Learning Center on Caribbean Weather and Climate

HOME    FUNDAMENTALS    CLIMATE    WEATHER    SOCIETY    SCHOOLS    ABOUT    NEW

Caribbean weather - Weather systems in the tropics

by Mr. Horace Burton, Chief Meteorologist, Caribbean Institute for Meteorology and Hydrology (CIMH).

Waves in the Easterlies

During the summer, a number of weather systems move westward across the tropics with wind fields which resemble the form of a wave. These systems, which are associated with specific patterns of cloud and precipitation, are referred to as waves or tropical waves. Most of the waves which affect the Caribbean originate over Africa and move westward across the Atlantic and the Caribbean Sea. Some dissipate or weaken before reaching the Lesser Antilles, while others continue uninhibited and develop into tropical storms or hurricanes. The waves move westward at a rate of about 25 km per hour and with a frequency of one every four to five days. These systems produce rainfall amounts ranging from a few millimetres for a weak system to 50 millimetres or more for an active or strong system.

The best known and most widely discussed wave model is the Easterly Wave model, which was developed by Riehl to describe the characteristics of the summer precipitation systems over the Caribbean (top view¹ and cross section² of an idealized easterly wave in the Caribbean). Riehl noted that periodic centres of rising and falling pressure were associated with shifts in the winds from the normal east northeasterly direction to an east southeasterly direction. The wave axis is the line along which the winds turn from a northerly to a southerly component. The area west of the wave axis is characterized by subsidence (sinking air) and fair weather while unstable weather with cloudiness, showers and thundershowers occurred in the area east of the wave axis. Ahead of the wave, trade wind cumulus gradually gives way to cumulus clouds with more vertical extent. Winds are from the east or the east northeast and there is no precipitation. Close to the wave axis cumulus of above average development along with some cirrus and altocumulus occurs, along with a few light showers. Winds are from the northeast and north northeast. As the wave axis passes, the sky becomes cloudy to overcast with frequent showers and thundershowers. The winds turn clockwise and are from the east and then the east south east and southeast. As the wave moves further west conditions gradually improve.

Satellite imagery and observations indicate other cloud and precipitation patterns occur with the waves in the easterlies and that unstable weather conditions are not restricted to the area east of the wave axis. Even though the broad features of the wind field associated with these waves appear similar to that of the Easterly wave, the other meteorological characteristics may be quite different. It is the variation of these characteristics that account for the differences in the distribution of cloudiness and precipitation.

¹from COMET, 1977: Tropical cyclogenesis and evolution. Tutorial in Satellite Meteorology: Case Studies Using GOES Imager Data.

²from Riehl, H., 1948: Waves in the easterlies. Univ. of Chicago, Misc. Report No. 17, 79 pp.

Back to top of this page

Copyright notice and disclaimer: In cases when the origin is not specifically indicated, the copyright of the material of this site resides at The Caribbean Institute for Meteorology and Hydrology (CIMH). The information contained within Cawedu.net is provided with the understanding that The CIMH makes no warranties, either expressed or implied, concerning the accuracy, completeness, reliability, or suitability. The public is urged to use the information freely with appropriate acknowledgement of its source, but shall not be modified in content and then presented as original material.

Send email to meteorartyahoo.com with questions or comments about this web site. Questions concerning material, copyright of which resides outside of CIMH, should be directed to the institutions in question.