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 August 1984

Description: This thesis addresses several aspects of the problem of determining the effect of the low-frequency eddy variability on the mean circulation of the Western North Atlantic. A framework for this study is first established by scale analysis of the eddy and mean terms in the mean momentum, vorticity, and heat balances in three regions of the Western North Atlantic -- the northern recirculation, the southern recirculation, and the mid-ocean. The data from the last decade of field experiments suggest somewhat different conclusions from the earlier analysis of Harrison (1980). In the momentum balance we confirm that the eddy terms are negligible compared to the lowest order mean geostrophic balance. The eddy term may be an 0(1) term in the vorticity balance only in the northern recirculation region where the mean flow is anisotropic. In the mean heat balance, if the mean temperature advection is scaled using the thermal wind relation, then the eddy heat flux is negligible in the mid-ocean, but it may be important in the recirculation areas. For all the balances the eddy terms are comparable to or an order of magnitude larger than the mean advective terms. We conclude from the scale analysis that the eddy field is most likely to be important in the Gulf Stream recirculation region. These balances are subsequently examined in more detail using data from the Local Dynamics Experiment (LDE). Several inconsistencies are first shown in McWilliams' (1983) model for the mean dynamical balances in the LDE. The sampling uncertainties do not allow us to draw conclusions about the long-term dynamical balances. However, it is shown that if we assume that the linear vorticity balance holds between the surface and the thermocline for a finite record, then the vertical velocity induced by the eddy heat flux divergence is non-zero. The local effect of the mesoscale eddy field on the mean potential vorticity distribution of the Gulf Stream recirculation region is determined from the quasigeostrophic eddy potential vorticity flux. This flux is calculated by finite difference of current and temperature time series from the Local Dynamics Experiment. This long-term array of moorings is the only experimental data from which the complete eddy flux can be calculated. The total eddy flux is dominated by the term due to the time variation in the thickness of isopycnal layers. This thickness flux is an order of magnitude larger than the relative vorticity flux. The total flux is statistically significant and directed 217° T to the southwest with a magnitude of 1.57 x 10 -5 cm/2s. The direction of the eddy flux with respect to the mean large scale potential vorticity gradient from hydrographic data indicates that eddies in this region tend to reduce the mean potential vorticity gradient. The results are qualitatively consistent with numerical model results and with other data from the Gulf Stream recirculation region. We find that the strength of the eddy transfer in the enstrophy cascade is comparable to the source terms in the mean enstrophy balance. The Austauch coefficient for potential vorticity mixing is estimated to be 0(107cm2/sec). An order of magnitude estimate of the enstrophy dissipation due only to the internal wave field shows that other processes must be important in enstrophy dissipation. The measured eddy potential vorticity fluxes are compared to the linear stability model of Gill, Green, and Simmons (1974). An earlier study (Hogg, 1984) has shown agreement between the empirical orthogonal modes of the data and the predicted wavenumbers, growth rates, and phase speeds of the most unstable waves. However, we show substantial disagreement in a comparison of the higher moments the eddy heat and potential vorticity fluxes. Because the critical layer of the model is located near the surface, the model predicts that most of the eddy potential vorticity and eddy heat flux should occur within about 300 meters of the surface. The data show much greater deep eddy heat flux than predicted by the model. It is suggested that the unstable modes in the ocean have a longer vertical scale because of the reduction in the buoyancy frequency near the surface. The evidence for in situ instability is also examined in the decay region of the Gulf Stream from an array of current and temperature recorders. Although there is vertical phase propagation in the empirical orthogonal modes for some of the variables at some of the moorings, there is not much evidence for a strong ongoing process of wave generation.

Description: This research has been conducted under NSF contract numbers OCE 77-19403, ATM 79-21431, and OCE 82-00154.

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

Description: Changes in morphology of the Marquesas Fracture Zone are correlated with small changes in Pacific-Farallon relative motion. The simple flexural signal of a locked fracture zone may be obscured by tectonic effects, and there is no evidence for the release of shear stress on the fracture zone by vertical slip after leaving the active transform. One such small change in plate motion is documented in the South ern Austral Island region of the South Pacific. A twelve degree clock wise change in Pacific-Farallon relative motion occurred around fifty million years ago. This Eocene change in spreading direction and rate is locally constrained with observations of magnetic anomalies and spreading fabric orientation. At the southeastern end of the Cook-Austral Island chain, multiple episodes of volcanism have left a diverse population of seamounts. Volume estimates from geophysical data and modeling show that one-half to two-thirds of the volcanic material is over thirty million years old, while the remainder is less than five million years old. Seismic and bathymetric data imply the presence of abyssal basalt flows in the flexural moat of the Austral Islands, probably associated with Austral Islands volcanism, which may contribute a significant amount of material to the archipelagic apron.

Description: The research presented in Chapter 2 was supported by National Science Founda tion grants OCE-9012949 and OCE-9012529. Chapters 3, 4 and 5 were supported by National Science Foundation grant OCE-9415930. A National Science Foundation graduate fellowship supported my first three years of graduate study.

