Current Situation and Outlook

3 MARCH 2006

World Meteorological Organization

EL NIСO/LA NIСA

UPDATE

Current Situation and Outlook

Since around the turn of the year, sea-surface temperatures (SSTs) in the central and eastern equatorial Pacific have ranged between half and one degree Celsius below normal. Combined with broader tropical Pacific ocean and atmosphere conditions, this is consistent with the early stages of a basin-wide La Niсa event. However, the development of basin-wide La Niсa conditions at this time of the year is highly unusual, hence there is some additional uncertainty over the extent to which typical La Niсa rainfall and temperature patterns will occur for this event. Furthermore, the La Niсa conditions are expected to be relatively short-lived, with a return to neutral conditions across the equatorial Pacific likely by mid-year or shortly thereafter.

At this time of the year, a continuation, or persistence, of prevailing conditions in the tropical Pacific is least likely, with the March to May period often witnessing marked transitions between phases of El Niсo/La Niсa. Although most computer models and expert interpretation project La Niсa conditions to continue, at least through the next couple of months, a return to neutral conditions is considered most likely by mid-year. As predictions are least reliable at this time of year, careful monitoring will be needed over the next several months for indications of La Niсa conditions persisting further into the year, or of the onset of a rapid evolution toward El Niсo conditions. Neither of these two scenarios is considered likely, but cannot be ruled out at the current time.

SSTs first became colder than normal close to the South American coast in September 2005. This likely influenced local climate patterns through the latter months of 2005, but did not at that time constitute a basin-wide La Niсa event. While conditions remained mostly neutral in the central equatorial Pacific, SSTs in the western equatorial Pacific were, and have continued to be, substantially warmer than normal. Combined with ocean patterns in the Indian Ocean, this situation continues to contribute to unusual and, in places, very damaging climate conditions in surrounding continental regions of the western Pacific and the Indian Oceans.

The recent development towards basin-wide La Niсa conditions has coincided with the approach of the annual cycle temperature maximum in the eastern equatorial Pacific. This is one reason why impacts of the current La Niсa conditions may not be typical of those associated with previous events, especially on the eastern side of the basin. These unusual factors emphasize the importance of considering regionally and locally specific seasonal climate forecasts provided by National Meteorological and Hydrological Services, and information on expected conditions should not rely solely on the presence of El Niсo or La Niсa.

In summary:

• Much of 2005 saw neutral conditions across the tropical Pacific, with no substantial swings toward basin-wide El Niсo or La Niсa conditions.

• The last few months of 2005 saw localized cooling close to the South American coast, sufficient to influence local climate patterns, but not widespread enough to constitute a basin-wide La Niсa.

• The unusually cold surface waters that then spread to the central equatorial Pacific, combined with complementary atmospheric changes across the basin, have led to conditions that resemble the early stages of a basin-wide La Niсa.

• It is unprecedented in the historical record for a La Niсa of substantial intensity or duration to develop so early in the year.

• In view of the unusual timing of this La Niсa development, caution is urged against inferring tendencies towards the typical weather patterns around the world associated with La Niсa events. In the present configuration of the climate patterns, the caution against general inferences is most applicable to the eastern side of the basin, while the western side of the basin continues to show conditions more typical of La Niсa.

• Most models and expert interpretations favour the event dissipating quite rapidly over the next 3 to 6 months. Nonetheless, neither a continuation of La Niсa beyond mid-year nor the development of El Niсo in the second