27 November 1997

El Niño predicted to cause more frequent tropical cyclones in South Pacific

Tropical cyclones are predicted to be more frequent in the South Pacific islands this summer, especially in the Cook Islands and French Polynesia, according to New Zealand climate scientists Reid Basher and Xiaogu Zheng at the Wellington-based National Institute of Water and Atmospheric Research (NIWA). "This is another result of the strong El Niño that is currently affecting the globe's climate" says Dr. Basher.

Cyclone Martin's devastating impacts on Manihiki in the northern Cook Islands at the end of October is an early sample of what might be expected over the coming cyclone season. "This cyclone was earlier than normal and further east than normal, which fits the classic El Niño behaviour."

During the last big El Niño, in 1982-83, a record 16 cyclones occurred. "Many of these struck in the Cook Islands and French Polynesia where ordinarily none occur. The number that reached "major hurricane" strength was also higher than usual that season." In a quiet season there may be only 5 cyclones in total across the whole of the South Pacific.

"Tropical cyclones only occur in specific tropical regions, where conditions are right for their initiation and development. By altering the normal patterns of climate, the El Niño phenomenon shifts the location of these favourable conditions. In the South Pacific, the warm tropical seas and areas of cloud and rain that nurture tropical cyclones expand eastward into those areas of Polynesia that normally do not experience cyclones."

In a recent research study of twenty years of past data, Dr. Basher and Dr. Zheng calculated the average chance of a cyclone affecting each place in the region (see table below for different island groups). This showed that there were systematic shifts in the regional pattern of risk for El Niño years. "During strong El Niño events, there were markedly greater chances of cyclones affecting Tuvalu, Samoa, Tonga, Cook Islands and French Polynesia. In addition, the overall risk for the whole region was higher by 28 %."

The tropical cyclone season typically lasts about five months, starting in November and finishing in April. The worst months are January, February and March. Cyclones tend to be more frequent around Vanuatu, New Caledonia, Fiji, and Tonga, but most South Pacific islands are exposed to some degree of risk and must be prepared for them.

"However, the risks while very real should not be exaggerated - the chances of any particular Pacific island town or resort being seriously affected is very small. For much of the time the damaging core of the cyclone will be over the ocean, and some cyclones will be relatively weak. For example, Fiji's Suva has remained unscathed for many years, despite numbers of cyclones affecting other parts of the country."

There is a plus side to the El Niño effect on cyclones-Melanesian countries in the far western part of the South Pacific region are likely to see fewer cyclones this season. The Solomon Islands are unlikely to have any cyclones, and Vanuatu and New Caledonia are likely to experience fewer than average. Fiji's chances remain about average (see table).

Based on 20 years of cyclone data, the annual chance of a cyclone occurring in any 
100 kilometer square for the main island groups are as follows:

                       Average for    Average for years	
Area                    all years     of strong El Niño  Comment

Solomon Islands           10 %                -          Reduced risk, cyclones unlikely
Vanuatu & New Caledonia   60 %               40 %        Reduced risk
Fiji                      50 %               50 %        Normal risk
Samoa                     20 %               40 %        Increased risk
Southern Tonga            40 %               70 %        Increased risk
Southern Cook Islands     30 %               75 %        Increased risk
Tuvalu                     5 %               25 %        Greatly increased risk
Tahiti, French Polynesia  15 %               50 %        Greatly increased risk
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. J. Climate, 8, 1249-1260.

For more information contact Dr. Reid Basher or Dr. Brett Mullan at NIWA, the National Institute for Water and Atmospheric Research, P.O. Box 14 901, Wellington, New Zealand. Tel: +64 4 386 0300 Fax: +64 4 386 0341

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