Adaptive Process of Bottom-Trapped Buoyant Coastal Current When Encountering a Protruding Coastal Headland

Abstract

Coastal current encountering a protruding headland is a ubiquitous phenomenon. Previous studies indicated that the coastal current either moves well around headland or separates offshore, leaving the upstream region unaffected. Yet, these studies often assumed a deep vertical coastal wall, and the coastal current was either of barotropic character or surface-advected, with weak interactions with the sloping topography. Here in this study, we conducted numerical experiments to investigate how a protruding headland regulates the “bottom-trapped” buoyant coastal current over a sloping coastal topography. It was found that at the initial stage, the coastal current separates at the sharp headland tip due to local increased centrifugal force, forming a secondary bulge on the lee side of the headland. Upstream of the headland, a countercurrent is formed shoreward of the front, which fills the space between front and coast, thus pushing the front offshore. This process persists as long as the cross-shelf scale of headland is larger than the baroclinic Rossby deformation radius. The final effect is that the front adapts its cross-shelf location to minimize the form drag induced by the headland, and consequently the separation on the lee side of the headland was reduced. Downstream of the headland, the plume front weakens and the alongshore propagation is slowed down, because more freshwater is stranded upstream. Such dynamics are distinct from the surface-advected buoyant coastal current, and may explain the fact that many buoyant coastal currents along zigzag coastline are wide and their alongshore extension distances are limited.