West Florida shelf response to local wind forcing: April 1998

Abstract

We compare west Florida shelf velocity and sea level data with a model simulation for April 1998. Responses for three upwelling and three downwelling favorable wind events are documented. Along-shelf jets accompanied by oppositely directed upper and lower layer across-shelf flows (with connecting vertical velocity) comprise the fully three-dimensional inner shelf responses, which are sensitive to stratification. With an initial density field representative of April 1998 the model simulates velocity and sea level variations in general agreement with the observations, whereas substantial mismatches occur without stratification. Despite the winds being the primary motive agent for the inner shelf the stratification dependence requires that model density fields be maintained through a combination of adequate initial conditions; surface, offshore, and land-derived buoyancy inputs; and data assimilation. Dynamical analyses define the inner shelf as the region where the surface and bottom boundary layers are important in the momentum balance. Kinematically, this is where surface Ekman layer divergence, fed by the bottom Ekman layer convergence, or conversely, sets up the across-shelf pressure gradient. Stratification causes a response asymmetry wherein the offshore scale and magnitude of the upwelling responses are larger than those for the downwelling responses. This asymmetry is attributed to thermal wind effects across the bottom Ekman layer. Buoyancy torque by isopycnals bending into the bottom adds constructively (destructively) with planetary vorticity tilting under upwelling (downwelling) favorable winds, and this may have important implications for nutrients and other material property distributions on the shelf. Copyright 2001 by the American Geophysical Union.