the top 4m of the water column. In the crosswind direction observed, observed shear seems to be indicative of an across shelf pressure gradient and intense near-surface mixing.

Description: Financial support for my work was from NSF grant OCE-87-16937.

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, 1979

Description: A major goal in the study of plate tectonics is the acquisition of a knowledge of the history of relative motion among the rigid plates of the earth's lithosphere. The three papers of this thesis contribute to this effort and demonstrate that studies of the stability and evolution of triple junctions and of the finite rotations of systems of three plates can yield significantly more accurate tectonic histories than can studies of the relative motions between two plates alone. Topographic and magnetic investigation of the Southwest Indian Ridge and reconstruction of the plate system of the Indian Ocean shows that both Africa and Antarctica are rigid plates and their pole of relative rotation has remained fixed near 8°N, 42°W since the Eocene. A detailed survey of the Indian Ocean triple junction reveals that the Indian Ocean plate motions have remained constant since 10 Ma. The stability conditions of the junction show that the general morphology of the Southwest Indian Ridge results from the evolution of the Indian Ocean triple junction. A method is presented for determining the finite rotations best reconstructing the past relative positions of three plates around a triple junction. The method is illustrated by reconstructions of the plates around the Labrador Sea triple junction at the times of anomalies 24 (56 Ma) and 21 (50 Ma). The region of uncertainty of the Greenland-North America finite pole is mapped for each reconstruction, and it demonstrates that consideration of the three plate system yields more well-constrained results than does a treatment of the two plates alone.

Description: This work was supported by the Office of Naval Research contract N00014-75-C-0291 with the Massachusetts Institute of Technology.

Description: Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution February 2007

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

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

Description: Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution August, 1978

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

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

Description: Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution August, 1978

Description: A linearized theory for the response of a circular pendulum spar in 2-dimensional waves and a uniform current is developed. The linear forces on the cylinder are predicted using an approximate potential flow theory for slender bodies. The dynamic equations are then amended to account for the wake effects of viscous bluff body flow by including a quadratic drag law and neglecting wave damping. A spectral model for the forces on a cylinder due to an oscillating wake, modeling the force as a frequency modulation process, is proposed. The non-linear equations of motion which result are then solved, assuming constant force coefficients, by linearization for use with a Gaussian random sea. The method of equivalent linearization is extended to include mean flow effects and a spatially distributed process. Some numerical experiments are then used to test the performance of the linearization. For a variety of environments, the linearization predicts the standard deviation of the simulation response to within 10% and the mean angle of inclination to within 30%. Results of the numerical experiments indicate that there is significant variation (order of magnitude changes) in both response and mean angle of inclination. Thus, significant changes are followed by the linearization. A laboratory experiment was carried out to test the linearized spar model in a realistic fluid environment. Only the low Keulegan Carpenter number regime was investigated. With some minimal manipulations, good agreement is obtained between the experiment and the linearized estimates. It appears that the drag coefficients for vortex induced in-line forces may be an order of magnitude larger than those reported in the literature, .5 instead of .06, and that the shedding of vortices due to steady flow may reduce the added mass coefficient significantly, as observed in oscillating flows with significant vortex shedding.

Description: The National Science Foundation provided tuition and stipend support under an NSF Graduate Fellowship for three years. I was fortunate to have been selected by the Board of Trustees of the Naval Postgraduate School Foundation as the first recipient of the Carl E. Menneken Fellowship for Scientific Research, which provided partial support during 1976-77.

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

Description: A key question in biological oceanography is how plankton populations maintain themselves in regions of favorable growth and survival in the face of horizontal transport by ocean currents. Plankton are thought to be retained on the highly productive Georges Bank by the clockwise flow, which intensifies with vernal warming. The extent to which plankton are transported off the bank to the southwest or transported northward and retained on the bank remains poorly understood. This thesis examined the relationship between plankton and physical properties in the southwest corner of the bank, the retention-loss region (RLR). Analysis of field data (Video Plankton Recorder, Acoustic Doppler Current Profiler, and satellite-tracked drifters) and modeling results was performed to quantify the relationships between plankton, hydrography, and currents and the fluxes through the RLR. Temperature-salinity-plankton diagrams and factor analysis revealed that most plankton taxa had characteristic relationships to the hydrography, with the exception of copepods which were everywhere abundant. The flux of plankton during a complete tidal cycle and in the de-tided current data indicated this region was not retentive to plankton, since the bulk of the flow remained to the southwest, despite the presence of a vernally warmed surface layer. A Lagrangian particle trajectory model was used to further examine transport of plankton through the RLR during late spring /early summer (June) when vernal stratification was established. Passive particles were used, since no die1 vertical migration by plankton was found in the data. The model revealed that the bulk of the plankton was carried out of the RLR through the southern and western boundaries. The modeling and data analysis show clearly that the plankton were lost from the bank to the southwest rather than being re-circulated to the north. These results have important implications for the plankton populations on Georges Bank and can be used in future modeling efforts that examine the factors controlling plankton populations in this region.

Description: Submitted in the 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, 1996

