Near-surface variations and surface fluxes over the northern Bay of Bengal during the 1999 Indian summer monsoon

Abstract

Detailed observations of the atmosphere and the ocean were carried out in the Bay of Bengal during the peak monsoon months of July and August 1999 under the Bay of Bengal Monsoon Experiment (BOBMEX). The emphasis in BOBMEX was on time series observations of the ocean and the atmosphere from fixed locations in the bay. Indian research vessels INS Sagardhwani and ORV Sagar Kanya were deployed at 13° N, 87°E and 17.5° N, 89° E in the southern and northern Bay of Bengal, respectively. This paper describes the near-surface characteristics during phase I (27 July to 6 August) and phase II (13-24 August) of BOBMEX observed at the northern location. During phase I, average surface pressure was 999 hPa, average winds were 10 m^s-1, and three monsoon systems developed. Sea surface temperature (SST) remained around 28.5°C. Phase I also saw the arrival of low-saline water plume, and the surface salinity decreased from 33 practical salinity units (psu) to less than 29 psu in a span of just 4 days. During phase II of BOBMEX, average surface pressure was 1006 hPa, average wind speed was lower at 7.5 m s^-1, and no monsoon system developed. There was a weak phase of convection characterized by low winds and clear-sky conditions, during which period SST increased from around 28° C to more than 29.5° C in 5 days. Convective available potential energy of the surface air decreased by 2-3 kJ kg^-1 following convection and recovered in a time period of 1 or 2 days. SST predicted by using observed surface fluxes showed that horizontal advection is important in the northern bay and that a one-dimensional heat balance model works sometimes, but not always. It is also shown that near-surface characteristics over the northern Bay of Bengal are different from those observed over the west Pacific warm pool.