Monsoon Trough - Role in Tropical Cyclogenesis

Role in Tropical Cyclogenesis

A monsoon trough is a significant genesis region for tropical cyclones. Vorticity-rich low level environments, with significant low level spin, lead to a better than average chance of tropical cyclone formation due to their inherent rotation. This is because a pre-existing near-surface disturbance with sufficient spin and convergence is one of the six requirements for tropical cyclogenesis. 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. This mirrors tropical cyclone genesis near these features, as genesis clusters in 2–3 weeks of activity followed by 2–3 weeks of inactivity. Tropical cyclones can form in outbreaks around these features under special circumstances, tending to follow the next cyclone to its poleward and west. This is different than the Atlantic Ocean, where tropical cyclones mainly form from tropical waves which move offshore Africa, although 2010's Tropical Storm Nicole may have been an exception to this rule. Eastern Pacific Ocean tropical cyclone formation shows a hybrid of these two mechanisms.

Whenever the monsoon trough on the eastern side of the summertime Asian monsoon is in its normal orientation (oriented east-southeast to west-northwest), tropical cyclones along its periphery will move with a westward motion. If it is reverse oriented, or oriented southwest to northeast, tropical cyclones will move more poleward. Tropical cyclone tracks with S shapes tend to be associated with reverse-oriented monsoon troughs. The South Pacific convergence zone and South American convergence zones are generally reverse oriented. The failure of the monsoon trough, or ITCZ, to move south of the equator in the eastern Pacific ocean and Atlantic ocean during the southern hemisphere summer is considered one of the reasons that tropical cyclones normally do not form in those regions. It has also been noted that when the monsoon trough lies near 20 degrees north latitude in the Pacific, the frequency of tropical cyclones is 2 to 3 times greater than when it lies closer to 10 degrees north.