La Niña is the cooler, dry phase that alternates with El Niño in the tropical Pacific.
During La Niña, sea-surface temperatures in the central and eastern equatorial Pacific become cooler than average. Stronger-than-usual trade winds help push warm surface water westward and allow cooler deep water to rise near South America.
This ocean pattern affects atmospheric circulation around the world. It can shift rainfall, temperature and storm tracks, but its regional effects differ by season and location. La Niña and El Niño are opposite phases of the El Niño–Southern Oscillation, or ENSO.
La Niña does not mean every region becomes cooler or wetter. Forecast centres identify an event using ocean-temperature and atmospheric indicators, rather than a single local weather observation.