La Niña conditions are expected to gradually strengthen and continue through the Northern Hemisphere winter 2011-12.
During September 2011, La Niña conditions strengthened as indicated by increasingly negative sea surface temperature (SST) anomalies across the eastern half of the equatorial Pacific Ocean. The weekly Niño indices continued their cooling trend and all are currently at or below –0.5oC . Consistent with this cooling, oceanic heat content (average temperature anomalies in the upper 300m of the ocean, remained below-average in response to a shallower thermocline across the eastern Pacific Ocean. Also, convection continued to be suppressed near the Date Line, and became more enhanced near Papua New Guinea. In addition, anomalous low-level easterly and upper-level westerly winds persisted over the central tropical Pacific. Collectively, these oceanic and atmospheric patterns reflect the continuation of La Niña conditions.
Currently, La Niña is not as strong as it was in September 2010. Roughly one- half of the models predict La Niña to strengthen during the Northern Hemisphere fall and winter. Of these models, the majority predict a weak La Niña. In addition, a weaker second La Niña winter has occurred in three of the five multi-year La Niñas in the historical SST record since 1950. However, the NCEP Climate Forecast System predicts a moderate-strength La Niña this winter and one model predicts a strong La Niña, which rivals last year’s peak strength.There is uncertainty as to whether this strong amplitude will be achieved. Thus, at this time, a weak or moderate strength La Niña is most likely during the Northern Hemisphere winter.
Across the contiguous United States, temperature and precipitation impacts associated with La Niña are expected to remain relatively weak during the remainder of the Northern Hemisphere early fall, and to strengthen during the late fall and winter. It is important to note that the strength of U.S. impacts is not necessarily related to the strength of La Niña across the equatorial Pacific. During October-December 2011, there is an increased chance of above-average temperatures across the mid-section of the country. Also, above-average precipitation is favored across the Pacific Northwest, along with a higher probability for drier-than-average conditions across much of the southern tier of the country