MIT-WHOI Joint Program: Academics: PO: Recent Dissertations and Theses

Selected Abstracts

On the Warm Bias Along the South-West African Coast in Coupled Models: An Oceanic Perspective
Jinbo Wang, S.M., 2008
Paola Rizzoli, Advisor

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

The Southern Ocean Meridional Overturning Circulation as Diagnosed from an Eddy Permitting State Estimate
Matthew Mazloff, Ph.D., 2008
Carl Wunsch, Advisor

A modern general circulation model of the Southern Ocean with one-sixth of a degree resolution is optimized to the observed ocean in a weighted least squares sense. Convergence to the state estimate solution is carried out by systematically adjusting the control variables (atmospheric state and initial conditions) using the adjoint model. A cost function compares the model state to in situ observations (Argo float profiles, CTD synoptic sections, SEaOS instrument mounted seal profiles, and XBTs), altimetric observations (ENVISAT, GEOSAT, Jason, TOPEX/Poseidon), and other data sets (e.g. infrared and microwave radiometer observed sea surface temperature and NSIDC sea-ice concentration). Costs attributed to control variable perturbations ensure a physically realistic solution. The state estimate is found to be largely consistent with the individual observations, as well as with integrated fluxes inferred from previous static inverse models.
The transformed Eulerian mean formulation is an elegant way to theorize about the Southern Ocean. Current researchers utilizing this framework, however, have been making assumptions that render their theories largely irrelevant to the actual ocean. It is shown that theories of the overturning circulation must include the effect of pressure forcing. This is true in the most buoyant waters, where pressure forcing overcomes eddy and wind forcing to balance a poleward geostrophic transport and allows the buoyancy budget to be closed. Pressure forcing is also lowest order at depth. Indeed, the Southern Ocean’s characteristic multiple cell overturning is primarily in geostrophic balance. Several other aspects of the Southern Ocean circulation are also investigated in the thesis, including an analysis of the magnitude and variability of heat, salt, and volume inter-basin transports.

Arctic Ocean Circulation in an Idealized Numerical Model
Peter Sugimura, S.M., 2008
Peter Winsor, Advisor

The mid-to-deep Arctic Ocean is generally characterized by a cyclonic circulation, contained along shelves and ridges. Here we analyze the general Arctic circulation using an idealized numerical model consisting of a circular basin with two channels acting as inflow and outflow. We analyze the circulation (direction, strength and sensitivity) for wind forcing with and without bathymetry (ridges), and with and without stratification. We find that the circulation is modified drastically by both bathymetry and wind direction, where an altered wind field can change both the direction of the horizontal basin circulation as well as the strength of the inflow and outflow. The idealized circulations imply that the Arctic circulation, and the associated export of freshwater, can easily switch states in a changing climate.

Observations of Turbulent Fluxes and Turbulence Dynamics in the Ocean Surface Boundary Layer
Gregory Gerbi, Ph.D., 2008
John Trowbridge, Advisor

This study presents observations of turbulence dynamics made during the low winds portion of the Coupled Boundary Layers and Air-Sea Transfer experiment (CBLAST-Low) in the ocean surface boundary layer. Observations include turbulent fluxes, turbulent kinetic energy, and the length scales of flux-carrying and energy-containing eddies. The observations of turbulent fluxes allowed the closing of heat and momentum budgets across the air-sea interface. The flux-carrying eddies are similar in size to those expected in rigid-boundary turbulence, but energy-containing eddies are smaller than those in rigid-boundary turbulence. The observations confirm previous speculation that surface wave breaking provides a surface source of turbulent kinetic energy that dissipates as it is transported to depth. A model that includes the effects of shear production, transport, and dissipation is able to reproduce the enhancement of turbulent kinetic energy near the ocean surface. The ocean surface boundary layer is observed to have small but finite temperature gradients that are related to the boundary fluxes of heat and momentum, as assumed by closure models. However, the turbulent diffusivity of heat in the surface boundary layer is larger than predicted by rigid-boundary closure models. This discrepancy can be explained by the addition of wave breaking to the rigid-boundary model.

