Air-sea interaction in the Bay of Bengal during the post-monsoon period

In September 1999 the Australian ship R/V Franklin operated in the Bay of Bengal, measuring ocean structure and air-sea fluxes, as part of the Joint Air-Sea Monsoon Experiment (JASMINE). The cruise track and methodology followed closely that adopted by scientists aboard the US research vessel Ronald H. Brown during May and early June 1999, at the onset of the Asian summer monsoon, as described elsewhere in this session. CTD casts to either 1000db or 500db were made every half-degree of latitude from 5°S to 12° N. A week was spent near 12° N, 88° E making ocean budget measurements around triangular tracks of side about 24 km. CTDs were taken at each vertex and at the mid-point of each side, with surface fluxes measured continuously along the entire track. Conditions were fine during this survey, with frequent scattered cloud but no rain.

Sea surface temperature warmed by almost 1°C during the 7-day period. Although advection is expected to be important, it appears that most of this warming was due to surface heating, with net surface heat fluxes of over 100 W/m2 being measured on each day of the survey. These large heat fluxes occurred in the presence of south-westerly winds averaging from 8 to 10 m/s at the beginning of the period, and increased significantly as winds abated to around 4 m/s at the end. In this paper we will present preliminary results from the study, and explore the implications of such rapid SST rise for sea-air coupling in this region. We will compare the climatic regime with that observed during the R/V Ronald H. Brown cruise earlier in the monsoon season.