"The likelihood of El Nino returning during 2003 is now considered very small," the Geneva-based WMO said in a statement.

But another phenomenon called La Nina, marked by an unusual cooling of the sea's surface, now looked more likely, the WMO warned.

"Cooling has continued during April and May at the surface in the eastern equatorial Pacific at a rate faster than generally expected, and surface temperatures in May were about one degree Celsius below normal," it said.

"Such cooling is consistent with development towards a La Nina event," it added.

The WMO predicted in January that El Nino would last until at least May

El Nino is marked by periodic shifts in sea temperatures in the main Pacific currents and it can have an impact on climate patterns around the world.

It causes drought or lighter-than-normal rainfall across southern Africa, Asia and Australia, but also an increase in rainfall and sometimes flooding in several south American countries.

La Nina and El Nino are associated even though they have opposite effects. A region that sees less rain due to El Nino will have strong rains once La Nina arrives.

The WMO said some of the conditions over the last two months across the tropical Pacific region had led some "expert interpretations to favour quite strongly the rapid move into a La Nina phase that would most likely prevail for the remainder of the year".

But the UN agency stressed that disagreement remained among forecast models with some experts saying that the recent rapid cooling may yet be a temporary development.

The coming two months will determine whether neutral conditions or those of La Nina prevail until the end of the year, the WMO added.

SPACE.WIRE