Annual and Interannual Variation of the Freshwater Budget in the Tropical Atlantic Ocean and the Caribbean Sea

Abstract

Annual and interannual variations of the moisture and freshwater budgets are examined in the tropical Atlantic Ocean and the Caribbean Sea. The seasonal moisture budget (E?P) is calculated by estimating precipitation from 11 years of outgoing longwave radiation data (1974?85), and subtracting evaporation estimated from surface data. Consistent with previous estimates, we find annual mean deficits of freshwater in the tropical Atlantic and Caribbean Sea. The seasonal cycle of freshwater flux in both regions has strong annual and semiannual variations caused by shifts of the intertropical convergence zone (ITCZ). In the tropical Atlantic 20°S?20°N, monthly rainfall varies by 3 cm/month with the strongest rainfall occurring in May and October. Significant inconsistencies between results of the present study and the seasonal rainfall estimates of Dorman and Bourke are found. Evaporation varies by half as much as rainfall, while runoff has little seasonal fluctuation. The annual cycle of the net moisture balance dominates most of the tropical Atlantic region except near the annual mean position of the ITCZ at 5°N. In the Caribbean Sea, the freshwater flux is greatest in June and September. The interannual variability of freshwater flux during the period 1974 to 1979 is examined. Seasonal or interannual variations of rainfall account for two-thirds of the variations of the freshwater flux in the tropical Atlantic. The least rainfall in the region during the 11-year record occurred in the Niño years of 1976?77 and 1982?83, while the wettest year was 1984.