A study of mean boundary-layer structures over the Arabian Sea and the Bay of Bengal during active and break monsoon periods

Observations from research ships that took part in the Indo-Soviet Monsoon Experiment of 1977 (MONSOON-77) and the International Monsoon Experiments (MONEX-79) over the central Arabian Sea and the north central Bay of Bengal were analyzed to study the mean wind and temperature structure of the monsoon boundary layer during active and break conditions. Mean profiles of wind speed and direction, along with virtual potential temperature obtained by averaging data from several research ships during 1977 and 1979, indicate that onset conditions were associated with substantial increases in wind speed over the Arabian Sea and a shift to strong southwest flow. Monsoon onset was also characterized by near-neutral to slightly unstable temperature profiles in the lowest kilometer. Break conditions in 1977, in which the monsoon trough moved northward and substantial (5-mb) pressure rises were noted over the Arabian Sea, show wind speeds typically decreasing from similar to 18 m sec super(-) super(1) during active conditions to similar to 8 m sec super(-) super(1) . Temperature profiles during break conditions are similar to those observed in premonsoon conditions in that the boundary layer is observed to be generally much more stable up to 900 mb. Above 900 mb, profiles of virtual potential temperature show little variation. Analysis of latent and sensible heat fluxes during June 1977 calculated by the bulk aerodynamic method indicates values of latent heat flux during active conditions to be similar to 2-3 times larger than those during break conditions. Sensible heat flux shows an increase from similar to 20 to 80 W m super(-) super(2) during the onset of the monsoon. Surface fluxes of water vapor indicate the importance of water vapor transport over the ship observation region in the central Arabian Sea during active conditions. The onset of the monsoon over the Arabian Sea is accompanied by an increase in the surface moisture flux by a factor of similar to 2. Time histories of precipitable water show decreases of similar to 15% from active to break periods.