Forecast: Rain, drought

By SIMON COLLINS science reporter

Weather forecasters are predicting a mix of tropical downpours and drought as the South Pacific feels the effects of global warming.

Pacific climatologists, who ended a four-day meeting in Auckland yesterday, said the warming was already producing more pronounced weather extremes.

A bigger part of each year's total rain is now falling on the four wettest days of the year. The effect is less rain and possibly droughts in the rest of the year.

Dr Blair Trewin, of the Australian Meteorological Office, said the extremes were strongest in the tropical parts of Southeast Asia and the Pacific Islands.

The trend is not yet clear in New Zealand, but Dr Jim Salinger of the National Institute of Water and Atmospheric Research (Niwa) said it was likely to occur.

"Climate change is going to be seen as increases in extremes first," he said. "This is very important in the Pacific, because an atoll like Aitutaki [in the Cook Islands] depends on what falls out of the sky for its water supplies.

"If it doesn't arrive, there are all sorts of problems. If there is too much high-intensity rainfall, things get washed away."

He said the islands - and possibly parts of New Zealand - would need to build bigger reservoirs to store water for the dry times.

MetService weather ambassador Bob McDavitt said northern New Zealand was in a tropical weather pattern this week.

Warm, muggy conditions built up into late-afternoon thunderstorms.

"We can expect that someone north of Taupo might get 25 to 50mm of rain in an hour, causing flash flooding for a few hours," he said.

An intense storm caused by the same conditions in Hamilton two weeks ago brought 56mm of rain in an hour, closing roads and flooding parts of the city in waist-deep water.

Mr McDavitt said such conditions were common at this time of year.

"We become subtropical during the summer months."

This week's meeting, the third Asia-Pacific workshop on climate variability and trends in Oceania, confirmed a continuing rise in average temperatures in East Asia and the Pacific in line with a worldwide increase of between 0.5C and 0.8C during the 20th century.

Carbon dioxide has increased from 280 to 361 parts a million in the air during the past 200 years.

Most scientists believe that the carbon dioxide from vehicle exhausts and other industrial pollution is causing the warmer weather through the "greenhouse effect", trapping energy from the sun's rays in the lower part of the atmosphere.

Niwa climatologist Dr Brett Mullan said warmer air could hold more water vapour than cold air.

This allowed much more water to build up in clouds before it condensed into rain.

Warmer air also rose more quickly than cold, again allowing more moisture to build up before turning into rain.

"That process builds up and results in heavier, extreme rains over often narrower, smaller regions," he said.

"Therefore the drier areas might expand as well, so you have the apparent contradiction of more heavy rain and more drought."

Dr Salinger said records showed that rising temperatures also raised average sea levels during the past century at an average rate of 1.4mm a year in Auckland, and 1.5mm a year in the Pacific Islands.

Dr Trewin said trends in East Asia and the Pacific over the past 50 years showed that minimum overnight temperatures generally rose faster than daytime maximums.

But the trends in New Zealand were generally weaker than in the tropics, and much of the South Island had cooled slightly.

Latest forecasts: nzherald.co.nz/weather

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