Planning for the Dry Season

The Caribbean experiences two dry seasons: one in the summer and the other between December and February of the following year. During both of these dry spells, rainfall is not usually adequate for activities such as planting of crops. Over the last three years researchers at the UWI have focused their attention on the dry season which occurs in summer because it is an interruption of the rainfall season which begins in May and ends in November.

A Search for Answers

It is important for us in the Caribbean to understand why the dip occurs in the summer and how to manage our water use to ensure there are sufficient supplies to last until the rains resume. The rainfall patterns are of particular interest to the region especially since this dry season falls within the hurricane season.

Studying the Wind

It is a known fact that wind speed is strongest during the summer dry season particularly over the western Caribbean Basin, affecting countries such as Jamaica, Haiti, Cuba and Central America. Scientists up to the time of the UWI study had not documented in any detail, a thorough understanding of the causes of the dry season or its relationship to wind speed. The UWI scientists were therefore motivated to fill this knowledge gap for the benefit of the numerous persons whose livelihood depends on water availability during the summer.

A Special Kind of Wind

Their first task was to study the Caribbean Low Level Jet (CLLJ). It is the strongest wind of the early summer found over the western Caribbean Basin (70° - 80° W) with an east-west axis along 15° N. It is confined to heights below 4.5 km (15,000 ft) and has maximum wind speeds approaching 16 m/s near the surface. The researchers have been able to show that there is year-to-year change in the strength and east-west extent of the CLLJ. They relate these changes to a difference between the surface temperatures over the eastern Pacific (near the equator) and the Atlantic (near the equator and towards the north). When the difference is driven by the Pacific (as in an El Niño event) the CLLJ winds are intensified to the north and in the south-western Caribbean. When the winds are driven by the Atlantic there is a uniform strengthening of the winds over the waters below Jamaica, around 15°.

Wind Linked to Rainfall

The CLLJ is also connected to a rainfall maximum along the Caribbean coast and near waters of Central America (up to 16° N) during June and more so, in July. There is evidence of variation in the extent of the wet zone over Central America, depending on whether the difference in sea surface temperatures is being driven by the Atlantic or Pacific Ocean. While Central America is experiencing this wetness, the main Caribbean basin usually experiences decreased rainfall.

Better Planning for the Dry Summer Months

Given the serious weather-related issues we encounter in the Caribbean, especially in the summer, understanding the dynamics of these winds is of great importance to governments of the region.

Ms. Felicia Whyte, a graduate student in the Department of Physics, UWI, Mona, and a member of the Climate Studies Group Mona undertook this research as part of her master’s thesis. She was supervised by Dr. Michael Taylor with inputs from Dr. Tannecia Stephenson and Mr. Jayaka Campbell also members of the Climate Studies Group Mona. michael.taylor@uwimona.edu.jm