Turbulent Convection in an Anelastic Rotating Sphere: A Model for the Circulation on Gas Giant Planets
Yohai Kaspi, Ph.D., 2008
Glenn Flierl, Advisor

This thesis studies the dynamics of a rotating compressible gas sphere, driven by internal convection, as a model for the dynamics on the giant planets. We develop a new general circulation model for the Jovian atmosphere, based on the MITgcm dynamical core augmenting the nonhydrostatic model. The grid extends deep into the planet's interior allowing the model to compute the dynamics of a whole sphere of gas rather than a spherical shell (including the strong variations in gravity and the equation of state). Dierent from most previous 3D convection models, this model is anelastic rather than Boussinesq and thereby incorporates the full density variation of the planet. We show that the density gradients caused by convection drive the system away from an isentropic and therefore barotropic state as previously assumed, leading to signicant baroclinic shear. This shear is concentrated mainly in the upper levels and associated with baroclinic compressibility eects. The interior ow organizes in large cyclonically rotating columnar eddies parallel to the rotation axis, which drive upgradient angular momentum eddy uxes, generating the observed equatorial superrotation. We also use simpler models: a barotropic annulus model, and a quasi- geostrophic baroclinic model to study dynamical mechanisms seen in the convection model.

The East Greenland Coastal Current: Its Structure, Variability and Large-Scale Impact
David Sutherland, Ph.D., 2008
Robert Pickart, Advisor

The subtidal circulation of the southeast Greenland shelf is described using a set of high-resolution hydrographic and velocity transects occupied in summer 2004. The main feature present is the East Greenland Coastal Current (EGCC), a low-salinity, high-velocity jet with a wedge-shaped hydrographic structure. The EGCC was observed along the entire shelf south of Denmark Strait, which when combined with evidence from chemical tracer measurements, implies the EGCC contains a significant Pacific Water signal, suggesting that the EGCC is an inner branch of the polar-origin East Greenland Current (EGC). A set of idealized laboratory experiments supported this hypothesis, showing that for a range of oceanic parameters, a buoyant current like the EGC could potentially influence EGCC formation. The volume transport of the combined EGC/EGCC system is roughly constant (~2 Sv) from 68°N to Cape Farewell, while the corresponding freshwater transport increases by ~60% over this distance (59 to 96 mSv). 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.

Dynamics of Marine Zooplankton: Social Behavior, Ecological Interactions and Physically-Induced Variability
Ariane Verdy, Ph.D., 2008
Glenn Flierl, Advisor

Marine ecosystems reflect the physical structure of their environment and of the biological processes they carry out. This leads to spatial heterogeneity and temporal variability, some of which is imposed externally, and some of which emerges from the biological processes themselves. The main focus of this thesis is on the formation of spatial patterns in the distribution of zooplankton arising from social interactions between individuals. In the Southern Ocean, krill often assemble in swarms and schools, the dynamics of which have important ecological consequences. I adopt a quantitative framework to describe the dynamics of predator and prey populations and to address the costs and benefits associated with social behavior. First, I formulate a model of resource utilization by a predator population with density-dependent reproduction. Second, I incorporate the predator-prey dynamics into a spatially-explicit model. Third, I derive a weakly nonlinear model for the spatial distribution of biomass and examine the formation of one-dimensional patterns driven by social tendencies. Fourth, I simulate the schooling behavior of zooplankton in a heterogeneous resource field. Finally, I consider two sources of temporal variability in ecosystem dynamics: transient amplification of small perturbations to stable equilibrium solutions, and climatic variability affecting the local biogeochemical environment.

