Scientists predict average rainfall, warmer winter

By SIMON COLLINS science reporter

Weather forecasters have good news for power users - average rainfall and relatively mild temperatures.

The National Institute of Water and Atmospheric Research (Niwa) says the abnormal El Nino weather pattern, which brought cold, dry weather for most of summer, has dissolved.

"We are heading towards neutral at the moment," said Niwa scientist Dr Jim Salinger.

That should mean average rainfall in the next three months over most of the country, including the crucial inland Otago and Southland regions, which supply the main hydro-electric power stations on the Waitaki and Clutha rivers.

The weather should also be warmer than usual because of relatively warm seas around New Zealand, reducing the need for heating.

But Niwa hydrologist Charles Pearson said the extremely dry weather in March and April, when rainfall was less than half the average over much of the country, meant the expected winter rain might still not be enough to stave off power cuts.

"When we do have rainfall on the Southern Alps, there is a chance that some of the precipitation will fall as snow. Even though it's a bit warmer than normal, some snow may be held in the catchments until snowmelt in summer.

"If we get a northeasterly, that tends to bring water, but if we get southerlies we'll get dumping of snow. We'll probably get a mix of those two systems."

New Zealand's long-term weather cycles are dominated by alternating patterns called El Nino and La Nina, known together as the El Nino Southern Oscillation.

In "neutral" conditions, trade winds blow westwards across the Pacific, piling up warm surface water so that sea levels around Indonesia are about 50cm higher than around South America.

Cool water wells up off the South American coast, bringing dry weather to the eastern Pacific, while the hot air in the west brings rain to New Zealand and Australia.

In El Nino years, the trade winds weaken, bringing warm, wet weather in the east but drought over Indonesia and Australia.

Conversely in La Nina years, the trade winds strengthen, and the pattern is a more intense version of the normal warm wet conditions around New Zealand. Scientists believe that about five years ago the world entered a 20 or 30-year cycle when there will be relatively more La Nina years, bringing relatively warm, wet weather.

In line with this prediction, the latest El Nino event, which began last May, proved short-lived, ending when the Australian drought broke in February and March.

Dr Salinger said the world was now in a period called the "predictability barrier" when it was unclear whether the next short-term pattern would be neutral or a new La Nina event. But most forecasters expected neutral conditions to last through the winter.

Mr Pearson said there was a 45 per cent probability that flows in the South Island hydro rivers would be normal, about a one-third probability that they would be less than normal and a 20 per cent probability of above-normal flows.