Geneva – A significant La Niña episode, which is affecting climate conditions in different regions of the world, continues in the tropical Pacific Ocean. Its strength is expected to decrease during the course of the coming four months, according to a new Update issued January 25 by the World Meteorological Organization (WMO).
Almost all forecast models predict a continuation of the current La Niña for at least the next 2-4 months, through the first quarter of 2011 and possibly into the second quarter (April or early May).
“The strength of the event is likely to decrease during the course of the coming 4 months,” says the El Niño/La Niña Update.
Beyond that time, the evolution of the El Niño/La Niña cycle is uncertain, it says.
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. Both events disrupt 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 quickly in June and July 2010, following the dissipation of the 2009/2010 El Niño.
Atmospheric indicators (in terms of sea-level pressure, winds, cloudiness, etc.) show this La Niña episode to be one of the strongest of the past century, while oceanic indicators have been at moderate to strong levels, with sea surface temperatures averaging around 1.5 degrees Celsius cooler than normal in the east-central tropical Pacific. There has been a robust ocean-atmospheric coupling, as evidenced by reduced cloudiness and stronger trade winds in association with cooler sea surface temperatures.
Below average sea level pressure and above average sea surface temperature in the western tropical Pacific and eastern Indian Ocean, in association with this La Niña, have led to much above average rainfall in parts of Australia, Indonesia and southeast Asia. This La Niña situation is also believed to be linked to above average rainfall in southern Africa, below average rainfall in eastern equatorial Africa, and below average rainfall in central southwest Asia and southeastern South America.
“Regions typically impacted by La Niña events are advised to take note of the expected continuation of moderate to strong La Niña conditions over the coming 1-2 months and weaker La Niña conditions during March and April,” says the Update.
The outlook is based on numerous numerical prediction models, the typical climatological patterns, and on the presence of above-normal subsurface sea temperatures in the western equatorial Pacific that are slowly moving eastwards.
While many recent regional climate impacts are consistent with the impacts typically associated with La Niña, some differ, and may continue to differ. It is, therefore, important to consult regional climate information and seasonal outlooks that consider both the prevailing La Niña conditions and other factors with potential influence on the local climate.
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.
Background
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.
Source: World Meteorological Organization update on La Niña dated January 25, 2011.