Description: Two-thirds of the Earth's surface is oceanic crust formed by magmatic and tectonic processes along mid-ocean ridges. Slow-spreading ridges, such as the Mid-Atlantic Ridge, are discontinuous and composed of ridge segments. Segments are thus fundamental units of magmatic accretion and tectonic deformation that control the evolution of the crust. The objective of this Thesis is to constrain the tectonic processes that occur at the scale of slowspreading segments, to identify the factors controlling segment propagation, and to provide constraints on lithospheric strength with laboratory deformation experiments. In chapter 2, bathymetry and gravity from various areas along the global mid-ocean ridge system are analyzed to quantify systematic variations at the scale of individual segments. There is a marked asymmetry in bathymetry and gravity in the vicinity of segment offsets. We develop a model of faulting to explain these observations. Low-angle faults appear to accommodate tectonic extension at the inside corners of ridge-offset intersections, and result in substantially uplifted terrain with thin crust with respect to that at the outside corners or centers of segments. Results from Chapter 3 indicate that the crust magmatically emplaced on axis is not maintained off-axis. This transition is revealed by both statistical and spectral analyses of bathymetry and gravity. Tectonic extension varies along the length of a segment, resulting in thinning and uplift of the crust at ridge-offset inside corners, and a decorrelation between bathymetry and gravity patterns. Tectonic deformation substantially reshapes the oceanic crust that is magmatically emplaced on-axis, and strongly controls the crustal structure and seafloor morphology off-axis. Satellite gravity data over the Atlantic shown in Chapter 4 reveal a complex history of ridge segmentation, and provides constraints on the processes driving the propagation of segments. The pattern of segmentation is controlled mainly by the geometry of the ridge axis, and secondarily by hot spots. Segments migrate primarily down regional gradients associated with hot spot swells. However, the lack of correlation between gradients and propagation rate, and the propagation up gradient of some offsets, suggest that additional factors control propagation (e.g., variations in lithospheric strength). Most non-transform offsets are short-lived and migrating, while transform offsets are long-lived and stable. Both the propagation of segments (Chapter 4) tectonism along a segment (Chapters 2 and 3) are controlled by the lithospheric rheology. In Chapter 5 I present results from laboratory deformation experiments on serpentinite. These experiments demonstrate that serpentinites are considerably weaker than peridotites or gabbros, display a non-dilatant style of brittle deformation, and strain is accommodated by shear cracking. Serpentinites may weaken the lithosphere, enhance strain localization along faults, and control the style of faulting.

Description: A fellowship from Caixa de Pensions "La Caixa" in Barcelona provided me with all the required financial support to come to WHOI. The work presented in this thesis was also supported by the National Science Foundation grants OCE-90l2576, OCE-930078, OCE-9313812, and Office of Naval Research grant N00014-9l-J-1433.

Description: Submitted in partial fulfillment of the requirements for the degree of Oceanographic Engineer at the Woods Hole Oceanographic Institution and the Massachusetts Institute of Technology and for the degree of Master of Science in Ocean Engineering at the Massachusetts Institute of Technology February 1979

Description: The development and application of an autonomous field instrumentation system consisting of four current meters and four wave gauges, along with a field monitor and digital recorder, is documented. The flow sensors are electromagnetic current meters, which employ the principle of electromagnetic induction to sense an induced electrical potential from the flow of water through an imposed magnetic field. The 10 cm diameter, discus-shaped sensor was tested in the laboratory under a wide variety of conditions, including both steady and oscillatory flow tests. The results of these tests indicate an excellent response in terms of linearity and horizontal cosine. The vertical cosine response is close to ideal in the region of ±30°, but beyond a negative angle of attack of approximately -30° the response is compromised by the onset of separation under dominantly steady flow conditions. The wave gauges are surface-piercing digital sensors, relying on the presence or absence of water at 128 individual sensing electrodes spaced 1.5 cm apart along the front surface of the wave gauge. On command, the instantaneous water surface elevation is measured, then telemetered digitally to the shorebased monitor and recorder. Field measurements of waves and currents at four stations across the width of the surf zone were made, using this system at a beach along the southern coast of Maine. Spilling breakers (approximately 1.0 m in height with an angle at breaking of about 8°), translated across the 30 m surf zone, generated an observed net longshore current during the four hour measurement period. The subsequently analyzed data from this experiment showed a strong longshore current which varied across the width of the surf zone, having a maximum of about 15 cm/ sec just inside the breaker line. A net offshore current was observed at all four stations, and averaged approximately 10 cm/sec to 15 cm/sec. Using a simplified force balance model for the generation of longshore currents on a plane, uniform beach, the data was further analyzed to investigate the validity and parameterization of the momentum flux forces and bottom friction forces within the surf zone. There was an observed shoreward loss in momentum flux across the width of the surf zone, from about -150,000 dynes/cm outside the breakers to near zero close to the shoreward extent of the surf zone. The computed friction coefficient from the balancing longshore current-induced bottom friction was found to be relatively unstable during periods of changing wave and current conditions, but was observed to be between 0.10 and 0.15 during more stable conditions.

Description: The support of the NOAA Sea Grant Program through the MIT Sea Grant Program, along with the MIT/WHOI Joint Research Seed Funds is acknowledged.

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

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

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

Description: Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution April 1977

Description: A 37 day long field program was carried out in March 1974 on the New England continental shelf break to study the current and hydrographic structure and variability on the shelf and in the shelf/slope front. A second experiment was conducted in the shelf break region for one week in January 1975 to study frontal exchange processes. The mean currents during the March 1974 experiment all had a westward alongshore component, increasing in magnitude progressing offshore from ~5 cm/sec to a maximum at the nearshore edge of the shelf/slope front of between 10 and 20 cm/ sec, and decreasing in magnitude with depth. The current structure was such that the velocity vector rotated clockwise with depth in the shelf waters inside the front. The mean alongshore transport of shelf water was on the order of 0.4 Sverdrups through a cross-shelf transect south of Block Island. About 30% of the transport occurred in the wedge-shaped region offshore of the 100 m isobath and inshore of the front. Comparison of the observed mean currents with those predicted by the steady frictional boundary layer model of Csanady (1976) indicates that the model captures most of the essential features of the shelf circulation. The low frequency currents contain approximately 30% of the total current variance. An empirical orthogonal modal analysis indicates that for low frequency alongshore motions the whole shelf together with the water above the front moves as a unit and that the on- offshore currents are characterized by opposing flows at surface and bottom. The alongshore wind stress component is the dominant forcing term for these low frequency motions and for the subsurface pressure field as well. For motion with periods longer than 33 hours, the time derivative term in the cross-shelf momentum balance is comparable with the Coriolis term while the advective terms are 2 to 10 times smaller, on the average. The semi-diurnal tide is barotropic over the shelf with current magnitudes that increase almost by a factor of two between the shelf break and the inshore mooring 70 km shoreward. At the shelf break one-dimensional continuity gives the correct relation between the surface tide and the semi-diurnal currents. The semi-diurnal tide is clockwise polarized. The diurnal tide is baroclinic, increasing somewhat toward the bottom, is less clockwise polarized than the semi-diurnal, and has tidal ellipses aligned with the isobaths. The diurnal tidal energy decreases toward shore. Inertial energy in the frontal zone is equal to the semi-diurnal tidal energy near the surface. The inertial energy decreases with depth and is an order of magnitude smaller further on the shelf. The inertial oscillations are shown to be highly correlated with the wind stress record, arising and decaying on a time scale of 3 to 4 days. The inertial oscillations are shown to be preferentially forced by wind stress events that have a large amount of clockwise energy at near inertial periods. The frontal zone is shown to be in near geostrophic balance with an anticipated vertical shear across the front of the order of 5 to 8 cm/sec. Thus, there is a wedge-shaped region of velocity deficit that is confined directly under the front and above ~200 m. Outside of this region the velocity is alongshore to the west. Low frequency motion of the front is shown to exist on time scales from 3 to 10 days although the complete nature of the motions is not known. An oscillation of the front about its mid-depth position at periods of 3 1/2 to 4 days was caused initially by an eastward wind stress event forcing the front offshore near surface and onshore along the bottom. This was accompanied by large temperature oscillations near the bottom at midshelf and current oscillations confined to those current meters near the front. The internal wave band is most energetic in the center of the front, is about half as energetic above the front where it is subject to variations associated with the wind stress, and is smaller and nearly constant below the front. The internal wave energy decreases shoreward reflecting the decreasing stratification shoreward of the wintertime hydrography. Linear internal wave theory seems to break down in the conditions of the frontal zone. A stability analysis of the front to small perturbations is carried out by extending the model of Margules frontal stability of Orlanski (1968) to include the steep bottom topography of the shelf break region. The study covers the parameter range pertinent to the New England continental shelf break region and indicates that the front is indeed unstable; however, the associated growth rates are so slow that baroclinic instability does not seem to be a viable explanation for the observed frontal motions. Application of the theory to the nearly flat topography of the shelf itself shows that the front would be at least 20 times more unstable there suggesting that the front would migrate offshore to the shelf break region until a stable equilibrium was established between frictional dissipation and the instabilities.

