It became clear by the beginning of the 1980's that the El Niño and Southern Oscillation are intimately related, with the large-scale warming (El Niño) coordinated 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 Niño) is a phenomenon that is inherently due to the coupling between the atmosphere and ocean, and that the crucial interactions between these media are in the tropical Pacific. Important observational studies led to empirical evidence that linked the tropical ENSO events to seasonal climate anomalies over Australia, South America 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 Niño 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), an international program devoted to understanding and predicting the interannual variability characteristic of ENSO. Progress in the second half of TOGA has been impressive and has gone a long way towards fulfilling the original goals of the program.
This paper will review the progress made in understanding ENSO during the last four years, in particular in the observing systems set up to see the evolving warm and cold phases in the tropical Pacific and in the modeling and theoretical advances devised to explain them. The paper will also briefly review the new field of ENSO prediction, in particular the progress of combining coupled atmosphere-ocean models with the real-time data provided by the TOGA Observing System. One of the truly exciting advances is the application of these forecasts for social and economic utility: some examples of current applications will be indicated.