Water Column Stability and the Role of Velocity Shear on a Seasonally Stratified Shelf, Mississippi Bight, Northern Gulf of Mexico

Water column stability is a critical component of coastal systems, regulating the vertical exchange of momentum, mass, and biogeochemical constituents. Observations of water column velocity and hydrographic conditions during the summer of 2010 were used to investigate aspects of the region of freshwater influence in the Mississippi Bight. Conductivity-temperature-depth surveys south of Mobile Bay indicated strong stratification (peak buoyancy frequencies 0.09-0.3s(-1)) across the inner and midshelf. Despite this strong stratification, mooring data from the 20-m isobath indicated that the bulk Richardson numbers (Ri) can span 3 orders of magnitude, O(1-100). Examination of the high-frequency (hourly) shear and buoyancy frequency time series revealed downwelling conditions, and near-inertial oscillations were primarily responsible for generating instances of relatively low stability (bulk Ri<1). Periods of intense near-inertial oscillations also intermittently drove synthetic gradient Ri to critical or near-critical levels within the pycnocline. While the near-inertial oscillations were dominated by a clockwise rotating two-layer structure, there was also a secondary three-layer structure, both of which interacted with the low-frequency upwelling to produce highly variable vertical structuring of the horizontal velocity. The resulting velocity shear on water column stability may have important implications for shelf hypoxia during the stratified season in the Mississippi Bight.