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Published September 20, 2011
El Niño Status and ForecastData Source(s): NOAA-Climate Prediction Center (CPC), International Research Institute for Climate and Society (IRI)
It’s official. La Niña is back after a brief, four-month hiatus. A La Niña Advisory recently was issued by the NOAA-Climate Prediction Center (CPC), signaling La Niña conditions have developed and are expected to persist for at least the next six months.
Sea surface temperatures continued to decline across the eastern equatorial Pacific Ocean during the past month, crossing the -0.5 degree Celsius threshold for a La Niña event. Weak atmospheric circulation patterns that began during last winter’s La Niña event have continued all summer and have helped drive the redevelopment of the current La Niña. Southern Oscillation Index (SOI) values remain low but are expected to strengthen in the coming months (Figure 12a).
Official forecasts issued by the International Research Institute for Climate and Society (IRI) put the chance of La Niña conditions persisting through the upcoming winter season at greater than 50 percent (Figure 12b). There is a 48 percent chance of ENSO-neutral conditions returning and only a remote chance of an El Niño event forming through the November–January period.
While forecast models are uncertain about the strength of the current La Niña event, most agree a weak event will persist through the winter at the very least. It is interesting to note that the chance of ENSO-neutral conditions returning by the January–March period is 49 percent, and only 1 percent greater than the chance of La Niña conditions continuing during this period. This is an indication of how weak the La Niña conditions are at this point, and how much uncertainty there is in the ability of this event to carry through until the spring.
Weak La Niña conditions are expected to expand and intensify drought conditions across the southern tier of the U.S., including Arizona and New Mexico. Official seasonal precipitation forecasts issued by the CPC indicate increased chances for below-average precipitation in the Southwest until the spring.
The first figure shows the standardized three month running average values of the Southern Oscillation Index (SOI) from January 1980 through August 2011. The SOI measures the atmospheric response to SST changes across the Pacific Ocean basin. The SOI is strongly associated with climate effects in the Southwest. Values greater than 0.5 represent La Niña conditions, which are frequently associated with dry winters and sometimes with wet summers. Values less than -0.5 represent El Niño conditions, which are often associated with wet winters.
The second figure shows the International Research Institute for Climate and Society (IRI) probabilistic El Niño-Southern Oscillation (ENSO) forecast for overlapping three month seasons. The forecast expresses the probabilities (chances) of the occurrence of three ocean conditions in the ENSO-sensitive Niño 3.4 region, as follows: El Niño, defined as the warmest 25 percent of Niño 3.4 sea-surface temperatures (SSTs) during the three month period in question; La Niña conditions, coolest 25 percent of Niño 3.4 SSTs; and neutral conditions where SSTs fall within the remaining 50 percent of observations. The IRI probabilistic ENSO forecast is a subjective assessment of current model forecasts of Niño 3.4 SSTs that are made monthly. The forecast takes into account the indications of the individual forecast models (including expert knowledge of model skill), an average of the models, and other factors.
For a technical discussion of current El Niño conditions, visit :
For more information about El Niño and to access graphics similar to the figures on this page, visit :
Southwest Climate Outlook Staff
- Michael Crimmins, UA Extension Specialist
- Stephanie Doster, Institute of the Environment Editor
- Dan Ferguson, CLIMAS Program Director
- Gregg Garfin, Founding Editor, Institute of the Environment
- Zack Guido, CLIMAS Associate Staff Scientist
- Gigi Owen, CLIMAS Assistant Staff Scientist
- Nancy J. Selover, Arizona State Climatologist
- Jessica Swetish, CLIMAS Publications Assistant
Please direct your Southwest Climate Outlook comments and suggestions to Zack Guido.
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