GENEVA, Sept 25 (Reuters) - An El Nino event, usually associated with significant changes in rainfall, is likely to develop this month and next in the Pacific, affecting global climate patterns, the World Meteorological Organization (WMO) said on Tuesday.

The phenomenon, characterised by unusually warm ocean temperatures in the tropical Pacific, has been linked previously to drier-than-normal conditions in Australia, Indonesia, the Philippines, northeastern Brazil, southeastern Africa and parts of Asia, the United Nations agency said.

"A weak El Nino may develop in September and October and last until the northern hemisphere winter," the WMO said in a statement.

El Nino is also associated with wetter-than-normal conditions in Ecuador, northern Peru, southern Brazil to central Argentina and parts of eastern Africa, it said.

El Nino winters tend to be mild over western Canada and parts of the United States and wet over the southern United States, it added.

La Nina, its opposite phenomenon which causes an abnormal cooling of waters, ended in April.

The WMO update is based on many different climate forecast models gathered from centres around the world.

"The majority of these climate forecast models say that there is a 'moderately high likelihood' of an El Nino. Having said that, it can't be ruled out that neutral conditions may continue," WMO spokeswoman Clare Nullis told a news briefing.

The Geneva-based WMO promotes cooperation among its 189 member states and their national meteorological and hydrological services and is the U.N. system's voice on weather, climate and water.

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