9 / 2023-07-22 15:55:09

Dynamics of the gap-leaping WBC: the Kuroshio and Agulhas Current

Western boundary current; Agulhas Current; Kuroshio; Gap-leaping dynamics

The dynamics of a gap-leaping western boundary current (WBC) has gained increasing attention due to its important role in the water exchange between the open ocean and marginal sea, and inter-ocean basins, resembling the Kuroshio and the Gulf Stream. However, if the downstream WBC of strong nonlinearity flows across the gap, what will happen? In this study, the influence of meridional shift of the oceanic subtropical front (STF) on the Agulhas Current (AC) regime shifts is studied. The satellite observations suggest the northward shift of the STF leads to the AC leap across the gap associated with little Agulhas leakage, and the southward shift of the STF mainly results in the AC intruding into the Atlantic Ocean in the forms of a loop current and an eddy-shedding paths, while there are three flow patterns of AC for the moderate latitude of STF. The ocean model suggests no hysteresis associated with the multiple equilibrium states exists in the AC system, but it reproduces the similar AC regimes depending on different gap widths as in the observations, and explains the observed Agulhas leakage well. We also present the parameter space of the critical AC strength that results in different AC flow patterns as a function of the gap width. The vorticity dynamics of the AC regime shift suggest that the β term is mainly balanced by the viscosity term for the AC in the leaping and loop current patterns, while the β and instantaneous terms are mainly balanced by the advection and viscosity terms for the AC in the eddy shedding pattern. These results help to explain the dynamics of the AC flowing across the gateway beyond the tip of Africa affected by the north-south shift of the STF in the leaping regime or the penetrating regime.