Prediction of Abnormally Heavy Precipitation over the Equatorial Pacific Dry Zone

Abstract

A simple objective method for using the barometric pressure changes associated with the Southern Oscillation to predict abnormally heavy rainfall over the Pacific equatorial dry zone was applied to 79 years of Darwin sea-level pressure data and precipitation data for a large number of small islands in the equatorial Pacific. It appears that this method, based on a consideration of pressure changes from one year to the next, could provide a prediction one to eight months in advance of heavy precipitation over most of the central and western part of the zone with about 76% effectiveness. There also appears to be some degree of correlation between the amount of pressure departure and the extensiveness (in time and space) of this heavy rainfall. A check into those cases where an associated El Niño occurred indicated that the El Niño usually sets in a few to several months earlier than the heavy precipitation over the central and western equatorial Pacific dry zone and prior to the obvious Darwin pressure deviation. The El Niño development degree does not appear to correlate with the magnitude of the Darwin pressure deviation. The early appearance of El Niño indicates the possibility that it and the variable magnitude of the pressure shift associated with the Southern Oscillation may both be influenced to some extent by other factors such as variations in the antarctic circumpolar atmospheric and oceanic circulation and interhemispheric interaction in the eastern tropical Pacific.