Correlation Structure of the El Nino/Southern Oscillation Phenomenon

Relationships among the atmospheric phenomena associated with the Southern Oscillation and El Nino are investigated, using the Comprehensive Ocean-Atmosphere Data Set (COADS) of marine surface observations from ships of opportunity and the World Monthly Surface [Land] Station Climatology (WMSSC) for the period 1950-79. Annual mean (April March) sea level pressure at Darwin, Australia is used as an index of the Southern Oscillation. Results are based on simple linear correlation techniques, stratified by season as in the Rasmusson and Carpenter (1982) composite. Correlations on the order of +0.9 are observed between Darwin pressure, sea surface temperature (SST) and rainfall in the equatorial central Pacific, and zonal wind in the equatorial western Pacific. Relations among these variables are strongest from July through November, when the month to month autocorrelation is also at its strongest. Sea surface temperature along the Peruvian coast and pressure in the eastern Pacific are also most strongly coupled to the Southern Oscillation during these months, which correspond to the cool season in that region. The amplitude of the tropical pressure and central Pacific SST anomalies associated with the Southern Oscillation appear to be just as large and the relationships between them just as coherent during positive excursions of the Southern Oscillation (cold episodes) as during negative excursions (warm episodes). Lead/lag relationships among climatic variables associated with the Southern Oscillation and El Nino events along the South American coast are also examined in the context of the same seasonal stratification. Our results are generally consistent with the traditional view that the Southern Oscillation is, to first order, a standing oscillation with geographically fixed nodes and antinodes.