Use of the Southern Oscillation in Weather Prediction

Abstract

Information and data recently received from Chilean meteorological authorities provide an improved atmospheric pressure record for Easter Island for 1912–13 and 1942–67. This study indicates that variations in the difference of mean monthly sea level pressure between Easter Island and Darwin, Australia, provide a much more effective aid to forecasting extended periods of abnormally heavy rainfall over the central and western equatorial Pacific than our earlier approach which used just the Darwin pressure trend. The principal advantages of using this forecasting aid over the earlier method are that. 1) it provides a physically significant index with regard to atmospheric and hydrospheric circulation over the Indo-Pacific region, and more clearly reflects fluctuations associated with the Southern Oscillation; 2) it provides a 1–2 month increase in lead time for forecasts of abnormally heavy equatorial Pacific rainfall; and 3) it is much more effective over the 1943–61 period (when the Darwin pressure trend method broke down). Considering these advantages, the equally successful performance for comparable periods prior to and following the break-down interval for the earlier method, plus the fact that a large part of the pressure difference is controlled by the Darwin pressure input, it appears that the use of this aid would also be at least as effective as the earlier method over the rest of its 79-year evaluation period.