Seasonal Variability of Eddy Kinetic Energy in the Greater Agulhas Current System

Abstract

Based on the novel energetic analysis tools, namely multiscale window transform and multiscale energy and vorticity analysis, this study investigates the seasonal variability of eddy kinetic energy (EKE) in the greater Agulhas Current system (GACS), which is divided into three subdomains based on the horizontal structures of background flows: the Mozambique Channel (MZC) subdomain, the northern Agulhas Current (NAC) subdomain, and the Agulhas retroflection (ARF) subdomain. Results show that the seasonality of EKE is spatially inhomogeneous. In the MZC subdomain, the EKE is strongest in spring and weakest in autumn, whereas in the NAC and ARF subdomains, the EKE level is highest in summer and lowest in winter. In all the three subdomains, the seasonal cycle of the barotropic instability of the mean flow corresponds well with that of the EKE. In contrast, the nonlocal transportation that mainly works on the redistribution of EKE is out of phase with the seasonality of EKE. Regarding the local wind forcing and baroclinic instability, they both have weak impacts on the EKE evolution, contributing power only about 10% of the barotropic instability. Moreover, neither of their seasonal cycles is consistent with the seasonality of EKE. Therefore, it is the barotropic instability of the mean flow that controls the seasonal variability of the EKE in the GACS.