Interaction of a Hysteresis Western Boundary Current with a Large-Scale Marginal Sea Circulation in a Gap-Leaping System

The influence of a large-scale circulation (LSC) in a marginal sea on a hysteresis western boundary current (WBC) flowing across a gap is studied using a nonlinear 1.5-layer ocean model. Results show that both single-gyre LSC and double-gyre LSC are able to induce the critical-state WBC transition from the eddy-shedding regime to the leaping regime, while only double-gyre LSC is able to induce the critical-state WBC transition from the leaping regime to the eddy-shedding regime. The dynamics of WBC transition suggests that the meridional advection enhanced by the perturbation of the LSC is responsible for the regime shift from penetration to leap and that the meridional advection reduced by the perturbation of the LSC is responsible for the regime shift from leap to penetration. We also present the parameter space of the critical LSC that can induce the regime shift of WBC far away from the critical state. When the WBC is in the eddy-shedding regime, the critical strength of the single-gyre LSC increases as the WBC transport decreases regardless of the island’s presence in the gap. The critical strength of the double-gyre LSC increases as the WBC transport decreases in the no-island case, while the critical parameter increases as the WBC transport at first decreases and then increases in the island case. When the WBC is in the leaping regime, the critical strength of the double-gyre LSC increases as the WBC transport increases. These results help to explain the observed fact that the Kuroshio flows across the Luzon Strait in the leaping regime or the penetrating regime.