Monsoon trough

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The Glossary of Meteorology defines a monsoon trough simply: A line in a weather map showing the locations of minimum sea level pressure in a monsoon region. ^[1] Several regions worldwide qualify as a monsoon region: western Mexico, the far western north Pacific Ocean and north Indian Ocean near southeast Asia, the far western south Pacific Ocean near Australia, central South America, and occasionally near the west coast of Africa.

If the trough is in its normal orientation, tropical cyclones along its periphery will move with a westward motion. If it is reverse oriented, tropical cyclones will move more poleward. Tropical cyclone tracks with S shapes tend to be associated with reverse-oriented monsoon troughs. ^[2] The South Pacific convergence zone and South American convergence zones are generally reverse oriented. ^[3]

August position

February position

1 Movement
2 Effect of wind surges
3 Embedded depressions
4 Role in tropical cyclogenesis
5 See also
6 References
7 External links

[edit] Movement

It reaches its zenith in latitude during the late summer when the wintertime surface ridge in the opposite hemisphere is the strongest, with the monsoon trough reaching as far as the 40th parallel in the Northern Hemisphere in August. Likewise, in the Southern Hemisphere, it is at its greatest extent in February. ^[3] On the charts on the left and right, the areas of convergence in the streamline field mark the climatological position of the monsoon trough for those respective months. Movements in the monsoon trough location can mark the beginning/ending of the wet or dry season.

Monsoon Depression near Bangladesh

[edit] Effect of wind surges

Increases in the relative vorticity of the monsoon trough is normally a product of increased wind convergence. Wind surges can lead to this increase in convergence. A strengthening or equatorward movement in the subtropical ridge can cause a strengthening of the monsoon trough as a wind surge moves southward towards its location. As fronts move through the subtropics of one hemisphere during their winter, wind surges can cross the equator in oceanic regions and enhance the monsoon trough in the other hemisphere's summer. A key way of detecting whether a wind surge has reached the monsoon trough is the formation of a burst of thunderstorms within the feature.

[edit] Embedded depressions

If a circulation forms within the monsoon trough is able to compete with the neighboring thermal low over the continent, a wind surge will occur at its periphery. Such a circulation which is broad in nature within a monsoon trough is known as a monsoon depression. Monsoon depressions are generally asymmetric, and tend to have their strongest winds on their eastern periphery. ^[4]

[edit] Role in tropical cyclogenesis

Worldwide, the monsoon trough is the main genesis region for tropical cyclones. ^[5] Vorticity-rich low level environments lead to a better than average chance of tropical cyclone formation.^[4] This is because a pre-existing near-surface disturbance with sufficient vorticity and convergence is one of the six requirements for tropical cyclogenesis. ^[6] The monsoon trough is one such area of strong low-level vorticity. There appears to be a 15-25 day cycle in thunderstorm activity associated with the monsoon trough, which is roughly half the wavelength of the Madden-Julian Oscillation, or MJO. ^[7] This mirrors tropical cyclone genesis near these features, as genesis clusters in 2-3 weeks of activity followed by 2-3 weeks of inactivity. ^[5] Indeed, tropical cyclones can form in outbreaks around these features under special circumstances, tending to follow the next cyclone to its north and west. ^[8] This is different than the Atlantic Ocean, where tropical cyclones mainly form from tropical waves which move offshore Africa. Eastern Pacific Ocean tropical cyclone formation shows a hybrid of these two mechanisms.