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Average Risk of Tropical Cyclones in SW Pacific

MEDIA RELEASE THURSDAY 6 OCTOBER 2005

Near Average Risk of Tropical Cyclones Across the Southwest Pacific for the November to May Cyclone Season

For most tropical South Pacific countries the chances of tropical cyclone activity are near normal for the coming season, according to NIWA climate scientist Dr Jim Salinger.

“The tropical cyclone season, from November – May, is likely to be near average intensity throughout much of the South Pacific with a normal frequency of occurrence expected in most areas this season, due to the El Niño/Southern Oscillation (ENSO) neutral conditions (i.e. no El Niño or La Niña) that are expected to persist in the tropical Pacific over spring and summer.

For the southwest Pacific, a tropical cyclone is a tropical low-pressure system intense enough to produce sustained gale force winds (at least 34 knots or 63 km/h). A “severe tropical cyclone” produces sustained hurricane force winds (at least 64 knots or 118 km/h), and corresponds to the hurricanes in the North Atlantic and typhoons of the North Pacific. Southwest Pacific tropical cyclones are grouped into classes ranging from 1 to 5, with 5 being the strongest. On average four per season reach at least class 4 with mean wind speeds of at least 64 knots or 118 km/h, while two usually reach class 5 with mean speeds in excess of 90 knots or 167 km/h.

In the USA, Hurricanes Katrina and Rita both reached class 5, their maximum sustained winds speeds reaching at least 135 knots or 250 km/h. In the 2004/05 season in the South Pacific Cyclones Meena, Nancy, Olaf and Percy, which battered the Cook Islands, all reached at least class 4 in strength or higher. These were very destructive to Samoa and the Cook Islands.

“About nine tropical cyclones on average can be expected over the entire Southwest Pacific region in an ENSO-neutral season. Tropical sea surface temperatures, which play an important role in the development of tropical cyclones, are presently above average over the seas to the north and east of the Date Line, but are near average elsewhere.”

“In the Southwest Pacific, tropical cyclones usually develop in the wet season, from November through April, but there can also be an occasional occurrence in May. Peak cyclone occurrence is usually during January, February and March” he said. “In seasons similar to the present, several tropical cyclones usually occur in the region around Vanuatu, New Caledonia, and the adjacent Coral Sea, some affecting other areas. For the entire region there is a 70% chance that at least one tropical cyclone will occur before 1 January, increasing to 97% by 1 February.”

“About half of the tropical cyclones that develop reach hurricane force (having) mean wind speeds at least 64 knots (118 km/h). For New Zealand the risk is slightly less than one passing somewhere near northern New Zealand, (an occurrence is likely in about 2 out of 3 seasons similar to the present) with the highest risk districts being Northland and Gisborne.”

ENDS

The full season: November to May

The following table shows the average number of tropical cyclones passing within 5º (550 km circle) of the main island groups of the Southwest Pacific over the full November through May period. (Based on 35 years of data, and for tropical cyclones having mean wind speeds over 34 knots*)

Area Average over Neutral ENSO Years Average over all years Comment
Fiji 2.4 2.3 Average risk
Tonga 2.3 2.1 Average risk
Niue 2.0 1.9 Average risk
Vanuatu 2.8 3.0 Average risk
New Caledonia 2.9 2.8 Average risk
Wallis and Futuna 1.8 1.7 Average risk
Southern Cook Islands 1.4 1.4 Average risk
Samoa 1.3 1.4 Average risk
Tuvalu 1.0 1.1 Average risk
Northern New Zealand 0.7 1.0 Average risk
Southern Papua-New Guinea 0.5 0.6 Average risk
Tokelau 0.5 0.7 Average risk
Society Islands/Tahiti 0.6 0.8 Average risk
Austral Islands 0.5 0.8 Average risk
Tuamotu 0.2 0.4 Average risk
Pitcairn 0.1 0.3 Average risk
Solomon Islands 0.9 1.4 Cyclones still likely
Northern Cook Islands 0.3 0.8 Cyclones still possible
Marquesas Less than 0.1 0.1 Cyclones unlikely

* For the southwest Pacific, a tropical cyclone is a tropical low-pressure system intense enough to produce sustained gale force winds (at least 34 knots or 63 km/h). A “severe tropical cyclone” produces sustained hurricane force winds (at least 64 knots or 118 km/h).

In the French language, the term "Cyclone tropicaux" refers to the hurricane phase (64 knots or 118 km per hour or more) but the “Island Climate Update” publication follows the English language definition of “Tropical cyclone” as defined in the World Meteorological Organisation Tropical Cyclone Operational Plan for the South Pacific and South-East Indian Ocean as follows “A non-frontal cyclone of synoptic scale developing over tropical waters and having a definite organised wind circulation with maximum 10-minute average wind speed of 34 knots (63 km per hour) or greater”.


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