Role of Frictional Processes in Mesoscale Eddy Available Potential Energy Budget in the Global Ocean

Mesoscale eddies in the global ocean are thought to be powered primarily through transfer from available potential energy stored in mean flows to eddy available potential energy (EAPE). Part of the EAPE is further converted to eddy kinetic energy (EKE) with the residue dissipated through diabatic processes. Currently, the effect of frictional processes on the EAPE budget has not been thoroughly analyzed. Using an eddy-resolving (1/10 degrees) global climate simulation, we demonstrate that frictionally driven mesoscale eddy flow plays an important role in the EAPE budget, accounting for more than 30% of the EAPE-to-EKE conversion in the global ocean and dominating its seasonal cycle. This role can be understood based on the turbulent thermal wind balance under which the frictional force induces an ageostrophic secondary circulation producing a prominent EAPE-to-EKE conversion in the winter surface boundary layer.