El Niño–Southern Oscillation (ENSO) is a coupled ocean–atmosphere phenomenon centered in the tropical Pacific, characterized by quasi-periodic fluctuations between warm (El Niño), cold (La Niña), and neutral phases. It arises from interactions between equatorial Pacific sea surface temperature anomalies, changes in thermocline depth, and variations in trade winds and the Walker circulation. ENSO modulates global atmospheric circulation, influencing precipitation patterns, temperature anomalies, and extreme weather frequency worldwide. It is a primary source of interannual climate variability and is represented and predicted in climate models using indices such as Niño 3.4 and the Southern Oscillation Index, with substantial implications for seasonal forecasting and climate impact assessments.
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