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 December 1985

Description: Samples from time-series sediment traps deployed in three distinct oceanographic settings (North Pacific, Panama Basin, and Black Sea) provide strong evidence for rapid settling of marine particles by aggregates. Particle water column residence times were determined by measuring the time lag between the interception of a flux event in a shallow trap and the interception of the same event in a deeper trap at the same site. Effective sinking speeds were determined by dividing the vertical offset of the traps (meters) by the interception lag time (days). At station Papa in the North Pacific, all particles settle at 175 m day-1, regardless of their composition, indicating that all types of material may be settling in common packages. Evidence from the other two sites (Panama Basin and Black Sea) shows that particle transport may be vertical, lateral, or a combination of directions, with much of the Black Sea flux signal being dominated by lateral input. In order to ascertain whether marine snow aggregates represent viable transport packages, surveys were conducted of the abundance of these aggregates at several stations in the eastern North Atlantic and Panama Basin using a photographic technique. Marine snow aggregates were found in concentrations ranging from ~1 mm3 liter-1 to more than 500 mm3 liter-1. In open ocean environments, abundances are higher near the surface (production) and decline with depth (decomposition). However, in areas near sources of deep input of resuspended material, concentrations reach mid-water maxima, reflecting lateral transport. A model is proposed to relate the observed aggregate abundances, time series sediment flux and inferred circulation. In this model, depthwise variations in sediment flux and aggregate abundance result from suspension from the sea floor and lateral transport of suspended aggregates which were produced or modified on the sea floor. Temporal changes in sediment flux result from variations in the input of fast-sinking material which falls from the surface, intercepts the suspended aggregates, and transports them to the sea floor. A new combination sediment trap and camera system was built and deployed in the Panama Basin with the intent of measuring the flux of marine snow aggregates. This device consists of a cylindrical tube which is open at the top and sealed at the bottom by a clear plate. Material lying on the bottom plate is illuminated by strobe lights mounted in the wall of the cylinder and photographed by a camera which is positioned below the bottom plate. Flux is determined as the number of aggregates arriving during the time interval between photographic frames (# area-1 time-1). Results show that essentially all material arrives in the form of aggregates with minor contributions of fecal pellets and solitary particles. Sinking speeds (m day-1), calculated by dividing the flux of aggregates (# m-2 day-1) by their abundance (# m-3), indicate that the larger (4-5mm) aggregates are flocculent and sink slowly (~1m day-1) while the smaller aggregates (1-2.5mm) are more compact and sink more quickly (~36m day-1). These large, slow-sinking aggregates may have been re-suspended from the sediment water interface at nearby basin margins.

Description: This research was supported by ONR contract numbers N00014-82-C-0019 and N00014-85-C-0001, NSF grant numbers OCE-83-09024, OCE-84-17106, and DPP-85-01152 and the WHO1 education office.

Description: Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution September 1986