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

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

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

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

Description: Two-thirds of the surface of the Earth is created at mid-ocean ridges where magmas rise from the mantle and cool to form the oceanic crust. The objective of this Thesis is to examine the influence of magma supply and eruptive processes on axial morphology, crustal construction, and the properties of crustal magma chambers at intermediate and fast spreading ridges. Variations in magma supply on time scales of ~100 Kyr generate along-axis changes in crustal thickness and temperature. Magma sill properties and hydrothermal activity are closely linked to spreading events which occur on much shorter time scales (ca. 10-100 yr) than the longer-term variations in magma supply reflected in along-axis changes in ridge morphology. The seismically constrained depths of ridge crest magma sills (〉1-2 km) are considerably deeper than the level of neutral buoyancy (100-400 m). The apparent inverse relationship between magma sill depth and spreading rate suggests that a thermally controlled permeability boundary, such as the solidus horizon, controls the depth at which magma ponds beneath mid-ocean ridges. Recent thermo-mechanical models predict that, at intermediate spreading rates, rift valley and magma sill formation are sensitive to small changes in crustal thickness and mantle temperature. Analysis of gravity at an intermediate spreading ridge shows that small differences in crustal thickness (300-700 m) and mantle temperature (10-15°C) are indeed sufficient to produce major changes in lithospheric strength and axial morphology. A stochastic model for the emplacement of dikes and lava flows with a bimodal distribution of lava flows is required to satisfy geological and geophysical constraints on the construction of the extrusive section. Most dikes are intruded within a narrow zone at the ridge axis. Short flows build up approximately half the extrusive volume. Occasional flows that pond at a considerable distance off-axis build up the remainder of the extrusive section. This Thesis underlines the importance of eruption dynamics in the emplacement of the uppermost volcanic layer of the crust and of the crustal thermal structure in controlling local variations in magma sill depth and ridge morphology.

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

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

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

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

Description: This thesis addresses the parameterization of the heat and momentum transporting properties of eddy motions for use in three-dimensional, primitive equation, z-coordinate atmosphere and ocean models. Determining the transport characteristics of these eddies is fundamental to understanding their effect on the large-scale ocean circulation and global climate. The approach is to transform the primitive equations to yield the altered 'transformed Eulerian mean' (TEM) equations. The assumption is made that the eddy motions obey quasigeostrophic dynamics while the mean flow obeys the primitive equations. With this assumption, the TEM framework leads to the eddies appearing as one term, which acts as a body force in the momentum equations. This force manifests itself as a flux of potential vorticity (PV) - a quantity that incorporates both eddy momentum and heat transporting properties. Moreover, the dynamic velocities are those of the residual mean circulation, a much more relevant velocity for understanding heat and tracer transport. Closure for the eddy PV flux is achieved through a flux-gradient relationship, which directs the flux down the large scale PV gradient. For zonal flows, care is taken to ensure that the resulting force does not generate any net momentum, acting only to redistribute it. Neglect of relative vorticity fluxes in the PV flux yields the parameterization scheme of Gent and McWilliams. The approach is investigated by comparing a zonally-averaged parameterized model with a three dimensional eddy-resolving calculation of flow in a stress-driven channel. The stress at the upper surface is communicated down the water column to the bottom by eddy form drag. Moreover, lateral eddy momentum fluxes act to strengthen and sharpen the mean flow, transporting eastward momentum up its large scale gradient. Both the vertical momentum transfer and lateral, upgradient momentum transfer by eddies, are captured in the parameterized model. The advantages of this approach are demonstrated in two further zonal cases: 1) the spin-down of a baroclinic zone, and 2) the atmospheric jet stream. The time mean TEM approach and the eddy PV flux closure are explored in the context of an eddy-resolving closed basin flow which breaks the zonal symmetry. Decomposition of eddy PV fluxes into components associated with advective and dissipative effects suggest that the component associated with eddy flux divergence, and therefore forcing of the mean flow, is mainly directed down the large scale gradient and can be parameterized as before. Thus, the approach can be used to capture eddy transport properties for both zonal mean and time mean flows. The PV flux embodies both the eddy heat and momentum fluxes and so presents a more unified picture of their transferring properties. It therefore provides a powerful conceptual and practical framework for representing eddies in numerical models of the atmsophere and ocean.

Description: The work in this thesis was funded by grants from NSF, (OCE-9634331, OCE- 9503895), ONR (NOOOI4-95-1-0967), and by a fellowship from the Joint Program on the Science and Policy of Global Change at MIT.

