Are Alabama Upwellings Broken? A Comparison of Burger Number and Velocity Structure

Upwelling events play a key role in the biological productivity of many areas by bringing colder, nutrient-rich water to the surface. The dynamics of common upwelling regions in eastern boundary current systems have been linked to the Burger number (S = αN/f, where α is the bottom slope, N is the buoyancy frequency, and f is the Coriolis parameter), but this association has not been extensively examined in other coastal systems. Hydrographic and velocity data collected from a long-term mooring on the 20 meter isobath off the shore of Alabama were used to examine upwelling dynamics on a wide, gently sloping continental shelf in a highly stratified system with intermittent wind forcing. Two upwelling events of different duration, short (~6 days) and long (~11 days), from the summer of 2006 were used to characterize structural differences in this system. Despite the two events having similar Burger numbers, their structures were different than expectations in several ways. In both events, the onshore return flow was through the bottom boundary layer, despite relatively high Burger Numbers (S~0.6-0.7). Furthermore, the boundary layer stratification significantly impacted the temporal evolution of the upwelling response during the peak of these events. The more stratified the bottom boundary layer, the more concentrated the region of onshore return flow. These results suggest that additional factors not considered in the Burger number are affecting the upwelling dynamics of this region.