Data Sources: NOAA-Climate Prediction Center (CPC), International Research Institute for Climate and Society (IRI)
Near-average sea surface temperatures (SSTs) continue in the tropical Pacific Ocean and the El Niño-Southern Oscillation (ENSO) remains neutral. Atmospheric pressure patterns reflected in the Southern Oscillation Index (SOI) and winds near the surface and at high altitudes are all very close to average for this time of year, which also reflect neutral conditions (Figure 12a).
Official sea-surface temperature outlooks issued jointly by the NOAA-Climate Prediction Center (CPC) and International Research Institute for Climate and Society (IRI) suggest the continuation of neutral conditions continuing into spring 2014. The CPC notes that dynamical models show a slight warming trend over the winter season, while statistical models indicate a slight cooling trend. In both cases, there are no dramatic shifts towards El Niño or La Niña. Combined, these models indicate a strong likelihood—greater than 80 percent—that neutral conditions will continue into the January–March period (Figure 12b). There are only small chances that an El Niño or La Niña will emerge. The chance that an El Niño event will develop rises to 30 percent by late spring, but confidence is low in this long outlook period.
ENSO-neutral conditions make it difficult to forecast winter precipitation. While La Niña and El Niño events tend to bring drier- and wetter-than-average conditions to the Southwest, respectively, neutral conditions are less definitive. Many parts of the Southwest have experienced three consecutive dry winters during which La Niña was present twice, and last winter experienced a neutral event. Another winter with below-average precipitation would further stress reservoir storage, which is very low on the Rio Grande and is approaching thresholds on the Colorado that trigger forced water conservation strategies.
The first figure shows the standardized three-month running average values of the Southern Oscillation Index (SOI) from January 1980 through August 2013. 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.