Monsoon effects in the Bay of Bengal inferred from profiling float-based measurements of wind speed and rainfall

Both rainfall and wind have acoustic signatures that can be discerned at depths in the ocean well below the sea surface. We examine observations of rainfall and wind speed collected at a depth of 600 m in the Bay of Bengal from a specially modified Argo profiling float. In addition to the normal Argo sensors, the float carried a passive acoustic listener sensor package that monitored the spectrum of acoustic noise along the float trajectory at intervals of a few minutes and used a set of existing algorithms to estimate the wind speed and rainfall rate from these noise spectra. A comparison of the acoustically derived wind speed and rainfall estimates with analogous satellite-derived data (rainfall from the Tropical Rainfall Measuring Mission and QwikScat winds) showed general agreement in both cases, although the float-based measurements were representative of conditions a few kilometers around the float, whereas the remotely sensed observations were smoothed over much larger length scales. The strong monsoon signal in the Bay of Bengal is clearly present in the float-based wind and rainfall data. The near-surface salinity measured by the float varied because of both rainfall events and the proximity to strong coastal runoff from major rivers. The float profiles of temperature and salinity in the upper ocean, and the effects of wind and rainfall, were simulated in a version of the Price-Weller-Pinkel mixed-layer model, which showed that the direct effects of most rainfall events are concentrated in the upper 20-30 m of the water column.