A senior scientist at the National Institute of Water and Atmospheric Research, Dr Jim Salinger, says La Nina has given New Zealand hot and dry summers in the south and south-west and wet weather elsewhere for the past few years. But now La Nina has virtually gone.
Scientists know there is at least a 50 per cent chance that El Nino will return in the months ahead.
But they will know for certain in about two months if our summer will be influenced by El Nino or a neutral weather pattern.
El Nino, which means "Christ Child," will create dry weather with westerly winds over the north and northeast, and wet weather in the west and southwest.
It has affected New Zealand before, around 1986 and 1997.
"It caused dramatic droughts in the east of the North Island and Marlborough," Dr Salinger said.
By mid-1998 about $3 million was distributed to drought-stricken farmers in Hawkes Bay, Wairarapa, Marlborough, South Canterbury and North Otago.
The climatic condition affected only our spring and summer, and although it had arrived every few years, it was not necessarily cyclical, Dr Salinger said.
MetService meteorologist Mick Rice says El Nino is caused by a warming of the Pacific Ocean and westerly currents flowing towards South America.
Deep, cold water cannot surface and warm water flows from the west to the east, instead of the reverse. It then creates a high-pressure system in the western Pacific, bringing high winds from the west. "Everything is backwards," Mr Rice said.
In 1997, El Nino caused floods, destructive waves, forest fires, which caused choking smog, and famine in Asia and the Pacific.
International climate scientists say they have already picked up the early signs of its return, such as a warming of the Pacific Ocean and early forest fires in Asia.
Satellite photographs show several forest fires are already burning on the Indonesian island of Sumatra.
While a limited number of fires occur every year, they do not usually begin until later in April, suggesting that El Nino may already be having an effect.
The Indonesian Government is making plans to import rice in anticipation of a drought like the one which devastated agriculture in 1997.
So far, the projections suggest that a new El Nino would be weaker than that of 1997, but the phenomenon is unpredictable.
Mr Rice said El Nino usually lasted about 18 months, but this could vary.
There were many theories about its causes - a slight change in climatic conditions could spark it. "It is a delicate balance."
Herald Online feature: Climate change
Intergovernmental Panel on Climate Change
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Summary: Climate Change 2001
United Nations Environment Program
World Meteorological Organisation
Framework Convention on Climate Change
