Why do I care? Like El Niño, La Niña impacts include drought conditions, or worsening of, below or above average temperature and precipitation fluctuations across the United States which can all be very harmful to crop growth and/ or human health even after the La Niña event is no longer occurring in the Pacific Ocean.

La Niña is essentially the below normal cooling of sea surface temperatures in the eastern tropical Pacific Ocean. La Niña usually follows an El Niño event, but not always. La Niña events, like El Niño events, will vary year to year and with each event.  La Niña can be thought of as opposite to El Niño.  Some scientists use El Viejo (Spanish for "old man") instead of La Niña in their work.

The eastern tropical Pacific Ocean cools during a La Niña event. This occurs because the easterly trade winds around the equator strengthen as the differences in pressure between the eastern and western tropical Pacific increase. Upwelling of colder ocean water in the eastern tropical Pacific from below the surface increases and more of this colder water moves to the west, pushed by the strong easterly trade winds. This causes the precipitation usually found over the western and central tropical Pacific to move farther to the west. This cools the average sea surface temperatures from the normal in the eastern tropical Pacific.

The polar jet stream during the winter in a La Niña event splits into two over the Pacific Ocean. This is because a persistent high pressure area resides south of Alaska and drives the upper portion of the polar jet stream to the north, while the lower portion swings around the high pressure from the south. Over Canada and the northern Great Plains, colder than normal conditions persist with the location of the upper portion of the polar jet. The southern portion of the polar jet brings warm, moist air into the northwestern United States causing much of this area to be wet. During the La Niña event, the southern states surrounding the Gulf of Mexico tend to remain warm and dry. Along the east coast of the United States, warm temperatures persist during the winter months.

Figure C shows the typical weather conditions felt across the US during the winter months in a La Niña event. This is typical of a moderate La Niña event. As seen from the map, much of the United States is affected by the cooling of the Pacific Ocean.

Activities to further explain this module: