An El Niño is a temporary change in the climate of the Pacific ocean, in the region around the equator.

You can see its effects in both the ocean and atmosphere, generally in Northern Hemisphere winter. Typically, the ocean surface warms up by a few degrees celsius. At the same time, the place where hefty thunderstorms occur on the equator moves eastward. Although those might seem like small differences, it nevertheless can have big effects on the world's climate. Scripps Institution of Oceanography

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