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 2000

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

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

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

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

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

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

Description: We analyze bathymetric and gravity anomalies at five plume-ridge systems to constrain crustal and mantle density structure at these prominent oceanic features. Numerical models are then used to explore the physical mechanisms controlling plume-ridge interaction and to place theoretical constraints on the temperature anomalies, dimensions, and fluxes of the Icelandic and Galapagos plumes. In Chapter 1 we analyze bathymetric and gravity anomalies along the hotspot-influenced Galapagos Spreading Center. We find that the Galapagos plume generates along-axis bathymetric and mantle-Bouguer gravity anomalies (MBA) that extend 〉500 km east and west of the Galapagos Islands. The along-axis MBA becomes increasingly negative towards the plume center, reaching a minimum of ~-90 mGal near 91°W, and axial topography shallows by ~1.1 km toward the plume. These variations in MBA and bathymetry are attributed to the combined effects of crustal thickening and anomalously low mantle densities, both of which are due to a mantle temperature anomaly imposed beneath the ridge by the Galapagos plume. Passive mantle flow models predict a temperature anomaly of 50±25°C is sufficient to produce the 2-4 km excess crust required to explain the along-axis anomalies. 70-75% of the along-axis bathymetric and MBA variations are estimated to arise from the crust with the remaining 25-30% generated by the anomalously hot, thus low-density mantle. Along Cocos-plate isochrons, bathymetric and MBA variations increase with increasing isochron age, suggesting the subaxial mantle temperature anomaly was greater in the past when the plume was closer, to the ridge axis. In addition to the Galapagos plume-ridge system, in Chapter 2 we examine alongisochron bathymetric and MBA variations at four other plume-ridge systems associated with the Iceland, Azores, Easter and Tristan hotspots. We show that residual bathymetry (up to 4.7 km) and mantle-Bouguer gravity anomalies (up to -340 mGal) are greatest at on-axis plumes and decreases with increasing ridge-hotspot separation distance, until becoming insignificant at a plume-ridge separation of ~500 km. Along-isochron widths of bathymetric anomalies (up to 2700 km) decrease with increasing paleo-spreading rate, reflecting the extent to which plume material flows along-axis before being swept away by the spreading lithosphere. Scaling arguments suggest an average ridgeward plume flux of -2.2x106 km/my. Assuming that the amplitudes of the MBA and bathymetric anomalies reflect crustal thickness and mantle density variations, passive mantle flow models predict maximum subaxial mantle temperature anomalies to be 150-225°C for ridge-center plumes, which decrease as the ridges migrate away from the plumes. The dynamics of mantle flow and melting at ridge-centered plumes are investigated in Chapters 3 using three-dimensional, variable-viscosity, numerical models. Three buoyancy sources are examined: temperature, melt depletion, and melt retention. The width W to which a plume spreads along a ridge axis depends on plume volume flux Q, full spreading rate U, buoyancy number B = (QΔρg)/(48η0U2), and ambient/plume viscosity contrast ϒ according to W=2.37(Q/U)l/2(Bϒ)0.04. Thermal buoyancy is first order in controlling along-axis plume spreading while latent heat loss due to melting, and depletion and retention buoyancy forces contribute second order effects. Two end-member models of the Iceland-Mid-Atlantic Ridge (MAR) system are examined. The first endmember model has a broad plume source of radius 300 km, temperature anomaly of 75°C, and volume flux of 1.2xl07 km3/my. The second model has a narrower plume source of radius 60 km, temperature anomaly of l70°C, and flux of 2.1 x106 km3/my. The first model predicts successfully the observed crustal thickness, topographic, and MBA variations along the MAR, but the second model requires substantial along-axis melt transport in order to explain the observed along-axis variations in crustal thickness, bathymetry, and gravity. We favor this second model because it predicts a mantle P-wave velocity reduction in the plume of ~2% as consistent with recent seismic observations beneath Iceland. Finally in Chapter 4 we use three-dimensional numerical models to investigate the interaction of plumes and migrating midocean ridges. Scaling laws of axial plume spreading width Ware derived first for stationary ridges and off-axis plumes, which yield results consistent with those obtained from independent studies of Ribe [1996]. Wand the maximum plume-ridge interaction distance Xmax again scale with (Q/U)l/2 as in the case of ridge-centered plumes and increase with ϒ and buoyancy number. In the case of a migrating ridge, Xmax is reduced when a ridge migrates toward the plume due to excess drag of the faster-moving leading plate, and enhanced when a ridge migrates away from the plume due to reduced drag of the slower-moving trailing plate. Thermal erosion of the lithospheric boundary layer by the previously ridge-centered plume further enhances Wand Xmax but to a degree that is secondary to the differential migration rates of the two plates. Model predictions are compared with observed along-isochron bathymetric and MBA variations at the Galapagos plume-ridge system. The anomaly amplitudes and widths, as well as the increase in anomaly amplitude with age are predicted with a plume source temperature anomaly of 80-120°C, radius of 80-100 km, and volume flux of 4.5x106 km3/m.y. Our numerical models also predict crustal production rates of the Galapagos Islands consistent with those estimated independently using the observed island topography. Predictions of the geochemical signature of the plume along the present-day ridge suggest that mixing between the plume and ambient mantle sources is unlikely to occur in the asthenosphere or shallow crust, but most likely deeper in the mantle possibly by entrainment of ambient mantle as the plume ascends through the depleted portion of the mantle from its deep source reservoir.

