JASMINE observations of upper ocean structure and variability in the Bay of Bengal during the southwest monsoon

Upper ocean observations of velocity, temperature and salinity fields were made during April-June and September 1999 as part of the Joint Air-Sea Monsoon Interaction Experiment (JASMINE) in the Bay of Bengal. The ocean component's observational goals were to document: the meridional structure of temperature, salinity and velocity fields, the mixed layer and barrier layer structures, and the upper ocean heat, freshwater and momentum budgets as they vary during active and break periods of the monsoon. The 1999 field work was a collaborative pilot study of air-sea fluxes, convection and the upper ocean response to atmospheric forcing. The work was conducted aboard the NOAA Ship Ronald H. Brown (April-June) and the R/V Franklin (September). Ocean observations focused on the Bay of Bengal sector near 88°E from 5°S to 16.5°N. During April-June five meridional surveys were conducted, including break periods of light winds and upper ocean warming, and active periods of strong winds, variable rainfall, and upper ocean cooling. Two 5-day periods of intense observations were conducted near 12°N, 88°E for detailed budget studies: one during a break period with upper-ocean heating and increasing salinity; the other during an active period with upper-ocean cooling and freshening. The data set documents the Bay of Bengal salinity front near 8°N, the intraseasonal reversal of near-equatorial currents, and the associated changes of the temperature and salinity fields (including mixed and barrier layer structures) during the southwest monsoon onset. The September data include a meridional survey along 88°E and an intensive 7-day budget survey near 12°N. The data were taken during conditions of no rain, moderate winds and sunshine, and daily net heat fluxes over 100 w/m² into the ocean resulting in an increase of near-surface (0-50 m) temperature from 28.2°C to 29.2°C over a week. The new data sets significantly expand our observational knowledge in the Bay of Bengal sector during the southwest monsoon period, and indicate the importance of circulation patterns and near-surface salinity in modifying mixed and barrier layer structure. The data suggest that realistic models need to include salinity as well as temperature, need to have high vertical and horizontal resolution to adequately resolve mixed layer and barrier layer structures effecting SST, and need to be forced by high-frequency winds and fluxes.