Tropical Cyclone Guidance
Dr Jim Salinger, Dr Jim Renwick, and Stuart Burgess
Higher risk of tropical cyclones for some South Pacific countries near and east of the Date Line this year.
Countries with increased risk over this period are Fiji, Wallis and Futuna, Niue, and the southern Cook Islands, and possibly the Solomon Islands, Tuvalu, Tokelau, Samoa, Tonga, and the Austral Islands of French Polynesia. Islands west of the Date Line are still likely to experience tropical cyclones, with a normal rate of occurrence expected (Table 1).
| Country | Average over all years | Average over weak ENSO years | Comment (Risk) |
|---|---|---|---|
| Fiji | 2.3 | 2.6 – 2.8 | Increased |
| Niue | 1.8 | 2.2 – 2.3 | Increased |
| Wallis and Futuna | 1.7 | 2.0 – 2.4 | Increased |
| Southern Cook Islands | 1.4 | 1.9 – 2.1 | Increased |
| Tonga | 2.0 | 2.1 – 2.3 | Average – Increased |
| Tuvalu | 1.1 | 1.2 – 1.8 | Average – Increased |
| Solomon Islands | 1.4 | 1.4 – 1.7 | Average – Increased |
| Samoa | 1.4 | 1.5 – 2.0 | Average – Increased |
| Austral Islands | 0.8 | 0.7 – 1.1 | Average – Increased |
| Tokelau | 0.7 | 0.6 – 1.2 | Average – Increased |
| Northern Cook Islands | 0.8 | 0.9 -1.0 | Average |
| Southern Papua New Guinea | 0.6 | 0.4 – 0.5 | Average |
| Society Islands | 0.8 | 0.4 – 0.8 | Average |
| Tuamotu Islands | 0.4 | 0.1 – 0.4 | Average |
| Pitcairn Island | 0.3 | 0.1 – 0.2 | Average |
| Vanuatu | 3.1 | 2.4 – 3.8 | Variable – Uncertain |
| New Caledonia | 2.8 | 2.1 – 3.1 | Variable – Uncertain |
| Northern New Zealand | 1.0 | 0.8 – 1.3 | Variable – Uncertain |
| Marquesas Islands | 0.1 | 0.0 – 0.1 | Cyclones unlikely |
Table 1 shows the average number of tropical cyclones passing within 555 km (a circle of radius equal to 5° of latitude) of the main island groups of the Southwest Pacific over the full November to May period. The uncertainty range occurs from analysing different sets of analogues for the coming season. (Based on 34 years of data, and for tropical cyclones having mean wind speeds over 34 knots)
This southeastward elongation of the normal pattern is expected because of weak El Niño conditions now affecting the tropical Pacific region, and the extra ocean warmth around and east of the Date Line (Figure 1). The seas are warmer than normal, and are expected to remain so throughout much of the cyclone season. The 2003/04 tropical cyclone season was unusually ‘quiet’, with only four occurrences. However, two of these tropical cyclones reached major hurricane force. One was very destructive, resulting in a national disaster, devastating the island of Niue, with loss of life and property.
Although South Pacific tropical cyclones usually develop over the wet season, between November and April, there can also be an occasional occurence in May. Peak cyclone occurrence is usually during January, February, and March. In seasons similar to the present the highest numbers occurred in the region between Vanuatu and Fiji in the South Pacific. Taken over the whole of the South Pacific, on average 9 tropical cyclones occur during the November to April season, but this can range from as few as 4 in 1994/95 and last season, to as many as 17 in 1997/98 during the very strong El Niño event.
On average, 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. On average, there is a 70% chance that at least one tropical cyclone will occur in the Southwest Pacific before 1 January, increasing to 97% by 1 February. About half of the tropical cyclones that develop reach hurricane force (with) mean wind speeds of at least 64 knots (118 km/h).
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 always be prepared.