Thursday, November 26, 2009

Intertropical Convergence Zone

The Intertropical Convergence Zone is an area near the Equator that circumnavigates the globe. Most of it lies between 10ºN and 10ºS. The trade winds, or the winds near the Equator, converge on this zone. Since the winds are rotated in different directions by the Coriolis Effect, no low pressure systems, and therefore no tropical cyclones, can form too near the Equator.

The Intertropical Convergence Zone over the East Pacific. Although no low pressure systems can form in the ITCZ, it is constantly marked by thunderstorm activity.

However, the boundary of the ITCZ, or the ITCZ Axis, is always shifting, and a tropical system can sometimes, but very rarely, from below 5ºN or above 5ºS. A notable example is Typhoon Vamei, which was a Western Pacific cyclone that reached tropical storm strength at a mere 1.5ºN in December of 2001, which is the closest formation to the Equator on record. Also notable is 2004's Cyclone Agni, which formed farther from the Equator than Vamei and moved towards it, eventually reaching the most southerly point of 0.7ºN before turning back northward. Claims on this system are disputed because it was not officially tracked until a few days after the record was set but a fair amount of evidence supports that Agni does indeed hold the record for closest cyclone to the Equator.

Cyclone Agni at record latitude, a mere 45 miles from the Equator.

Although the winds over the ITCZ hinder tropical cyclones from forming within the region, they do provide an important factor for tropical cyclone development: tropical waves. A vast majority of tropical cyclones form from tropical waves, which are areas of convection that typically move west over the ITCZ. After the wave moves northward, it then can develop a low pressure system, and eventually become a tropical system. Therefore, the ITCZ is an important factor in tropical cyclone development, even if tropical cyclones can't form in the area itself.

A map of the formation and progression of tropical waves before becoming tropical cyclones.

No comments: