Eddy mixing and transport over continental slopes (Sep 2015-Aug 2019)

Continental slopes support some of the most influential currents in the world ocean circulation, such as the strong poleward western boundary currents 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. 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. The central aim of this project was to develop a parametrization of eddy mixing and transport over continental slopes, i.e. a representation of the effect of eddies over continental slopes that can be implemented in ocean circulation models that would otherwise be unable to simulate those eddies.

(Click here or on the image below to see an animated version.)

A 3D rendering of the potential temperature in our shelf/slope process model. The model is forced by winds directed along the ocean surface with the shallower water to their left, and the potential temperature is continuously restored to a fixed profile at the northern boundary.

Intellectual Merit: To achieve this goal we developed a suite of high-resolution, process-oriented simulations of eddies over idealized continental shelves and slopes (see Figure). These simulations allowed us to probe the behavior of these eddies and their efficiency in transferring heat and material across continental slopes. We first characterized the dynamics of eddies over continental slopes, and performed a careful comparison of the representation of these eddies between different ocean simulation codes. We then demonstrated that existing parameterizations of open-ocean eddies can be modified to capture the behavior of eddies over continental slopes by imposing an additional dependence on the steepness of the sea floor. Our new parameterization is applicable to only half of the various continental shelf/slope current systems found in nature; extension of the parameterization to cover the full range of global continental shelf/slope current systems is underway. To facilitate further development and testing of parameterizations of continental shelf/slope eddies in the oceanographic community, this project additionally supported the development of a new latitudinally-averaged model of subtropical eastern boundary currents, and a new high-resolution regional simulation of the northwestern Atlantic.

Broader Impacts: Our new parameterization is an adaptation of existing parameterizations of open-ocean eddies that are currently implemented in ocean general circulation models, and is therefore ready for implementation and testing. Our parameterization has the potential to substantially improve the representation of oceanic boundary currents and cross-slope exchanges in these models, which would be of direct benefit to the wide range of disciplines that make use of such comprehensive models. This project directly and indirectly supported the training of two graduate students, a postdoctoral researcher, and seven undergraduate students. The suite of model simulations has been made available to the public via an online repository, to facilitate further scientific investigation of continental shelf/slope eddies. Key insights into the behavior of continental shelf/slope eddy behavior have been reported via scientific publications and presentations at national scientific meetings.

A link to the online repository of model simulations may be found here.

This work was supported by the National Science Foundation, grant number OCE-1538702.