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Average risk of tropical cyclones in South Pacific

NIWA Media Release 26 September 2008

Average risk of tropical cyclones across the South Pacific

Neutral El Niño/Southern Oscillation (ENSO) conditions are likely to give an average chance of tropical cyclone activity for most tropical South Pacific countries over coming months. Communities should remain alert and prepared.

For the coming tropical cyclone season, from November 2008 – May 2009, we are likely to see a normal risk of occurrence over much of the South Pacific. A reduced risk of tropical cyclones is likely in parts of French Polynesia east of the Date Line.

Climate forecasting organisations in the Pacific are in general agreement that there are neutral ENSO conditions are likely to persist from spring into summer in the Southern Hemisphere. As a result, the likelihood is for a normal risk of tropical cyclones. There is a good chance that the first tropical cyclone of the coming season in the South Pacific region may occur before the end of December, which is normal during neutral seasons. On average eight to ten tropical cyclones can be expected over the entire South Pacific region during a neutral ENSO season.

In the South Pacific, tropical cyclones usually develop in the wet season, from November through April, but occasionally occur in October and May. Peak cyclone occurrence is usually from January to March. In seasons with similar climate backgrounds, several tropical cyclones usually occur in the region between Vanuatu, New Caledonia, Fiji, and Tonga, while a few affect other areas. In an average season about half of the tropical cyclones that develop reach hurricane force with mean wind speeds of at least 64 knots (118 km/h).

For New Zealand, the neutral ENSO conditions increases the risk of experiencing an ex-tropical cyclone. There is just over a 4 out of 5 chance of an ex-tropical cyclone passing within 500 km of the country sometime between November and May, with the highest risk districts being Northland and Gisborne. By the time such systems reach New Zealand they are no longer classified as tropical cyclones, but can still cause strong winds and heavy rainfall. The most common months for ex-tropical cyclones to affect New Zealand are January to March.

South Pacific tropical cyclones are grouped into classes ranging from 1 to 5, with 5 being the most dangerous. In an average season, four are likely to reach class 4 with mean wind speeds of at least 64 knots or 118 km/h, and one to two, class 5, with mean speeds in excess of 90 knots or 167 km/h.

Futher notes (PDF)

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