Predictability of Droughts and Floods due to El Nino and La Nina Episodes

The possibility of predicting rainfall anomalies in the Philippines due to El Niño and La Niña is investigated. The data used in the investigation consist of rain gauge observations, satellite-derived rainfall and sea surface temperatures in the Central Pacific Ocean. The investigation is carried out in two steps. First, the evolution of patterns of rainfall anomalies associated with the El Niño episodes of 1982-83, 1987-88 and 1997-98 is examined by using satellite-derived rainfall. It is found that the patterns of rainfall anomalies vary from one episode to another. This finding indicates that it is difficult to predict rainfall anomalies by extrapolation from patterns of previous El Niño episodes.

Next, correlations between sea surface temperatures and rainfall anomalies are analyzed for the synoptic stations in the Philippines. Simultaneous correlations between annual anomalies for temperatures and rainfall are computed. Non-simultaneous or lagged correlations are also computed for seasonal anomalies (temperature values preceding rainfall values). The highest correlations are obtained for the station of Dumaguete. The highest value of the variance in rainfall explained by sea surface temperature for this station is only about 46 percent. The lowest values of the variance explained are found for stations in Luzon. The results of the correlation analysis are used to develop regression equations for predicting rainfall from the sea surface anomalies. Tests of the equations using dependent data show that it is not possible to predict the occurrence of droughts or floods from the sea surface temperatures, except possibly for the station of Dumaguete. In general, one can conclude that El Niño sea surface temperatures alone are not sufficient for predicting droughts in the Philippines.