Description: Funds for 'the field program and the data analysis of the New England Shelf Dynamics Experiment have been provided by the National Science Foundation through grants GA-4l075 and DES 74-03001.

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

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

Description: Submitted in partial fulfillment of the requirements of the degree of Doctor of Philosophy at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution June, 1970

Description: Measurements of ocean currents were made by the author in the Western Mediterranean Sea at five depths for two months during early 1969. In terms of the dominant and persistent presence of inertial oscillations, circularly polarized currents having periods of a half pendulum day, the data are among the most striking ever collected. Two contemporary theories have been adapted for interpretation of this data. On the basis of a ray or short-wave-length theory, energy arriving at the observing site is found to fall into two categories, that making direct arrival from the, surface where it is assumed to have been generated, and that which undergoes one or more reflections. To the extent that the former dominates, it is found that the Algerian Coast about 130 km. to the south would cast a shadow to the north, the precise shape of which would be highly dependent on small variations in frequency. The nature of this frequency dependence implies a gradual increase in frequency with depth at the observing latitude. Although the data show a measurable shift (about 3%) towards higher frequencies, which is roughly the required amount, the lack of progressive frequency change with depth does not support the shadow hypothesis. In addition, the data is interpreted in terms of normal mode theory, where the nearby coast is seen to force a discrete modal structure to the solutions. The observed variation of current phase with depth indicates that a single internal mode dominates over a large portion of the data, while variations of both current amplitude and phase with depth are consistent this being the third internal vertical mode. Existence of a normal mode is also consistent with the long time, on the order of three weeks, for which the oscillations were observed to persist and with the dimensions of the Mediterranean Basin.

Description: This work was supported by the National Science Foundation under contract GA10208 and by the Office of Naval Research under contract NONR 241-11.

Description: Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy at the Woods Hole Oceanographic Institution and the Massachusetts Institute of Technology October 1979

Description: An investigation was carried out to observe the geologic effects of steady bottom currents on sediments of East Katla Ridge on the southern insular rise of Iceland. Near-bottom southwest to west-flowing currents exceeded 20 cm sec-1 for two weeks over a 25-kilometer wide section of the ridge flank between approximately 1400 and 1800 meters water depth; maximum density and minimum temperature were observed at 1800 meters. Total transport of Iceland-Scotland Overflow Water was calculated to be 5.0 x 106 m3 sec-1; suspended sediment transport is approximately 0.4 x 106 grams sec-1, with a net deposition of 10 to 15 cm/1000 years estimated from the flux difference in and out of the station array. Sediment distribution patterns indicate that the current axis, where flow exceeds approximately 15 cm sec-1, is a site of erosion and winnowing (sand layer formation) while the current margin is a site of rapid accumulation (from observed Holocene rates of 25 to 35 cm per 1000 yr to estimated rates of greater than 100 cm/1000 yr based on 3.5 kHz echo-sounder records). Holocene silty turbidites are locally thick in a sub-marine channel; sandy turbidites and current-winnowed 'sandy contourites' are present in the axis of the major submarine canyon. ‘ Sandy contourite' deposits beneath the axis of the Iceland-Scotland Overflow Current are very poorly sorted muddy sands lacking primary sedimentary structures. Bioturbation is inferred to cause the unique characteristics of these deposits, as well as the absence of fine silt laminae in 'muddy contourites' at the current margin.

Description: Financial support for shipboard operations and most of the post-cruise data analysis was provided by NSF Grant OCE76-Sl49l to Dr. Charles Hollister. Sediment trap and hydrocast operations received partial support under ONR Contract N00014-74-C-0262.

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

Description: Published

Description: Published

Description: Industrial tuna fishing

Description: Except for the presence in most localities of a shallow homogeneous surface layer and of a relatively homogeneous and deeper bottom layer, the oceans of the temperate and tropical regions are stratified and vertically stable at all depths. Due to the opacity of water for long-wave radiation and to the damping of vertical turbulence by the stability, there is no potent mechanism for altering the potential density of any water element below the layer of direct surface influences. Hence there can be no flow of major proportions across surfaces of constant potential density. For these reasons it is now generally accepted that flow takes place essentially parallel to these surfaces. It follows that the major sources for the water on each surface of constant potential density are to be found along its intersection with the sea surface in higher latitudes.

