La Niña is El Niño’s cooler counterpart in the tropical Pacific Ocean. Whereas El Niño exhibits abnormally warm ocean temperatures and a strong atmospheric circulation across the equator, La Niña represents abnormally cold water. The cooler sea surface temperature pattern enhances the circulation in the tropics, called the Walker circulation.
The Walker circulation tends to dominate the weather across the equatorial Pacific. Air flows west toward Indonesia, where water is typically the warmest, and rises. This creates lots of thunderstorms and rain. During El Niño, this circulation is disrupted. The warmest water sloshes to the eastern side of the Pacific near South America. Air ends up rising closer to South America, and it sinks over Indonesia.
La Niña is the exact opposite. It sends the circulation into overdrive.
“During La Niña events … when waters in the western Pacific are even warmer than normal and waters in the eastern Pacific are even colder, it is like someone turned the normal Walker Circulation ‘up to 11,'” writes climate.gov’s Tom Di Liberto. “Warm, moist air rises even more over the Maritime Continent and South America leading to above-average rainfall. In the eastern Pacific, where colder than average waters exist, an enhanced downward branch of the Walker Circulation helps to further reduce the region’s already small rainfall totals.”
In its forecast, Columbia University’s International Research Institute for Climate and Society has increased the likelihood of La Niña to 65 percent by early fall, and a 70 percent chance by next winter. This is up from 50 percent last month.
NOAA will “declare” a La Niña when temperatures across the eastern side of the Pacific have cooled to a temperature departure of 0.5 degrees Celsius below normal, and when the Walker circulation strengthens like we would expect it to during a true La Niña.