La Niña shifts the pattern of tropical cyclones this season

Higher than usual risks of tropical cyclones are predicted for the melanesian countries of Solomon Islands, Vanuatu and New Caledonia in the coming tropical cyclone season, according to climate scientist Reid Basher of New Zealand’s National Institute of Water and Atmospheric Research (NIWA). "This is because of the La Niña conditions which have begun to affect the South Pacific region’s climate this year" says Dr Basher.

19 October 1998

But while these countries and the Coral Sea are likely to see more tropical cyclones than average, Fiji and Polynesian neighbours in the eastern part of the South Pacific are likely to experience fewer tropical cyclones than average.

[See table below for risks for different island groups.]

Tropical cyclones brew up in the South Pacific region over the wet season, which normally starts in November. Peak numbers occur during the months of January, February and March, but by the end of April the tropical cyclone season has usually finished. Risks are generally higher for the region around Vanuatu, New Caledonia, Fiji and the adjacent Coral Sea. As many as 16 tropical cyclones can occur in a season, or as few as 6.

"Tropical cyclones only occur in specific parts of the globe’s tropical oceans, where conditions are right for their initiation and development. By altering the normal patterns of climate, the El Niño and the La Niña phenomena shift the location of these favourable conditions."

"In the South Pacific basin, the areas of warm seas and cloud and rain that nurture tropical cyclones contract westward during a La Niña, lowering the chance of tropical cyclones in Polynesia, but raising the risk in Melanesia. There is also a southward shift in the pattern in eastern areas of the tropical cyclone region which helps to reduce risks for Tuvalu and Samoa."

According to Dr Basher, no repeat is expected of last year’s dramatic season in which the Cook Islands and French Polynesia in the eastern part of the South Pacific region were badly hit and record numbers of tropical cyclones occurred. "This was due to the strong 1997-98 El Niño, which has now faded away and has been replaced by its opposite sibling, the La Niña. The La Niña has been strengthening and is expected to remain in place throughout the coming tropical cyclone season," says Dr Basher.

The predictions are based on a research study funded by New Zealand’s Public Good Science Fund, in which Dr Basher and colleague Dr Xiaogu Zheng used twenty years of past data to calculate the average chance of a tropical cyclone affecting each place in the region. They showed that there were systematic shifts in the regional pattern of risk according to whether it was a La Niña year or El Niño year.

Major tropical cyclones bring extreme winds, heavy rainfall and sea surges that cause river and coastal flooding, landslides, and extensive damage to crops, trees, houses, power lines, ports and roads. Many lives can be lost. For a small South Pacific island country the whole economy can be severely affected.

Tropical cyclones are rather unpredictable; so most South Pacific islands are exposed to some degree of risk and must be always prepared. "However, travellers should not be too concerned - the chances of any particular Pacific island town or resort being seriously affected is relatively small. Often the damaging core of the tropical cyclone will be over the ocean, and some will be small and weak. For example, Fiji’s Suva has remained unscathed for many years, despite numbers of tropical cyclones affecting other parts of the country" says Dr Basher.

Table of percentage risk for the Pacific Islands follows:

Average chance of a tropical cyclone occurring in any 100 km square
for the main island groups of the South Pacific

(Based on 20 years of data, and for tropical cyclones having winds over 34 knots)

Area	Average over all twenty years	Average over the five La Niña years	Comment
Solomon Islands	10 %	30 %	Greatly increased risk.
New Caledonia	60 %	100 %	Greatly increased risk.
Vanuatu	60 %	80 %	Increased risk
Niue	45 %	40 %	Average risk
Southern Tonga	40 %	30 %	Reduced risk
Fiji	45 %	20 %	Greatly reduced risk
Southern Cook Islands	30 %	10 %	Greatly reduced risk
Tahiti	15 %	5 %	Greatly reduced risk.
Samoa	20 %	5 %	Greatly reduced risk.
Tokelau	5 %	–	Reduced risk, cyclones unlikely
Tuvalu	5 %	–	Reduced risk, cyclones unlikely

The original research may be found in the paper: Basher, R E and X Zheng, 1995, "Tropical cyclones in the Southwest Pacific: spatial patterns and relationships to Southern Oscillation and sea surface temperature", in the Journal of Climate, Vol. 8, Pages 1249-1260.

For further information:

In Pacific islands — contact your local Meteorological Service.