Summer might see return to cold south-westerlies

Press, 12 July 1985, Page 21

Summer might see return to cold south-westerlies

By

TESSA WARD

Wellington meteorologists will know by September whether the summer will be marked by cold, southwesterly airstreams.

The Southern Oscillation Index (5.0.1.) reading for June was negative, and negative readings for July and August would indicate a likely significant change in weather by early spring, said a Wellington meteorologist, Dr Brett Mullan. If the July and August readings do prove to be negative, the cold, southerly airstreams that prevailed during the 1982-83 summer could be expected, said Dr Mullan. “The east coast tended to be dry and there was more rain in the southern and western regions that summer.”

Sun-spot activity has been one explanation put forward so far to account for last summer’s drought in the eastern regions, particularly in South Canterbury and North Otago. Dr Mullan said that he had no particular explanation to offer for last summer’s weather patterns.

“What we do know is that the 1982-83 weather patterns definitely correlate with a negative 5.0.1. reading,” he said.

In fact, the Wellington Meterorological Office re-

ports that New Zealand has been having a higher frequency of westerlies and south westerlies since the late 1970’s than at any time since 1940’5.

These westerly winds have brought above-average rain in western areas and persistent droughts in some areas. Some of the West Coast has had serious coastal erosion while eroded material has built up on the east coast of both islands. Big spring avalanches in the Southern Alps, a shortage of snow at some skifields east of the Main Divide, and below-average sunshine in areas exposed to the south-west have been other results of the prevailing westerly winds. Since early 1984 the Franz Josef Glacier has advanced significantly as another outcome of the increased precipitation on the West Coast, according to the report. “The shift in weather patterns is not noticeable day to day or even month to month,” the report says. But there is clearly a tendency for more precipitation in western areas and less in eastern areas during periods of increased westerlies. The 5.0.1. reading is a measurement of the pressure between two main influential weather zones in

the southern hemisphere. One is a huge high-pres-sure zone over the tropical Pacific Ocean, north-east of New Zealand, and the other is a low-pressure zone over northern Australia and Indonesia. Although they are thousands of kilometres apart these two zones interact.

As wind from the highpressure zone travel westwards towards the low-pres-sure zone the high-pressure zone weakens in intensity. A negative 5.0.1. reading indicated a particularly weak zone of high pressure over the Pacific Ocean.

“This means that the winds moving westwards, called the south-easterly trade winds, are also weak with a smaller difference in the intensity between the two zones,” Dr Mullan said. “The result is an increase in the frequency of westerly and south-westerly winds over New Zealand, generally bringing cold temperatures.

“We use weather stations at Darwin and Tahiti to gain our readings and we also receive data on the strngth of the trade winds from various parts of the Pacific Ocean.”