The Role of the ITCZ in Generating Tropical Depressions
On or near the equator, where average solar radiation is greatest, air is warmed at the surface and rises. This creates a band of low air pressure, centered on the equator. This rising air comprises one segment of a circulation pattern called the Hadley Cell. The rising air is replaced by the Trade winds approaching the equator from north and south. As the trade winds meet near the equator, surface convergence and uplift take place. For this reason the equatorial band of low pressure is called the Equatorial Trough, Intertropical Convergence Zone, or the ITCZ.
Fig. 1. Global belts of high and low pressure, and the major prevailing winds on Earth.
The ITCZ is a region of light winds, which lends it the name the doldrums. The convergence of the Southeast and Northeast Trade Winds, within the doldrums, creates a zone of Cumulus clouds and attendant shower activity. Cumulus clouds often build up to great heights. Aircraft reports have estimated tops of Cumulonimbus to be as high as 12,000 m. The ITCZ varies from 20 miles to as much as 300 miles in width, and typically has an undulating conformation.
Seasonal Meandering of the ITCZ. We are interested in the ITCZ because, under certain circumstances, tropical depressions on the ITCZ intensify to hurricanes. It may seem puzzling that the ITCZ can produce cyclones, when the Coriolis force is at its weakest near the equator. The answer to this puzzle lies in the fact that the ITCZ is not stationary on the equator, but migrates north and south with the seasons. The ITCZ moves north during the high-sun season of the Northern Hemisphere, and south during the high-sun season in the Southern Hemisphere. These movements are not perfectly symmetrical above and below the equator, because of the influence of land masses, among other factors.
Fig. 2. The seasonal meandering of the ITCZ.
When the ITCZ is near the equator, the convergence of surface winds along the ITCZ is likely to take the form of parallel flow, with easterly wind approaching the doldrums from both north and south. When the Trade Winds converging at the ITCZ are weak and nearly parallel to it, the ITCZ tends to narrow in width, and show little shower activity. This condition is shown below.
Fig. 3. Trade winds converging parallel to the ITCZ.
When the ITCZ lies south of the Geographic Equator, as it does in the Western Pacific and Indian Oceans, the Northeast Trade Winds acquire a northwesterly direction after crossing the equator (because the Coriolis force changes direction below the equator!) In this situation the convergence is strong and favors the formation of a Tropical Depression. (In the diagram below, note that the direction of the winds entering the low encourages clockwise rotation, which is cyclonic in the Southern Hemisphere).
Fig. 4. A tropical depression on the ITCZ south of the geographical equator.
When the ITCZ lies north of the Geographic Equator, the Southeast Trade Winds acquire a southwesterly direction after crossing the equator and again the convergence pattern favors the formation of a Tropical Depression.
Fig. 5. A tropical depression on the ITCZ north of the geographical equator.
Tropical depressions tend to move from east to west at a rate of about 10 to12 knots. In most cases these disturbances are shortlived. Surface winds attending a tropical depression usually do not exceed 25 knots. The enclosing isobar has an elongated oval shape, the width of the ITCZ is greater in the region enclosed by the isobar(s), and the ITCZ exhibits a deflection away from the Geographic Equator.
Importance of the Seasonal Position of the ITCZ. There is a definite connection between the seasonal position of the Equatorial Trough and zones of hurricane formation, which is borne out by the fact that no hurricanes occur in the South Atlantic (where the trough never lies south of 5 deg S) or in the southeast Pacific (where the trough remains north of the equator). On the other hand, satellite photographs over the northeast Pacific show an unexpected number of cyclonic vortices in summer, many of which move westwards near the trough line about 10 deg -1 deg N.