The University of Arizona

La Niña Redux?

August 16, 2011
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Mounting evidence points to a possible return of La Niña this fall. This is not good news for the Southwest where severe to exceptional drought conditions already cover much of Arizona and New Mexico (see drought map below).

Map of drought conditions in AZ and NM through August 2. Credit: CLIMAS and U.S. Drought Monitor

La Niña events historically deliver dry conditions to the Southwest, and this past winter was no exception. A very strong La Niña event developed in July, 2010 and persisted in a strong state until late last spring when it finally waned. During that time, La Niña controlled the weather pattern across the western and central U.S., driving a persistent and strong jet stream right through the middle of country, partitioning cool and wet conditions to the north and warm and dry conditions to the south. La Niña’s fingerprints are all over current precipitation maps like the percent of normal precipitation over the last 12 months (see map below--reds and oranges indicated below-average precipitation).

Percent of normal precipitation across the contiguous U.S. from August 2010 through July 2011. Credit: High Plains Regional Climate Center

While the monsoon rains have not made much of a dent in drought conditions across the region, there is growing concern that La Niña may make a repeat performance as early as this fall. The latest El Niño-Southern Oscillation (ENSO) Advisory produced by the NOAA Climate Prediction Center (NOAA-CPC) notes that several different recent observations across the equatorial Pacific Ocean, including a large area of cooler than average water just below the surface, are signs that we may be in store for a ‘double-dip’ La Niña event. The atmosphere has been slow to catch on that the recent event ended early this summer, as sea surface temperatures in the eastern Pacific returned to near-average levels. Large-scale circulation patterns across the Pacific Ocean, including enhanced easterly winds along the equator consistent with a La Niña event, are still persisting. The atmosphere often lags behind strong El Niño or La Niña events, but this was expected to end early this summer. Instead, this weak but persistent La Niña-like pattern in the atmosphere may help spur on the return of a real La Niña this fall in the form of below-average sea surface temperatures  across the eastern Pacific Ocean. This would, in turn, strengthen the atmospheric circulation pattern that, in large part, brought on the record drought conditions across the southern third of the U.S. this past spring.

It’s not a done deal, however. Forecast models are mixed, but a growing number suggest a ‘double-dip’. NOAA-CPC notes that key forecast model, in particular, is now showing a rapid return to at least weak La Niña conditions by this fall. This has prompted a ‘La Niña Watch’ by NOAA-CPC to meaning that ‘conditions are favorable for the development of La Niña conditions within the next six months.’

Is the potential for a rapid return to La Niña conditions surprising? Maybe not if you look at past strong La Niña events. Dr. Klaus Wolter with the NOAA Earth Systems Research Laboratory noted even last spring that this event may return the following fall season even if it weakened or disappeared during the summer as this one did. His insight, detailed in presented public talks and in his experimental forecast discussion, lies in looking at past La Niña events that had rapid onsets and overall deep temperature anomalies.  He notes that this most recent event tracked well with two other ‘double-dip’ events occurring in 1954-1955 and 1974-1975-both periods with serious and deep drought conditions across the Southwest.

The fact that La Niña may be back this fall may not be surprising, but that is little comfort for those facing the prospect of record drought conditions potentially deepening and expanding across the southern tier of the U.S. this upcoming winter season.