Wind-modulated buoyancy circulation over the Texas-Louisiana shelf

Abstract

This numerical investigation examines buoyancy-driven circulation on the Texas-Louisiana shelf modulated by seasonal winds. In wintertime, with downcoast (in the direction of Kelvin wave propagation) wind forcing, the Mississippi-Atchafalaya River plume exhibits a bottom-advected pattern. The plume is fairly wide and the horizontal density gradients span almost across the entire shelf inshore of 50 m. Within the plume, vertical shear of alongshore flow is in thermal wind balance with the cross-shore density gradient, and the shear causes a slight reversal of alongshore flow near the bottom. An alongshore flow estimated by the thermal wind relation, along with an assumption of zero bottom velocity, generally well agrees with the actual flow near the surface in regions deeper than 20 m. In spring and summer, the thermal-wind-balance-derived flow deviates from the actual alongshore flow, and an Ekman flow driven by strong onshore wind makes the major contribution to the deviation. Besides, the summertime upcoast wind component transforms the plume to a surface-advected state, resulting in reduced cross-shore density gradients and increasing the relative importance of wind-driven, barotropic alongshore flow, which contributes to the remaining deviation. Key Points Freshwater plume on the shelf is modulated by alongshore wind A thermal wind flow well explains the alongshore flow near the surface in winter Wind-driven flows are important components of the summertime alongshore flow