Thursday 19 October 2017

Heavy rains bring flooding to south and central Trinidad.

Central and southern parts of the Caribbean island of Trinidad have suffered widespread flooding after a period of heavy rains and thunderstorms that lasted about 24 hours across Wednesday 18 and Thursday 19 October 2017. There are no reports of any casualties associated with this flooding, but transport networks have been severely disrupted and many homes inundated.

Flooding on Trinidad on Wednesday 18 October 2017. Loop.

Like other parts of the Caribbean, Trinidad has suffered a series of flooding events this summer, associated with a series of particularly severe hurricanes. However this weeks flooding is not associated with any hurricane, but rather appears to have been caused by the movement of the Inter-Tropical Convergence System, a permanent low pressure system that circles the globe approximately on the equator. 

This system is caused by heat from the Sun, which is greater at the equator then elsewhere, causing air over the equator to rise, and drawing in air currents from the Northern and Southern Hemispheres, forming the trade winds. This updraughts pull up water vapour from the sea surface high into the atmosphere, where it eventually precipitates out of the cooler, less dense atmosphere, falling back as rain. 

Flooding on Trinidad on Wednesday 18 October 2017. Loop.

Importantly, while the actual equator is fixed, the tilt of the Earth relative to the Sun means that the point at which the Sun is directly overhead moves northward in the northern summer and south in the southern summer, creating a thermal equator, which the Inter-Tropical Convergence System follows, moving north in the north and south with the seasons (and since  the sea heats and cools more slowly than the land, this movement is more pronounced over land than sea). The upshot of this is a system of storms which circulates on the equator at the equinoxes, but which moves north and south to the tropics around the solstices. Furthermore this system moves further away from the equator in warmer years, and is predicted as being likely to do so more with global warming. It is this  Inter-Tropical Convergence System which has passed over Trinidad this week, causing storms and flooding over the island.

The approximate position of the Inter-Tropical Convergence System in July and January.
Mats Halldin/

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