La Niña is a coupled ocean-atmosphere phenomenon that is the counterpart of El Niño as part of the broader El Niño-Southern Oscillation climate pattern. During a period of La Niña, the sea surface temperature across the equatorial Eastern Central Pacific Ocean will be lower than normal by 3–5 °C. In the United States, an episode of La Niña is defined as a period of at least 5 months of La Niña conditions. The name La Niña originates from Spanish, meaning "the girl", analogous to El Niño meaning "the boy".
Changes to the atmosphere and ocean circulation during La Niña events include:
- Cooler than normal ocean temperatures across the central and eastern tropical Pacific Ocean.
- Increased convection or cloudiness over tropical Australia, Papua New-Guinea, and Indonesia.
- Stronger than normal (easterly) trade winds across the Pacific Ocean (but not necessarily in the Australian region).
- High (positive) values of the SOI (Southern Oscillation Index).