Description: A study of the remineralization of organic carbon was conducted in the organic-rich sediments of Buzzards Bay, MA. Major processes affecting the carbon chemistry in sediments are reflected by changes in the stable carbon isotope ratios of dissolved inorganic carbon (ΣCO2) in sediment pore water. Six cores were collected seasonally over a period of two years. The following species were measured in the pore waters: ΣCO2, δ13C-ΣCO2, PO4, ΣH2S, Alk, DOC, and Ca. Measurements of pore water collected seasonally show large gradients with depth, which are larger in summer than in winter. The δ13C (PDB) of ΣCO2 varies from 1.3 o/oo in the bottom water to approximately -10 o/oo at 30 cm. During all seasons, there was a trend towards more negative values with depth in the upper 8 cm due to the remineralization of organic matter. There was a trend toward more positive values below 8 cm, most likely due to biological irrigation of sediments with bottom water. Below 16-20 cm, a negative gradient was re-established which indicates a return to remineralization as the main process affecting pore water chemistry. Using the ΣCO2 depth profile, it was estimated that 67-85 gC/m2 are oxidized annually and 5 gC/m2-yr are buried. The amount of carbon oxidized represented remineralization occurring within the sediments. This estimate indicated that approximately 20% of the annual primary productivity reached the sediments. The calculated remineralization rates varied seasonally with the high of 7.5 x 10-9 mol/L-sec observed in August 84 and the low (0.6 x 10-9) in December 83. The calculated remineralization rates were dependent on the amount of irrigation in the sediments; if the irrigation parameter is known to ±20%, then the remineralization rates are known to this certainty also. The amount of irrigation in the sediments was estimated using the results of a seasonal study of 222Rn/22R6a disequilibria at the same study site (Martin, 1985). Estimates of the annual remineralization in the sediments using solid-phase data indicated that the solid-phase profiles were not at steady-state concentrations. The isotopic signature of ΣCO2 was used as an indicator of the processes affecting ΣCO2 in pore water. During every month, the oxidation of organic carbon to CO2 provided over half of the carbon added to the ΣCO2 pool. However, in every month, the δ13C of ΣCO2 added to the pore water in the surface sediments was greater than -15 o/oo, significantly greater than the δ13C of solid-phase organic carbon in the sediments (-20.6 o/oo). The δ13C of ΣCO2 added to the pore water in the sediments deeper than 7 cm was between -20 and -21 o/oo, similar to the organic carbon in the sediments. Possible explanations of the 13C-enrichment observed in the surface sediments were: a) significant dissolution of CaC0, (δ13C = + 1.7 o/oo), b) the addition of significant amounts of carbonate ion from bottom water to pore water, c) an isotopic difference between the carbon oxidized in the sediments and that remaining in the sediments. The effect of CaCO3 dissolution was quantified using measured dissolved Ca profiles and was not large enough to explain the observed isotopic enrichment. An additional source of 13C-enriched carbon was bottom water carbonate ion. In every month studied, there was a net flux of ΣCO2 from pore water to bottom water. The flux of pore water ΣCO2 to bottom water ranged from a minimum of 10 x 10-12 mol/cm2-sec in December 83 to a maximum of 50 x 10-12 mol/cm2-sec in August 84. However, because the pH of bottom water was about 8 while that of the pore water was less than or equal to 7, the relative proportion of the different species of inorganic carbon (H2CO*3, HCO-3, CO2-3 was very different in bottom water and pore water. Thus, while there was a net flux of ΣCO2 from pore water to bottom water, there was a flux of carbonate ion from bottom water to pore water. Because bottom water ΣCO2 was more 13C-enriched than pore water ΣCO2, the transfer of bottom water carbonate ion to pore water was a source of 13C-enriched carbon to the pore water. If the δ13C of CO2 added to the pore water from the oxidation of organic carbon was -20.6 o/oo, then the flux of Co2-3 from bottom water to pore water must have been 10-30% of the total flux of ΣCO2 from pore water to bottom water. This is consistent with the amount calculated from the observed gradient in carbonate ion. Laboratory experiments were conducted to determine whether the δ13C of CO2 produced from the oxidation of organic carbon (δ13C-OCox) was different from the δ13C of organic carbon in the sediments (δ13C-SOC). In the laboratory experiments, mud from the sampling site was incubated at a constant temperature. Three depths were studied (0-3, 10-15, and 20-25 cm). For the first study (IE1), sediment was stirred to homogenize it before packing into centrifuge tubes for incubation. For the second study (IE2), sediment was introduced directly into glass incubation tubes by subcoring. The second procedure greatly reduced disturbance to the sediment. Rates of CO2 production were calculated from the concentrations of ΣCO2 measured over up to 46 days. In both studies, the values of Rc in the deeper intervals were about 10% of the surface values. This was consistent with the field results, although the rates decreased more rapidly in the field. In all cases, the remineralization rates during the beginning of IE1 were much greater than those at the beginning of IE2. The sediment for IE1 was collected in February 84. The measured value of Rc in the surface sediment of the laboratory experiment (24 x 10-9 mol/L-sec) was much greater than the value of Rc observed in the field in another winter month, December 83 (.62 x 10-9). The sediment for IE2 was collected in August 85. The measured values of Rc in the surface sediment (6.6-12 x 10-9 mol/L-sec) were consistent with the field values from August 84 (7.5 x 10-9). The ΣCO2 results indicated that IE2 reproduced field conditions more accurately than IE1 did. The isotopic results from the experiments strongly suggested that δ13C-OCox in the surface sediments (-17.8 o/oo ± 1.9 o/oo) was greater than δ13C-SOC (-20.6 ± 0.2 o/oo). The magnitude of the observed fractionation was small enough that the observed values of δ13C-ΣCO2 in the pore waters could be explained by fractionated oxidation coupled with the diffusion of carbonate ion from bottom water to pore water. The observed fractionation was most likely due to the multiple sources of organic carbon to coastal sediments. A study of the natural levels of radiocarbon In these sediments indicated that the carbon preserved in the sediments is approximately 30% terrestrial while the rest is from phytoplankton.

