By Stephanie Nebehay
GENEVA (Reuters) - La Nina, a weather phenomenon typically linked to flooding in the Asia-Pacific, African drought and a more intense hurricane season over the Atlantic, could occur in a weak form this year, the World Meteorological Organization said Thursday.
A borderline La Nina situation has developed in recent weeks in the tropical part of the Pacific Ocean, where sea surface temperatures have cooled slightly, but there is an equal chance of neutral conditions returning, the United Nations agency said.
"If a La Nina event does occur, current indications are that it would be considerably weaker than the moderate to strong 2010-2011 episode, which ended in May 2011," the WMO said in a statement calling for continued close monitoring.
But there is virtually no prospect of El Nino, its opposite weather phenomenon which warms the Pacific, occurring this year, it said in an assessment based on data from climate prediction centers and experts worldwide.
La Nina, a natural cooling of the Pacific Ocean, occurs every 2 to 7 years, causing major climate fluctuations including altered tropical rainfall patterns, according to WMO expert Rupa Kumar Kolli.
The 2010-2011 La Nina episode was linked to disastrous flooding in parts of Australia, Indonesia, Southeast Asia, and parts of South America including Colombia, the WMO said.
It also contributed to drought in parts of the Horn of Africa, southeastern South America and the southern United States, and weaker winter monsoons in Sri Lanka and southern India.
CAUSE FOR ADDITIONAL ALERT
"The last La Nina situation, which was moderate to strong in intensity, was believed to have caused the drought conditions over the greater Horn of Africa which we all know is undergoing a famine type of situation," Kolli said, referring to parts of Somalia which have been declared famine zones.
"So for them this can be considered to be cause for additional alert," he said.
But the climate over eastern Africa is strongly influenced by the Indian Ocean, which can create opposite effects, so both factors must be taken into account, he added. Experts were now meeting in Entebbe, Uganda to chart a regional outlook.
La Nina weather could return to delay planting of Brazil's grain crop again, forcing Chinese buyers to rely on U.S. Gulf port soybeans longer and putting Brazil's corn output at risk, grain specialists said Tuesday.
Brazil's coffee belt may face more weather risks if rainfall is erratic in the world's top coffee-growing country, as some forecasters fear, during the critical flowering phase that will define next year's crop.
La Nina is also closely associated with a more intense hurricane season over the tropical North Atlantic, Kolli said.
"At this stage either a normal (hurricane) season or a slightly more intense season, that's what we can expect based on a neutral to La Nina type of outlook available to us today."
A low-pressure system pushing northwest through the Gulf of Mexico has a strong chance of becoming a tropical cyclone in the next two days and threatening southern U.S. states, the National Hurricane Center said Thursday.
The warning came after Irene battered the eastern U.S. coast with up to 15 inches of rain at the weekend.
(Editing by Mark Heinrich)