[1] The Caribbean rainfall season runs from May through November and is distinctly bimodal in nature. The bimodality allows for a convenient division into an early season (May-June-July) and a late season (August-September -October). Evidence suggests that interannual variability in the early season is influenced strongly by anomalies in the sea surface temperatures of the tropical North Atlantic, with positive anomalies over a narrow latitudinal band (0°-20°N) being associated with enhanced Caribbean rainfall. The coincidence of this band with the main development region for tropical waves suggests a modification of the development of the waves by the warmer tropical Atlantic. The strong influence of the tropical North Atlantic wanes in the late season, with the equatorial Pacific and equatorial Atlantic becoming more significant modulators of interannual variability. The spatial pattern of significant correlation suggests strongly the influence of the El Niño/La Niña phenomenon, with a warm Pacific associated with a depressed late season and vice versa. There additionally seems to be a robust relationship between late season Caribbean rainfall and an east-west gradient of sea surface temperature (SST) between the two equatorial oceanic basins. Oppositely signed SST anomalies in the NINO3 region and the central equatorial Atlantic (0°-15°W, 5°S-5°N) are well correlated with Caribbean rainfall for this period.

Singular value decomposition (SVD) techniques are used to deduce a relationship between rainfall over the Caribbean basin and oppositely signed sea-surface temperature anomalies in the Pacific and Atlantic. The analysis is done for four 3 month seasons. The first two seasons: November-January (NDJ) and February-April (FMA) encompass the Caribbean dry period, and the other two, May-July (MJJ) and August-October (ASO), include the early and late Caribbean rainy seasons. The first SVD mode for all seasons represents variability due to El Niño-southern oscillation (ENSO) and, with the exception of the later wet season, the second SVD mode represents variability due to tropical North Atlantic sea-surface temperatures. ENSO has the greatest impact during the late rainfall season (ASO) and the early dry season (NDJ), whereas the tropical Atlantic controls variability in the early rainfall season (MJJ). The configuration of concurrent but oppositely signed sea-surface temperature anomalies in the tropical Pacific and Atlantic basins is only associated with rainfall modification in the late Caribbean rainfall season (ASO) and the early Caribbean dry season (NDJ).

Interbasin and intrabasin gradients play an important role as a part of a regional system of Caribbean climate drivers, which include the Atlantic warm pool (AWP) and the Caribbean low‐level jet (CLLJ). When the Caribbean is conditioned to be wet between May and November, near‐surface geopotentials in the Caribbean are lower than in the nearby eastern tropical Pacific and east tropical Atlantic. As a result, there is vertical ascent in the Caribbean through to the middle troposphere which connects to zonal circulations with both the eastern tropical Pacific and the eastern tropical Atlantic. The Caribbean Sea is also warm, and there is a moderate easterly flow regime, indicating a weakening of the trade winds. Deviations from this state caused by changes in one or both sides of the Pacific‐Caribbean and Caribbean‐Atlantic circulations (and diagnosed by changes in their geopotential gradients) reasonably track the transition of the Caribbean from wet to dry and vice versa on intraseasonal and interannual time scales. The study also uses changes to the gradients to offer insight into why the Caribbean region is projected to be drier during its traditional rainy season in the face of warmer surface temperatures under global warming. The Caribbean seemingly enters into a “July” mode, which persists for the duration of the boreal summer. The mode is characterized by higher (lower) geopotentials in the Caribbean (Pacific and Atlantic), a stronger CLLJ, and anomalous descent in the Caribbean in spite of the warmer surface temperatures.

The Caribbean rainfall season is best characterized by its bimodal nature, with an initial peak in May-June and a second more prominent one in September-October. This allows for a convenient division into an early and a late rainfall season. In this study we examine the rainfall patterns of the early rainfall season (mid April to July) for links with El Niño-Southern Oscillation (ENSO) events. Whereas traditionally ENSO events have been identified with dry conditions during the later Caribbean rainfall season, recent research suggests a second signal that manifests itself as a wet early rainfall season of the year of ENSO decline (the El Niño+1 year). Two leading empirical orthogonal function modes of early season Caribbean rainfall are examined for evidence of this. Strong correlations are shown to exist between the first mode and wintertime equatorial Pacific anomalies. The first mode explains nearly half of the early season variability. The idea that the wintertime Pacific anomalies alter the early Caribbean rainfall season via the warm spring sea surface temperature anomalies they induce in the north tropical Atlantic is also investigated. An atmospheric general circulation model is also used to show that, when warm/cold anomalies exist across the north tropical Atlantic, this results in a large-scale atmospheric circulation that is more/less favourable to rainfall production over the Caribbean.

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