WMO: La Niña reappears - still weak, but expected to slightly strengthen
Geneva - La Niña conditions have re-emerged in the tropical Pacific since August 2011, according to the latest Update from the World Meteorological Organization. This La Niña is expected to persist through the end of this year and into early 2012, possibly strengthening to moderate intensity. However, it is likely to be considerably weaker than the recent episode that was linked to flooding and drought in different parts of the world.
The Update is based on input from climate prediction centres and experts around the world and is an authoritative source of information on a phenomenon which has such a widespread impact on weather and climate – and lives and livelihoods – around the globe.
La Niña is characterized by unusually cool ocean surface temperatures in the central and eastern tropical Pacific. It is the opposite of El Niño, which is marked by unusually warm ocean surface temperatures. Both are strongly coupled to the atmospheric circulation in the tropics and are major – but not the only - determinants of the seasonal and year-to-year fluctuations of our climate.
The present La Niña follows closely behind the moderate to strong La Niña that started in September 2010 and ended with neutral conditions established in May 2011, when ocean temperatures, tropical rainfall patterns, and atmospheric winds over the equatorial Pacific Ocean returned to near the long-term average.
The neutral conditions subsequently gave way to a re-emergence of La Niña. By the end of October the La Niña had slowly strengthened to a weak-to-moderate level.
El Niño ruled out before April 2012
Historical precedents 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 unlikely to reach conditions as strong as those of the 2010-11 La Niña event, taking into account forecasts from a large number of computer models.
The Update points to a return to a neutral state during 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.
It is always important to recognize that several other factors influence seasonal climatic patterns in addition to El Niño and La Niña. One noteworthy aspect is the current warmer than normal sea surface temperatures in the western equatorial Indian Ocean, which is not commonly observed 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.
The El Niño/La Niña Update is a consensus-based product prepared by WMO in close collaboration with the International Research Institute for Climate and Society (IRI), USA.
For more information, please contact the World Meteorological Organization:
Clare Nullis, Press Officer
Tel: +(41 22) 730 8478; +(41 79) 7091397 (cell),
E-mail: cnullis[at]wmo.int