El Niño and La Niña are part of the El Niño-Southern Oscillation (ENSO), a natural fluctuation in oceanic sea surface temperatures (SSTs) and surface air pressure between the east and west tropical Pacific Ocean. During an El Niño event, easterly trade winds weaken, allowing warmer surface water from the western tropical Pacific Ocean to flow eastward. During a La Niña event, these trade winds intensify, preventing warmer water in the west from moving east, and stacking warm surface water in the west. Large areas of lower surface air pressure and convective precipitation follow the warmer water as it migrates across the tropical Pacific Ocean, altering broad-scale atmospheric circulation patterns (e.g., the Walker Circulation), which can influence weather around the world (Fig. 1).
Figure: El Niño events cause the winter path of the jet streams to move over Southwest, usually delivering more winter rain and snow in the region. Image modfied from the National Oceanic and Atmospheric Administration (NOAA).