Description: Financial support was provided by the Education Office of the Massachusetts Institute of Technology/Woods Hole Oceanographic Institution Joint Program In Oceanography, by an Andrew W. Mellon Foundation grant to the Coastal Research Center, WHOI, and by the National Science foundation under grant NSF OCE83-15412.

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 1975

Description: The influence of natural short-term fluctuations in environmental parameters on three components of transient benthic invertebrate community structure: abundance of individuals and species, biomass of individuals, and species diversity, were investigated in this study. The effect of low dissolved-oxygen on transient benthic community structure was studied with samples from Golfo Dulce, an intermittently anoxic basin off the west coast of Costa Rica and the Posa de Cariaco, an anoxoic trench off the north coast of Venezuela. Periodic fluctuations in oxygen concentration were accompanied by a community numerically dominated by a single polychaete species and low species diversity. As the frequency of fluctuations in oxygen concentration decreased, the number of species and individuals in the community increased with a corresponding increase in species diversity. In contrast to fluctuating oxygen conditions which eliminated many species from the community, fluctuating amounts of suspended matter in the bottom water allowed one species to proliferate while maintaining the total species list length. High rates of terrigenious sedimentation occurring naturally off the Spanish Sahara coast produced conditions which apparently hampered the feeding mechanisms of a spionid polychaete. Further offshore, where the diversity should be expected to increase, the spionids were able to flourish. The result was greater numerical abundance and biomass offshore and a lower transient diversity value. Results of simulation of catastophic burial by in situ burial of small isolated portions of Buzzards Bay sediment indicated that sedimentation rates recorded off Spanish Sahara would not eliminate species by burial. However, the small size of the organisms found off Spanish Sahara is probably a result of the constant expenditure of energy for escape. In regions of fluctuating environmental conditions, diversity values are low, principally because of dominance by a single species. Increasingly stable conditions, even though stressful, result in a more even distribution of individuals among the species present and a correspondingly high transient value.

Description: This work was funded by a Woods Hole Oceanographic Institution Predoctoral Fellowship, N.S.F. Grant GA-3655l, and N.S.F. Grant GA-33502.

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 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 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.

