El Nino, La Nina, and California's weather

In a normal year, steady trade winds blow across the tropical Pacific Ocean from east to west, piling up warm surface water near Asia and Australia, while cold, nutrient-rich water rises up, upwelling, off the coast of South America to replace it. That upwelling supports huge numbers of fish, since the nutrients feed the tiny organisms at the bottom of the ocean food web.

Every few years, those trade winds weaken or even reverse for months at a time, a pattern called El Nino, and the pile of warm water sloshes back east across the Pacific instead of staying near Asia. That warm water suppresses the normal upwelling off South America, disrupts fisheries, and shifts storm tracks, often bringing wetter winters to California. The opposite pattern, La Nina, is when trade winds strengthen, upwelling increases, and California winters often turn drier than average. Both patterns show how ocean temperature and atmospheric wind are deeply connected.