Feature article
Slightly Higher Risk of Tropical Cyclones for South Pacific Countries near the Date Line
Dr Jim Salinger, Dr Jim Renwick and Stuart Burgess
For some South Pacific countries near the Date Line the chances of tropical cyclone activity are slightly higher than normal for the November 2003 – April 2004 season.
The last few tropical cyclone seasons were relatively ‘quiet’ except last year, with only six occurrences during 2000/01, five in 2001/02, but increasing to ten last season (near the long-term average). Taken over the whole of the South Pacific, on average nine tropical cyclones can occur during the November to April season, but this can range from as few as four in 1994/95, to as many as seventeen in 1997/98, during the last very strong El Niño.
This season, ENSO conditions are expected to remain neutral (no El Niño or La Niña) and total cyclone numbers are expected to be near normal. However, some Pacific Island countries near the Date Line may experience a slightly higher risk of tropical cyclone occurrence than usual, based on analysis of previous ENSO neutral cyclone seasons. Countries with increased risk for the 2003/2004 cyclone season (see Table and Figure below) are Fiji and Tonga.
In the South Pacific tropical cyclones develop in the South Pacific over the wet season, usually from November through April. Peak cyclone occurrence is usually during January, February and March. In seasons similar to the present the highest numbers occur in the region between Vanuatu and Fiji in the South Pacific. Those countries with the highest risk include Vanuatu, New Caledonia, Fiji, Tonga and Niue.
Tropical cyclones require huge amounts of energy to survive, and will form only over specific regions of the globe’s tropical oceans, where conditions are right for their formation and development. The La Niña and El Niño phenomena alter the patterns of climate, altering the risk of a cyclone in different parts of the South Pacific.
Major tropical cyclones bring extremes of wind, rainfall and sea surges, resulting in 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. Individual tropical cyclones are, however, rather unpredictable; so most South Pacific islands are exposed to some degree of risk every year and must be always prepared. In addition, whether it is an active year or a quiet year for tropical cyclones, it only takes one major tropical cyclone striking an island to cause widespread devastation. Therefore island countries must always be prepared.
Table 1. Average number of tropical cyclones occurring within 100 km square for the main island groups of the South Pacific for October – June.
| Country over | Average all years | Average over Neutral ENSO | Probability Occurrences |
|---|---|---|---|
| Fiji | 2.4 | 2.7 | Slightly increased risk |
| Tonga | 2.0 | 2.4 | Slightly increased risk |
| Niue | 1.7 | 2.0 | Slightly increased risk |
| Papua New Guinea | 0.6 | 0.5 | Average risk |
| Vanuatu | 3.2 | 2.9 | Average risk |
| New Caledonia | 2.9 | 2.7 | Average risk |
| Wallis and Futuna | 1.7 | 1.8 | Average risk |
| Samoa | 1.4 | 1.3 | Average risk |
| Tokelau | 0.7 | 0.5 | Average risk |
| Southern Cook Islands | 1.5 | 1.4 | Average risk |
| Tuvalu | 1.2 | 1.0 | Average risk |
| Society Islands | 0.7 | 0.5 | Average risk |
| Northern Cook Islands | 0.8 | 0.3 | Average risk |
| Austral Islands | 0.8 | 0.5 | Average risk |
| Northern NZ | 1.0 | 0.7 | Average risk |
| Tuamotu Islands | 0.5 | 0.2 | Average risk |
| Pitcairn Island | 0.3 | 0.1 | Average risk |
| Solomon Islands | 1.4 | 0.9 | Reduced risk, but cyclones still likely |
| Marquesas Islands | 0.1 | Less than 0.1 | Cyclones unlikely |
(Based on 32 years of data, and for tropical cyclones having mean wind speeds over 34 knots*)
*For the southwest Pacific, “tropical cyclone” is a tropical low-pressure system with an organised wind circulation intense enough to produce sustained gale force winds (at least 34 knots or 63 km/h) near its centre. A “severe tropical cyclone” produces sustained hurricane force winds (at least 64 knots or 118 km/h), and corresponds to the hurricanes or typhoons of other parts of the world.
Figure 1 Tropical cyclone occurrence, neutral ENSO periods October – June, 1970/71 – 2001/02.