Cross-Shelf Circulation and Momentum and Heat Balances over the Inner Continental Shelf Near
Martha's Vineyard, Massachusetts
Melanie Fewings, Ph.D., 2007
Steve Lentz, Advisor

The water circulation and temperature over the inner continental shelf are investigated using six years of observations. During small waves, cross-shelf rather than along-shelf wind stress is the dominant mechanism driving cross-shelf circulation. During large waves and onshore winds the wind- and wave-driven shears cancel. During large waves and offshore winds the velocity is strongly vertically sheared because the wind- and wave-driven shears have the same sign. The subtidal cross-shelf momentum balance is a combination of geostrophy and coastal set-up by the cross-shelf wind. The estimated wave radiation stress gradient is also large. The dominant along-shelf momentum balance is between the wind stress and pressure gradient. The fluctuating along-shelf pressure gradient is a local sea level response to wind forcing, not a remotely generated pressure gradient. In summer, the water is cooled by an upwelling circulation. The cross-shelf heat flux nearly balances the surface heating in summer, so the water temperature is almost constant. The along-shelf heat flux divergence is apparently small. In winter, the change in water temperature is closer to that expected due to the surface cooling. Heat transport due to surface gravity waves is substantial.

Ocean Circulation and Dynamics on the West Antarctic Peninsula Continental Shelf
Carlos Moffat-Varas, Ph.D., 2007
Robert Beardsley, W. Brechner Owens, Advisors

Observations of current velocity, temperature, salinity and pressure from a 2-year moored array deployment and four hydrographic cruises conducted by the United States Southern Ocean GLOBEC program on the western Antarctic Peninsula continental shelf are used to characterize the ocean circulation and its connection to fresh water and heat fluxes on the shelf. Marguerite Trough, a large bathymetric feature connecting the shelf-break to Marguerite Bay, plays a critical role in determining the circulation. At time-scales of 5 to 20 days, the observations reveal the presence of bottom-trapped topographic Rossby waves in Marguerite Trough. The subtidal circulation in the trough has a significant wind-driven component, with downwelling-favorable winds forcing cross-shelf flow on the northern side of Marguerite Trough and along the shore on the outer shelf. Upwelling-favorable winds force roughly the opposite circulation. The cyclonic circulation on the trough helps advect blobs of salty, warm and nutrient-rich water across the shelf. Finally, the first description of the Antarctic Peninsula Coastal Current (APCC) is provided. The APCC is a seasonal and forced by freshwater fluxes associated with large glacier melt and precipitation rates in the region.

Air-Sea Interaction at Contrasting Sites in the Eastern Tropical Pacific: Mesoscale Variability and Atmospheric Convection at 10°N
J. Thomas Farrar, Ph.D., 2007
Robert Weller, Advisor

The role of ocean dynamics in driving air-sea interaction is examined at two contrasting sites on 125ºW in the eastern tropical Pacific Ocean using satellite data and data from two air-sea interaction moorings. Analysis reveals marked differences in the role of ocean dynamics in modulating sea surface temperature (SST). At a near-equatorial site (3ºS), the 1997-1998 El Nino event dominated the evolution of SST and surface heat fluxes, and it is found that wind-driven southward Ekman transport was important in the local transition from El Nino to La Nina conditions. At a 10ºN site near the summertime position of the Inter-tropical Convergence Zone, oceanic mesoscale motions played an important role in modulating SST at intraseasonal (50- to 100-day) timescales. The characteristics and possible generation mechanisms of this mesoscale variability are examined. Focusing on 10ºN in the eastern tropical Pacific, the hypothesis that mesoscale oceanic SST variability can systematically influence cloud properties is investigated using several satellite data products. A statistically significant relationship between SST and columnar cloud liquid water and surface solar radiation is identified within the wavenumber-frequency band corresponding to oceanic Rossby waves.

