Press Release No. 916
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Geneva, 23 May 2011 (WMO) – The La Niña episode, which caused disastrously wet conditions in certain regions and drought in others, is coming to an end, according to the latest Update issued by the World Meteorological Organization. Development of El Niño or re-development of La Niña is not considered likely for the middle part of the year, but the outlook at this time is not clear for the second half of 2011.
Near-neutral conditions - with the ocean temperatures, tropical rainfall patterns, and atmospheric winds over the equatorial Pacific Ocean being near the long-term average - are considered the most likely scenario for mid-year 2011, it said, but cautioned that this time of year is known to be particularly marked by low forecast skill.
La Niña is characterized by unusually cool ocean surface temperatures in the central and eastern tropical Pacific. It is the opposite of El Niño, which is characterized by unusually warm ocean surface temperatures in the same area. Both events drive the large-scale ocean-atmosphere circulation patterns in the tropics and have important consequences for weather and climate around the globe. Once established, they typically last for 9 months or more.
The current La Niña developed in mid-July 2010 and peaked in January 2011. Sea surface temperatures in the central equatorial Pacific were about 1.5 degrees Celsius colder than average from September 2010 to early March 2011, indicating a moderately strong La Niña event. However, the atmospheric aspects (changes in sea-level pressure, winds, cloudiness, etc) were among the strongest of the last century.
As a result of these atmospheric conditions, parts of northern and eastern Australia, Indonesia, Southeast Asia, and portions of northern South America (e.g. Colombia) suffered from extremely heavy rains in late 2010 and/or early 2011.
The La Niña episode was also linked to below average rainfall in eastern equatorial Africa, and below average rainfall in central southwest Asia and southeastern South America.
- A La Niña event of moderately strong intensity continued through the first quarter of 2011 in the oceans, and very strong intensity in the atmosphere.
- The La Niña event has been weakening in the oceans since about February, but is only now in the process of ending in mid-May. The atmospheric aspects of the event remained very strong through the end of April, and only now are weakening.
- In considering expected climate over coming months, it is important to recognize that atmospheric patterns typical of La Niña may in some regions continue for a couple of months after the disappearance of the cool waters in the tropical Pacific. Detailed seasonal forecasts should be consulted with these possible residual climate effects in mind.
- Looking ahead beyond mid-year 2011, there are currently no clear indications for enhanced risk of El Niño or La Niña in the second half of 2011. The ocean-atmosphere system has a rather low predictability at this time of year. Monitoring for another 1 to 2 months is required to more firmly establish the evolution of the system. Accordingly, near-neutral conditions are currently considered the most likely scenario for the second half of 2011.
The El Niño/La Niña Update is a consensus-based product prepared by WMO in close collaboration with the International Research Institute for Climate and Society (IRI), USA, based on input from climate prediction centres and experts around the world.
Generally, during La Niña episodes rainfall is increased across the western equatorial Pacific, including northern Australia and Indonesia during December-February and the Philippines during June-August and is nearly absent across the eastern equatorial Pacific. Wetter than normal conditions also tend to be observed during December-February over northern South America and southern Africa, and during June-August over South Asia and southeastern Australia. Drier than normal conditions are generally observed along coastal Ecuador, northwestern Peru and equatorial eastern Africa during December to February, and over southern Brazil and central Argentina during June-August.
La Niña episodes also contribute to large-scale temperature departures throughout the world, with most of the affected regions experiencing abnormally cool conditions. These include: below-normal temperatures during December-February over southeastern Africa, Japan, southern Alaska and western/central Canada, and southeastern Brazil; cooler than normal conditions during June-August across India and southeastern Asia, along the west coast of South America, across the Gulf of Guinea region, and across northern South America and portions of central America; and warmer than normal conditions during December-February along the Gulf coast of the United States.
La Niña is also known to be associated with a relatively more active hurricane season in tropical North Atlantic, during June to November.
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