Higher risk of Tropical Cyclones for South Pacific countries east of the Date Line
By Dr Jim Salinger, Dr Jim Renwick and Stuart Burgess
For some South Pacific countries east of the date line the chances of tropical cyclone activity are higher than normal for the November 2002 – January 2003 period.
The last few tropical cyclone seasons were relatively ‘quiet’, with only six occurrences during 2000/01 and five in 2001/02. However, this season some Pacific Island countries east of the date line are likely to experience a higher risk of tropical cyclone occurrence than is usual. This eastward elongation of the normal pattern is expected because of now well-established weak to moderate El Niño conditions presently affecting the tropical Pacific region. The Southern Oscillation Index is negative, and is expected to remain so throughout the cyclone season. Countries with increased risk over the November 2002 to January 2003 period (see Table 1and Figure 1) are: Wallis and Futuna, Samoa, Tokelau, Niue, and the southern Cook Islands. Tropical cyclones are still very likely about and west of the date line, but a lower than normal frequency of occurrence is expected.
The South Pacific tropical cyclone season coincides with southern hemisphere wet season, usually from November through April. Peak cyclone occurrence is usually during January, February and March. On average, the highest numbers occur in the region around Vanuatu, New Caledonia, and the adjacent Coral Sea.
In seasons similar to the present during November to January, at least one tropical cyclone usually occurs in that region. Taken over the whole of the South Pacific, on average four tropical cyclones can occur in the early part of the season, but this can range from as few as zero in 2000/01, to as many as eleven in 1997/98 (the last El Niño).
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.
Probability of tropical cyclones affecting South Pacific Islands during November 2002 to January 2003
| Area | Average no of cyclones 1970-2001 | Average over Moderate El Niño | Probability of Occurrences |
|---|---|---|---|
| Wallis & Futuna | 0.9 | 1.4 | Increased |
| Samoa | 0.8 | 1.2 | Increased |
| Tokelau | 0.5 | 0.8 | Increased |
| Niue | 0.9 | 1.2 | Increased |
| Southern Cook Islands | 0.7 | 1.4 | Increased |
| New Caledonia | 1.4 | 1.1 | Reduced |
| Vanuatu | 1.6 | 1.1 | Reduced |
| Fiji | 1.1 | 0.9 | Average |
| Solomon Islands | 0.5 | 0.3 | Average |
| Tuvalu | 0.7 | 0.9 | Average |
| Tonga | 0.9 | 0.8 | Average |
| Northern Cook Islands | 0.4 | 0.4 | Average |
| Society Islands/Tahiti | 0.3 | 0.3 | Average |
| Austral Islands | 0.3 | 0.4 | Average |
| Northern New Zealand | 0.4 | 0.2 | Average |
| Southern PNG | 0.1 | Less than 0.1 | Unlikely |
| Tuamotu | 0.2 | Less than 0.1 | Unlikely |
| Pitcairn Island | 0.1 | Less than 0.1 | Unlikely |
Average number of Tropical Cylone occurrences, Moderate El Niño November – January periods, 1970–2001
The December issue of the ICU will provide an update on information relating to any occurrences and further probability of tropical cyclones in our forecast region of the South West Pacific over the remainder of the wet season.