Description: Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution April, 1976

Description: Two numerical applications of two-level quasigeostrophic theory are used to investigate the interrelationships of the mean and mesoscale eddy fields in a closed-basin ocean model. The resulting techniques provide a more accurate description of the local dynamics, origins, and parametric dependences of the eddies than that available in previous modelling studies. First, we propose a novel and highly efficient quasigeostrophic closed-domain model which has among its advantages a heightened resolution in the boundary layer regions. The pseudospectral method, employing an orthogonal expansion in Fourier and Chebyshev functions, relies upon a discrete Green's function technique capable of satisfying to spectral accuracy rather arbitrary boundary conditions on the eastern and western (continental) walls. Using this formulation, a series of four primary numerical experiments tests the sensitivity of wind-driven single and double-gyred eddying circulations to a transition from free-slip to no-slip boundary conditions. These comparisons indicate that, in the absence of topography, no-slip boundaries act primarily to diffuse vorticity more efficiently. The interior transport fields are thus reduced by as much as 50%, but left qualitatively unchanged. In effect, once having separated from the western wall, the internal jet has no know1edge, apart from its characteristic flow speed, of the details of the boundary layer structure. Next, we develop a linearized stability theory to analyze the local dynamic processes responsible for the eddy fields observed in these idealized models. Given two-dimensional (x, z) velocity profiles of arbitrary horizontal orientation, the resulting eigenfunction problems are solved to predict a variety of eddy properties: growth rate, length and time scales, spatial distribution, and energy fluxes. This simple methodology accurately reproduces many of the eddy statistics of the fully nonlinear fields; for instance, growth rates of 10-100 days predicted for the growing waves by the stability analysis are consistent with observed model behavior and have been confirmed independently by a perturbation growth test. Local energetic considerations indicate that the eddy motions arise in distinct and recognizable regions of barotropic and baroclinic activity. The baroclinic instabilities deîend sensitively on the vertical shear which must exceed 0(5 cm sec-1) across the thermocline to induce eddy growth. As little as a 10% reduction in |uz|, however, severely suppresses the cascade of mean potential energy to the eddy field. In comparison, the barotropic energy conversion process scales with the horizontal velocity shear, |uy|, whose threshold values for instability, a (2 x 10-6 sec-1), is undoubtedly geophysically realizable. A simple scatter diagram of |uy| versus |uz| for all the unstable modes studied shows a clear separation between the regions of barotropic and baroclinic instability. While the existence of baroclinic modes can be deduced from either time mean or instantaneous flow profiles, barotropic modes cannot be predicted from mean circulation profiles (in which the averaging process reduces the effective horizontal shears). Finally, we conduct a separate set of stability experiments on analytically generated jet profiles. The resulting unstable modes align with the upper level velocity maxima and, although highly sensitive to local shear amplitude, depend much less strongly on jet separation and width. Thus, the spatial and temporal variability of the mesoscale statistics monitored in the nonlinear eddy simulations can be attributed almost entirely to time-dependent variations in local shear strength. While these results have been obtained in the absence of topography and in an idealized system, they yet have strong implications for the importance of the mid-ocean and boundary layer regions as possible eddy generation sites.

Description: This research has been made possible by National Science Foundation grant OCE74-03001 A03, formerly DES73-00528, and the National Science Foundation funded National Center for Atmospheric Research.

Description: Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution September, 1976

Description: The temporal and spatial variability of low frequency moored temperature and velocity observations, obtained as part of the Mid-Ocean Dynamics Experiment (MODE), are analyzed to study the kinematics and energetics of mesoscale eddies in the ocean. The temporal variability of the low frequency motions is characterized by three regimes: very low frequencies with periods greater than 200 days, an eddy energy containing band of 80 to 120 day periods, and high frequencies wìth periods less than 30 days. At very low frequencies, the zonal kinetic energy exceeds the meridional at all depths. In the thermocline, the very low frequency zonal flow dominates the total kinetic energy. The greatest contribution to the kinetic and potential energy in the MODE region, except for the thermocline zonal flow, is from an eddy energy containing band of 80 to 120 day periods. Eddy scale kinetic energy spatial variations are confined to this band. At high frequencies, the kinetic and potential energy scale with frequency as ω-2.5 and with depth in the WKB sense. Energy at high frequencies is partitioned evenly between zonal kinetic, meridional kinetic and potential energy and is homogeneous over 100 km. Using the technique of empirical orthogonal expansion, the vertical structure of the energetically dominant eddies is described by a few modes. The displacement is dominated by a mode with a thermocline maximum and in phase displacements with depth, while the kinetic energy is dominated by an equivalent barotropic mode. A smaller portion of the kinetic and potential energy is associated with out of phase thermocline and deep water currents and displacements. The dynamics of the mesoscale eddies are very nonlinear. Using the vertical veering of the current at MODE Center, the estimated horizontal advection of heat contributes significantly to the low frequency thermal balance. The observed very low frequency anisotropic flow is consistent with the nonlinear eddy spindown models, dominated by cascades of vorticity and energy. At high frequencies, the spectral similarity is consistent with advected geostrophic turbulence.

Description: The National Science Foundation supported the work through grants GX29034 and IDO-75-03998 and a graduate fellowship.

Description: Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution September 1976

Description: This thesis is a collection and analysis of seafloor magnetic anomalies, bathymetry, and the paleomagnetism of DSDP sediments and basalt in the West Philippine Basin, in an attempt to resolve questions about its origin as a marginal basin. Our results suggest that this basin was formed in an Eocene pulse of rapid spreading (v1/2 = 41-44 mm/yr) in a direction (N 21°E) significantly different from later pulses which opened the more eastern basins of the Philippine Sea. The Central Basin Fault appears to be intimately associated with this spreading by nature of its structure and trend, and it may be a remanent of a former ridge system. Our preliminary calculation of paleopole positions also suggests that there was a large amount (60°) of clockwise rotation between this basin and the magnetic pole. This is consistent with rotations of the Pacific plate with respect to the magnetic pole and current directions of Philippine- Pacific'relative rotations. Basement depths of 6 km in the West philippine Basin imply that its crustal and/or lithospheric structure is different from Pacific structure of the same age.

