Impact of river discharge on the hydrography and circulation on the inner shelf of coastal Alabama

River discharge is a key player in the marine system on the Alabama shelf. Opportunistic observations as well as long-term data sets of shelf hydrography, circulation, and ocean color are used to capture the coupled estuarine-shelf interactions of the Alabama coastal region during periods of low and high river discharge. The system response to discharge at the tidal, synoptic, seasonal, and interannual timescales is demonstrated. The major changes generated by individual discharge events from Mobile Bay are shown using approximately two weeks of observations from a surface advected plume. During this time period differences in shelf circulation were directly linked to the discharge plume as a ‘bulge’ region with anticyclonic circulation identified at times throughout the event. The influence of wind forcing, even during very low wind (<3.75 m s^-1) and large outflow (~7,000 m³ s^-1) conditions, was apparent, as a result of the shallow and wide characteristics of the plume. At seasonal timescales the cumulative effect of discharge events was primarily observed west of Mobile Bay as a thin surface layer consistent with the event-based observations. The seasonal expression of discharge appears to be primarily limited to the spring/summer and limited data on interannual patterns suggest the seasonal signal is negligible during a spring season with very low discharge. Consequently, the river discharge plays a key role in the hydrography and circulation at a range of time scales and as a result has significant implications on the transport of materials in the coastal zone.