11.9.1 Key Processes
Climate change scenarios for small islands of the Caribbean Sea, Indian Ocean and Pacific Ocean are included in the AR4 for a number of reasons. Ocean-atmosphere interactions play a major role in determining the climate of the islands and including their climate in the projections for neighbours with larger landmasses would miss features peculiar to the islands themselves. Many small islands are sufficiently removed from large landmasses so that atmospheric circulation may be different over the smaller islands compared to their larger neighbours (e.g., in the Pacific Ocean). For the Caribbean, which is close to large landmasses in Central America and northern South America, some islands partly share climate features of one, while others partly share features of the other. At the same time, the Caribbean islands share many common features that are more important than are those shared with the larger landmasses, such as the strong relationship of their climate to SST.
The Caribbean region spans roughly the area between 10°N to 25°N and 85°W to 60°W. Its climate can be broadly characterised as dry winter/wet summer with orography and elevation being significant modifiers at the sub-regional scale (Taylor and Alfero, 2005). The dominant synoptic influence is the North Atlantic subtropical high (NAH). During the winter, the NAH is southernmost and the region is generally at its driest. With the onset of the spring, the NAH moves northward, the trade wind intensity decreases and the equatorial flank of the NAH becomes convergent. Concurrently, easterly waves traverse the Atlantic from the coast of Africa into the Caribbean. These waves frequently mature into storms and hurricanes under warm SSTs and low vertical wind shear, generally within a 10°N to 20°N latitudinal band. They represent the primary rainfall source and their onset in June and demise in November roughly coincide with the mean Caribbean rainy season. In the coastal zones of Venezuela and Columbia, the wet season occurs later, from October to January (Martis et al., 2002). Interannual variability of the rainfall is influenced mainly by ENSO events through their effect on SSTs in the Atlantic and Caribbean Basins. The late rainfall season tends to be drier in El Niño years and wetter in La Niña years (Giannini et al., 2000, Martis et al., 2002, M. Taylor et al., 2002) and tropical cyclone activity diminishes over the Caribbean during El Niño summers (Gray, 1984). However, the early rainfall season in the central and southern Caribbean tends to be wetter in the year after an El Niño and drier in a La Niña year (Chen and Taylor, 2002). The phase of the NAO modulates the behaviour of warm ENSO events (Giannini et al., 2001). A positive NAO phase implies a stronger than normal NAH and amplifies the drying during a warm ENSO. On the other hand, a negative NAO phase amplifies the precipitation in the early rainfall season in the year after an El Niño.