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 September 2007.

Description: The water circulation and evolution of water temperature over the inner continental shelf are investigated using observations of water velocity, temperature, density, and bottom pressure; surface gravity waves; wind stress; and heat flux between the ocean and atmosphere during 2001-2007. When waves are small, cross-shelf wind stress is the dominant mechanism driving cross-shelf circulation. The along-shelf wind stress does not drive a substantial cross-shelf circulation. The response to a given wind stress is stronger in summer than winter. The cross-shelf transport in the surface layer during winter agrees with a two-dimensional, unstratified model. During large waves and onshore winds the cross-shelf velocity is nearly vertically uniform, because the wind- and wave-driven shears cancel. During large waves and offshore winds the velocity is strongly vertically sheared because the wind- and wave-driven shears have the same sign. The subtidal, depth-average cross-shelf momentum balance is a combination of geostrophic balance and a coastal set-up and set-down balance driven by the cross-shelf wind stress. The estimated wave radiation stress gradient is also large. The dominant along-shelf momentum balance is between the wind stress and pressure gradient, but the bottom stress, acceleration, Coriolis, Hasselmann wave stress, and nonlinear advection are not negligible. The fluctuating along-shelf pressure gradient is a local sea level response to wind forcing, not a remotely generated pressure gradient. In summer, the water is persistently cooled due to a mean upwelling circulation. The cross-shelf heat flux nearly balances the strong surface heating throughout midsummer, so the water temperature is almost constant. The along-shelf heat flux divergence is apparently small. In winter, the change in water temperature is closer to that expected due to the surface cooling. Heat transport due to surface gravity waves is substantial.

Description: My last three years of thesis work were supported by National Aeronautics and Space Administration Headquarters under the Earth System Science Fellowship Grant NNG04GQ14H, and by WHOI Academic Programs Fellowship Funds. I also benefited from the freedom of a Clare Boothe Luce Fellowship during my first year in the Joint Program, which allowed me more time than is usual to explore different research topics before choosing an advisor. This research was also funded by the National Aeronautics and Space Administration under grant NNG04GL03G and the Ocean Sciences Division of the National Science Foundation under grants OCE-0241292 and OCE-0548961. The Martha's Vineyard Coastal Observatory is partly funded by the Woods Hole Oceanographic Institution and the Jewett/EDUC/Harrison Foundation. The ADCP deployments at CBLAST site F were funded by National Science Foundation Small Grant for Exploratory Research OCE-0337892. Ship time for deployment and recovery of the F ADCP was provided by Robert Weller through Office of Naval Research contracts N00014-01-1-0029 and N00014-05-10090 for the Low-Wind Component of the Coupled Boundary Layers Air-Sea Transfer Experiment.

Description: Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution June 2003

Description: Inertial terms dominate the single-gyre ocean model and prevent western-intensification when the viscosity is small. This occurs long before the oceanically-appropriate parameter range. It is demonstrated here that the circulation is controlled if a mechanism for ultimate removal of vorticity exists, even if it is active only in a narrow region near the boundary. Vorticity removal is modeled here as a viscosity enhanced very near the solid boundaries to roughly parameterize missing boundary physics like topographic interaction and three dimensional turbulence over the shelf. This boundary-enhanced viscosity allows western-intensified mean flows even when the inertial boundary width, is much wider than the frictional region because eddies flux vorticity from within the interior streamlines to the frictional region for removal. Using boundary-enhanced viscosity, western-intensified calculations are possible with lower interior viscosity than in previous studies. Interesting behaviors result: a boundary-layer balance novel to the model, calculations with promise for eddy parameterization, eddy-driven gyres rotating opposite the wind, and temporal complexity including basin resonances. I also demonstrate that multiple-gyre calculations have weaker mean circulation than single-gyres with the same viscosity and subtropical forcing. Despite traditional understanding, almost no inter-gyre flux occurs if no-slip boundary conditions are used. The inter-gyre eddy flux is in control only with exactly symmetric gyres and free slip boundaries. Even without the inter-gyre flux, the multiple-gyre circulation is weak because of sinuous instabilities on the jet which are not present in the single-gyre model. These modes efficiently flux vorticity to the boundary and reduce the circulation without an inter-gyre flux, postponing inertial domination to much smaller viscosities. Then sinuous modes in combination with boundary-enhanced viscosity can control the circulation.

