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. The study of the climate system of the earth necessarily involves investigation into the processes that govern the interaction of the ocean and the atmosphere at the sea surface. In recent decades our ability to observe the largescale ocean, and hence to observe this interaction, has improved greatly, with global, nearly real-time measurements of sea-surface temperature, wind, rainfall, and ocean color available. Detailed planning is presently underway to measure sea-surface salinity from space. Thus, the most basic parameters necessary for examining large-scale ocean atmosphere interaction are, or soon will be, in place. Our ability to examine the subsurface ocean on global scales and in real time has also improved, owing to the advent of the international Argo program in the earliest years of the 21st century (Roemmich et al. 2004). The goal of this program has been to deploy approximately 3000 profiling floats over the world ocean that collect vertical profiles of temperature and salinity in the upper 2000 m of the ocean, and then transmit the measurements to shorebased centers in real time, with the data then publicly available within a day of collection. With the satellite data and the concomitant Argo observations, we now have a global, real-time observing system that can monitor the state of the lower atmosphere and upper ocean, which should eventually lead to vastly improved understanding of atmosphere–ocean interaction and improved climate models. The monsoons of the Indian Ocean are an example of particularly strong ocean–atmosphere interaction over basin scales. As satellite-derived wind data clearly show (Fig. 1), the large-scale wind field in the northern Indian Ocean reverses between boreal summer and winter, leading to changes in the basin-scale ocean circulation. In the Arabian Sea, the Bay of Bengal, and along the coast of east Africa significant changes in the wind field drive strong modulations in the ocean currents in these regions. At the same time, the seasonal cycles of heating, evaporation, and precipitation lead to large-scale changes in the temperature and salinity of the upper ocean, as can be seen in Fig. 2; this figure has been constructed by using all available near1 Corresponding author ([email protected]).