Nature of Caribbean Rainfall
Characteristics, Seasonal Patterns, and Trends
The location and diverse topography of the Caribbean influence the amount of annual rainfall and its pattern. At least 70-80 percent of the region's rainfall is realized during the wet season (Enfield and Alfaro 1999), with high variability in the onset, duration, and quantum of rainfall during wet and dry seasons. From The Bahamas and Belize in the west to Trinidad and Tobago in the southeast, the wet season begins around May or June and ends around November or December. The remainder of the year largely represents the dry season. North of around 18°N, wet season rainfall exhibits a bimodal peak interposed by a distinct drier episode, colloquially referred to as a "mid-summer drought" (Gamble et al. 2008). With respect to monthly rainfall totals, much of the region shows a primary maximum in the latter half of the wet season—September to November. This is true in terms of the chance and the intensity of rainfall on any given day (Figure 20.1).
Smoothed rainfall seasonality in four select stations from CARICOM countries. The left column depicts the chance of a wet day (i.e., a calendar day with >0.85 mm), whereas the right column depicts the average rainfall intensity on a wet day for each Julian day of the year. The remainder of the year, the Caribbean dry season, shows a rainfall deficit, particularly when compared to evaporative losses. Rainfall declines through December continues until late February/early March, the peak of the dry season. Rainfall totals between April and May tend to be highly variable from year to year, with May being among the wettest months of the year in some years and virtually dry, with relatively high potential evaporation rates in other years. These dry months of May extend the impacts of the dry season.
In northern Guyana and Suriname, however, two wet and two dry seasons are experienced per year.
Variability in the intensity and timing of the events mentioned above often results from the El Nino southern oscillation (ENSO) and the gradient in SST between the Pacific and Atlantic (Enfield and Alfaro 1999; Giannini et al. 2000, 2001; Taylor et al. 2002, 2011;) Stephenson et al. 2008, the North Atlantic oscillation (Charlery et al. 2006) and North Atlantic high pressure cell (Gamble et al. 2008), decadal fluctuations (Taylor et al. 2002), the Madden Julian oscillation (MJO) (Martin and Schumacher 2011), and the Caribbean low-level jet (Cook and Vizy 2010; Taylor et al. 2012). Increasing attention is also given to the role of the Saharan air layer in reducing Caribbean rainfall (Prospero and Lamb 2003; Prospero and Nees 1986; Rodriguez 2013). However, the ENSO is likely the single most important factor in the interannual rainfall variability in the Caribbean.