A modeling study of water and salt exchange for a micro-tidal, stratified northern Gulf of Mexico estuary

Abstract

A three-dimensional hydrodynamic model is applied to the Mobile Bay system to study water and salt exchange with the northern Gulf of Mexico via Main Pass (MP) and eastern Mississippi Sound via Pass-aux-Herons (PaH). On average, more water leaves the Bay through MP than through PaH, and the Bay gains salt through MP and loses about the same amount through PaH. However, the volume discharge rate Qf and salt transport rate FS vary greatly in response to wind and river discharge with the range of variation 1–2 orders of magnitude larger than the corresponding mean. Stratification plays a key role for salt transport through MP. During periods of large river discharge, the landward shear dispersive transport FE peaking during equatorial tides and the landward tidal oscillatory transport FT peaking during tropic tides, respectively, balance the seaward advective transport QfS0. During periods of relatively weak stratification, FS at MP is almost entirely determined by QfS0 and its variability is well correlated with north–south (along-estuary) wind, associated with the barotropic (water level) adjustment. At the shallow, weakly stratified PaH, FS is almost identical to QfS0, and Qf is well correlated with east–west wind, with the correlation becoming stronger during the dry period.