Monsoon depression

From Glossary of Meteorology

monsoon depression

A depression that forms within the monsoon trough.

The term is most frequently used to describe weak cyclonic disturbances that form over the Bay of Bengal and generally track northwestward over the Indian subcontinent. These occasionally intensify into tropical cyclones if they remain over warm ocean water long enough. The term is also used to describe depressions that form within the monsoon trough near Australia and in the western North Pacific region. The term has gained ascendancy in use to refer to a broad tropical cyclonic vortex characterized by 1) its large size, where the outermost closed isobar may have a diameter on the order of 600 n mi (1000 km); 2) a loosely organized cluster of deep convective elements, which may form an elongated band of deep convection in the east semicircle; 3) a low- level wind distribution that features a 100 n mi (200 km) diameter light-wind core, which may be surrounded by a band of gales or contain a highly asymmetric wind field; and 4) a lack of a distinct cloud system center. Most monsoon depressions that develop in the western North Pacific eventually acquire persistent central convection and accelerated core winds, marking their transitions into conventional tropical cyclones.