El Niño and La Niña are opposite phases of a natural climate pattern across the tropical Pacific Ocean that swings back and forth every 3-7 years on average. El Niño and La Niña alternately warm and cool large areas of the tropical Pacific—the world’s largest ocean—which significantly influences where and how much it rains there.

This shift disrupts the atmospheric circulation patterns that connect the tropics with the middle latitudes, which in turn modifies the mid-latitude jet streams. By modifying the jet streams, ENSO can affect temperature and precipitation across the United States and other parts of the world. The influence on the U.S. is strongest during the winter (January-March), but it lingers into the early spring.

These maps illustrate the typical impacts of El Niño and La Niña on U.S. winter weather. During El Niño, the southern tier of Alaska and the U.S. Pacific Northwest tend to be warmer than average, whereas the U.S. southern tier of states—from California to the Carolinas—tends to be cooler and wetter than average.  During La Niña, these deviations from the average are approximately (but not exactly) reversed.

These impacts have been associated with El Niño and La Niña events in the past. But “associated with” doesn’t mean that all of these impacts happen during every El Niño episode. They may happen as often as 80 percent of the time, or as infrequently as 40 percent of the time.  In other words, the influence of El Niño on U.S. winter climate is a matter of probability, not certainty.

These maps are adapted from originals produced by NOAA Climate Prediction Center.