Parametrizing ocean eddy transfer over continental slopes

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Continental slopes support some of the most influential currents in the world ocean circulation, such as the strong poleward western boundary currents like the Gulf Stream found in the subtropics. However, the circulation across continental slopes is arguably of comparable importance, as they separate shallow coastal waters from the water masses of the deep ocean. Water mass exchanges across continental slopes facilitate upwelling of nutrients to support coastal marine ecosystems, transport heat towards marine-terminating glaciers around Greenland and Antarctica, and control the outflow of dense water masses such as Mediterranean Water and Antarctic Bottom Water. Recent studies point to the importance of mesoscale eddies in modulating both the structure and transport of coastal currents and the cross-slope transfer of water masses and properties. Predictive ocean and climate models are currently unable to resolve these eddies, and are unlikely to routinely resolve the mesoscale globally for at least a couple of decades. Yet despite major recent advances in parametrization strategies for eddies over a flat ocean bed, there have been no concerted attempts to develop analogous strategies for continental slopes. This project aims to close the current gap in understanding mesoscale turbulence over steep slopes, and thereby to develop a parametrization of eddy mixing and transport over continental slopes. This project will provide a graduate student researcher with three years of training in numerical and analytical modeling as well as working knowledge of observational datasets. It also supports an early-career faculty member. A key objective of this project is to produce a set of recommendations for the treatment of mesoscale eddy transport over continental slopes, which will be disseminated widely among the oceanographic and climate communities. Adoption of these practices could substantially improve the representation of shelf and slope processes in large-scale ocean and climate models. The project's model output will be made available to the wider oceanographic community for the purpose of testing and validation. The principal investigator will also participate in outreach activities via existing frameworks supported by his institution.A central goal of this project is to explore the fundamental properties of equilibrated turbulence over continental slopes across a range of forcing conditions, ocean stratifications and slope geometries. The work will characterize the slope?s impact on the eddy life cycle and generation mechanisms, cross- and along-slope eddy transfer of mass and tracers, the routes to dissipation of surface-input momentum and energy, sources and generation of eddy kinetic energy in slope currents, and constraints on the inverse energy cascade and jet formation. In strongly baroclinic coastal currents there is an abundance of potential energy available for the generation of mesoscale eddies, but the availability of energy may be offset by the steep topography, which tends to suppress energy conversion to mesoscale eddy motions. The relative roles of these compensating effects in setting rates of eddy transfer and mixing remains poorly understood. The principal investigator and a graduate student researcher will develop an eddy-resolving process model and create a suite of continental shelf and slope test cases, the output of which will be made available to the wider oceanographic community. This suite of simulations will be used to characterize key properties of slope turbulence, such as the budgets of energy and momentum, and the eddy life cycle. Guided by these findings, the research team will explore several promising theoretical approaches toward the construction of a parametrization of eddy transfer over steep topography. The parametrization will be tested and validated against the suite of eddy-resolving simulations, and against existing databases of satellite-derived sea-surface height data and coastal float release data. The findings of this work will be used to create a set of recommendations and best practices for parametrizing mesoscale eddies over continental slopes, which will be disseminated widely to ocean and climate modelers.