Wind, Sea Ice, Inertial Oscillations and Upper Ocean Mixing in Marguerite Bay, Western Antarctic Peninsula: Observations and Modeling
Jason Hyatt, Ph.D., 2006
Robert Beardsley, W. Brechner Owens, Advisors

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

Overflows and Upper Ocean Interaction: A Mechanism for the Azores Current
Shinichiro Kida, Ph.D., 2006
Jiayan Yang, Jim Price, Advisors

The oceanic response to overflows is explored using a two-layer isopycnal model. Overflows are a major source of the dense water of the global deep ocean and when they enter the open ocean as dense gravity currents down a continental slope, they entrain upper oceanic water. The upper ocean must balance this mass loss and the vortex stretching associated with this entrainment. Overflows represent an intense localized mass and PV forcing for the upper ocean.
The simulations show that entrainment forces a cyclonic circulation along bathymetric contours in the upper layer and when baroclinic instability develops, a double gyre also forms near the strait. These circulations are topographic beta-plumes and have transports as large as the overflows. For the Mediterranean overflow, the topographic beta-plume becomes a basin scale flow and establishes two trans-Atlantic zonal jets, analogous to the Azores Current and the Azores Countercurrent. The presence of eddies near the steep slope near Cape St. Vincent allows the topographic beta-plume to connect to the open ocean.
This thesis shows that overflows can induce a significant circulation in the upper ocean, and for the Mediterranean overflow, this circulation is a basin scale flow.

Infragravity Waves Over Topography: Generation, Dissipation, and Reflection
James Thomson, Ph.D., 2006
Steve Elgar, Advisor

Ocean surface infragravity waves (periods from 20 to 200 s) observed along the southern California coast are shown to be sensitive to the bottom topography of the shelf region, where propogation is linear, and of the nearshore region, where nonlinearity is important. Infragravity waves exchange energy with swell and wind waves (periods from 5 to 20 s) via conservative nonlinear interactions that approach resonance with decreasing water depth. Consistent with previous results, it is shown here that as waves shoal into water less than a few meters deep, energy is transfered from swell to infragravity waves. In addition, it is shown here that the apparent dissipation of infragravity energy observed in the surfzone is the result of nonlinear energy transfers from infragravity waves back to swell and wind waves. The energy transfers are sensitive to the shallow water bottom topography. On nonplanar beach profiles the transfers, and thus the amount of infragravity energy available for reflection from the shoreline, change with the tide, resulting in the tidal modulation of infragravity energy observed in bottom-pressure records on the continental shelf. The observed wave propagation over the shelf topography is dominated by refraction, and the observed partial reflection from, and transmission across, a steep-walled submarine canyon is consistent with long-wave theory. A generalized regional model incorporating these results predicts the observed infragravity wave amplitudes over variable bottom topography.

Production and Analysis of a Southern Ocean State Estimate
Matthew R. Mazloff, SM, 2006
Carl Wunsch, Advisor

Evolution of the Irminger Current Anticyclones in the Labrador Sea from Hydrographic Data
Tatiana Rykova, SM, 2006
Fiamma Straneo, Advisor

The continuous supply of heat and fresh water from the boundaries to the interior of the Labrador Sea plays an important role for the dynamics of the region and in particular, for the Labrador Sea Water formation. Thus, it is necessary to understand the factors governing the exchange of properties between the boundary and interior. A significant fraction of heat and fresh water, needed to balance the annual heat loss and to contribute to the seasonal freshening of the Labrador Sea, is thought to be provided by coherent long-lived anticyclonic eddies shed by the Irminger Current. The population, some properties, rates and direction of the propagation of these anomalies are known but the evolution and the mechanism of their decay are still far from obvious. In this work I investigated their water mass properties and evolution under the strong wintertime forcing using hydrographic data from 1990-2004 and a 1-dimensional mixed layer model. There were 50 eddies found in the hydrographic data record, 48 of which were identified as anticyclones. Vertical structure of the eddies was investigated, leading to the categorization of all the anticyclones into three classes: 12- with a fresh surface layer and no mixed layer, 18- without a fresh layer and at least one mixed layer, and 18 with ambiguous vertical structure. Four eddies of the second group appeared to have cores extending to as deep as 1500m vertically and an isopycnal displacement of 400-600m. A number of eddies without a fresh water cap contained Labrador Sea Water from the previous year at mid-depths.