Description: Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution April, 1977

Description: A total of four moorings from POLYMODE Array I and II were analyzed in an investigation of internal wavefield-mean flow interactions. In particular, evidence for wave-mean flow interaction was sought by searching for time correlations between the wavefield vertically-acting Reynolds stress (estimated using the temperature and velocity records), and the mean shear. No significant stress-shear correlations were found at the less energetic moorings, indicating that the magnitude of the eddy viscosity was under 200 cm2/sec, with the sign of the energy transfer uncertain. This is considerably below the 0(4500 cm2/sec) predicted by Müller (1976). An extensive error analysis indicates that the large wave stress predicted by the theory should have been clearly observable under the conditions of measurement. Theoretical computations indicate that the wavefield "basic state" may not be independent of the mean flow as assumed by Müller, but can actually be modified by large-scale vertical shear and still remain in equilibrium. In that case, the wavefield does not exchange momentum with a large-scale vertical shear flow, and, excepting critical layer effects, a small vertical eddy viscosity is to be expected. Using the Garrett-Munk (1975) model internal wave spectrum, estimates were made of the maximum momentum flux (stress) expected to be lost to critical layer absorption. Stress was found to increase almost linearly with the velocity difference across the shear zone, corresponding to a vertical eddy viscosity of -100 cm2 s -1. Stresses indicative of this effect were not observed in the data. The only significantly non-zero stress correlations were found at the more energetic moorings. Associated with the 600 m mean velocity and the shear at the thermocline were a positively correlated stress at 600 m, and a negatively correlated stress at 1000 m. These stress correlations were most clearly observable in the frequency range corresponding to 1 to 8 hour wave periods. The internal wavefield kinetic and potential energy were modulated by the mean flow at both levels, increasing by a factor of two with a factor of ten in the mean flow. The observed stress correlations and energy level changes were found to be inconsistent with ideas of a strictly local eddy viscosity, in which the spectrum of waves is only slightly modified by the shear. When Doppler effects in the temperature equation used to estimate vertical velocity were considered, the observations of stress and energy changes were found to be consistent with generation of short (0.4 to 3 km) internal waves at the level of maximum shear, about 800 m. The intensity of the generated waves increases with the shear, resulting in an effective vertical eddy viscosity (based on the main thermocline shear) of about +100 cm2 s-1 The stresses were not observable at the 1500 m level, indicating that the waves were absorbed within 500 m of vertical travel. The tendency for internal wave currents to be horizontally anisotropic in the presence of a mean current was investigated. Using the Garrett- Munk (1975) model internal wave spectrum, it was found that critical layer absorption cannot induce anisotropies as large as observed. A mechanical noise problem was found to be the cause of large anisotropies measured with Geodyne 850 current meters. It could not be decided, however, whether or not the A.M.F. Vector Averaging Current Meter is able to satisfactorily remove the noise with its averaging scheme.

Description: The research reported here was provided by Office of Naval Research Contract Numer N00014-76-C-0197 NR 083-400.

Description: Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution January, 1971

Description: Observations of the ocean in the vicinity of Bermuda on two different occasions show systematic distortions of the isotherms close to the island and an area of intensive mixing on the northern coast. Two mechanisms are investigated and each produces some agreement with data from different flow regimes. Firstly, the island is modeled as a circularly symmetric obstacle with steep sides and a small aspect ratio. A steady, rotating, and stratified flow which, far from the island, is uniform in the horizontal and a linear function of the vertical coordinate is taken to be flowing past the island. Neglecting circulation effects, the problem is solved to first order in a small parameter, α, which measures the steepness of the island and a small Rossby number, ε. This allows a calculation of the depth contours of isotherms to 0(ε2,εα). For one set of data the flow is such that the slope effect of 0(εα) predominates while for another period of observation both slope and Rossby number influences are of the same magnitude. In both cases qualitative agreement between fact and theory is remarkably good. In addition, it is shown that the north slope (for a west-east current) is the most favored area for mixing as there the Richardson number is a minimum and the flow is most likely to separate from the boundary. A second means of producing isotherm distortion and mixing areas close to the island concerns the nonlinear effects of shoaling internal gravity waves. For normal incidence on a two-dimensional beach the Reynolds stresses produced by the fundamental wave motion are shown to force a mean Eulerian current which is equal hut opposite in sense to the Stokes drift. This causes the mean Lagrangian current to vanish so that the physical constraint that there be no net motion of fluid particles along isopycnals into the beach is satisfied. In addition, isotherms are distorted in a fashion analogous to the surface set-down produced by shoaling surface waves. The mean isopycnal shift can be as much as 10m where the theory has some validity. Distortions of the predicted form are observed in the data from a period when the mean currents were small. Consideration of the oblique incidence problem shows that this generalization has little effect on the expected magnitude of the shifts but that a significant longshore current can be forced by the breaking of the waves.

Description: This study was supported by the Office of Naval Research under contracts Nonr 1841(74) and Nonr 3963(31) with the Massachusetts Institute of Technology. Additional support came from the National Science Foundation in the form of a summer fellowship and computer time under contract NSF GJ-133 with 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 January, 1977

Description: This thes is is a collection of papers on the paleomagnetics of samples from several Deep Sea Drilling Project (DSDP) sites in the Indian Ocean. These papers present the basic paleomagnetic data, discuss the statistical methods for analyzing such data from DSDP cores, and examine the implications of the paleolatitudes for the origin of the Ninetyeast Ridge and the northward motion of India. Rarely do DSDP paleolatitudes approach the reliability of good continental pole positions. However, the reliability of such paleolatitudes can be markedly improved by using comparisons with paleolatitudes of different ages from the same site, paleolatitudes of similar ages from different sites on the same plate, estimates of paleolatitude from the skewness of marine magnetic anomalies, and continental paleopole. positions. Using such comparisons, a new paleomagnetic pole of upper Cretaceous age has been defined for the Pacific plate. A middle Cretaceous pole has been defined for the Wharton Basin plate, and it suggests that there may have been left lateral motion between Australia and the Wharton Basin. Paleolatitudes from the Ninetyeast Ridge are consistent with the pole position for the Deccan Traps. These data indicate that India and the Ninetyeast Ridge moved northwards with respect to the South Pole at 14.9 ± 4.5 cm/yr from 70 to 40 mybp and at 5.2 ± .8 cm/yr from 40 mybp until the present. However, when this paleomotion is compared to the Australian paleomagnetic data (by removing the relative motion components), a major inconsistency appears between 40 and 50 mybp. The Australian data indicate that India should be 13° further north than the positions implied by the Ninetyeast Ridge data. Basal paleolatitudes on the Ninetyeast Ridge indicate that its volcanic source was approximately fixed in latitude near 50°S, supporting the hypothesis that the ridge is the trace of the Kerguelen hotspot on the northward moving Indian plate. There is considerable geologic evidence in favor of such an hypothesis, and there is none to contradict it.

