The impact of wind forcing on the thermal wind shear of a river plume

Abstract

A 38-day long time series obtained using a combination of moored Wirewalkers equipped with conductivity-temperature-depth profilers and bottom-mounted and subsurface acoustic Doppler current profilers provided detailed high-resolution observations that resolved near-surface velocity and vertical and cross-shelf density gradients of the Chesapeake Bay plume far field. This unprecedented data set allowed for a detailed investigation of the impact of wind forcing on the thermal wind shear of a river plume. Our results showed that thermal wind balance was a valid approximation for the cross-shelf momentum balance over the entire water column during weak winds (|𝜏w 𝑦 | < 0.075 Pa), and it was also valid within the interior during moderate downwelling (−0.125< 𝜏w 𝑦 < −0.075 Pa). Stronger wind conditions, however, resulted in the breakdown of the thermal wind balance in the Chesapeake Bay plume, with thermal wind shear overestimating the observed shear during downwelling and underestimating during upwelling conditions. A momentum budget analysis suggests that viscous stresses from wind-generated turbulence are mainly responsible for the generation of ageostrophic shear.