Hysteresis of a Western Boundary Current Leaping across a Gap

Abstract

An idealized problem of a western boundary current of Munk thickness LM flowing across a gap in a ridge is considered using a single-layer depth-averaged approach. When the gap (of width 2a) is narrow, a ? 3.12LM, viscous forces alone restrict penetration of the current through the gap. However, the gap is ?leaky? in the linear case and some very weak flow still passes through. For larger gap width, the boundary current may leap across the gap due to inertia, characterized by the Reynolds number, completely choking off water exchange between the two basins. For a ≥ 4.55LM the flow may be in one of two regimes (penetrating or leaping) for the same parameters, depending on previous evolution. The penetrating branch solutions become unsteady with eddies forming west of the gap between the two counterflowing zonal jets. As the boundary current slowly accelerates, transition from the penetrating to leaping regime happens when the width of a zonal jet near the gap becomes comparable with a, implying the Reynolds number ReP ? (a/LM)3. On the other hand, as the boundary current slowly decelerates, the leaping regime persists while the meridional advection dominates the ? effect in a wiggle of the current core within the gap, implying that the leaping regime breaks at ReL ? a/LM. Thus hysteresis occurs over the range of Reynolds numbers ReL < Re < ReP. This behavior is analogous to the well-known teapot effect.