206 / 2024-09-11 14:09:13

Rhythms of the Agulhas Current within the Framework of Eddy Energetic Anisotropy

meander,acceleration,mean flow strain,eddy anisotropy

The Agulhas Current (AC) is the strongest western boundary current in the Southern Hemisphere. The variabilities in its strength and meandering behavior influence the local marine environment and interbasin exchange. 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 southern 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, the merging of cyclones with the AC plays a role in the generation of meanders. This study first demonstrates that upstream cyclones reduce the horizontal potential vorticity gradient, facilitating eddies to traverse the AC. The current envelops these cyclones and flows in pulse-like meandering patterns. The places where these meanders form are not exclusive to the Natal Bight.

Meandering also occurs locally due to the barotropic instability of the AC, indicating a downscaled conversion of kinetic energy. We further diagnose the kinetic energy conversion within the framework of energetic anisotropy, 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, 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.