Lower risk of tropical cyclones east of the date line but all islands should remain vigilant

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Weak La Niña conditions are likely to alter the chances of tropical cyclone activity for several tropical South Pacific countries over coming months. NIWA and Pacific meteorological organisations are urging island communities to remain prepared as there is always a risk.

Average annual number of Tropical Cyclones, Nov-May periods, all seasons, from 1969/70-2006/07

21 September 2007

“For the coming tropical cyclone season, from November 2007 – May 2008, we are likely to see an average risk of occurrence for those areas of the South Pacific near the Date Line. These countries include Tuvalu, Fiji, Wallis and Futuna, Tonga, Niue, and northern regions New Zealand. However, a reduced risk of tropical cyclones is likely in several parts of the South Pacific east of the Date Line, such as The Cook Islands, French Polynesia, and Pitcairn Island,” said NIWA climate scientist Dr Jim Salinger. “Islands west of the Date Line are still likely to experience tropical cyclones, with a near normal rate of occurrence.”

Tropical sea surface temperatures, which play an important role in the development of tropical cyclones, are presently below average along the equator across the entire Pacific Basin east of the Date Line. “Climate forecasting organisations in the Pacific are in general agreement that we are seeing the development of a weak La Niña, although the situation is still evolving. Atmospheric and oceanic conditions already show La Niña characteristics in the eastern Pacific, but conditions in the western Pacific do not yet exhibit this behaviour. This reduces the risk of tropical cyclones east of the Date Line,” said Dr Salinger. On average six or seven tropical cyclones can be expected over the entire Southwest Pacific region during a weak La Niña season. This compares with an average of nine or ten over all seasons.

“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 in both neutral and La Niña seasons,” said Dr Salinger. In the Southwest Pacific, tropical cyclones usually develop in the wet season, from November through April, but occasionally occur in May. Peak cyclone occurrence is usually from January to March. In seasons similar to the present, several tropical cyclones usually occur in the region between Vanuatu, New Caledonia, Fiji, and Tonga, with some affecting other areas. In an average season about half of the tropical cyclones that develop reach hurricane force with mean wind speeds at least 64 knots (118 km/h).

For New Zealand, the predicted weak La Niña conditions will not have much effect on the likelihood of experiencing an ex-tropical cyclone. There is an 80% 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 affecting New Zealand are January to March.

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.

Last season (2006/07) there were seven tropical cyclones in the South Pacific. Cyclone Xavier was particularly severe, reaching class 5 in strength, fortunately occurring over the seas east of the Solomon Islands, and tracking between Vanuatu and Fiji. Damaging cyclones were: Arthur (class 5) and Zita (class 3) which produced wind related damage and coastal flooding in parts of Southern French Polynesia, cyclone Becky (class 5) with fallen trees and damage to crops in parts of Vanuatu, and cyclone Cliff (class 4) producing minor wind damage and minor to moderate flooding in eastern parts of Fiji.

 
Class Description Mean wind speed (km/h) Central Pressure (hPa)
1 Tropical Depression <62 >995
2 Gale 62-88 995-986
3 Storm 89-117 985-975
4 Hurricane 117-167 <975
5 Major Hurricane >167 <945

Classification of tropical cyclones*

* Thompson, C., Ready, S., & Zheng, X., 1992: Tropical Cyclones in the Southwest Pacific Nov. 1979 - May 1989,  N.Z. Met. Service

For further information:

In the Pacific islands – contact your local Meteorological Service.

In New Zealand – contact: Dr Jim Salinger, NIWA, Auckland

Tel: + 64 9 375 2053 (Bus) or  + 64 27 521 9468 (mobile)

Acknowledgements

This tropical cyclone information has been prepared as a collaborative effort between NIWA and Meteorological Services around the Pacific.  It has been prepared based on contributions and climate information received from the Meteorological Services of Australia (Bureau of Meteorology), Cook Islands, Fiji, French Polynesia, Kiribati, New Caledonia, Niue, Papua New Guinea, New Zealand (Meteorological Service of New Zealand), Samoa, Solomon Islands Tonga, Tuvalu, Vanuatu and in the United States National Oceanic and Atmospheric Administration (NOAA) and the International Research Institute for Climate and Society (IRI).

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 37 seasons of data, and for tropical cyclones having mean wind speeds over 34 knots*)

 
Area Average over all years Average over Weak La Niña
seasons
Comment
Tuvalu 1.1 0.8 Average risk
Fiji 2.3 2.1 Average risk
Wallis and Futuna 1.8 1.8 Average risk
Tonga 2.0 2.1 Average risk
Tokelau 0.8 0.6 Average risk
Northern New Zealand 0.9 0.8 Average risk
Solomon Islands 1.3 0.7 Variable risk – uncertain
Vanuatu 3.0 2.4 Variable risk – uncertain
New Caledonia 2.7 2.4 Variable risk – uncertain
Samoa 1.5 1.1 Variable risk – uncertain
Niue 1.9 1.3 Variable risk – uncertain
Southern Papua New Guinea 0.6 0.3 Reduced risk
Northern Cook Islands 0.8 0.2 Reduced risk
Southern Cook Islands 1.5 0.6 Reduced risk
Society Islands/Tahiti 0.8 0.3 Reduced risk
Tuamotu 0.4 0.0 Reduced risk
Austral Islands 0.8 0.1 Reduced risk
Pitcairn 0.3 0.0 Reduced risk
Marquesas 0.1 0.0 Cyclones unlikely
Western Kiribati 0.0 0.0 Cyclones unlikely
Eastern Kiribati 0.0 0.0 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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Average annual number of Tropical Cyclones, weak Nov-May La Niña periods, from 1969/70-2006/07.