What it is and its role in Trinidad and Tobago’s Rainfall
The Inter Tropical Convergence Zone or ITCZ as it is commonly called is one of the main climatological features of the global atmosphere.
It is a zone (belt) of low atmospheric pressure that forms where the northeast trade winds meet the southeast trade winds (zone of convergence) near the earth’s real equator.
When the two trade wind regimes meet, moist warm air is forced upward. This causes water vapour to condense as the air cools and rises, resulting in the formation of a band of clouds with heavy rainfall.
The location of the ITCZ varies throughout the year and dramatically affects rainfall in areas where it is located.
It is also called the thermal or climate equator because the zone of low pressure associated with the ITCZ migrates with the changing position of the thermal equator.
The thermal equator is that zone that receives the most intense heat from the sun and hence moves with the movement of the sun. The ITCZ, therefore, appears to move north and south with the overhead sun, because where the sun is overhead is likely to be the hottest area.
As such, the movement of the thermal equator shifts the belts of planetary winds and low pressure zone to the north and to the south annually.
When it moves north over Trinidad and Tobago it brings wet weather and significant rainfall, with thunderstorms regularly occurring directly within the associated band of cloud mass. Land areas heat up a lot more quickly than the sea, so you will find that the ITCZ is particularly active over land areas during the day and more active over coastal and sea areas during the night.
Trinidad receives around 1643 mm of rainfall per wet season.
Trinidad has a twin-peak rainfall regime, with a major peak in June-July-August and a smaller peak in November as the graph above shows for Piarco, Trinidad.
This twin peak in rainfall occurs because the ITCZ moves northward in the early part of the wet season, which brings much rainfall with it. As it moves southward– as the sun migrates later in the season– it brings another peak in the rainfall totals.
Tobago has average rainfall totals per wet season of around 1250 mm and, as the graph below shows, Crown Point in Tobago receives an early peak in rainfall in July when the ITCZ is furthest north and its main peak in November when the ITCZ is on its way southward.
It is important to note that the ITCZ is not the only feature affecting rainfall during the wet season.
Usually, the wet season accounts for 75 to 80% of the annual rainfall total in Trinidad and Tobago.
While the ITCZ is not the only main rainfall-producing feature of our local wet season, it does account for a significant part of the annual rainfall. This is because when it is in the area, rainfall activity associated with it, although not always continuous, can span one to three days or sometimes more.
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