A Study of the Adiabatic Dynamics of Buoyancy-Driven Eastern Boundary Currents in an Ocean with Eddies

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Boundary currents are found in the ocean on the eastern flanks of all the world's major subtropical gyres. These eastern boundary current (EBC) regions are extremely biologically productive relative to their small area. Their subsequent economic impact on coastal countries has made understanding their dynamics a high priority for the past several decades. Most EBC regions are subjected to equatorward winds for at least part of the year. These winds drive water offshore, leading to the upwelling of nutrient-rich water near the coast and the formation of equatorward currents along the upwelling fronts. The equatorward surface currents are often associated with poleward undercurrents whose dynamics are still not entirely clear. Due to its magnitude and ubiquity, wind-driven coastal upwelling has become the dominant paradigm for understanding the dynamics of EBCs. In contrast, poleward boundary currents not subject to longshore wind are also found along eastern boundaries, with the Leeuwin Current off West Australia as the prime. These currents form in response to the large-scale meridional gradient of surface buoyancy, which drives an eastward surface current via thermal wind. This fluid downwells when it impinges on the eastern boundary; the associated isopycnal deflection drives a poleward geostrophic current. Since the meridional surface buoyancy gradient is a global and persistent feature, this buoyancy driven downwelling may compete with wind-driven upwelling along eastern boundaries subjected to upwelling-favorable winds; possibly dominating the dynamics of these systems during seasons without upwelling-favorable winds. There several examples of wind-driven EBC systems that reverse direction and develop poleward surface currents when upwelling-favorable winds are absent. The Davidson Current develops off the Californian coast when equatorward winds relax in fall and winter and a similar poleward current forms off the west coast of India when the winds weaken in winter. This project will contribute to understanding eastern boundary currents, leading to improvements in the numerical and conceptual models used to make decisions about coastal management and those used to forecast events such as dead zones or regional climate change. One of the main foci is to characterize the mechanisms responsible for trapping eastern boundary currents near the coast in the eddying regime. This is a long-standing, but still unresolved, problem in coastal oceanography as the current hypotheses fail to explain the trapping of boundary currents in the absence of strong vertical mixing and coastal topography. In addition, this project may contribute to the understanding of inhomogeneous oceanic turbulence. In addition, the graduate student involved in the project will be trained in oceanography, modern approaches to the numerical modeling of oceanic flows on large-scale computational resources, and analysis of model output.Along their eastern boundaries, oceans can have both wind-driven and buoyancy-driven currents acting in opposite directions. When upwelling-favorable winds blow, the wind-driven equatorward current may simply stack on top of the persistent buoyancy-driven poleward current- this provides a possible mechanism for the formation of coastal undercurrents. This project seeks to quantify the extent to which the buoyancy-driven and wind-driven circulations interact and how the strength of this interaction depends on external factors such as wind, buoyancy forcing, and geometry. First, the dynamics of buoyancy-driven EBCs in isolation must be more fully understood. It is not clear how the poleward currents are trapped near the eastern boundary and what determines their width. Laminar models of buoyancy-driven EBCs require either topographic features or unrealistically large diapycnal diffusivities to trap the boundary currents, but quasi-adiabatic, eddy-resolving models form trapped EBCs even without coastal topography. This indicates that eddy momentum fluxes play an important role in trapping the boundary current. Specifically, the investigators plan to determine the role of eddy momentum fluxes in trapping buoyancy-forced EBCs. An important component of this objective is the development of a representation of the effect of eddy momentum fluxes on the EBC suitable for use in analytical or reduced-complexity numerical models. Second, they will seek to quantify how the width, speed, and transport of the EBC depend on external factors such as the large-scale buoyancy gradient, diapycnal diffusivity, rotation, the β effect, and coastal topography. In addition, the nature of the transient response of the EBC to changes in the large-scale buoyancy gradient on seasonal and interannual timescales will be examined. Finally, the investigators will focus on understanding how the buoyancy-driven circulation interacts with the wind-driven circulation under upwelling-favorable winds and determining whether changes in the large-scale buoyancy gradient mainly influence the undercurrent, or directly affect the strength and structure of the equatorward upwelling jet.