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 I
ndian 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.
A convection of the summer monsoon circulation of the western North Pacific characterized by 1) a very large nearly circular low-level cyclonic vortex (not the result of the expanding wind field of a
preexisting monsoon depression or tropical cyclone) that has an outermost closed isobar with a diameter on the order of 1200 n mi (2500 km); 2) a cloud band bordering the southern through eastern periphery of the vortex/surface low; and 3) a relatively long (two week) life span. Initially, a subsequent regime exists in its core and western and northwestern quadrants with light winds and scattered low cumulus clouds; later, the area within the outer closed isobar may fill with deep convective cloud and become a isobar or tropical cyclone. Note: a series of midget tropical cyclones may emerge from the ?head? or leading edge of the peripheral tropical cyclone of a monsoon gyre.
A seasonal low found over a continent in the summer and over the adjacent sea in the winter. Examples are the lows over the southwestern US and India in summer and those located off lower California a
nd in the Bay of Bengal in winter. Palmer (1951) points out that, while the winter and summer monsoon lows appear similar on mean charts, they are dynamically quite different. Compare thermal low.
The temporary extension of deep monsoon flow into a region not normally dominated by persistent monsoon flow. This temporary extension or surge may last from a few days to three weeks. These surges mo
st commonly occur eastward across the Philippine Sea into the western North Pacific and east of Australia into the western South Pacific. The establishment of a reverse-oriented monsoon trough is accompanied by an eastward surge in the monsoon flow. Monsoon surges are often precursors to the development of tropical cyclones.