Trapping of Wind Momentum in a Salinity‐Stratified Ocean
Dipanjan Chaudhuri, Debasis Sengupta, Eric A. D’Asaro, S. Shivaprasad
Abstract
Abstract We study the nature of ocean currents forced by summer monsoon winds in the Bay of Bengal using data from two cruises within a pool of low‐salinity (<32 psμ) river water, and one cruise outside the low‐salinity pool. Upper ocean (5–70 m) temperature, salinity, and velocity profiles were measured at sub‐kilometer horizontal resolution along the 800–1800‐km‐long cruise tracks. We find that beneath the river water, density stratification at 5–15 m depth can be 10 times stronger than typically seen in much of the subtropical or tropical ocean. The shallow salinity‐dominated stratification suppresses shear‐induced turbulence, thereby “trapping” momentum input from monsoon winds in the uppermost 20 m of the ocean. Estimates of directly wind‐forced ocean currents are consistent with Ekman depth scaling as , where N 0 is stratification at the base of the mixed layer and f is Coriolis frequency. In the presence of river water, cruise‐mean wind‐forced currents resemble tightly surface trapped Ekman‐like spirals: The shallowest measured current at 5 m depth has a speed of nearly 0.2 m/s, directed 80° to the right of the wind. Outside the pool of river water, wind momentum penetrates to 40 m depth.