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

Description: Observations and inverse models suggest that small-scale turbulent mixing is enhanced in the Southern Ocean in regions above rough topography. The enhancement extends 1 km above the topography suggesting that mixing is supported by breaking of gravity waves radiated from the ocean bottom. In other regions, gravity wave radiation by bottom topography has been primarily associated with the barotropic tide. In this study, we explore the alternative hypothesis that the enhanced mixing in the Southern Ocean is sustained by internal waves generated by geostrophic motions flowing over bottom topography. Weakly-nonlinear theory is used to describe the internal wave generation and the feedback of the waves on the zonally averaged flow. A major finding is that the waves generated at the ocean bottom at finite inverse Froude numbers drive vigorous inertial oscillations. The wave radiation and dissipation at equilibrium is therefore the result of both geostrophic flow and inertial oscillations and differs substantially from the classical lee wave problem. The theoretical predictions are tested versus two-dimensional and three-dimensional high resolution numerical simulations with parameters representative of the Drake Passage region. Theory and fully nonlinear numerical simulations are used to estimate internal wave radiation from LADCP, CTD and topography data from two regions in the Southern Ocean: Drake Passage and the Southeast Pacific. The results show that radiation and dissipation of internal waves generated by geostrophic motions reproduce the magnitude and distribution of dissipation measured in the region.

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 2001

Description: A comparison of monthly biogeochemical measurements made from 1993 to 1995, combined with hydrography and satellite altimetry, was used to observe the impacts of nine eddy events on primary productivity and particle flux in the Sargasso Sea. Measurements of primary production, thorium-234 flux, nitrate+nitrite, and photosynthetic pigments made at the US JGOFS Bermuda Atlantic Time-series Study (BATS) site were used. During the three years of this study, four out of six high thorium- 234 flux events over 1000 dpm/m2/d occurred during the passage of an eddy. Primary production nearly as high as the spring bloom maximum was observed in two modewater eddies (May 1993 and July 1995). The 1994 spring bloom at BATS was suppressed by the passage of an anticyclone. Distinct phytoplankton community shifts were observed in mode-water eddies, which had an increased percentage diatoms and dinoflagelletes, and in cyclones, which had an increased percentage cyanobacteria (excluding Prochlorococcus). The difference in the observations of mode-water eddies and cyclones may result from the age of the eddy, which was very important to the biological response. In general, eddies that were one to two months old elicited a large biological response; eddies that were three months old may show a biological response and were accompanied by high thorium flux measurements; eddies that were four months old or older did not show a biological response or high thorium flux. Our conceptual model depicting the importance of temporal changes during eddy upwelling and decay fit the observations well in all 7 upwelling eddies. Additional information is needed to determine the importance of deeper mixed layers and winter mixing to the magnitude of the eddy impacts. Also, sampling generally captured only the beginning, end, and lor edge of an eddy due to the monthly to semi-monthly frequency of the measurements made at BATS. Lagrangian studies, higher resolution time-series, and/or more spatial coverage is needed to provide additional information for improved C and N budgets in the Sargasso Sea and to complete our understanding of the temporal changes that occur in an eddy.

Description: Funding for this work was provided by NASA and NSF through the JGOFS Synthesis and Modeling Program.