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

Description: The formation of new oceanic crust is the result of a complex geodynamic system in which mantle rises beneath spreading centers and undergoes decompression melting. The melt segregates from the matrix and is focused to the rise axis, where it is eventually intruded and/or erupted to form the oceanic crust. This thesis combines surface observations with laboratory studies and geodynamic modeling to study this crustal-production system. Quantitative modeling of the crustal and mantle contributions to the axial gravity and topography observed at the East Pacific Rise shows that the retained melt fraction in the mantle is small (〈3%) and is focused into a narrow column extending up to 70 km beneath the ridge axis. Consistent with geochemical constraints, the extraction of melt from the mantle therefore appears to be efficiently focus melt toward the ridge axis. A combination of laboratory and numerical studies are used to constrain the pattern of mantle flow beneath highly-segmented ridges. Even when the buoyant component of mantle flow is constrained to be two-dimensional, laboratory studies show that a segmented ridge will drive three-dimensional mantle upwelling. However, using reasonable mantle parameters in numerical models, it is difficult to induce large-amplitude three-dimensional mantle upwelling at the relatively short wavelengths of individual segments (~50 km). Instead, a simple model of three-dimensional melt migration shows that the observed segment-scale variations in crustal thickness can be explained by focusing of melt as it upwells through a more two-dimensional mantle flow field. At the Reykjanes Ridge, the melt appears to accumulate in small crustal magma chambers, before erupting in small batches to form numerous overlapping hummocky lava flows and small volcanoes. This suggests that crustal accretion, particularly at slow-spreading centers, may be a highly discontinuous process. Long-wavelength variations in crustal accretion may be dominated by variations in mantle upwelling while short-wavelength, segment-scale variations are more likely controlled by a complex three-dimensional processes of melt extraction and magma eruption.

Description: During my first three years in the Joint Program, I was supported by an National Science Foundation Graduate Student Fellowship. Other support has been derived from National Science Foundation grants OCE-9296017, OCE-9224738, OCE-9215544, and EAR grant 93-07400.

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

Description: With Lagrangian and hydrographic data taken in the deep Brazil Basin, we identify several submesoscale coherent vortices (SCVs). These features contrast with SCV paradigms in that float data indicate approximately equal populations of cyclonic and anticyclonic vortices, and hydrographic data suggest that roughly half exhibit the convex lens shape generally associated with SCVs, while half are instead shaped like a concave lens, with isopycnal surfaces pinched together. There is some evidence that the vortex cores may be enriched in warm, salty North Atlantic Deep Water, suggesting formation in the north or northwest regions of the basin. Data is available from 153 floats which were ballasted for 2500 and 4000 db pressures. They tracked 34 eddies, which are believed to be roughly 30 km in diameter, and rotate with apparent periods of about 30 days. Many floats experienced formation or entrainment events, and destruction or detrainment events, near seamounts.

Description: This work was supported by the National Science Foundation, grants OCE 99-11148 and OCE 94-15509.

