INFLUENCE OF THE MONSOON TROUGH ON AIR-SEA INTERACTION IN THE HEAD OF THE BAY OF BENGAL DURING THE SOUTHWEST MONSOON OF 1990 (MONSOON TROUGH BOUNDARY LAYER EXPERIMENT-90)

The analysis of 3-hourly time-series data on surface meteorological parameters collected at 20° N, 89° E in the head of the Bay of Bengal during the southwest monsoon period (18 August–19 September) of 1990 under the MONTBLEX-90 programme reveals considerable temporal variability in sea-level pressure, sea-surface temperature (SST) and the fluxes of heat and momentum at the air-sea interface. This variability is related closely to the north-south movement of the monsoon trough and the formation and development of synoptic weather systems during this period. A rapid increase in wind speed, cloudiness, instability, momentum flux, sensible heat flux and moisture flux (by 80 Wm-2), and a decrease of SST (by 0.3 °C) and net surface heat flux by 80 Wm-2, was associated with the development of a depression when the monsoon trough moved southwards. At the peak of the depression, values of the latent heat flux and evaporation reached up to 270 Wm-2 and 1.0 cm day-1 respectively. During the depression period the heat loss across the air-sea interface matched well with the heat loss in the upper (100 m) ocean. With the northward movement of the monsoon trough, the momentum and surface heat fluxes decreased rapidly while the sea surface gained heat energy at rates up to 195 Wm-2.