Observations of mixed layer restratification by onshore surface transport following wind reversal in a coastal upwelling region

Abstract

Observations, from the Oregon continental shelf, describe the slumping of a coastal upwelling front in response to a reversal of winds from upwelling‐ to downwelling‐favorable. Initially, the front outcropped in a surface mixed layer of depth 10–20 m with a pronounced cross‐shelf density gradient. Following wind reversal, both the unbalanced cross‐shelf pressure gradient and wind‐driven Ekman transport drove a rapid onshore movement of near‐surface water, causing the mixed layer to restratify. At the surface, the cross‐shelf density gradient steepened to become a discontinuous front, which propagated onshore at 0.43–0.60 m s−1 as the head of a buoyant gravity current. Internal waves were driven ahead of the front on the base of the former mixed layer. An injected dye tracer revealed that surface water from inshore of the strongest frontal gradient detached from the surface as the gravity current passed over the top of it. This water largely retained the low potential vorticity signature that it had taken on in the mixed layer as it spread across the shelf in a weakly stratified, subsurface layer. Restratification and frontogenesis were most likely gravity‐driven in the first hours following wind reversal, with the contribution from Ekman transport becoming increasingly significant.