Temperature and Salinity Variability in Thermohaline Staircase Layers
David Stuebe, SM, 2005
Ray Schmitt, Advisor

A moored profiler record from the western tropical North Atlantic provides the first continuous time series of temperature, salinity and velocity profiles in a thermohaline staircase. Variations in the intensity of layering and the evolution of layer properties are well documented during the 4.3 month record. Such staircases are the result of strong salt fingering at the interfaces between the mixed layers, and these data provide unique insights into the dynamics of salt fingers. In particular, a striking linear correlation between the temperature and salinity of the layers may be interpreted as resulting from vertical salt finger flux divergences. Data from this record allow new interpretations of previous work on this topic by McDougall (1991).

Subduction in an Eddy-Resolving State Estimate of the Northeast Atlantic Ocean
Geoffrey Gebbie, Ph.D., 2004
Carl Wunsch, Advisor

Are eddies an important contributor to subduction in the eastern subtropical gyre?
Here, an adjoint model is used to combine a regional, eddy-resolving numerical model with observations to produce a state estimate of the ocean circulation. The estimate is a synthesis of a variety of in-situ observations from the Subduction Experiment, TOPEX/POSEIDON altimetry, and the MIT General Circulation Model. The adjoint
method is successful because the Northeast Atlantic Ocean is only weakly nonlinear.

The state estimate provides a physically-interpretable, eddy-resolving information source to diagnose subduction. Estimates of eddy subduction for the eastern subtropical gyre of the North Atlantic are larger than previously calculated from parameterizations in coarse-resolution models. Furthermore, eddy subduction rates have typical magnitudes of 15% of the total subduction rate. Eddies contribute as much as 1 Sverdrup to water-mass transformation, and hence subduction, in the North Equatorial Current and the Azores Current. The findings of this thesis imply that the inability to resolve or accurately parameterize eddy subduction in climate models would lead to an accumulation of error in the structure of the main thermocline, even in the relatively-quiescent eastern
subtropical gyre.

Constraining the North Atlantic Circulation with Transient Tracer Observations
Xingwen Li, Ph.D. 2003
Carl Wunsch, Advisor

The capability of transient tracers to constrain the ocean circulation in the North Atlantic is explored. Study of an idealized tracer shows that inferences of circulation properties from transient state distributions are impacted by uncertainties in the time-varying boundary conditions and sparse data coverage. Comparison of CFC, tritium, temperature and salinity (T-S) observations with model results in the North Atlantic shows that regions of important model-data disagreements in the transient tracer fields can also be readily identified in the T-S distributions. In the model, excessive vertical penetration of convective adjustment, leads to problematic production and outflow of the NADW, again appearing in both transient tracer and T-S fields.

Sensitivities of the model fields are determined using the adjoint model. In the dual solutions, CFC-11, CFC-11/CFC-12 ratio age, and T - (β/α)S (α and β are thermal and haline expansion coefficients, respectively) exhibit the major ventilation pathways and the associated timescales, in the model. High sensitivity fields are candidates for providing the most powerful constraints in the corresponding inverse problems. Assimilation of both CFC and tritium data, with different input histories, sampling distributions, and radioactive decay constants, shows that by adjusting only initial-boundary conditions of CFCs and tritium, a 1° x 1° offline model and the transient tracer data can be brought into near-consistency, in the domain between 4.5°S and 39.5°N of the North Atlantic.

Constraining a GCM with transient tracers is thus fully practical. However, the large uncertainties in the time-varying boundary conditions of transient tracer concentrations, and in their interior distributions, renders the transient tracers less-effective in determining the circulation than are more conventional steady tracers, and known oceanic dynamics.

