Nonlinear surface Ekman effects on cyclonic and anticyclonic vortices

Abstract

The transfer of momentum between the atmosphere and oceanic motions affected by the Earth's rotation occurs through the thin surface Ekman layer. The exchange depends on the surface wind stress, which produces the Ekman pumping of fluid to the ocean upper layer. The Ekman pumping mainly depends on: (i) the curl of the wind stress and (ii) the advection of vorticity due to the Ekman transport. The wind stress is usually parametrised as a quadratic function of the relative speed between the wind and the ocean currents, providing a feedback mechanism between the two fluids. Under steady and spatially uniform wind conditions over mesoscale vortices, the first mechanism generates vertical motions that induce the vortex decay (top drag), while the second promotes the horizontal advection of vorticity in the Ekman transport direction. This study examines the nonlinear effects of both mechanisms in cyclonic and anticyclonic vortices. The analyses consist of simple analytical approximations and nonlinear numerical simulations of quasi-two-dimensional vortices. When considering only the top drag mechanism, it is found that anticyclones decay faster than cyclones. By considering only the vorticity-advection effect, the vortices acquire horizontal momentum and drift; furthermore, anticyclones are reinforced while cyclones are weakened. The joint action of both mechanisms and the possible consequences on vertical transport properties are also discussed.