Research papers Spatial variability of flow over a river-influenced inner shelf in coastal Alabama during spring Brian Dzwonkowski a,n , Kyeong Park b , Jungwoo Lee b,1 , Bret M. Webb c , Arnoldo Valle- Levinson d a School of Marine Sciences, University of Maine, Orono, ME 04469, USA b Department of Marine Sciences, University of South Alabama, Dauphin Island Sea Lab, Dauphin Island, AL 36528, USA c Department of Civil Engineering, University of South Alabama, Mobile, AL 36688, USA d Department of Civil and Coastal Engineering, University of Florida, Gainesville, FL 32611, USA article info Article history: Received 8 July 2013 Received in revised form 26 November 2013 Accepted 9 December 2013 Available online 19 December 2013 Keywords: Velocity structure Discharge Current asymmetry Coastal current Alabama Gulf of Mexico abstract Spring-time water column velocity data in 2011 and density data from a series of spring-time hydro- graphic 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. & 2013 Elsevier Ltd. All rights reserved. 1. Introduction Transport and circulation in the coastal zone are affected by spatial variations in the velocity field that are derived from a range of physical processes (e.g. land-induced changes of the wind field, and magnitudes and locations of discharge sources) as well as the physical geometry of the coastal system (e.g. coastline orientation and bathymetric irregularities). Density gradients, in particular, can produce spatial variability in inner shelf currents. For example, Tilburg and Garvine (2003) found the along and across-shelf spatial scales associated with a buoyancy intrusion on the New Jersey shelf to be on the order of 20 km and 3–5 km, respectively. Other examples of the impacts of density gradients on spatial structure of shelf circulation abound and result from a number of processes such as wind-driven across-shelf advection in boundary layers (e.g. Li and Weisberg, 1999a, 1999b; Weisberg et al., 2000; Kirincich et al., 2005), differential heating/cooling of water across a sloping shelf (e.g. He and Weisberg, 2002, Weisberg et al., 2005; Lentz, 2008a), and/or buoyancy discharge from riverine/estuarine sources (e.g. Chao, 1988; Chant et al., 2008). In the last case, pulses of fresher water often form buoyant coastal currents that flow down-shelf (in the direction of Kelvin-wave propagation) adjacent to the coast due to the influence of Coriolis force (Whitney and Garvine, 2005). In regions heavily influenced by riverine and estuarine discharge, such as the northern Gulf of Mexico, these buoyant coastal currents are expected to have a prominent role in controlling the transport of material on the shelf (Wiseman and Garvine, 1995; Walker et al., 2005; Chant et al., 2008; Pringle et al., 2011). While the seasonal effects of buoyant discharge plumes are often concentrated in the surface layer of the water column, near-shore mixing of freshwater can lead to the development of an across-shelf density gradient affecting flow throughout the water column (e.g. Lentz, 2008b). Contents lists available at ScienceDirect journal homepage: www.elsevier.com/locate/csr Continental Shelf Research 0278-4343/$ - see front matter & 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.csr.2013.12.005 n Corresponding author. Tel.: þ1 207 581 4391; fax: þ1 207 581 4990. E-mail address: briandz@maine.edu (B. Dzwonkowski). 1 Present address: Environmental Engineering Research Division, Water Resources & Environmental Research Department, Korea Institute of Construction Technology, Korea. Continental Shelf Research 74 (2014) 25–34