Subtidal circulation patterns in a shallow, highly stratified estuary: Mobile Bay, Alabama

Abstract

Mobile Bay is a wide (25–50 km), shallow (3 m), highly stratified estuary on the Gulf coast of the United States. In May 1991 a series of instruments that measure near‐surface and near‐bed current, temperature, salinity, and middepth pressure were deployed for a year‐long study of the bay. A full set of measurements were obtained at one site in the lower bay; all but current measurements were obtained at a midbay site. These observations show that the subtidal currents in the lower bay are highly sheared, despite the shallow depth of the estuary. The sheared flow patterns are partly caused by differential forcing from wind stress and river discharge. Two wind‐driven flow patterns actually exist in lower Mobile Bay. A barotropic response develops when the difference between near‐surface and near‐bottom salinity is less than 5 parts per thousand. For stronger salinity gradients the wind‐driven currents are larger and the response resembles a baroclinic flow pattern. Currents driven by river flows are sheared and also have a nonlinear response pattern. Only near‐surface currents are driven seaward by discharges below 3000 m3/s. At higher discharge rates, surface current variability uncouples from the river flow and the increased discharge rates drive near‐bed current seaward. This change in the river‐forced flow pattern may be associated with a hydraulic jump in the mouth of the estuary.