Understanding and predicting ENSO

Throughout the 1960's and 1970's, oceanographers referred to the large-scale warming of the equatorial eastern and central Pacific as El Nino. This anomalous warming was later shown to be associated with anoma­ lies in the upper ocean thermal structure throughout the equatorial Pacific Ocean. At about the same time, scientists realized that the Southern Oscillation was in­ timately related to the large-scale changes in the trop­ ical Pacific Ocean. In brief, the Southern Oscillation represents the variability in the Indo-Pacific Walker cir­ culation in the tropics, and is manifest as a displacement of the convection over the maritime continent, and is as­ sociated with large-scale anomalies in the surface wind and sea level pressure throughout the tropical Pacific (and into the Indian Ocean) basin. It became clear by the beginning of the 1980's that the El Nino and Southern Oscillation are intimately re­ lated, with the large-scale warming (El Nino) coordi­ nated with a relaxation of the trade winds throughout Pacific Ocean and a displacement of the atmospheric convection into the central Pacific from the maritime continent. Scientists coined the acronym ENSO to uniquely describe this large-scale, interannual climate phenomenon. Throughout the 1980's, theoretical and modeling studies demonstrated that ENSO (hence, El Nino) is a phenomenon that is inherently due to the cou­ pling between the atmosphere and ocean, and that the crucial interactions between these media are in the trop­ ical Pacific. Important observational studies led to em­ pirical evidence that linked the tropical ENSO events to seasonal climate anomalies over Australia, South Amer­ ica and into the tropical Indian ocean, and to anomalies in the midlatitude N. Hemisphere climate, especially over the N. Pacific Ocean and over central and North America during winter. Rasmusson and Wallace [1983] provide a succinct presentation of the salient features of the tropical ENSO phenomenon, with a more recent and comprehensive overview of the tropical and global characteristics of the ENSO phenomenon available in the volume edited by Glanz et al. [1991]. Finally, a report on El Nino and climate that is appropriate for the general public is found in Wallace [1994]. The last four years have coincided with the second half of TOGA (Tropical Ocean/Global Atmosphere),