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Influence of an Island on Hysteresis of a Western Boundary Current Flowing across a Gap

We investigate impact of an island on hysteresis of a western boundary current (WBC) flowing across a gap using a nonlinear 1.5-layer ocean model. The results of hysteresis curves show the island in the middle of the gap facilitates the WBC intrusion. The inserted (removed) island in the middle of the gap promotes the WBC to shed eddy (leap across the gap) when the WBC path transits from the periodic penetrating (leaping) to the leaping (periodic penetrating) regime without (with) an island. Vorticity balance analysis reveals that the WBC transition from the eddy-shedding (leaping) to the leaping (eddy-shedding) regime is induced by increased (decreased) meridional advection. Moreover, the critical Reynolds number of the WBC at the Hopf bifurcation is not sensitive to the size and location of the island when the total gap width is fixed. The critical Reynolds number of the WBC translating from the eddy shedding to the leaping regime increases when either the total gap width increases or the island’s meridional length increases; however, the critical Reynolds number is inversely proportional to the width of the southern gap with fixed total gap width and enlarged island length. The island promotes the WBC to shed eddy except when the island is near the northern barrier. The influence of an eastward-shifted island on the WBC transition from the eddy-shedding to the leaping regime is gradually reduced when the island is east of the Munk layer.