WMO El Niño/La Niña Update

17 November 2011

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Current Situation and Outlook

La Niña conditions were re-established in the tropical Pacific during August, after a brief period of neutral conditions following the significant La Nina of 2010-11. Model forecasts and expert interpretation suggest that La Niña should remain in place until at least the end of 2011, with some further strengthening possible. The current La Niña is likely to be considerably weaker than the La Niña event of 2010-11. Over the coming months, National Meteorological and Hydrological Services and other agencies will continue to monitor and predict Pacific Basin climate patterns in order to assess the strength and longevity of this La Niña event.

Neutral conditions, i.e., neither El Niño nor La Niña, had prevailed in the tropical Pacific between May and August 2011, following the moderate to strong La Niña event of 2010-11. The neutral conditions subsequently gave way to a re-emergence and gradual strengthening of La Niña conditions prevailing at the present time. The key indicators of this have been a consistently positive Southern Oscillation Index (a measure of large-scale fluctuations in monthly or seasonal mean surface air pressure between western and eastern tropical Pacific; high positive index is associated with La Niña and high negative index with El Niño), stronger than average trade winds and cooling across the central and eastern tropical Pacific Ocean, especially below the surface. By the end of October, the La Niña conditions had slowly reached a weak-to-moderate level.

Historical precedence and the latest outputs from forecast models suggest that peak intensity of this La Niña will be reached in late 2011 or early 2012, and that it is very unlikey to reach conditions as strong as those of the 2010-11 La Niña event. A return to a neutral state could then be expected during the typical transition period of the phenomenon from March to May 2012. However, given the uncertainty in the range of varying model outputs, close monitoring is required for firmer signs of the peak strength and likely duration of this event. Development of El Niño is considered highly unlikely prior to the typical transition period of March to May.

Importantly, several other factors influence seasonal climatic patterns apart from El Niño and La Niña. At the regional level, seasonal outlooks will need to assess the relative impacts of both the current La Niña and other relevant factors. A current noteworthy example is the area of warmer than normal sea surface temperatures in the western equatorial Indian Ocean; an uncommon observation during La Niña events. This requires careful monitoring, along with conditions in the tropical eastern Indian Ocean, as these can influence surrounding continental climate patterns. Locally applicable information should therefore be consulted in detailed regional/national seasonal climate outlooks, such as those produced by WMO Regional Climate Centres (RCCs), Regional Climate Outlook Forums (RCOFs) and National Meteorological and Hydrological Services (NMHSs).

In summary:

Weak La Niña conditions re-emerged in August 2011 following a short period of neutral conditions;
Weak to moderate La Niña conditions are expected to persist through the remainder of 2011 and possibly early 2012. This La Niña event is expected to be considerably weaker than the 2010-11 La Niña event;
Development of El Niño is considered highly unlikely before April 2012.

The situation in the tropical Pacific will continue to be carefully monitored. More detailed interpretations of regional climate fluctuations will be generated routinely by the climate forecasting community over the coming months and will be made available through the National Meteorological and Hydrological Services. For web links of the National Meteorological Services, please visit:

http://www.wmo.int/pages/members/members_en.html