Mesoscale Eddy Kinetic Energy Budgets and Transfers between Vertical Modes in the Agulhas Current

Abstract

Abstract Western boundary currents are hotspots of mesoscale variability and eddy–topography interactions, which channel energy toward smaller scales and eventually down to dissipation. Here, we assess the main mesoscale eddies energy sinks in the Agulhas Current region from a regional numerical simulation. We derive an eddy kinetic energy ( ) budget in the framework of the vertical modes. It accounts for energy transfers between energy reservoirs and vertical modes, including transfers channeled by topography. The variability is dominated by mesoscale eddies (barotropic and first baroclinic modes) in the path of intense mean currents. Eddy–topography interactions result in a major mesoscale eddy energy sink, along three different energy routes, with comparable importance: transfers toward bottom-intensified time-mean currents, generation of higher baroclinic modes, and bottom friction. The generation of higher baroclinic modes takes different forms in the Northern Agulhas Current, where it corresponds to nonlinear transfers to smaller vertical eddies on the slope, and in the Southern Agulhas Current, where it is dominated by a (linear) generation of internal gravity waves over topography. Away from the shelf, mesoscale eddies gain energy by an inverse vertical turbulent cascade. However, the Agulhas Current region remains a net source of mesoscale eddy energy due to the strong generation of eddies, modulated by the topography, especially in the Southern Agulhas Current. It shows that the local generation of mesoscale eddies dominates the net budget, contrary to the paradigm of mesoscale eddies decay upon western boundaries.