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

Description: Interaction between the Antarctic Circumpolar Current and the continental slope/shelf in the Marguerite Bay and west Antarctic Peninsula is examined as interaction between a wind-driven channel flow and a zonally uniform slope with a bay-shaped shelf to the south. Two control mechanisms, eddy advection and propagation of topographic waves, are identified in barotropic vortex-escarpment interactions. The two mechanisms advect the potential vorticity (PV) perturbations in opposite directions in anticyclone-induced interactions but in the same direction in cyclone-induced interactions, resulting in dramatic differences in the two kinds of interactions. The topographic waves become more nonlinear near the western(eastern if in the Northern Hemisphere) boundary of the bay, where strong cross-escarpment motion occurs. In the interaction between a surface anticyclone and a slope penetrating into the upper layer in a two-layer isopycnal model, the eddy advection decays on length scales on the order of the internal deformation radius, so shoreward over a slope that is wider than the deformation radius, the wave mechanism becomes noticeably significant. It acts to spread the cross-isobath transport in a much wider range while the transport directly driven by the anticyclone is concentrated in space. A two-layer wind-driven channel flow is constructed to the north of the slope in the Southern Hemisphere, spontaneously generating eddies through baroclinic instability. A PV front forms in the first layer shoreward of the base of the topography due to the lower-layer eddy-slope interactions. Perturbed by the jet in the center of the channel, the front interacts with the slope/shelf persistently yet episodically, driving a clockwise mean circulation within the bay as well as crossisobath transport. Both the transports across the slope edge and out of the bay are comparable with the maximum Ekman transport in the channel, indicative of the significance of the examined mechanism. The wave-boundary interaction identified in the barotropic model is found essential for the out-of-bay transport and responsible for the heterogeneity of the transport within the bay. Much more water is transported out of the bay from the west than from the east, and the southeastern area is the most isolated region. These results suggest that strong out-of-bay transport may be found near the western boundary of the Marguerite Bay while the southeastern region is a retention area where high population of Antarctic krill may be found.

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 Science at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution August 1987

Description: Several problems connected by the theme of thermal forcing are addressed herein. The main topic is the stratification and flow field resulting from imposing a specified heat flux on a fluid that is otherwise confined to a rigid insulating basin. In addition to the traditional eddy viscosity and diffusivity, turbulent processes are also included by a convective overturning adjustment at locations where the local density field is unstable. Two classes of problems are treated. The first is the large scale meridional pattern of a fluid in an annulus. The detailed treatment is carried out in two steps. In the beginning (chapter 2) it is assumed that the fluid is very diffusive, hence, to first approximation no flow field is present. It is found that the convective overturning adjustment changes the character of the stratification in all the regions that are cooled from the top, resulting in a temperature field that is nearly depth independent in the northernmost latitudes. The response to a seasonal cycle in the forcing, and the differences between averaging the results from the end of each season compared to driving the fluid by a mean forcing are analyzed. In particular, the resulting sea surface temperature is warmer in the former procedure. This observation is important in models where the heat flux is sensitive to the gradient of air to sea surface temperatures. The analysis of the problem continues in chapter 5 where the contribution of the flow field is included in the same configuration. The dimensionless parameter controlling the circulation is now the Rayleigh number, which is a measure of the relative importance of gravitational and viscous forces. The effects of the convective overturning adjustment is investigated at different Rayleigh numbers. It is shown that not only is the stratification now always stable, but also that the vigorous vertical mixing reduces the effective Rayleigh number; thereby the flow field is more moderate, the thermocline deepens, and the horizontal surface temperature gradients are weaker. The interior of the fluid is colder compared to cases without convective overturning, and, because the amount of heat in the system is assumed to be fixed, the surface temperature is warmer. The fluid is not only forced by a mean heat flux, or a seasonally varying one, but its behavior under permanent winter and summer conditions is also investigated. A steady state for the experiments where the net heat flux does not vanish is defined as that state where the flow field and temperature structure are not changing with time except for an almost uniform temperature decrease or increase everywhere. It is found that when winter conditions prevail the circulation is very strong, while it is rather weak for continuous summer forcing. In contrast to those results, if a yearly cycle is imposed, the circulation tends to reach a minimum in the winter time and a maximum in the summer. This suggests that, depending on the Rayleigh number, there is a phase leg of several months between the response of the ocean and the imposed forcing. Differences between the two averaging procedures mentioned before are also observed when the flow field is present, especially for large Rayleigh numbers. The circulation is found to be weaker and the sea surface temperature colder in the mean of the seasonal realizations compared to the steady state derived by the mean forcing. As an extension to the numerical results, an analytic model is presented in chapter 4 for a similar annular configuration. The assumed dynamics is a bit different, with a mixed layer on top of a potential vorticity conserving interior. It is demonstrated that the addition of the thermal wind balance to the conservation of potential vorticity in the axially symmetric problem leads to the result that typical fluid trajectories in the interior are straight lines pointing downward going north to south. The passage of information in the system is surprisingly in the opposite sense to the clockwise direction of the flow. A model for water mass formation by buoyancy loss in the absence of a flow field is introduced in chapter 3. The idea behind it is to use the turbulent mixing parameterization to generate chimney-like structures in open water, followed by along-isopycnal advection and diffusion. This model can be applied to many observations of mode water. In particular, in this work it is related to the chimneys observed by the MEDOC Group (1970), and the Levantine Intermediate Water in the Eastern Mediterranean Basin. An analytic prediction of the depth of the water mass is derived and depends on the forcing and initial stratification. It suggests that the depth of shallow mode water like the 18°C water or the Levantine Intermediate Water would not be very sensitive to reasonable changes in atmospheric forcing. Similar conclusions were also reached by Warren (1972) by assuming that the temperature in the thermocline decreases linearly with depth, and by approximating the energy balance in a water column by a Newtonian cooling law.

