By ANNE BESTON AND NZPA
Scientists expect the dry South Island conditions that caused this year's power crisis to last another 20 years.
Dr Jim Salinger, a senior climatologist at the National Institute of Water and Atmospheric Research (Niwa), said scientists from Australia, New Zealand and the Pacific were now convinced that the Interdecadel Pacific Oscillation (IPO) weather phenomenon switched to its opposite phase in 1998.
"This means slightly wetter weather in the north and east of the North Island, and drier conditions in the south and west of the South Island. We're certainly seeing that now ... in the Southern hydro lakes," he said.
Drastically low levels in the lakes caused the Government to launch a 10-week energy conservation campaign over winter.
Energy Minister Pete Hodgson called off the campaign on September 10 after 6 1/2 weeks, when it appeared lake levels had stopped shrinking. They have since started falling again.
Ralph Matthes, executive director of the Major Energy Users Group, which includes big users such as forestry mills and the Glenbrook steel refinery, said he was "obviously concerned" about the findings, but the industry was now waiting for Mr Hodgson's review of the power crisis.
"We're hoping he will come up with some recommendations by Christmas."
The IPO was first identified in the late 1990s, and its importance only recently recognised.
It was a giant weather pattern, said Dr Salinger, that switched between cooler and warmer sea temperatures over the entire Pacific basin.
If it is warmer in the northern Pacific Ocean, it will be cooler in the east. It swings pendulum-like between the two phases approximately every 20 years, but scientists are still puzzling over exactly how it works.
But they were in no doubt the pendulum had swung into a new phase said Dr Salinger, meaning that prevailing winds changed from south-westerly to north-easterly.
There would be more of the volatile La Nina weather patterns in the north of the country, bringing warmer temperatures but cloudier, wetter summers and more chance of tropical storms.
The north and north-east of the Pacific would be drier. This was already apparent in Tuvalu and Western Kiribati's massive droughts, and wetter conditions in Fiji, Tonga and New Caledonia.
Meteorologists from Pacific island and rim countries meeting in Auckland this week compared notes on their weather, leading to confirmation of the IPO shift.
Federated Farmers national vice-president Tom Lambie said that although farmers were receiving conflicting advice on long-term weather trends, if the IPO shift were confirmed it would at least allow them to plan.
"It would allow us to look at how we harvest water, what irrigation systems might need to be in places they have not traditionally been."
Mr Lambie said the east coast of the South Island had good rainfall over the spring, but it was still dry in pockets from the Mackenzie Country to central Otago.
Dr Salinger said the IPO was in a positive phase from 1922 to 1944, negative from 1946 to 1977, then positive from 1978 to 1998. "Now it's back to negative, and that change is likely to last for the next 25 to 30 years."
In the mid-1950s, when the IPO was in a negative phase, there were power blackouts in the South Island.
A spokeswoman for Energy Minister Pete Hodgson was not sure if the minister knew about the findings because he was overseas.
Intergovernmental Panel on Climate Change (IPCC)
United Nations Environment Program
World Meteorological Organisation
Framework Convention on Climate Change
Executive summary: Climate change impacts on NZ
IPCC Summary: Climate Change 2001