Rhythms of the Agulhas Current Within the Framework of Energetic Anisotropy

Abstract

This study investigates the mechanisms driving the acceleration and meandering of the Agulhas Current (AC), focusing on the role of eddy-mean flow interactions. The analysis revealed that anticyclones originating from the Mozambique Channel and south of Madagascar played pivotal roles in accelerating the AC. Simultaneously, when anticyclones collide with the AC, they undergo processes of rotating and elongating into ellipses. In addition to the previously suggested barotropic instability induced by anticyclones, this study revealed that the merging of cyclones with the AC plays a role in the generation of meanders. Upstream cyclones reduce the horizontal potential vorticity gradient, facilitating eddies to traverse the current. The AC envelops these cyclones and flows in meandering patterns. The places where these meanders form are not exclusive to the Natal Bight. In addition, we further diagnose the kinetic energy conversion to reveal the interaction between eddy anisotropy (i.e., eddy deformation and orientation) and mean flow strain (i.e., stretching and shearing). The results suggest that the anisotropy of anticyclonic and cyclonic eddies prompts downscale KE transfer and the growth of meanders, establishing a positive feedback loop. Contrary to the findings of previous hypotheses, the acceleration of AC in turn leads to a decrease in the mean flow strain rate, exerting negative feedback on energy conversion and inhibiting the development of meanders. These two feedback mechanisms work together to determine the fate of AC meandering. The energetic anisotropy diagnosis holds potential applicability to other western boundary current systems.