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

Description: The thesis develops and demonstrates methods of classifying ocean processes using an underwater moving platform such as an Autonomous Underwater Vehicle (AUV). The "mingled spectrum principle" is established which concisely relates observations from a moving platform to the frequency-wavenumber spectrum of the ocean process. It clearly reveals the role of the AUV speed in mingling temporal and spatial information. For classifying different processes, an AUV is not only able to jointly utilize the time-space information, but also at a tunable proportion by adjusting its cruise speed. In this respect, AUVs are advantageous compared with traditional oceanographic platforms. Based on the mingled spectrum principle, a parametric tool for designing an AUVbased spectral classifier is developed. An AUV's controllable speed tunes the separability between the mingled spectra of different processes. This property is the key to optimizing the classifier's performance. As a case study, AUV-based classification is applied to distinguish ocean convection from internal waves. The mingled spectrum templates are derived from the MIT Ocean Convection Model and the Garrett-Munk internal wave spectrum model. To allow for mismatch between modeled templates and real measurements, the AUVbased classifier is designed to be robust to parameter uncertainties. By simulation tests on the classifier, it is demonstrated that at a higher AUV speed, convection's distinct spatial feature is highlighted to the advantage of classification. Experimental data are used to test the AUV-based classifier. An AUV-borne flow measurement system is designed and built, using an Acoustic Doppler Velocimeter (ADV). The system is calibrated in a high-precision tow tank. In February 1998, the AUV acquired field data of flow velocity in the Labrador Sea Convection Experiment. The Earth-referenced vertical flow velocity is extracted from the raw measurements. The classification test result detects convection's occurrence, a finding supported by more traditional oceanographic analyses and observations. The thesis work provides an important foundation for future work in autonomous detection and sampling of oceanographic processes.

Description: This thesis research has been funded by the Office of Naval Research (ONR) under Grants NOOOl4-95-1-1316, NOO0l4-97-1-0470, and by the MIT Sea Grant College Program under Grant NA46RG0434.

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

Description: A new, global inversion is used to estimate the large scale oceanic circulation based on the World Ocean Circulation Experiment and Java Australia Dynamic Experiment hydrographic data. A linear inverse "box" model is used to combine consistently the transoceanic sections. The circulation is geostrophic with an Ekman layer at the surface and oceanic layers defined by neutral surfaces. Near-conservation of mass, salt and top-to-bottom silica is required and, in addition, heat and the phosphate-oxygen combination (170[P04]+[02]) are conserved in layers that are not in contact with the surface. A globally-consistent solution is obtained for a depth-independent adjustment to the thermal wind field, freshwater flux divergenees, the Ekman transport, and the advective and diffusive dianeutral fluxes between layers. A detailed error budget permits calculation of statistical uncertainties, taking into account both the non-resolved part of the solution and the systematic errors due to the temporal oceanic variability. The estimated water mass transports during the WOCE period (1985-1996) are generally similar to previous published estimates. However, important differences are found. In particular, the inflow of bottom waters into the Pacific Ocean is smaller than in most previous estimates. Utilization of property anomaly conservation constraints allows the estimation of significant dianeutral diffusivities in deep layers, with a global average of 3 ± lcm2s- 1 north of 30°S. Dianeutral transfers indicate that about 20 Sv of bottom water is formed in the Southern Ocean. Significant ocean-atmosphere heat fluxes are found, with a global heating of 2.3 ± 0.4PW in the tropical band and a corresponding cooling at high latitudes. The signature of a large-scale average export production is found for nutrients in several temperate regions. Despite the large uncertainties, the production magnitudes are consistent with independent measurements from sediment traps and isotopic data. Net nutrient sources or sinks are found in several regions, suggesting either transport of dissolved organic matter or a seasonal alias. Oxygen indicates large exchanges with the atmosphere, with intake at high latitudes and outgassing/remineralization at low latitudes.

