Upwelling at a Shelf Break Caused by Buoyancy Shutdown


The adjustment of an initially uniform along-shelf flow is examined over a stratified shelf break, where a flat shelf intersects a steep slope. Over the shelf, friction drives an offshore Ekman transport in the bottom boundary layer for a current flowing in the direction of Kelvin wave propagation. Over the slope, downslope Ekman advection of buoyancy tilts the isopycnals and leads to vertical shear in the geostrophic flow. This process reduces the bottom stress and is known as buoyancy shutdown of the Ekman transport. Near the shelf break, convergence in the offshore Ekman transport induces Ekman pumping. Ekman pumping drives a secondary circulation that accelerates the along-shelf flow over the slope and forms a jet near the shelf break. Scalings are identified for the strength of Ekman pumping near the shelf break, the length scale over which it occurs, and the time scale for the flow acceleration over the slope. The scalings are tested in a series of numerical simulations with application to flows near the Middle Atlantic Bight shelf break.