El Niño Status and Forecast

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.