The Summer Atmospheric Circulation over the Arabian Sea

Abstract

The summer heat low system extending from Somalia across southeast Arabia to northwest India is the most extensive and intense on earth. Although it develops in the normal way over desert regions in response to the sun's zenithal march, it is maintained and intensified through the summer by subsidence of air originally lifted and warmed by the release of latent heat in monsoon rain systems to the east and south. The subsidence not only dominates West Pakistan, Arabia, and Somalia, but severely restricts low cloud formation over the central and western Arabian Sea. The heat low exports cyclonic vorticity in the middle and upper troposphere to the northern Arabian Sea. When a deep layer of moist air is present over the eastern part of this area, subtropical cyclogenesis occurs, producing a burst of west Indian monsoon rains. This in turn, by increasing subsidence above the heat low, intensifies the heat low and its associated low-level monsoon circulation. When the supply of moist air is cut off, the subtropical cyclone fills, the heat low weakens, and a break takes place in the monsoon rains. With renewal of the moisture supply, the sequence is repeated.