Asymmetry in the Zonal Drift of Mesoscale Vortices: From Single Vortex to Vortex Clusters

Abstract For single-vortex evolution on the β plane, an asymmetry in the zonal drift velocity is known to exist in shallow waters, with the anticyclone speeding up and the cyclone slowing down as the vortex amplitude increases. However, there seems no clear evidence for such an asymmetry in the statistics of mesoscale vortices. This problem is studied using a hierarchical modeling approach that considers single-vortex physics, vortex cluster simulations permitting interactions but without forcing, and satellite-based eddy trajectory products. The objective is to trace the asymmetry signals through the hierarchy. To be consistent with satellite observations, a single-vortex, force balance theory is first extended to a vortex-following control volume. A correction for nonnegligible boundary fluxes is proposed. A parametric relationship between the zonal drift velocity and amplitude, serving as an indicator of the asymmetry, is also established and interpreted. We then ask if the amplitude sensitivity in both vortex cluster simulations and satellite observations is consistent with this parametric relationship. The analyses yield a coherent result: When anticyclones and cyclones are examined separately, the individual responses cannot be explained by the single-vortex theory. However, their difference in amplitude sensitivity falls within the range of theoretical estimates. The differences in amplitude-bin-averaged drift velocity also agree between the simulations and observations. The result therefore provides evidence for the existence of the asymmetry in vortex clusters where vortices interact. Possible causes for the unexplained trend in the individual responses are discussed.