The Inner Shelf Response to Wind-Driven Upwelling and Downwelling

Abstract

A two-dimensional numerical model is used to study the response to upwelling- and downwelling-favorable winds on a shelf with a strong pycnocline. During upwelling or downwelling, the pycnocline intersects the surface or bottom, forming a front that moves offshore. The characteristics of the front and of the inner shelf inshore of the front are quite different for upwelling and downwelling. For a constant wind stress the upwelling front moves offshore at roughly a constant rate, while the offshore displacement of the downwelling front scales as t because the thickness of the bottom layer increases as the front moves offshore. The geostrophic alongshelf transport in the front is larger during downwelling than upwelling for the same wind stress magnitude because the geostrophic shear is near the bottom in downwelling as opposed to near the surface in upwelling. During upwelling, weak stratification is maintained over the inner shelf by the onshore flux of denser near-bottom water. This weak stratification suppresses vertical mixing, causing a small reduction in stress at mid depth that drives a weak cross-shelf circulation over the inner shelf. For constant stratification, the inner shelf stratification and cross-shelf circulation are stronger. During downwelling on an initially stratified shelf, the inner shelf becomes unstratified because the very weak cross-shelf circulation forces lighter water under denser, driving convection which enhances the vertical mixing. As a result the stress is nearly constant throughout the water column and the cross-shelf circulation is slightly weaker than in the initially unstratified case. The downwelling response is essentially the same for the constant stratification and the two-layer cases. Model runs including the evolution of a passive tracer indicate that the inner shelf region acts as a barrier to cross-shelf transport of tracers from the coastal boundary to farther offshore and vice versa, due to strong vertical mixing and weak cross-shelf circulation in this region.