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 1998

Description: A freshwater plume often forms when a river or an estuary discharges water onto the continental shelf. Freshwater plumes are ubiquitous features of the coastal ocean and usually leave a striking signature in the coastal hydrography. The present study combines both hydrographic data and idealized numerical simulations to examine how ambient currents and winds influence the transport and mixing of plume waters. The first portion of the thesis considers the alongshore transport of freshwater using idealized numerical simulations. In the absence of any ambient current, the downstream coastal current only carries a fraction of the discharged fresh water; the remaining fraction recirculates in a continually growing "bulge" of fresh water in the vicinity of the river mouth. The fraction of fresh water transported in the coastal current is dependent on the source conditions at the river mouth. The presence of an ambient current augments the transport in the plume so that its freshwater transport matches the freshwater source. For any ambient current in the same direction as the geostrophic coastal current, the plume will evolve to a steady-state width. A key result is that an external forcing agent is required in order for the entire freshwater volume discharged by a river to be transported as a coastal current. The next section of the thesis addresses the wind-induced advection of a river plume, using hydrographic data collected in the western Gulf of Maine. The observations suggest that the plume's cross-shore structure varies markedly as a function of fluctuations in alongshore wind forcing. Consistent with Ekman dynamics, upwelling favorable winds spread the plume offshore, at times widening it to over 50 km in offshore extent, while downwelling favorable winds narrow the plume width to a few Rossby radii. Near-surface current meters show significant correlations between cross-shore currents and alongshore wind stress, consistent with Ekman theory. Estimates of the terms in the alongshore momentum equation calculated from moored current meter arrays also indicate an approximate Ekman balance within the plume. A significant correlation between alongshore currents and alongshore wind stress suggests that interfacial drag may be important. The final section of the thesis is an investigation of the advection and mixing of a surface-trapped river plume in the presence of an upwelling favorable wind stress, using a three-dimensional model in a simple, rectangular domain. Model simulations demonstrate that the plume thins and is advected offshore by the crossshore Ekman transport. The thinned plume is susceptible to significant mixing due to the vertically sheared horizontal currents. The first order plume response is explained by Ekman dynamics and a Richardson number mixing criterion. Under a sustained wind event, the plume evolves to a quasi-steady, uniform thickness. The rate of mixing slowly decreases for longer times as the stratification in the plume weakens, but mixing persists under a sustained upwelling wind until the plume is destroyed. Mixing is most intense at the seaward plume front due to an Ekman straining mechanism in which the advection of cross-shore salinity gradients balances vertical mixing. The mean mixing rate observed in the plume is consistent with the mixing power law suggested by previous studies of I-D mixing, in spite of the two-dimensional dynamics driving the mixing in the plume.

Description: This research was funded by a National Science Foundation graduate fellowship, and Gulf of Maine Regional Marine Research Program grants UM-S227 and UM-S276.

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 2011

Description: Eastern oceanic boundary currents are subject to hydrodynamic instability, generate small scale features that are visible in satellite images and may radiate westward into the interior, where they can be modified by the large-scale circulations. This thesis studies the stability of an eastern boundary current with and without the large-scale flow influence in an idealized framework represented by barotropic quasi-geostrophic dynamics. The linear stability analysis of a meridional current with a continuous velocity profile shows that meridional eastern and western boundary currents support a limited number of radiating modes with long meridional and zonal wavelengths and small growth rates. However, the linearly stable, long radiating modes of an eastern boundary current can become nonlinearly unstable by resonating with short trapped unstable modes. This phenomenon is clearly demonstrated in the weakly nonlinear simulations. Results suggest that linearly stable longwave modes deserve more attention when the radiating instability of a meridional boundary current is considered. A large-scale flow affects the short trapped unstable mode and long radiating mode through different mechanisms. The large-scale flow modifies the structure of the boundary current to stabilize or destabilize the unstable modes, leading to a meridionally localized maximum in the perturbation kinetic energy field. The shortwave mode is accelerated or decelerated by the meridional velocity adjustment of the large-scale flow to have an elongated or a squeezed meridional structure, which is confirmed both in a linear WKB analysis and in nonlinear simulations. The squeezed or elongated unstable mode detunes the nonlinear resonance with the longwave modes, which then become less energetic. These two modes show different meridional structures in kinetic energy field because of the different mechanisms. In spite of the model simplicity, these results can potentially explain the formation of the zonal jets observed in altimeter data, and indicate the influence of the large-scale wind-driven circulation on eastern boundary upwelling systems in the real ocean. Studies with more realistic configurations remain future challenges.