The "El Nino" phenomenon, which sparks climate extremes around the globe, is likely to take hold in the Pacific Ocean by the end of the year and could even do so within weeks, the UN said on Thursday.
There was an 80 per cent likelihood that El Nino could start between October and November and 60 per cent that it would do so between now and end of August, said the UN's weather agency, the World Meteorological Organisation.
Australia's Bureau of Meteorology rates the chance of an El Nino forming by the southern sprint as a 70 per cent prospect.
The El Nino phenomenon - which can lead to extremes including droughts and heavy rainfall across the globe - occurs every two to seven years, when the prevailing trade winds that circulate over surface waters in the tropical Pacific start to weaken.
The phenomenon, which last occurred between June 2009 and May 2010, can play havoc for farmers and global agricultural markets.
It leaves countries like India, Indonesia and Australia drier, increasing chances of wildfires and lower crop production, while leading to heavier rainfall in the eastern Pacific and South American nations, raising the spectre of floods and landslides.
"El Nino leads to extreme events and has a pronounced warming effect," said WMO chief Michel Jarraud.
El Nino drags precipitation across the Pacific, leaving countries including India and Indonesia drier.
India's monsoon rains have already arrived five days later than normal, and the prospect of a weak rainy season has raised fears of lower crop production and rising food prices.
In Indonesia, there are concerns that dry conditions could fan wildfires caused by slash-and-burn techniques used to clear land quickly and cheaply.
Fires on Indonesia's western Sumatra island last June already caused the worst haze in southeast Asia for more than a decade, affecting daily life for millions and sparking a heated diplomatic row with neighbouring Singapore and Malaysia.
Australia also tends to be drier during an El Nino, increasing the risk of bushfires.
El Nino-year winters are generally drier in the north of the United States and Europe.
El Nino also causes heavier-than-normal rainfall in the eastern Pacific and South America - raising the spectre of floods and landslides, as well as shifting nutrient-rich ocean currents that lure fish.
In the past, that has battered local fishing industries and caused diplomatic battles over shifting fishing zones.
The southwest United States and southern Africa, meanwhile, tend to be drier, while east Africa faces heavy rainfall.