The asymmetric Ekman decay of cyclonic and anticyclonic vortices

Abstract

Recent studies have shown differences in the behaviour of cyclonic and anticyclonic quasi-two-dimensional vortices in laboratory experiments in a rotating fluid. In this paper, the role of dissipative effects due to bottom topography is investigated as a possible cause for the asymmetry in the spin-down of both types of vortices. The basic mechanism of Ekman friction in 2D mathematical models is the presence of a linear damping term in the vorticity equation, which produces the flow decay. Here, an extended 2D formulation including nonlinear Ekman corrections is considered. The aim is to show that nonlinear Ekman effects are responsible for the different decay of cyclonic and anticyclonic vortices, while the conventional formulation (only containing the linear friction term) predicts a symmetric decay for both cases. In order to illustrate the role of nonlinear Ekman effects, axisymmetric vortices are simulated numerically. The relatively simple structure of such vortices allows a better understanding of their evolution. The main difference in the spin-down process of cyclones and anticyclones is the decay rate, which is faster for cyclonic motion. Furthermore, it is shown that the basic mechanism for such a difference is the outward advection of fluid in cyclones and inward in anticyclones, both effects due to Ekman pumping and suction, respectively. The results derived here intend to provide a physical interpretation which could be applied for more general, non-axisymmetric structures.