El Niño–Southern Oscillation (ENSO)

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The largest year-to-year swing in the climate system, and the clearest case of ocean circulation and atmosphere driving each other. In the normal state, trade winds pile warm water in the western Pacific and let cold water rise in the east. When the trades weaken, the warm pool slides east, the thermocline in the eastern Pacific deepens, and coastal upwelling off Peru brings up warm nutrient-poor water instead of cold nutrient-rich water — the anchoveta fishery collapses and rainfall patterns shift across the tropics. NOAA indexes the state by sea surface temperature anomalies in the Niño-3.4 region, 5°N–5°S and 170°W–120°W. As of the Climate Prediction Center discussion of 9 July 2026 an El Niño Advisory was in effect, with El Niño conditions present and forecast to continue.