THE EFFECTS OF EL-NINO-SOUTHERN OSCILLATION ON PRECIPITATION IN SAN-JOSE, COSTA-RICA

The effects of interannual variations in El Nino-Southern Oscillation (ENSO) upon annual, seasonal and monthly precipitation totals of San Jose, Costa Rica, over the period 1882-1987 are analysed using both a Southern Oscillation index (SOI) and a classification of El Nino and anti-El Nino years. The analyses confirm the posited regional positive relationship between annual precipitation and the SOI and suggest that estimates of the probabilities of annual rainfall could be made successfully by using a mixed probability distribution that incorporates changes in both the mean and variance of precipitation under each of the three classes of El Nino activity. Strong seasonality is identified in the effects of ENSO, particularly in the months of July, August, and October, whereas the month of September seems unaffected. Monthly lead lag analyses imply that there may also be a reversal of the nature of association between the SOI and precipitation in the following year.