called anomalies, are differences in the state of the climate system from normal conditions (averaged over many years) for that time of year. Short-term climate variations are periods of a few months up through a few years that are unusually warm or cool (or wet or dry). The TOGA (Tropical Oceans and Global Atmosphere) Program was the first organized effort to study, understand, and predict the year-to-year variations of the climate system.

Figure 1. Schematic of ENSO. Contours indicate isotherms, with the increasingly darker fields near the equator showing increases in temperature. The top panel illustrates normal conditions in the tropical Pacific Ocean, while the lower one illustrates the warm phase of ENSO. (Courtesy of M. McPhaden, NOAA/PMEL.)

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