Spatial variability of flow over a river-influenced inner shelf in coastal Alabama during spring

Abstract

Spring-time water column velocity data in 2011 and density data from a series of spring-time hydrographic surveys from 2008 to 2011 were used to examine the spatial variability of the circulation over the inner shelf of the Mississippi Bight off Mobile Bay. Spring-time depth-averaged currents were eastward at all sites, but the vertical profiles were different. East of Mobile Bay the along-shelf flow was eastward, with an offshore component at the surface and an onshore component at depth, indicative of upwelling circulation. West of Mobile Bay the along-shelf flow was also eastward, with a characteristic region of negative vertical shear in the upper layer of the water column. The deeper site had an across-shelf flow structure similar to the east sites, while the shallower site exhibited onshore flow throughout the water column. These spatial differences are attributed, in part, to the seasonally averaged effects of local wind forcing and discharge. In terms of wind forcing, the depth-averaged along-shelf current responded to along-shelf wind asymmetrically in favor of upwelling (more transport in upwelling than during downwelling). Thus, weak seasonal downwelling favorable wind conditions did not inhibit the velocity profiles from having upwelling circulation. West of Mobile Bay, negative vertical shears in the upper portion of the velocity profiles were consistent with the influence of freshwater discharge. This freshwater influence is supported with available chlorophyll-a data (as a freshwater proxy), which showed an enhanced freshwater influence west of Mobile Bay. In addition, across-shelf density data showed a shallow lens of freshwater west of Mobile Bay. These findings have implications for understanding the transport of river-derived nutrients on the Mississippi–Alabama shelf.