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El Niño Outlook
The World Meteorological Organization

Produced in collaboration with
International Research Institute
for Climate Prediction (IRI)

27 March 2002


ClimatePatterns in the Pacific

Research conducted overthe past few decades has thrown considerable light on the important role playedby interactions between the atmosphere and ocean in the tropical belt of thePacific Ocean in altering global weather and climate patterns. During El Niñoevents, for example, sea temperature at the surface in the central and easterntropical Pacific Ocean becomes substantially warmer than normal. During La Niñaevents, the sea surface temperatures in these regions become colder than normal.These temperature changes can drive major climate fluctuations around the globeand once initiated, such events can last for 12 months or more. The last El Niñoevent occurred during 1997-1998 and was followed by a prolonged La Niña phasethat extended from mid-1998 to early 2001. El Niño events change the likelihoodof particular climate patterns around the globe, but the events are neverexactly the same.

CurrentSituation and Outlook

Historical records showthe approximate March-June period to be a favoured one for transitions to El Niñoor La Niña, and hence considerable discussion can be expected around this timeof the year regarding possible developments in the tropical Pacific. This yearthe discussion is especially intense and while most expert interpretationsindicate that the likelihood of an El Niño onset in 2002 is the largest it hasbeen since the 1997/98 El Niño event, the prediction of El Niño is still amaturing science. There is currently a range of expert opinion on just howcertain continuing El Niño development is, with a few believing thatdevelopment into a Pacific basin-wide El Niño is most likely but the majoritybelieving such development is only slightly more likely than not, and thereforeemphasizing that uncertainty still exists.

During February, asanticipated in the last statement (issued 6 February2002), warm water emerged at the surface along the coast of South Americaand the easternmost parts of the Equatorial Pacific. This has strongly impactedclimate patterns in surrounding areas including enhanced precipitation incoastal regions of Peru and Ecuador. For example, OCHA reported that heavyrainfall caused floods and landslides, with loss of life and substantial impactson agriculture and power supply.

The question forecastersare now considering is whether this warming will spread over the whole centraland eastern tropical Pacific and lead to the development of a Pacific basin-wideEl Niño. A further westerly wind burst in the Equatorial western Pacificoccurred during February and is likely to kick the ocean further toward El Niñoconditions, though the burst was not as strong as the one in December, that wasbelieved to be responsible for the current warming off the coast of SouthAmerica.

Indications continue to beuncertain at this time from forecast models on whether the situation willdevelop further over the coming months into an El Niño event. The currentconditions beneath the tropical Pacific are still generally thought to beinsufficient alone to guarantee onset of an El Niño, and further developmentswill be watched for in the next few weeks and months. Existing unusually warmconditions in the Equatorial Pacific near the dateline continue to be a factorthat could contribute to developments.

Even if El Niñoconditions do not develop, it is still possible for significant climatefluctuations to occur in the next several months in different parts of theglobe, including the Pacific. For example, sea surface temperatures (SST) alongthe equator around the dateline are currently warmer than normal, which ishaving an effect on the atmospheric circulation in this region and furtherafield. Furthermore, seasonal climate fluctuations have many causes, involvingpatterns of SST beyond the Pacific and factors other than sea-surfacetemperature. For example, regional climate fluctuations can be driven by SSTpatterns in the tropical Atlantic and tropical Indian Oceans. However, forecastsof SST patterns in these ocean basins currently have very limited skill. This islargely due to inadequate observations of conditions beneath the ocean surface,and the lack of understanding of the mechanisms of systematic SST changes inthese ocean basins.

 In summary

  • As anticipated in the last statement, warm water has appeared at the surface close to the coast of South America and is now also evident extending along the Equator into the easternmost parts of the Pacific Ocean.
  • The large-scale situation remains favourable for El Niño, and a westerly wind burst during February in the Equatorial western Pacific is a further favourable factor.
  • Unusually warm waters over a large area already exist near the dateline and are influencing tropical circulation there.
  • Different computer models still vary on whether the situation will develop further into what is commonly referred to as an El Niño event.
  • While most expert opinions see some increase in the likelihood of El Niño since the last statement on 6 February 2002, most emphasize that uncertainty exists, though with the development of an El Niño in 2002 now slightly more likely than not.
  • No evidence of substance is available on the likely strength of El Niño, should it develop. It should be remembered that the 1997/98 event was by many measures, the strongest event during the 20th Century.
  • More information on the risk of El Niño development for 2002 is expected to become available over the next couple of months.

Thesituation in the tropical Pacific will therefore continue to be carefullymonitored and further advisories will be issued. More detailed interpretationsfor regional climate fluctuations are likely to be generated routinely by theclimate forecasting community over the coming months and will be made availablethrough National Meteorological Services.

Monitoringand Forecasting the El Niño/La Niña Phenomenon

The forecasting of PacificOcean developments is undertaken in a number of ways. Complex computer modelsproject the evolution of the tropical Pacific Ocean from its currently observedstate. Statistical forecast models can also capture some of the precursors ofsuch developments. Expert analysis of the current situation adds further value,especially in interpreting the implications of the evolving situation below theocean surface. All forecast methods try to incorporate the effects ofocean-atmosphere interactions within the climate system.

The meteorological andoceanographic data that allow El Niño and La Niña episodes to be monitored andforecast are drawn from national and international observing systems. Theexchange and processing of the data are carried out under programmes coordinatedby the World Meteorological Organization.


This El Niño Outlookstatement has been prepared as a collaborative effort between the WorldMeteorological Organization and the International Research Institute for ClimatePrediction (IRI) as a contribution to the United Nations Interagency Task Forceon Natural Disaster Reduction. It has drawn on contributions from the AustralianBureau of Meteorology, China Meteorological Administration, European Centre forMedium Range Weather Forecasts, IRI, Japan Meteorological Agency, NationalInstitute of Water and Atmospheric Research in New Zealand, Met Office UnitedKingdom, and the Climate Variability and Predictability (CLIVAR) Project of theWorld Climate Research Programme.


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