As floods driven by heavy downpours in South Africa, Australia and Sri Lanka swallowed vast tracts of land in the past few weeks, almost every news report blamed La Niña - described as "the ocean-atmosphere phenomenon" by the US government’s National Aeronautics and Space Administration (NASA).
IRIN takes a look at La Niña, what it is, how it affects weather across the world, and whether it can be linked to the current extreme weather events.
What is La Niña?
It is the name given to the cooling of the surface of the central and eastern Pacific Ocean that occurs every two to five years, first observed by the coastal residents of Peru.
“In the first months of each year, a warm southward current usually modified the cool waters [of the eastern Pacific Ocean],” noted science writer Bob Henson and scientist Kevin Trenberth of the US-based University Corporation for Atmospheric Research.
The residents also noticed that every few years the warming started early in December, it was stronger, and lasted as long as a year or two. They called it El Niño, “The Christ Child”.
Then the waters would turn cooler again, and this phenomenon was dubbed “La Niña” or “The Girl Child”.
Researchers later discovered that the phenomenon took place on a much bigger scale, affecting most of the tropical Pacific Ocean. The Pacific is the largest body of water on earth and has a tremendous influence on the global climate.
The impact of El Niño-La Niña on the atmosphere was only realized gradually as bits and pieces of the effect of the interaction between the ocean and the air on the earth’s rainfall patterns and weather began to be discovered.
It began with the work of two scientists studying the collapse of the monsoon rains in India, followed by a drought, in the early 1900s.
Sir Gilbert Walker found a seesaw variation in pressure between the eastern and western Pacific Ocean, said the Earth Observatory, the public arm of NASA.
“Walker found that when air pressure was high at Darwin, Australia, (western Pacific) it was low at Tahiti (eastern Pacific), and when air pressure was low at Darwin, it was high at Tahiti.” He called this pressure seesaw the Southern Oscillation (SO).
In the 1960s, Jacob Bjerknes, a Norwegian meteorologist, recognized that the pressure variations affected rainfall patterns and the weather. He reasoned that the difference in pressure pushed winds westwards, carrying moisture from the ocean and dumping heavy monsoon rains in Indonesia. He named this phenomenon the Walker circulation, after Sir Gilbert.
Bjerknes observed that during El Niño conditions, when the waters off northern Peru were warmer, the surface air pressure was lower as a result. The pressure difference between east and west weakened, and so did the westward trade winds, the US National Academy of Sciences notes on its website.
"As the winds falter, warm moist air rises over the central Pacific instead of farther west, effectively stealing the monsoon rains from India and Indonesia and spawning rainstorms that strike the west coasts of North and South America."
During the La Niña period, the reverse happens - the trade winds become stronger, bringing more rain to Asia.
Collectively the phases of the air-sea interaction in the region are referred to as the "El Niño/Southern Oscillation (ENSO)".
What impact does it have globally?
La Niña and El Niño are still being studied, but scientists have managed to map the weather patterns observed during each episode.
However, neither phenomenon can be said to have caused an individual extreme weather event. One has to consider the influence of other factors, such as local weather anomalies in the atmosphere or sea conditions at the time.
But, generally, during La Niña episodes rainfall increases across the western equatorial Pacific region, which covers northern Australia and Indonesia, from December to February, and the Philippines from June to August, according to the World Meteorological Organization (WMO). South Asia and southeastern Australia also receive above-normal rainfall.
On the other hand, rain is nearly absent in the eastern equatorial Pacific region. From June to August, southern Brazil and central Argentina experience drier conditions, and between December and February, so do coastal Ecuador and northwestern Peru.
Equatorial eastern Africa also usually records drier than normal conditions in the December-February period.
Northern South America and Southern Africa, however, tend to have wetter than normal seasons between December and February.
Over the years, scientists have also discovered that La Niña episodes feed into large-scale temperature deviations from the norm throughout the world, with most of the affected regions experiencing unusually cool conditions.
Below-normal temperatures from December to February have been recorded over southeastern Africa, Japan, southern Alaska, western and central Canada, and southeastern Brazil, while the Gulf Coast of the United States has warmed up.
In the tropical north Atlantic a more active hurricane season was recorded from June to November.
India and southeastern Asia experienced cooler summers in the June-August period. The west coast of South America, the Gulf of Guinea region in West Africa, northern South America and portions of Central America also become cooler.
Can current extreme weather events be linked to La Niña?
2010 has been Australia’s wettest year since 2000, and the third wettest on record, according to the Australian Bureau of Meteorology, which has linked the recent spate of heavy rains to La Niña. The government said the cost of recovery from the resulting floods would run into billions.
The heavy rains in Indonesia, the Philippines and Thailand were also typical of La Niña, the WMO noted in a brief prepared in collaboration with the International Research Institute for Climate and Society at Columbia University in the US.
There was no clear link between La Niña and the recent heavy rains in Sri Lanka, where parts of the country received record rainfall, causing floods and landslides which affected more than 835,575 people, according to the government's Disaster Management Centre.
The UK Met Office, casting some doubt on the link between La Niña and the heavy rainfall in Sri Lanka, noted in a statement that the "current La Niña extends further west than usual, and this is associated with a westward shift in rainfall patterns in the region. Sri Lanka is on the very western edge of this rain."
The La Niña phenomenon is associated with rainfall deficiency in equatorial eastern Africa, and the "current drought in Somalia and northern Kenya is believed to be due to La Niña influence," said the WMO.
In South Africa, floods, lightning and storms, accompanied by heavy downpours in the past few weeks have claimed 41 lives and affected thousands across 33 municipalities. Cobus Oliver, a scientist at the South African Weather Service, said the heavier than usual torrents of rain were "mostly because of the La Niña". He expected the impact to be felt for another six months locally.
WMO projected that the global impact of La Niña would linger into the second quarter of 2011, until April or early May, but the strength of the event was likely to wane over the next four months.
Is this the strongest La Niña?
The strength of a La Niña occurrence is measured by a WMO consensus index - a three-month average of departures from the norm in sea-surface temperatures in an area identified with ENSO. According to WMO, the current La Niña is one of the strongest recorded in the past century.
The Australian Bureau of Meteorology noted that the current episode was exceeded by the La Niña of 1917-18, with the 1975-76 occurrence ranked third. Several other indices have also indicated that the La Niña events of 2010-11, 1975-76, 1917-18, 1955-56 and possibly 1988-89, were among the strongest on record.