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Published December 19, 2011
El Niño Status and ForecastData Source(s): NOAA-Climate Prediction Center (CPC), International Research Institute for Climate and Society (IRI)
Below-average sea surface temperatures (SSTs) across the equatorial Pacific Ocean continued during November, with the most recent weekly SST in the Niño 3.4 region measuring about -1.0 degrees Celsius below average. The cooler-than-average temperatures indicate a weak to moderate La Niña event. Stronger-than-average easterly winds along the equator and suppressed convection in the eastern Pacific are also occuring. The three-month average of the Southern Oscillation Index (SOI) value is currently +1.3, indicating that the atmosphere is fully engaged with the current La Niña SST pattern (Figure 12a). Both the atmospheric conditions and the presence of a large pool of cooler-than-average temperatures in the upper 300 meters of the ocean suggest that La Niña conditions will continue, according to the NOAA-Climate Prediction Center (NOAA-CPC).
Forecasts issued by the International Research Institute for Climate and Society (IRI) indicate La Niña conditions have more than a 60 percent chance of persisting through the February–April period (Figure 12b). Chances for a return of neutral conditions increase to almost 60 percent by the March–May period. There is still some uncertainty about the final strength of the event. About half of the forecast models suggest La Niña will reach moderate strength, while the other half indicate it will remain weak. Most of the models project it will peak in intensity between December and January.
It is expected that a La Niña will bring dry conditions to the Southwest, and seasonal precipitation forecasts issued by NOAA-CPC reflect this. Outlooks call for increased chances for below-average precipitation in all of Arizona and New Mexico through the February–April period (see page 14).Notes:
The first figure shows the standardized three month running average values of the Southern Oscillation Index (SOI) from January 1980 through November 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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