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

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

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

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

Description: The global mid-ocean ridge system is one of the most striking geological features on the surface of the Earth. In this system, the East Pacific Rise (EPR) is the fastest spreading ridge and is thus considered as the most active magmatically among the plate boundaries. In January and February of 1988, an extensive survey by the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution was conducted along the EPR between 9°05' and 9°55'N to study the crustal structure of the axial region. This thesis, the result of that cruise, comprises four main topics: (1) characterization of normal faulting from Sea Beam bathymetric data, (2) application of mechanical models to explore the hypothesis that buoyancy arising from crustal magma chambers and gravitational spreading of the upper crust are the principal processes leading to the initiation and development of normal faults, (3) investigation of seafloor magnetization anomalies to constrain upper crustal structure, and (4) analysis of gravity anomalies to examine possible correlations between observed variations in seafloor manifestations of volcanism and deformation and underlying structure. Thus, each topic focuses on different levels of the mid-ocean ridge. Together with the results of seismic and other observations, the findings are woven into a better understanding of the tectonic processes and structure of fastspreading mid-ocean ridges. First, to understand the characteristics of normal faults at fast-spreading ridges, we utilized swaths of Sea Beam bathymetry and estimated the distribution and geometry of normal fault zones using the slope of the seafloor as the criterion for a faulted surface. In our survey area, nonnal fault activity begins 2-8 km off-axis and continues at least to 30-40 km from the axis, as indicated by an increase in the total and average throws of normal fault zones versus distance from the axis. There appears to be no significant difference in the plan-view area of inward- and outward-facing nonnal fault zones. The distance from the rise axis to the nearest large-offset fault zone (throw 〉 20 m) on either side of the axis is approximately symmetric to the north of 9°23'N, but the midpoint between nearest largeoffset fault zones is offset 2-3 km to the west of the bathymetric axis to the south of 9°23'N. The continued growth of nonnal fault zones suggests that significant extensional stress persists to greater distances from the axis than previously thought and that the rise axis possesses a finite strength. The argument that the rise axis has finite strength is consistent with recent evidence for solidified axial dikes along magmatically active portions of the EPR from near-bottom seismic refraction experiments, which suggests that, while eruption of magma at the rise axis weakens the axis, the persistence of such weak zones is short-lived and the emplacement zones at any given time are localized along the axis. We examined how the presence of a low-density, low-strength magma chamber within the crust and gravitational spreading of a mechanically strong upper crust over an underlying substrate contribute to the fonnation of faults at a fast spreading mid-ocean ridge by comparing the predicted stress field with the observed pattern of normal fault zones. We employed boundary element methods to incorporate buoyancy and gravitational spreading as body forces in an elastic medium, and we detennined stress and strain fields for a variety of rise axis conditions and a range of possible sets of material properties for different parts of the mid-ocean ridge. Our results show that the strength of the rise axis is one of the most crucial factors governing the near-axis stress field. If the rise axis is mechanically weak, the maximum extensional stress from buoyancy occurs at shallow depth off the rise axis. A weak rise axis may result from recent magmatism such as the intrusion of dikes into the upper crust. On the other hand, if the rise axis is mechanically strong, which may result after solidification and cooling of the dike zone, the maximum surface extensional stress occurs on the rise axis. However, the reduction in size of a magma chamber that would accompany cessation of dike injection would lead to less buoyancy and thus a lower likelihood of stress levels sufficient for faulting. For a given set of material strengths and a given magnitude of buoyancy force, the flexural rigidity of the upper crust plays an important role in detennining if a zone of extension will develop off axis and, if so, the position and horizontal extent of that zone. A thin or mechanically weak upper crust is more likely to develop a zone of extension than one that is thick or mechanically strong. The stress field resulting from gravitational spreading is similarly affected by the strength of the rise axis. While buoyancy can explain a consistent distance at which normal faults initiate off-axis, gravitational spreading can account for continued activity on normal faults to a greater distance from the axis than can buoyancy. The existence of a magma lens can play an important role in reducing the magnitude of the stress field for a weak rise axis, as the crust above the magma lens can slide and thus relieve the thickness-averaged extensional stress. Next, we inverted surface ship measurements of the scalar magnetic field along the EPR between 9°10' and 9°50'N. We examined whether the axial magnetization high, which increases in amplitude to the south in our area, can best be explained by variations in the thickness or in the magnetization intensity of the source layer. The variation in axial magnetization is too large to be explained solely by the variations in the depth to the top of the axial magma chamber indicated by reflection seismology. For a magnetic source layer that is 500 or 750 m thick, the observed along-axis variations in FeO and Ti02 explain only 36 and 60%, respectively, of the total variance of axial magnetization anomalies. Therefore, a combination of variations in magnetic layer thickness and in intensity of magnetization (by variations in the FeO and Ti02 contents of the source rock or by other mechanisms) is needed to explain the along-axis variation of axial magnetization. In addition to the increase in amplitude to the south, the axial magnetization high exhibits at least three marked changes in magnitude and offsets in its along-axis linearity ('magnetic devals') (at 9°25', 9°37', and 9°45'N) which appear to be related to boundaries or offsets between the segments of the axial summit caldera (ASC). Because the amplitudes of the axial magnetization anomalies are highest at the midpoints of the ASC segments, we speculate that midpoints of the ASC segments are the loci of more frequent lava eruptions, and the seafloor basalts at the midpoints are thus younger and more magnetic, than at the segment ends. The magnetization shows distinct short-wavelength (~ 5 km) banding to the north of 9°25'N over a region that does not appear to have been affected by an overlapping spreading center. Among the possible explanations for these off-axis magnetization anomalies are short geomagnetic reversal events within the Brunhes epoch, variations in the paleointensity of the Earth's field, variations in the magnetization intensity of the source rock due to variability in the magmatic supply, and variations in the degree of hydrothermal alteration at the rise axis. On the basis of comparisons of forward models and observations, short geomagnetic reversal events appear to be the most likely explanation of these anomalies. The analysis of sea-surface gravity field measurements shows an axial residual mantle Bouguer gravity anomaly too large to be explained by the anomalous temperature of the mantle or by changes in the thickness of the crust. The broad axial residual gravity low is interpreted as a signal arising largely from the upper mantle, presumably by presence of partial melt along the rise axis. A northward increase in the width of the low implies a greater melt fraction in the region to the north than to the south, especially on the Pacific plate side. The residual gravity anomaly also shows several short-wavelength local lows along the axis (e.g., 9°21', 9°32', and 9°42'N) which correlate with along-axis variations in axial magnetization and tomographic images of mid-crustal seismic velocities. Along axis the local lows have an amplitude of 1.5-3 mGal and appear at a nearly regular spacing (10- 15 km). Across the axis, however, the local lows show a greater variation (3-5 mGal), suggesting that there is an additional gravity anomaly signal arising from a low-density structure that is approximately continuous along the axis. The anomalous masses producing the local lows are interpreted as zones of relatively high melt concentration, formed within the crust by recent replenishment of magma from the upper mantle, that are surrounded by a region of lesser melt concentration corresponding to the low-velocity volume imaged by seismic tomography. If the zone of high melt concentration are modeled as circular rods of radius 1 km, along-axis length 10 km, and center of mass 2.25 km below the seafloor, density contrasts of 200-350 kg/m3 are needed to match the observed anomalies. For larger anomalous mass volumes, the density contrasts would be lower. The findings of this study support the hypothesis that the axis of the EPR can be divided into segments 10-15 km in length, with each segment defmed by the locus and timing of most recent emplacement of magma in the axial crust. The segments in the study area appear to be in different phases of a magmatic cycle, but the period of such a magmatic cycle is not known. By this view, the discrete emplacement of magma bodies gives rise to along-axis variations in crustal structure manifested as short-wavelength residual gravity anomalies and magnetic devals. Another consequence of a rise axis at which magma is emplaced at discrete locations is that the mechanical strength of the axial upper crust varies with position along the axis and over time. During active magmatism, the rise axis acts as a weak zone and the buoyancy of the axial magma chamber and surrounding low-velocity volume can lead to initiation of off-axis normal faulting. However, for a long segment of the rise bounded by transform faults, the axis will have sections with a solidified rucial injection zone as well as sections undergoing active magmatism, and thus the rise overall may appear to have finite strength. If such a finitestrength ridge axis is subject to significant extensional stress as a result of gravitational spreading, mantle convective tractions, or differential cooling, then continued normal fault activity would extend over a broad region to distances of at least several tens of kilometers from the spreading axis.

