Waiting for El Niño

Indicators in the Pacific Ocean suggest that an El Niño event may be on the way, though it is not clear yet whether any substantial development will occur later in 2002.

In early March 2002, the United States National Atmospheric and Oceanographic Administration (NOAA) announced that sea surface temperatures had warmed by two degrees Celsius in the eastern equatorial Pacific close to the South American coast during the previous month. Other data confirm the assessment that an El Niño Southern Oscillation (ENSO) event may be in its early stages. Rainfall has increased over the region that has warmed and, off Peru, the cold-water anchovy has been replaced by tropical species.

Forecasters are cautious, though, about what happens next. The ocean may be warming but there is, as yet, no sign of any significant El Niño signal in the large-scale Pacific wind and pressure patterns.

According to an assessment published by the International Research Institute for Climate Prediction in February, between one third and one half of the current forecast models predict El Niño conditions developing by mid-2002. Most of the remaining predictions indicate near-neutral conditions during the remainder of the year. Very few forecasts suggest a return to cold conditions.

These models are known to perform poorly in predicting El Niño development during the early months of the year and this may well account for the lack of any clear consensus.

The situation should become clearer over coming months. By June, the warming may or may not have reached the point where ocean-atmosphere interaction favours rapid development during the remainder of the year.

The possibility that an El Niño event might develop this year provided an appropriate backdrop for a recent workshop, Forecasting El Niño and La Niña in Indochina, held January 21st-25th 2002 in Hanoi, Vietnam.

The aim of the meeting was to strengthen regional climate forecasting capacity, building on existing institutional expertise and taking advantage of cooperation both within the region and further afield.

Vietnam, Laos and Cambodia are the main focus for this initiative, which is being organized by the Indochina Global Change Network with support from the Asia-Pacific Network for Global Change Research, NOAA in the United States and the Netherlands Foundation for Advancement of Tropical Research. Myanmar was also represented at the meeting. A team of international experts took part in the workshop, adding their specific skills and knowledge to the discussions and assessments.

Organized by the Indochina Global Change Network, the workshop was an important step forward in a series of collaborative activities aimed at dealing with climate impacts within this region. It was notable in bringing together forecast producers and forecast users to ensure that the science was “user-driven” as well as in its focus on the transfer and use of appropriate techniques that would build on existing strengths. To support this approach, a thorough review was undertaken of regional strengths. This highlighted the fact that, though weaknesses may exist in some nations, they can often be met by strengths in others.

Sharing experience proved an important aspect of the meeting. The promotion of ENSO science in Vietnam during the 1990s and the rapid development of public and political awareness of El Niño’s significance during those years provided a sound model for other nations of the region. (World Meteorological Organization Publication UNUP-1063, Once Burned, Twice Shy? Lessons learned from the 1997-98 El Niño, reviews experience in Vietnam and other countries world-wide.)

Participants developed plans for a set of follow-up activities. These fell into three main categories:

past and present: analysis of historical climate data to identify the ENSO signal in the region and monitoring current climate trends;
medium-term future: use of statistical techniques based on persistence and continuity to predict local effects one to three months ahead; and,
the longer-term: use of seasonal and longer-term forecasts based on dynamical models and other techniques issued internationally to provide warning of emerging trends.

The interpretation of ENSO predictions was a recurrent theme at the workshop, both in terms of technical issues such as accuracy and skill and of the words commonly used when forecasts are issued, often with little thought of linguistic and cultural diversity.

The results of one exercise were enlightening, and rather disturbing. Participants were asked to assign a probability of occurrence to terms such as “probable,” “possible,” “likely” and “unlikely.” In the case of “likely,” the mean probability that emerged was close to 60 per cent. But this masked three distinct groups of responses. Some participants interpreted “likely” as meaning a probability of occurrence of 30-40 per cent; some around 60 per cent; and others as high as 75 per cent. The potential for serious, if not disastrous, confusion needs not be emphasized.

During the final plenary session, the workshop participants concluded that regional cooperation had an important role to play in ensuring that the nations of Indochina are better prepared when El Niño next comes to visit.

On the Web
The Tiempo Climate Cyberlibrary lists web sites that contain regular updates of information regarding the development of ENSO events at: www.cru.uea.uk/tiempo/floor0/theme/theme.htm. The Tiempo Climate Cyberlibrary Newswatch service has the latest news.