Description: National Science Foundation (Grant DES-74-22552).

Description: During the past four years a deliberate effort has been made at the Woods Hole Oceanographic Institution to devise methods of kinematic observation generally suited to the needs of oceanographers. One result of this work, the electromagnetic method, has been brought from the experimental stage to one of useful maturity. Many of the theoretical potentialities of the method are still to be explored and developed. Nevertheless it seems likely that this remaining work may be done more soundly if present developments of the theory and instrumentation are made available for use and evaluation by, others. These studies in methods of kinematic observation have been supported mainly under the provisions of Bureau of Ships Contract NObs-2083, and Office of Naval Research Contract N6onr-277-1. This support and the assistance of the Naval Ordnance Laboratory, the Hydrographic Office (Oceanographic Division), the United States Coast Guard, and the David Taylor Model Basin of the United States Navy is gratefully acknowledged.

Description: The purpose of this paper is to discuss the nature of the electrical field induced in the ocean by particular types of velocity distribution. It is believed that these examples will be helpful in the interpretation of measurements by towed electrodes in the sea. The electrical field induced by waves and tidal streams, originally predicted by Faraday (1832), was first measured experimentally by Young, Gerrard and Jevons (1920), who used both moored and towed electrodes in their observations. Recently, the technique of towed electrodes has been developed by von Arx (1950, 1951) and others into a useful means of detecting water movements in the deep ocean. While the method has been increasingly used, the problem of interpreting the measurements in terms of water movements has become of great importance. Two of the present authors have made theoretical studies (Longuet-Higgins 1949, Stommel 1948) dealing with certain cases of velocity fields, and Malkus and Stern (1952) have proved some important integral theorems. There seems, however, to be a need for a more extended discussion of the principles underlying the method, and for the computation of additional illustrative examples. This is all the more desirable since some of the theoretical discussions published previously have been misleading.

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, 1975

Description: The Mid-Atlantic Ridge is one of the most well known and yet poorly understood spreading centers in the world. A detailed investigation of the Mid-Atlantic Ridge crest near 37°N (FAMOUS) was conducted using a deeply towed instrument package. The objective was to study the detailed structure and spreading history of the Mid-Atlantic Ridge median valley, to explore the roles of volcanism and faulting in the evolution of oceanic crust, and to study the morphologic expression and structural history of the zone of crustal accretion. In addition, microearthquake surveys were conducted using arrays of free-floating hydrophones. The most recent expression of the accreting plate boundary in the Famous Rift is an alternating series of linear central volcanoes and depressions 1.5 km wide which lie within the inner floor. This lineament is marked by a sharp maximum in crustal magnetization only 2-3 km wide. Magnetic studies indicate that over 90% of the extrusive volcanism occurs within the rift inner floor, a zone 1 to 12 km wide, while volcanism is extremely rare in the rift mountains. Volcanoes created in the inner floor are transported out on, block faults, becoming a lasting part of the topography. Magnetic anomaly transition widths vary from 1 km to 8 km with time and appear to reflect a bi-stable median valley structure. The valley has either a wide inner floor and narrow terraces, in which case the volcanic zone is wide and magnetic anomalies are poorly recorded (wide transition widths); or it has a narrow inner floor and wide terraces, the volcanic zone is then narrow and anomalies are clearly recorded (narrow transition widths). The median valley of any ridge segment varies between these two structures with time. At present the. Famous Rift has a narrow inner floor and volcanic zone (1-3 km) while the south Famous Rift is at the opposite end of the cycle with a wide inner floor and volcanic zone (10-12 km). Over 95% of the large scale (〉2 km) relief of the median valley is accounted for by normal faults dipping toward the valley axis. Normal faulting along fault planes dipping away from the valley begins just past the outer walls of the valley. Outward facing normal faulting accounts for most of the decay of median valley relief in the rift mountains while crustal tilting accounts for less than 20%. The pattern of normal faulting creates a broad, undulating horst and graben relief. Volcanic features contribute little to the large scale relief, but contribute to the short wavelength (〈2km) roughness of the topography. Spreading in the Famous area is highly asymmetric with rates twice as high to the east as to the west. At 1.7 m.y.b.p. the sense of asymmetry reverses in direction with spreading faster to the west, resulting in a gross symmetry when averaged through time. The change in spreading asymmetry occurred in less than 0.15 m.y. Structural studies indicate that the asymmetric spreading is accomplished through asymmetric crustal extension as well as asymmetric crustal accretion. Spreading in the Famous area is 17° oblique. Even on a fine scale there is no indication of readjustment to an orthogonal plate boundary system. Spreading has been stably oblique for at least 6 m.y., even through a change in spreading direction. Magnetic studies reveal that the deep DSDP hole at site 332 was drilled into a magnetic polarity transition, and may have sampled rocks which recorded the earth i s field behavior during a reversal. The presence of negative polarity crust within the Brunhes normal epoch in the inner floor has been determined, and may be due to old crust left behind or recording of a geomagnetic field event. Crustal magnetization decays to lie of its initial value in less than 0.6 m.y. The rapid decay may be facillitated by very intense crustal fracturing observed in the inner floor. Microearthquake, magnetic and structural studies indicate that both the spreading and transform plate boundaries are very narrow (1-2 km) and well-defined for short periods, but migrate over zones 10-20 km wide through time.