Description: The work in this thesis was supported by the National Science Foundation under grants OCE-8615797, OCE-8615892, and OCE-9000177.

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

Description: The objective of this Thesis was to interpret the structural development of slowspreading ridge segments by: 1) delineating the nature, magnitude, and relative importance of primary tectonic and volcanic processes that control crustal morphology, 2) investigating the spatial and temporal variability of these processes, and 3) examining how rheological variations in the lithosphere control its structural configuration. To that end, this Thesis provides detailed documentation of faults and volcanoes (seamounts) at the Mid-Atlantic Ridge from 25°25'N to 27°10'N and extending from zero-age crust at the ridge axis to -29 Ma crust on the ridge flank. This information was used to analyze the evolution of ocean crust from initial formation in the rift valley to degradation by aging processes on the ridge flank. Accumulation of sediments affects the seafloor morphological expression of ocean crustal structure, and sediment thicknesses were also mapped to facilitate study of the morphological record of crustal accretion and tectonism. In addition, deformation conditions in the lithosphere were analyzed by study of microstructure and geothermometry of abyssal peridotite mylonites recovered from fault zones at slow-spreading ridges.

Description: Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution March 1992

Description: Rotating baroclinic and barotropic boundary currents flowing around a corner in the laboratory were studied in order to discover the circumstances under which eddies were produced at the corner. Such flows are reminiscent of oceanic coastal flows around capes. When the baroclinic currents, which consisted of surface flows bounded by a density front, encountered a sharp corner, immediately downstream of the corner an anticyclone grew in the surface layer for an angle of greater than 40 degrees. Varying the initial condition of the flow or the depth of the lower layer did not noticeably affect the gyre's properties except for its growth speed, which was greater when the lower layer was shallower. The barotropic currents were pumped along a sloping bottom, and also formed anticyclonic gyres which quickly attained an approximately steady state. For a given topography, the size of the gyre was proportional to the inertial radius u/f. Volume flux calculations based on the surface velocity revealed vertical shear which increased with gyre size. Hydraulic models were also applied to flow around gently curving topography to determine the critical separation curvature as a function of upstream parameters.

Description: This work was supported by the National Science Foundation grant OCE 89-15408.

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 1988

Description: Heat flow, tectonic subsidence and crustal thickness distributions in the Ligurian Basin are best explained by asymmetric lithospheric thinning mechanisms. Over 150 heat flow measurements are made on several transects between Nice, France and Calvi, Corsica on continental slope and rise settings. Thermal gradient determinations are improved using an optimization technique. Piston core data and surface sediment 3.5 kHz reflectivity patterns help constrain thermal conductivity obtained from over 100 in situ stations. Plio-Quaternary stratigraphy is revised using new seismic reflection profiles: a boundary fault system associated with postrift margin uplift, a Pleistocene-age Var Fan construction, and recent diapirism of Messinian salt are indicated. After assessing local thermal disturbances (mass-wasting, microtopography, and salt refraction), positive heat flow corrections are made for multi-lithologic sedimentation histories and glacial paleotemperatures. Using boundary-layer cooling models, equilibrium heat flow estimates support geologic evidence for Oligocene and early Miocene rifting. Heat flow maxima correlate well with two "oceanic" sub-basins, suggesting that the southeastern trough near Corsica is ~5 Myr younger, consistent with the southeastern progression of volcanism and back arc rifting in the Western Mediterranean. Tectonic subsidence-crustal thickness trends indicate lithospheric stretching, with heat flow supporting asymmetric sub-crustal lithospheric thinning during the conjugate margin formation.

