Higher risk of tropical cyclones near and east of the Date Line
For some South Pacific countries near and east of the Date Line the chances of tropical cyclone activity are higher than normal for the November – May season, according to NIWA’s National Climate Centre.
"Countries with increased risk over this period are Fiji, Wallis and Futuna, Niue, and the southern Cook Islands, and possibly Tuvalu, Tokelau, Samoa, Tonga, and the Austral Islands of French Polynesia" said Dr Jim Salinger. "Islands west of the Date Line are still likely to experience tropical cyclones, with a normal rate of occurrence expected."
This south eastward elongation of the normal pattern is expected because of weak El Niño conditions presently affecting the tropical Pacific region, and the extra ocean warmth around and east of the Date Line. 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 three occurrences. However, two of the three 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.
Dr Salinger said "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 occurred in the region between Vanuatu and Fiji in the South Pacific. Taken over the whole of the South Pacific, on average nine tropical cyclones occur during the November to April season, but this can range from as few as four in 1994/95 and last season, to as many as seventeen in 1997/98 during the very strong El Niño event."
"In the South Pacific, tropical cyclones usually develop over the wet season, from November through April, but there can also be an occasional occurrence in May. Peak cyclone occurrence is usually during January, February and March" he said. "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 before 1 January, increasing to 97% by 1 February."
"About half of the tropical cyclones that develop reach hurricane force having mean wind speeds at least 64 knots (118 km/h). For New Zealand we can expect a tropical cyclone to pass somewhere near northern New Zealand in about two out of every three weak El Niño years. If such a cyclone does occur this season, the districts at highest risk are Northland and Gisborne."
For more information, contact your local Meteorological Service.
The following table 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 through 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*)
| Area | Average over all years | Average over Weak ENSO Years | Comment |
|---|---|---|---|
| Fiji | 2.3 | 2.6-2.8 | Increased risk |
| Niue | 1.8 | 2.2-2.3 | Increased risk |
| Wallis and Futuna | 1.7 | 2.0-2.4 | Increased risk |
| Southern Cook Islands | 1.4 | 1.9-2.1 | Increased risk |
| Tonga | 2.0 | 2.1-2.3 | Average – increased risk |
| Tuvalu | 1.1 | 1.2-1.8 | Average – increased risk |
| Solomon Islands | 1.4 | 1.4-1.7 | Average – increased risk |
| Samoa | 1.4 | 1.5-2.0 | Average – increased risk |
| Austral Islands | 0.8 | 0.7-1.1 | Average – increased risk |
| Tokelau | 0.7 | 0.6-1.2 | Average – increased risk |
| Northern Cook Islands | 0.8 | 0.9-1.0 | Average risk |
| Southern Papua-New Guinea | 0.6 | 0.4-0.5 | Average risk |
| Society Islands/Tahiti | 0.8 | 0.4-0.8 | Average |
| Tuamotu | 0.4 | 0.1-0.4 | Average |
| Pitcairn | 0.3 | 0.1-0.2 | Average |
| Vanuatu | 3.1 | 2.4-3.8 | Variable risk – uncertain |
| New Caledonia | 2.8 | 2.1-3.1 | Variable risk – uncertain |
| Northern New Zealand | 1.0 | 0.8-1.3 | Variable risk – uncertain |
| Marquesas | 0.1 | 0.0-0.1 | Cyclones unlikely |
*For the southwest Pacific, "tropical cyclone" is a tropical low-pressure system intense enough to produce sustained gale force winds (at least 34 knots or 63 km/h). 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. In the French language, the term "Cyclone tropicaux" refers to the hurricane phase (64 knots or 118 km per hour or more) but the "Island Climate Update" publication follows the English language definition of "Tropical cyclone" as defined in the World Meteorological Organisation Tropical Cyclone Operational Plan for the South Pacific and South-East Indian Ocean as follows "A non-frontal cyclone of synoptic scale developing over tropical waters and having a definite organised wind circulation with maximum 10-minute average wind speed of 34 knots (63 km per hour) or greater".