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, 1976

Description: Measurements of horizontal and vertical current by propeller cluster current meters and temperature by thermistors mounted on a rigid array 8 m high and 20 m long moored in the oceanic main thermocline near Bermuda are interpreted in terms of thermocline-trapped internal wave modes in the presence of temperature and density fine-structure. Two turning-point uniformly valid asymptotic solutions to the internal wave equation are developed to describe the wave functions. Mode decay beyond the turning point in depth or frequency produces a sharp cutoff in vertical current spectra above the local buoyancy frequency N(z). An internal wave wavenumber-frequency spectral model Ε(α,ω) = E(ω/No)-2 (α./α0)-2 describes vertical current spectra and potential energy to horizontal kinetic energy ratios. The red wavenumer shape suppresses peaks in both these quantities at frequencies near N(z). The data are consistent with time-averaged horizontal isotropy of the wave field. A dip in the vertical current spectra at 0.5 cph not predicted by the model appears related to the bottom slope. Temperature fine-structure is modeled as a passive vertical field advected by internal waves. Quasi-permanent fine-scale features of the stratification and vertically small-scale internal waves are indistinguishable in this study. The model of McKean (1974) is generalized to include fine-structure fields specified by their vertical wavenumber spectra as well as different Poisson-distributed layer models. Together with the trapped internal wave model, moored temperature spectra, temperature vertical difference spectra, and coherence over vertical separations are described using a fine-structure vertical wavenumber spectrum PT(k) =ATk-5/2 which agrees with other spectra made using vertical profiling instruments in the range 0.1 to 1.0 cpm. Horizontal current fine-structure is also modeled as a passive field advected vertically by long internal waves. The model describes moored horizontal current spectra (least successfully at frequencies near N(z)) and finite-difference vertical shear spectra. Contours of temperature in depth versus time indicate possible mixing events. These events appear concurrently with high shear and Richardson numbers O. 25≤ R ≤ 1.0. Over 7 m a cutoff in Ri at 0.25 is observed, indicating saturation of the internal wave spectrum. Spectra of finite-difference approximations to shear and buoyancy frequency are dominated by fine-structure contributions over nearly the whole internal wave range, suggesting that breaking is enhanced by fine-structure. Breaking appears equally likely at all frequencies in the internal wave range.

Description: This research was supported by Office of Naval Research contract N00014-67-0204-0047 and continuation contract NOOOl4-75-C-0291.

Description: The present investigation may be regarded as a part of a systematic effort to introduce into meteorology and physical oceanography methods and results which for a number of years have contributed to the rapid growth and increasing practical significance of experimental fluid mechanics. This science has recognized that the exact character of the forces controlling the motion of a turbulent fluid is not known and that consequently there is very little justification for a purely theoretical attack on problems of a practical character. For this reason fluid mechanics has been forced to develop a research technique all of its own, in which the theory is developed on the basis of experiments and then used to predict the behavior of fluids in cases which are not accessible to experimentation. In oceanography it has long been regarded as an axiom that the movements of the water are controlled by three forces, the horizontal pressure gradient, the deflecting force, and the frictional force resulting from the relative motion of superimposed strata. It is significant that thirty-five years of intensive theoretical work on this basis have failed to produce a theory capable of explaining the major features of the observed oceanic circulation below the pure drift current layer. The present investigation considers a force which has been completely disregarded by theoretical investigators although its existence has been admitted implicitly by practically everyone who has approached physical oceanography from the descriptive side, namely the frictional force resulting from large-scale horizontal mixing. The intro- . duction of this force permits us to see how motion generated in the surface layers may be diffused and finally dissipated without recourse to doubtful frictional forces at the bottom of the ocean.

Description: This paper is a discussion of possible discrepancies in computations of ocean currents (based on horizontal variations of dynamic topography calculated from arbitrary deep lying reference surfaces), because of time variations of temperature and salinity at fixed depths in the sea (illustrated for a 24-hour period at "Atlantis" Station 2639). The results contained herein, while based chiefly on information from the western North Atlantic, are of general applicability, since time variations of the same order of magnitude have been observed over extensive areas of the Atlantic ocean. In selecting material for analysis of dynamic situations in the region concerned, consideration has been given only to those favorably located stations from which the structural features could most conveniently be obtained for illustrating the points in question.

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, 1973

Description: This thesis presents an investigation of the dynamics of bottom boundary currents in the ocean. The major emphasis is to develop simple mathematical models in which various dynamical features of these complex geophysical flows may be isolated and explored. Two separate models are formulated and the theoretical results are compared to observational data and/or laboratory experiments. A steady flow over a constant sloping bottom is treated in each model. A streamtube model which describes the variatlon in average cross-sectional properties of the flow is derived to examine the interaction between turbulent entrainment and bottom friction in a rotating stratified fluid. Empirical laws are used to parameterize these processes and the associated entrainment and friction coefficients (Eo,K) are evaluated from data for two bottom currents: the Norwegian Overflow and the Mediterranean Outflow. The ability to fit adequately all observations with the solutions for a single parameter pair demonstrates the dynamical consistency of the streamtube model. The solutions indicate that bottom stresses dominate the frictional drag on the dense fluid layer in the vicinity of the source whereas relatively weak entrainment slowly modulates the flow properties in the downstream region. The combined influence of entrainment and ambient stratification help limit the descent of the Mediterranean Outflow to a depth of approximately 1200 m. while strong friction acting over a long downstream scale allows the flow of Norwegian Sea water to reach the ocean floor. A turbulent Ekman layer model with a constant eddy viscosity is also formulated. The properties of the flow are defined in terms of the layer thickness variable d(x,y), whose governing equation is judged intractable for the general case. However, limiting forms of this equation may be solved when the layer thickness is much less than (weak rotation) or greater than (strong rotation) the Ekman layer length scale. In the weak rotation limit, a similarity soltition is derived which describes the flow field in an intermediate downstream range. Critical measurements in a laboratory experiment are used to establish distinctive properties of rotational perturbations to the viscous flow, such as the antisymmetric corrections to the layer thickness profile and the surface velocity distribution, which depend on downstream distance like y2/7. The constraint of weak rotational effects precludes a meaningful comparison with oceanic bottom currents. The analysis of the strong rotation limit leads to the prediction of an Ekman flux mechanism by which dense fluid is drained from the lower boundary of the thick core of the current and the geostrophic flow is extinguished. The form of a similarity solution for the downstream flow is derived subject to the specification of a single constant by the upstream boundary condition. The results of some exploratory experiments are sufficient to confirm some qualitative aspects of this solution, but transience of the laboratory flow limits a detailed comparison to theory. Some features of the Ekman flux mechanism are noted in the observational data for the Norwegian Overflow.

