February 2009

No large La Niña or El Niño event expected in first half of this year


Although sea-surface temperatures in the Pacific cooled in December 2008, indicating La Niña-like conditions, the latter are already weakening and meteorologists do not expect a prolonged basin-wide La Niña event. Forecasts suggest that the conditions will most likely dissipate over the next couple of months, returning the tropical Pacific to neutral conditions by March-May 2009. Some regions, however, may continue experiencing significant changes in their climate patterns over the coming months.

La Niña is characterized by unusually cold ocean temperatures in the eastern equatorial Pacific, which disrupt the normal patterns of tropical precipitation and atmospheric circulation. During La Niña events, lower-than-normal atmospheric pressure tends to occur over Indonesia and northern Australia and higher-than-normal pressure over the eastern tropical Pacific. El Niño is the opposite condition, characterized by unusually warm ocean temperatures in the central and eastern equatorial Pacific.

The global average temperature for 2008 was slightly lower than that for the previous years of the 21st century, due in part to La Niña. The first quarter of 2008 was characterized by a La Niña event of moderate-to-strong intensity, which began in the third quarter of 2007 and prevailed through May 2008. La Niña conditions have gradually weakened from their peak strength in February 2008.

In December 2008, however, unusually cold sea-surface temperatures developed in the central and eastern equatorial Pacific, with temperatures more than 0.5°C below normal. A coupling of atmospheric wind patterns with these cooler temperatures led to La Niña-like conditions.

As these conditions weaken, the most likelyl outlook for March-May 2009 is for “neutral” conditions . Forecasts for the remainder of 2009 are uncertain at this stage, and the likelihood of El Niño conditions developing is no higher than for those of La Niña.

Some recent global climate patterns have been impacted by the La Niña-like conditions in the tropical Pacific. The impacts are expected to subside over the next couple of months, but a tendency for climate patterns consistent with La Niña may continue over the same period in some regions.

WMO prepares these updates in collaboration with the International Research Institute for Climate and Society, by consulting climate prediction centres around the world and facilitating the development of a consensus. WMO Members will continue to carefully monitor the situation in the tropical Pacific. Over the coming months, the climate forecasting community will provide detailed interpretations of regional climate fluctuations through the National Meteorological and Hydrological Services.

Full update of 17 February 2009

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