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 1992

Description: A pair of simple models representing the interaction of a continuously stratified f-plane quasigeostrophic lens with a uniform external shear flow is examined. The study is motivated by the desire to understand the processes that affect Mediterranean Salt Lenses and other mesoscale lenses in the ocean. The first model represents the eddy as a pair of quasigeostrophic 'point potential vortices' in uniform external shear, where the two point vortices are imagined to represent the top and bottom of a baroclinic eddy. While highly idealized, the model succeeds in qualitatively reproducing many aspects of the behavior of more complex models. In the second model the eddy is represented by an isolated three dimensional patch characterized by quasigeostrophic potential vorticity linear in z, in a background flow with constant potential vorticity. The boundary of the lens may be deformed by interactions with a uniform background shear. A family of linearized analytical solutions representing such a vortex is discussed in Chapter 3. These solutions represent lens-like eddies with trapped fluid cores, which may propagate through the surrounding water when there is external vertical shear. The analysis predicts the possible forms of the boundary deformation in a specified external flow, and the precession rate of normal mode boundary perturbations in the absence of external flow. The translation speed of the lens with respect to the surrounding fluid is found to be a simple function of the external vertical shear and the core baroclinicity. A numerical algorithm which is a generalization of the contour dynamics technique to stratified quasigeostrophic flow is used to extend the linear results into the nonlinear regime. This numerical analysis allows a determination of the range of environmental conditions (e.g., the maximum shear and/ or core baroclinicity) in which coherent vortex solutions can be found, and allows the stability of the steadily translating solutions to be examined directly. It is found that the solutions are stable if neither the external shear nor the core baroclinicity is too large, and that the breakdown of the unstable solutions is characterized by the loss of an extrusion of core fluid to the surrounding waters. The translation speeds of the large amplitude numerical solutions are found to have the same functional dependence on the external vertical shear and the core baroclinicity that was found in the linear analysis, and it is demonstrated that the solutions translate at a rate which is equal to the background flow speed at the center of potential vorticity of the lens. As a test of the model results, new data from a recent SOFAR float experiment are presented and compared with the model predictions. The data show that the cores of two different Mediterranean Salt Lenses are tilted, presumably as a result of interactions with external flows. Both the sense of the tilt and its relation to the translation of the lens are in qualitative agreement with the model solutions.

Description: I am happy to acknowledge the support of the Office of Naval Research (grant N00014-89-J-1182) and the National Science Foundation (grants OCE 8916446 and OCE 87-00601).

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 1992

Description: This study concerns the barotropic interactions between a mesoscale eddy and a straight monotonic bottom topography. Through simple to relatively complicated modeling effort, some of the fundamental properties of the interaction are investigated. In chapter two, the fundamental aspects of the interaction are examined using a simple contour dynamics model. With the simplest model configuration of an ideal vortex and a step topography, the basic dynamical features of the observed oceanic eddy-topography interaction are qualitatively reproduced. The results consist of eddy-induced cross-topography exchange, formation of topographic eddies, eddy propagation and generation of topographic waves. In chapter three, a more complicated primitive equation model is used to investigate a mesoscale eddy interacting with an exponential continental shelf/slope topography on both f and β-planes. The f-plane model recasts the important features of chapter two. The roles of the eddy size and strength and the geometry of topography are studied. It is seen that the multiple anticyclonic eddy-slope interactions strongly affect the total cross-slope volume transport and the evolution of both the original anticyclone and the topographic eddy. Since a cyclone is trapped at the slope and eventually moves on to the slope, it is most effective in causing perturbation on the shelf and slope. The responses on the shelf and slope are mainly wavelike with dispersion relation obeying that of the free shelf-trapped wave modes. On the β-plane, the problem of an eddy colliding onto a continental shelf/slope from a distance with straight or oblique incident angles is investigated. It is found that the straight eddy incident is more effective in achieving large onslope eddy penetration distance than the oblique eddy incident. The formation of a dipole-like eddy pair consisting of the original anticyclone and the topographic cyclone acts to suppress the eddy decay due to long Rossby wave radiation. A weak along-slope current near the edge of the slope is found, which is part of a outer slope circulation cell originated from the Rossby wave wake trailing the propagating eddy. Model-observation comparisons in_chapter four show favorable qualitative agreement of the model results with some of the observed events in the eastern U.S. continental margins and in the Gulf of Mexico. The model results give dynamical interpretations to some observed features of the oceanic eddy-topography interactions and provide enlightening insight into the problem.

