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Published August 23, 2011
El Niño Status and ForecastData Source(s): NOAA-Climate Prediction Center (CPC), International Research Institute for Climate and Society (IRI)
Sea surface temperatures across the central and eastern equatorial Pacific Ocean were close to average again this month, signaling the continuation of ENSO-neutral conditions. Southern Oscillation Index (SOI) values are also near zero, additional evidence for neutral conditions (Figure 14a). However, similar to last month, the NOAA-Climate Prediction Center (CPC) states there are several weak atmospheric circulation patterns characteristic of La Niña that continue to linger from last winter’s La Niña event, including slightly enhanced easterly winds along the equator and suppressed convection in the eastern Pacific.
The La Niña-like conditions in the atmosphere, albeit weak, and a pool of below-average ocean temperatures below the surface in the eastern Pacific have caused models to increase probabilities for a return of La Niña as early as this fall. Forecasts issued by the International Research Institute for Climate and Society (IRI) indicate a 44 percent chance that La Niña conditions will develop during the November–January period, an increase from 26 percent issued last month. It is important to note that the probability of neutral conditions continuing this fall is 54 percent; the chance for the development of El Niño is very low at 2 percent (Figure 14b).
Seasonal precipitation forecasts for the Southwest reflect in part the idea that a weak La Niña may return and persist through the fall and winter season. An increased chance of below-average precipitation across southern Arizona and New Mexico shows up in seasonal precipitation forecasts issued by the NOAA-CPC for December 2011 through April 2012. This is consistent with dry conditions characteristic of a La Niña event and would be the continuation of a very dry pattern for a second year in a row in the Southwest. The prospect of back-to-back dry winters is bad news for drought-stricken areas in Arizona and New Mexico.Notes:
The first figure shows the standardized three month running average values of the Southern Oscillation Index (SOI) from January 1980 through July 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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