La Niña Beginning to Wane, Forecast Center Says

La Niña conditions, which have played a key role in influencing recent winter weather in the U.S. and other parts of the world, are beginning to wane, and will likely be gone by early to mid-summer, according to the latest outlook from forecasters at the International Research Institute for Climate and Society (IRI). La Niña is a natural climate phenomenon that is characterized by cooler than average waters in the equatorial Pacific Ocean. Like its sibling El Niño, La Niña influences global weather patterns by altering air pressure and predominant winds over the Pacific, which have ripple effects — known to meteorologists as “teleconnections” — in far flung locations. It can make certain  conditions — such as drought in the American Southwest — more likely to take place, while lessening the odds of other outcomes.

Historical sea surface temperature index for part of the equatorial Pacific Ocean. Red shading indicates above average water temperatures, denoting El Niño events, while blue indicates below average temperatures, indicative of La Niña.  

Some events that are typically associated with La Niña — including heavy rainfall in northeastern Australia and wetter than average conditions in the Pacific Northwest — have come to pass this year. However, the winter of 2010/11 has featured some eccentricities that were more closely tied to other climate and weather variables. For example, the repeat blizzards in the Mid-Atlantic and Northeast in December and January are not typical for a La Niña winter. Instead, forecasters think they were more closely linked to an unusual configuration of the Arctic Oscillation and the related North Atlantic Oscillation (NAO).

According to IRI's latest update, the moderate to strong La Niña conditions were observed from mid-August 2010 until this month, when La Niña weakened to moderate strength, meaning Pacific waters have warmed slightly. The likelihood of La Niña conditions still being present is quite high until the summer, dropping from 94 percent during the February to April period to 34 percent during May to July. Typically, La Niña events reach their peak during the winter, and fade as summer approaches.

La Niña is one of the biggest determinants of interseasonal climate variability, particularly in the Northern Hemisphere. This distinguishes it from long-term climate change, which takes place over many decades.