The southern oscillation, Equatorial Pacific anomalies and El Niño

Abstract

Certain pressure indices (differences in sea level atmospheric pressure be tween Easter Island and Darwin, Australia and between Juan Fernández Island and Darwin), plotted as 12-month running mean values, were previously used for characterizing the Southern Oscillation and for monitoring and predicting significant equatorial Pacific ocean/atmosphere changes, including El Niño occurrences. The value of using additional pressure indices for these purposes is explored here. Since the primary interest was in changes over the Pacific, Darwin was used to represent the Indonesian equatorial low pressure center, but additional sites (Totegegie, Rapa and Tahiti) were used along the South Pacific subtropical ridge. In general, there is remarkable consistency between the trends of the various indices; however, on some occasions inflection points can be noted several months earlier when using one ridge site rather than another, and the amplitude of peaks and troughs in the indices is often much greater when using a particular ridge site. Three- and 6-month running means of the indices, which retain the regular annual cycle as well as the irregular interannual fluctuation, are used here to show the importance of phase relations between the two fluctuations in determining the intensity of anomalous developments. In the case of the unusual 1972 El Niño, the peaks of the two fluctuations were in phase in early 1971 and their troughs were in phase in mid-I972 so that a 14 mb drop in the 3-month running mean value of the Faster-Darwin index took place over an 18-month period. This indicated an extreme weakening of the southeast trade wind system, which we believe to be a causal factor for the severe 1972 event. In the subsequent 1975 event, the Southern Oscillation period shortened, and although the peaks of the two fluctuations were in phase the troughs were not; hence the degree of relaxation was limited and a weak event resulted. Additional evidence is presented to support a close relationship between the southeast trade system, as modified by the Southern Oscillation, and anomalous meteorological and oceanographic conditions in the equatorial Pacific.