Shelf Currents, Ice and Wind: A Numerical Modeling Study
Sarah Russell, Ph.D., 2003
W. Brechner Owens, Paola Rizzoli, Advisors

The effects of sea ice, downwelling favorable winds and barotropic background currents on shelf fronts are examined using numerical models.

The first part of the thesis uses a three dimensional, primitive equation model to examine the behavior of a shelf front under steady, along shelf winds and barotropic currents. The wind stress generates shoreward surface Ekman transport and the barotropic current generates an offshore bottom Ekman transport. In both cases, the Ekman transport causes the creation of mixed layers and a relationship describing the mixed layer thickness is derived relating the cross shelf flux of density to the along shelf flux of density.

The second part of the thesis focuses on the ice-ocean interaction using a simple, two layer, one dimensional toy model. In the presence of a current and the absence of wind, the ice is transported downstream with the current. In the presence of wind and the absence of a current, the net ice-ocean transport is perpendicular to the wind, as is expected for Ekman layer theory. The two layer system acts like a poorly resolved Ekman spiral: the ice has down wind and shoreward transport while the ocean has up wind and shoreward transport.

A Laboratory Study of Localized Boundary Mixing in a Rotating Stratified Fluid
Judith Wells, Ph.D., 2003
Karl Helfrich, Advisor

Oceanic observations indicate that abyssal mixing occurs near rough topography. How locally mixed fluid interacts with the ambient fluid is an open question. Laboratory experiments explore localized vertical boundary mixing within a body of linearly stratified, rotating fluid. A single oscillating bar produces a small region of turbulence along the wall at middepth. Mixed fluid quickly reaches a steady state height set by a turbulent-buoyant balance, independent of rotation.

Initially, the bar is exposed on three sides. Mixed fluid intrudes directly into the interior rather than forming a boundary current. The circulation patterns suggest a model of unmixed fluid being laterally entrained into the turbulent zone. In accord with the model, observed outflux is constant, independent of stratification and restricted by rotation.

Later the bar is laterally confined between two walls, which form a channel opening into the basin. A small percentage of mixed fluid enters a boundary current, which exits the channel. The bulk forms a cyclonic circulation in front of the bar, which blocks the channel and restricts horizontal entrainment. In the confined case, the volume flux of mixed fluid decays with time.

Eddies and Friction: Removal of Vorticity from the Wind-Driven Gyre
Baylor Fox-Kemper, Ph.D., 2003
Joseph Pedlosky, Paola Rizzoli, Advisors

Inertia dominates the single-gyre ocean model with small viscosity. Western-intensification doesn't occur with parameters approproaching the ocean's. However, ensuring a mechanism for ultimate removal of vorticity can control the circulation.

I model vorticity removal as a viscosity enhanced very near the solid boundaries parameterizing missing boundary physics. Boundary-enhanced viscosity allows western-intensification even with an inertial layer wider than the frictional region because of the eddy fluxes.

Thereby western-intensified calculations are possible with lower interior viscosity than in previous studies. Interesting behaviors result: a novel boundary-layer, promise for parameterization, gyres rotating opposite the wind, and temporal complexity including basin resonances.

Multiple-gyre calculations have weaker mean circulation than single-gyres with the same parameters. Despite traditional understanding, almost no inter-gyre flux occurs with no-slip boundary conditions. Only with exactly symmetric gyres and slip boundaries is the inter-gyre eddy flux important with small viscosity.

The multiple-gyre circulation is weakened by sinuous instabilities not present in the single-gyre. They efficiently flux vorticity to the boundary and reduce the circulation without an inter-gyre flux, postponing inertial domination to smaller viscosities. In combination with boundary-enhanced viscosity they then control the circulation.

On the Pathways of the Return Flow of the Meridional Overturning Circulation in the Tropical Atlantic
Markus Jochum, Ph.D., 2002
Paola Rizzoli, Advisor

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.