WMO El Niño/La Niña Update

Neutral conditions have prevailed over the tropical Pacific, following the dissipation of the 2010-11 La Niña event by early May 2011. However, a few weak remnants of La Niña have persisted, particularly in some atmospheric features, and in recent weeks the surface as well as sub-surface waters in the eastern and central tropical Pacific Ocean have also cooled. Model forecasts and expert interpretation suggest persistence of near-neutral conditions or the re-emergence of La Niña conditions as the possible scenarios for the remainder of 2011. If a La Niña event does indeed occur, current indications are that it would be considerably weaker than the 2010-11 La Niña event. Development of El Niño, however, is considered very unlikely. National Meteorological and Hydrological Services will continue to monitor Pacific Ocean conditions closely over the coming months to assess the likelihood of any re-emergence of La Niña conditions.

A La Niña event of moderate to strong intensity dominated in the central equatorial Pacific during the period of September 2010 to February 2011. Between February and April 2011 a gradual weakening of the oceanic aspects of the La Niña event occurred. Since May 2011 the ocean and atmosphere have been in a neutral state, though some La Niña features persisted in the tropical Pacific atmospheric circulation – particularly, enhanced upper level westerly wind anomalies. From about March to July 2011, tropical Pacific subsurface heat content became slightly warmer than normal, with implications for a possible transition to warm-neutral or even weak El Niño conditions for the remainder of the year. However, this positive heat content anomaly has now ended and reversed to slightly cooler than normal heat content for this time of the year. A pocket of moderate negative sea temperature anomaly has now re-developed at deeper levels in the east-central tropical Pacific, associated with a decrease in the thermocline depth. Sea surface temperatures in the east-central tropical Pacific returned to near-average during June and early July, but in the last several weeks have cooled to slightly below average levels, and are now near the borderline of ENSO-neutral and weak La Niña.

Although the period of April-June is historically favoured for transitions and development of new El Niño and La Niña events, new events can also develop during the third quarter of the year. Presently, rapid development of such an event appears unlikely. However, observations during the recent weeks have drifted toward the cool side of neutral in terms of atmospheric circulation, sea surface temperature, and subsurface ocean temperature. Together, these changes indicate a shift in the odds of weak La Niña forming during the forthcoming months. Overall, the complete set of forecast models indicates a range of possible outcomes for the remainder of 2011. Their consensus continues to favour near-neutral conditions, but only by a small margin over weak La Niña conditions. Given the current uncertainty, close monitoring is required for any firmer signs of possible future developments.

It is always important to recognize that other factors influence seasonal climatic patterns in addition to El Niño and La Niña. There is a need for detailed regional evaluations of prevailing conditions, combining expected El Niño/La Niña influences with influences from other geographic regions as well, to arrive at the best estimates of the weather patterns to expect regionally and locally over the coming months. One noteworthy aspect is the current warmer than normal sea surface temperatures in the western equatorial Indian Ocean. This requires careful monitoring, along with conditions in the tropical eastern Indian Ocean, as these can strongly influence surrounding continental climate patterns. Locally applicable information should therefore be consulted in detailed regional/national seasonal climate outlooks, such as those produced by Regional Climate Outlook Forums (RCOFs) and National Meteorological and Hydrological Services (NMHSs).

In summary:

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 National Meteorological and Hydrological Services.

This El Niño/La Niña Update has been prepared through a collaborative effort between the World Meteorological Organization (WMO) and the International Research Institute for Climate and Society (IRI) as a contribution to the United Nations Inter-Agency Task Force on Natural Disaster Reduction. It has been prepared based on contributions from the African Centre of Meteorological Applications for Development (ACMAD), Asia-Pacific Economic Cooperation (APEC) Climate Centre (APCC), Australian Bureau of Meteorology (BoM), Australian Centre for Sustainable Catchments of the University of Southern Queensland, Badan Meteorologi Klimatologi dan Geofisika (BMKG) – the Meteorological, Climatological and Geophysical Agency of Indonesia, Centro Internacional para la Investigación del Fenómeno El Niño (CIIFEN), China Meteorological Administration (CMA), Climate Prediction Center (CPC) of the National Oceanic and Atmospheric Administration (NOAA) of the United States of America (USA), Climate Variability and Predictability (CLIVAR) project of the World Climate Research Programme (WCRP), Comisión Permanente del Pacífico Sur (CPPS), El Comité Multisectorial encargado del Estudio Nacional del Fenómeno El Niño (ENFEN) of Peru, European Centre for Medium Range Weather Forecasts (ECMWF), Météo-France, IGAD (Inter-Governmental Authority on Development) Climate Prediction and Applications Centre (ICPAC), Instituto Nacional de Meteorologia e Hidrologia (INAMHI) of Ecuador, International Research Institute for Climate and Society (IRI), Japan Meteorological Agency (JMA), Korea Meteorological Administration (KMA), Mauritius Meteorological Services (MMS), Met Office in the United Kingdom (UKMO), National Center for Atmospheric Research (NCAR) of the USA, Southern African Development Community Climate Services Centre (SADC-CSC), Tasmanian Institute of Agricultural Research (TIAR, a Joint Venture between the University of Tasmania and the Tasmanian Government, Australia) and the University of Colorado of USA.