La Niña (or “little girl,” and often called El Viejo, anti-El Niño, or simply “a cold event” or “a cold episode”) is characterized by unusually cold ocean temperatures in the Equatorial Pacific Ocean, compared to El Niño that is characterized by unusually warm ocean temperatures (see above). El Niño and La Niña events tend to alternate about every three to seven years, but the time from one event to the next can vary from one to ten years.
A high pressure system over the Pacific Ocean results in a variation of pressure that causes a Walker circulation (named after British statistician and physicist Sir Gilbert Walker [1868–1958]). When the Walker circulation weakens, an El Niño results. A stronger Walker circulation brings about a La Niña.