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

Description: The age of the Nazca plate where it enters the Peru and northern Chile trenches varies from 30 Ma in the north to 45 Ma in the south as its dip beneath the South American continent steepens from 13° to 30°. If the elastic thickness Te of oceanic lithosphere depends only on its age, and therefore thermal state, we would expect that Te determined from fitting the flexure of the lithosphere over the outer rise as revealed in the depth and geoid anomalies would increase from the Peru Trench in the north to the northern Chile Trench further south. We find that just the opposite is true: the lithosphere appears stiffer outboard of the Peru Trench than it does further south, and the isotherm controlling the elastic/ductile transition must be 600°C or greater if the thermal structure of the plate is that predicted by the standard thermal plate model. Because the decrease in plate stiffness to the south is correlated with a decrease in the minimum radius of curvature of the flexed plate over the outer rise and outer trench wall, we interpret our result in terms of inelastic yielding of the oceanic lithosphere when bent to high strains. The fact that the more highly bent segment of subducting lithosphere also dips at a steeper angle at greater depth beneath the continent might suggest that the amount of inelastic weakening of the lithosphere could be predicted from seismic images of the down going slab, but we find little support for this correlation worldwide. The forces and moments controlling the shallow deformation of the plate seaward of the trench do not appear to be linked to upper mantle processes which impose the dip at greater depth. Finally, we consider the possibility that the elastic thickness of the lithosphere would be reduced for trenches that are highly arcuate in map view, again due to inelastic yielding. If such a relationship exists, the effect for oceanic lithosphere is much smaller than what is documented for continental plates where they underthrust highly arcuate fold belts.

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 November 1990

Description: One and two layer models are used to study flow over axisymmetric isolated topography. Inviscid or nearly inviscid flow in which non-linear effects have order one importance is considered, and both the effects of β and finite topography are included. A one-layer quasi-geostrophic model is used to find the shape of Taylor columns on both the f-plane and the β-plane in the inviscid limit of the frictional problem. In this limit, the boundary of the Taylor column is a streamline, and the velocity in both directions vanishes on the boundary. The fluid within the Taylor column is stagnant, corresponding to the solution that Ingersoll (1969) found for flow over a right circular cylinder on the f-plane. In this case, the Taylor column is circular. An iterative boundary integral technique is used to find the solutions for flow over a cone on the f-plane. In this case the Taylor column has a tear drop shape. Solutions are also found for flow on the β-plane over a cylinder, and the Taylor column is approximately elliptical for westward flow with the major axis in the x direction, while it is slightly elongated in the y direction for eastward flow. The stagnation point of the Taylor column is located on the edge of the topography for all the solutions found. It was not possible to find solutions for smooth topographic shapes. Steady solutions for flow over a right circular cylinder of finite height are studied when the quasi-geostrophic approximation no longer applies. The solution consists of two parts, one which is similar to the quasi-geostrophic solution and is driven by the potential vorticity anomaly over the topography and the other which is similar to the solution of potential flow around an cylinder and is driven by the matching conditions on the edge of the topography. When the effect of β is large, the transport over the topography is enhanced as the streamlines follow lines of constant background potential vorticity. For eastward flow, the Rossby wave drag can be much larger than predicted from quasi-geostrophic theory. A two-layer model over finite topography using the quasi-geostrophic approximation is developed. The topography is a right circular cylinder which goes all of the way through the lower layer and an order Rossby number amount into the upper layer, so that the quasi-geostrophic approximation can be applied consistently. This geometry allows description of flow in which an isopycnal intersects the topography. The model is valid for a different regime than existing models of steady flow over finite topography in a continuously stratified fluid in which the bottom boundary is an isopycnal surface. The solutions contain the two components that are found in the the barotropic model of flow over finite topography. The model breaks down when the interface goes above the topography which occurs more easily when the stratification is weak. Closed streamlines occur more readily over the topography when the stratification is weak, whereas in traditional quasi-geostrophic theory they occur more readily when the stratification is strong. Near the topography, the interface is depressed to the right and raised to the left (looking downstream). A hierarchy of time-dependent models is used to examine the initial value problem of flow initiation over topography on the f-plane. A modified contour dynamics method is developed that extends the range of problems to which contour dynamics can be applied. The method allows boundary and matching conditions to be applied on a circular boundary. A one-layer quasi-geostrophic model is used to show that more fluid that originates over the topography remains there when the flow is turned on slowly than when it is turned on quickly. Flow over finite topography in a one-layer model shows a variety of different behaviors depending on the topographic height. When the topography has moderate height, two cyclonic eddies are created; when the topography fills up most of the water column, the fluid oscillates on and off the topography as it moves around the topography in a clockwise direction, and none of the fluid is shed downstream. Two quasi-geostrophic stratified models are considered, one in which the topography is small, and the other in which it is finite. In the small topography model, an eddy is shed which is cyclonic, warm-core, and bottom-trapped. In contrast, the shed eddy is cyclonic, cold-core, and surfaceintensified in the finite depth model using the geometry described above.

Description: This study was supported by the Office of Naval Research Program N00014- 90-J-1839.

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 1999

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

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