Description: This work was supported in part by the Jet Propulsion Laboratory/CALTECH (contract #958125), and by gifts from Ford, General Motors, and Daimler-Chrysler to MIT's Climate Modelling Initiative.

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 1988

Description: Inverse methods are applied to historical hydrographic data to address two aspects of the general circulation of the Atlantic Ocean. The method allows conservation statements for mass and other properties, along with a variety of other constraints, to be combined in a dynamically consistent way to estimate the absolute velocity field and associated property transports. The method is first used to examine the exchange of mass and heat between the South Atlantic and the neighboring ocean basins. The Antarctic Circumpolar Current (ACC) carries a surplus of intermediate water into the South Atlantic through Drake Passage which is compensated by a surplus of deep and bottom water leaving the basin south of Africa. As a result, the ACC loses .25±.18x1015 W of heat in crossing the Atlantic. At 32°S the meridional flux of heat is .25±.19x1015 W equatorward, consistent in sign but smaller in magnitude than other recent estimates. This heat flux is carried primarily by a meridional overturning cell in which the export of 17 Sv of North Atlantic Deep Water (NADW) is balanced by an equatorward return flow equally split between the surface layers, and the intermediate and bottom water. No "leak" of warm Indian Ocean thermocline water is necessary to account for the equatorward heat flux across 32°S; in fact, a large transfer of warm water from the Indian Ocean to the Atlantic is found to be inconsistent with the present data set. Together these results demonstrate that the Atlantic as a whole acts to convert intermediate water to deep and bottom water, and thus that the global thermohaline cell associated with the formation and export of NADW is closed primarily by a "cold water path," in which deep water leaving the Atlantic ultimately returns as intermediate water entering the basin through Drake Passage. The second problem addressed concerns the circulation and property fluxes across 24°and 36°N in the subtropical North Atlantic. Conservation statements are considered for the nutrients as well as mass, and the nutrients are found to contribute significant information independent of temperature and salinity. Silicate is particularly effective in reducing the indeterminacy of circulation estimates based on mass conservation alone. In turn, the results demonstrate that accurate estimates of the chemical fluxes depend on relatively detailed knowledge of the circulation. The zonal-integral of the circulation consists of an overturning cell at both latitudes, with a net export of 19 Sv of NADW. This cell results in a poleward heat flux of 1.3±.2x1015 Wand an equatorward oxygen flux of 2900±180 kmol S-l across each latitude. The net flux of silicate is also equatorward: 138±38 kmol s-1 and 152±56 kmol s -1 across 36°and 24° N, respectively. However, in contrast to heat and oxygen, the overturning cell is not the only important mechanism responsible for the net silicate transport. A horizontal recirculation consisting of northward flow of silica-rich deep water in the eastern basin balanced by southward flow of low silica water in the western basin results in a significant silicate flux to the north. The net equatorward flux is thus smaller than indicated by the overturning cell alone. The net flux of nitrate across 36°N is n9±35 kmol 8- 1 to the north and is indistinguishable from zero at 24°N (-8±39 kmol 8-1 ), leading to a net divergence of nitrate between these two latitudes. Forcing the system to conserve nitrate leads to an unreasonable circulation. The dominant contribution to the nitrate flux at 36°N results from the correlation of strong northward flow and relatively high nitrate concentrations in the sub-surface waters of the Gulf Stream. The observed nitrate divergence between 24°and 36°N, and convergence north of 36°N, can be accounted for by a shallow cell in which the northward flow of inorganic nitrogen (nitrate) in the Gulf Stream is balanced by a southward flux of dissolved organic nitrogen in the recirculation gyre. Oxidation of the dissolved organic matter during its transit of the subtropical gyre supplies the required source of regenerated nitrate to the Gulf Stream and consumes oxygen, consistent with recent observations of oxygen utilization in the Sargasso Sea.

Description: This research was supported by NASA under contract NAG5-534 and NSF under contract OCE-8521685.