Description: Submitted in partial fulfillment of the requirements of the degree of Ocean Engineer at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution and Master of Science in Ocean Engineering at the Massachusetts Institute of Technology August, 1972

Description: A bottom mounted electromagnetic current meter measures the vertically-averaged conductivity-weighted velocity. This measurement complements free-fall relative velocity profiles and is valuable for transport determination and dynamics studies. Such an instrument has been designed to measure the three components of the electric field, Ex, Ey, and Ez. Salt bridges used with switched electrodes permit the induced e1ectromotive forces to be measured with only a short baseline; eight foot arms are planned. The first part of this report covers the theory behind the bottom mounted electric field meter. The second part discusses the design of the instrument as well as a brief description of the prototype bottom mounted electric field meter.

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, 1975

Description: A set of vertical profiles of horizontal ocean currents, obtained by electro-magnetic profilers in the Atlantic Ocean southwest of Bermuda in the spring of 1973, has been analyzed in order to study the vertical structure and temporal behavior of internal waves, particularly those with periods near the local inertial period. An important feature of the observed structure is the polarization of horizontal velocity components in the vertical. This polarization, along with temporal changes of the vertical wave structure seen in a time series of profiles made at one location, has been related to the direction of vertical energy flux due to the observed waves. Whereas the observed vertical phase propagation can be affected by horizontal advection of waves past the point of observation, the use of wave polarization to infer the direction of vertical energy propagation has the advantage that it is not influenced by horizontal advection. The result shows that at a location where profiles were obtained over smooth topography, the net energy flux was downward, indicating that the energy sources for these waves were located at or near the sea surface. An estimate of the net, downward energy flux (~ .2 - .3 erg/cm2/sec) has been obtained. Calculations have been made which show that a frictional bottom boundary layer can be an important energy sink for near-inertial waves. A rough estimate suggests that the observed, net, downward energy flux coul d be accounted for by energy losses in this frictional boundary layer. A reflection coefficient for the observed waves as they reflect off the bottom has been estimated. In contrast, some profiles made over a region of rough topography indicate that the rough bottom may also be acting to generate near-inertial waves which propagate energy upward. Ca1culations of vertical flux of horizontal kinetic energy, using an empirical form for the energy spectrum of internal waves, show that this vertical flux reaches a maximum for frequencies 10% - 20% greater than the local inertial frequency. Comparison with profiler velocity data and frequency spectra supports the conclusion that the dominant waves had frequencies 10% - 20% greater than the inertial frequency. The fact that the waves were propagating energy in the vertical is proposed as the reason for the observed frequency shift. Finally, energy spectra in vertical wave number have been calculated from the profiles in order to compare the data with an empirical model of the energy density spectrum for internal waves proposed by C. Garrett and W. Munk (1975). The result shows that although the general shape and magnitude of the observed spectrum compares well with the empirica1 model, the two-sided spectrum is not symmetric in vertical wave number. This asymmetry has been used to infer that more energy was propagating downward than upward. These calculations have also been used to obtain the coherence between profiles made at the same location, but separated in time (the so-called dropped, lagged, rotary coherence). This coherence is compared with the aforementioned empirical model. The coherence results show that the contribution of the semidiurnal tide to the energy of the profiles is restricted to long vertical wave lengths.

Description: Support for the experiment which is described in this report was provided by the Office of Naval Research under contracts N00014-66-C-0241, NR 083-004 and N00014-74-C-0262, NR 083-004.

Description: Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution July, 1975

Description: The local dynamics of low-frequency motions in the MODE region are investigated from three arrays of moored measurements of current and temperature. Tests for lowest-order balances of horizontal momentum, mass, heat, heat and vorticity within established errors are carried out. Geostrophic comparisons of four-day averaged observed and geostrophic current differences from the MODE-l array indicate that ageostrophic balance within estimated errors is the lowest-order horizontal momentum balance. The discrepancy between observed and geostrophic current differences has a standard deviation of 1.9 cm/see which is 26% as large as the standard deviation of the current differences. In the mass balance comparisons of estimates of δυ/δχ and δν/δγ from the MODE-O Array l indicate that within estimated errors the low frequency currents are horizontally nondivergent. The standard deviation of horizontal divergence, which is the discrepancy from horizontal nondivergence, is .22 x 10 6 sec 1 which is 36% as large as the standard deviation of the estimates of horizontal derivatives of velocity. These tests significantly increase the observational basis for geostrophy and horizontal nondivergence and confirm the validity of the error estimates. In the heat balance, estimates of horizontal advection of temperature balance local time changes of temperature within estimated errors for the IWEX observations. These estimates have small errors because a representation of horizontal advection of temperature in terms of the speed and turning about the vertical of the horizontal current is used. The errors are so small that from future measurements it may be possible to estimate the sum of local change plus horizontal advection of temperature and from this sum it may be possible to estimate vertical velocity. This balance between local change and horizontal advection demonstrates that horizontal advection of spatially varying features is an important cause of local time changes. The horizontal advection could not be explained in terms of advection by the long time-averaged flow field. This suggests that the local dynamics of low-frequency motions in the MODE region are strongly nonlinear. An indication of energy transfer, which occurs in nonlinear processes, is found in a phase lag such that estimates of horizontal advection lead local changes of temperature. In the context of the baroclinic instability model this phase lag is consistent with the growth of perturbation wave energy by conversion of potential energy contained in the forty-day averaged flow field. In the vorticity balance, estimates of planetary advection account for only half the local time change of vorticity for MODE-0 Array 1 measurements. Within estimated errors these two terms do not balance, so these observations cannot be explained as manifestations of barotropic Rossby waves alone. Estimates of vortex stretching and horizontal advection of relative vorticity could not be made. A phase lag such that estimates of planetary advection lead local changes of vorticity is consistent in the context of the instability model with an increase in perturbation wave enstrophy, which must occur when the perturbation wave grows, due to the conversion of planetary enstrophy. Because of the importance of the vorticity balance for understanding the dynamics of low-frequency motions an experiment is suggested to estimate accurately all terms in the lowest-order vorticity balance. From such measurements the energy transfer and enstrophy conversion could also be estimated.

Description: Support to carry out this thesis work was provided by the Office of Naval Research under contracts N00014-66-C0241 and C0262 NR 083-004 and by the National Science Foundation Office of the International Decade of Ocean Exploration under grant IDO75-03962.