La Niña - more cyclones from the tropics possible


The chances of a remnant tropical cyclone crossing northern New Zealand is raised this summer and autumn, according to research by NIWA climate scientists. And there is a very real chance that some part of New Zealand will see either the high winds or heavy rainfall these systems produce.

"During La Niña episodes the risk of a cyclone coming south and reaching New Zealand from the tropics increases," said senior NIWA climate scientist Dr Jim Salinger. "On average, Northland experiences two direct hits per decade, or one chance in five in any given cyclone season.

"In La Niña years the risk increases to one chance in two for Northland. In past La Niña years, at least one cyclone seems to have affect some part of New Zealand. So it is fair to expect some area of New Zealand to receive their severe weather effects during the period between January and April," he said.

"On average, about 11 tropical cyclones form in the South West Pacific in any one season, which runs from November to April. Of these, only one is likely to have major hurricane intensity winds of 166 km/h or more," said Dr Salinger. "Once these cyclones leave the tropics and head south they may change into a strong mid-latitude storm, often with high winds and heavy rain. It is almost impossible that a cyclone of Hurricane Mitch intensity would affect New Zealand.

"During La Niña events, the atmospheric and oceanic conditions are more favourable for the birth of tropical cyclones in the Coral Sea. Tropical cyclones get their energy from the warm tropical ocean. This increases the chance of finding tropical cyclones in our neighbourhood this summer.

"This summer a strong La Niña event is expected to affect New Zealand. In the weak La Niña summer of 1996/97 Cyclone Fergus brought over 300 mm of rain in the far north and 242 mm at Whitianga. During Cyclone Drena wind gusts as high as 124 km/h occurred at Mokohinau Island in the Hauraki Gulf," he added.

During previous La Niña seasons, 4 cyclones affected New Zealand in 1996/97, 2 in 1988/89, 2 in 1975/76, 2 in 1973/74 and 1 in 1970/71.

Last month NIWA announced that because of the La Niña event, more tropical cyclones would occur in Melanesia this year, but fewer would occur in Polynesia.



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Archived on 15 April 2019