Interannual variability of Caribbean rainfall, ENSO, and the Atlantic Ocean

The large-scale ocean-atmosphere patterns that influence the interannual va riability of Caribbean-Central American rainfall are examined. The atmosphe ric circulation over this region is shaped by the competition between the N orth Atlantic subtropical high sea level pressure system and the eastern Pa cific ITCZ, which influence the convergence patterns on seasonal and intera nnual timescales. The authors find the leading modes of interannual sea level pressure (SLP) and SST variability associated with Caribbean rainfall, as selected by cano nical correlation analysis, to be an interbasin mode, linking the eastern P acific with the tropical Atlantic, and an Atlantic mode. North Atlantic SLP affects Caribbean rainfall directly, by changing the patterns of surface h ow over the region, and indirectly, through SST anomalies. Anomalously high SLP in the region of the North Atlantic high translates into stronger trad e winds, hence cooler SSTs, and less Caribbean rainfall. The interbasin mod e, which manifests itself as a zonal seesaw in SLP between the tropical Atl antic and the eastern equatorial Pacific, is correlated with ENSO. When SLP is low in the eastern equatorial Pacific. it is high in the tropical Atlan tic: the surface atmospheric Row over the basin is divergent, to the west, toward the eastern Pacific ITCZ, and to the east, toward the tropical North Atlantic. A weakened meridional SLP gradient in the tropical North Atlanti c signifies weaker trade winds and the opportunity for SSTs to warm up, rea ching peak intensity 2-4 months after the mature phase of an ENSO event. Th is SST anomaly is particularly evident in the Caribbean-western Atlantic ba sin. The tendency is for drier-than-average conditions when the divergent atmosp heric flow dominates, during the rainy season preceding the mature phase of a warm ENSO event. The dry season that coincides with the mature phase of ENSO is wetter than average over the northwestern section of the basin, tha t is, Yucatan, the Caribbean coast of Honduras, and Cuba, and drier than av erage over the rest of the basin, that is, Costa Rica and northern South Am erica. The following spring, as the atmospheric circulation transitions to normal conditions, the positive SST anomaly that has built up in the basin takes over, favoring convection The positive precipitation anomaly spreads southeastward, from the northwest to the entire basin. At the start of a ne w rainy season, it is especially strong over the Greater Antilles.