The Caribbean rainy season spans the months of May through November and is bimodal by nature. The principal maximum in rainfall totals occurs late in the season in October, whilst the secondary, smaller peak occurs earlier in June. Whereas some studies have been done to elucidate the time variability of the late season (August - November) rain, (due to its coincidence with the period of peak Atlantic hurricane activity) little is known about the variability of the early season (May - July) rainfall. This study attempts to link anomalous early season rain in the Caribbean to El Niño events.
Rain events in the Caribbean occur due to the passage of amplified tropical waves over the region. The two most significant factors affecting the amplification process are tropical Atlantic SST and the vertical shear of the surface winds. Tropical Atlantic SST's are known to lag those of the eastern Pacific by 4-6 months, whilst an increase in the wind shear over the Caribbean, via the enhancement of the 200 mbar westerly winds, is known to be correlated with El Niño events. Whereas the former effect (i.e. warm SST) serves to enhance the amplification of tropical waves, the latter and seemingly more dominant effect does precisely the opposite. In MJJ of an El Niño+1 year however (i.e. the year following peak Pacific temperatures) tropical Atlantic SST remains high whilst the shear is almost back to its climatological value due to the rapidly declining El Niño event. This results in favorable conditions for wave amplification and an increase in early season Caribbean rainfall.
This study therefore investigates whether the presence of warm Atlantic SST’s during MJJ of an El Niño+1 year is sufficient to induce significant increases in Caribbean precipitation for the same period.