Feature article

Tropical cyclone guidance for the 2005/06 season

Dr Jim Salinger, Stuart Burgess and Dr Jim Renwick

Figure 1: Average annual number of Tropical Cyclones, Neutral ENSO periods, Nov-May from 1970/71 to 2004/05. (Click for enlargement)

The tropical cyclone season, from November – May, is likely to be near average intensity throughout much of the South Pacific, with a normal frequency of occurrence expected in most areas this season, due to the El Niño/ Southern Oscillation (ENSO) neutral conditions (i.e. no El Niño or La Niña) that are expected to persist in the tropical Pacific over spring and summer (see Figure 1).

For the southwest Pacific, a 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 in the North Atlantic and typhoons of the North Pacific. Southwest Pacific tropical cyclones are grouped into classes ranging from 1 to 5, with 5 being the strongest. On average, four per season reach at least class 4, with mean wind speeds of at least 64 knots or 118 km/h, while two usually reach class 5, with mean speeds in excess of 90 knots or 167 km/h.

In the USA, Hurricanes Katrina and Rita both reached class 5, their maximum sustained winds speeds reaching at least 135 knots or 250 km/h. In the 2004/05 season in the South Pacific, Cyclones Meena, Nancy, Olaf and Percy, which battered the Cook Islands, all reached at least class 4 in strength or higher. These were very destructive to Samoa and the Cook Islands.

About nine tropical cyclones on average can be expected over the entire Southwest Pacific region in an ENSO-neutral season. Tropical sea surface temperatures, which play an important role in the development of tropical cyclones, are presently above average over the seas to the north and east of the Date Line, but are near average elsewhere. In the Southwest Pacific, tropical cyclones usually develop in 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.

In previous seasons similar to the present, several tropical cyclones usually occur (red) in the region around Vanuatu, New Caledonia, and the adjacent Coral Sea, some affecting other areas. For the entire region 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 of at least 64 knots (118 km/h).

For further information: In the Pacific Islands - contact your local Meteorological Service

The full season: November to May

The following table shows the average number of tropical cyclones passing within 5° (550 km circle) of the main island groups of the Southwest Pacific over the full November through May period. Based on 35 years of data, and for tropical cyclones having mean wind speeds over 34 knots.

Country Average over Neutral ENSO years Average over all years Comment
Fiji 2.4 2.3 Average risk
Tonga 2.3 2.1 Average risk
Niue 2.0 1.9 Average risk
Vanuatu 2.8 3.0 Average risk
New Caledonia 2.9 2.8 Average risk
Wallis and Futuna 1.8 1.7 Average risk
Southern Cook Islands 1.4 1.4 Average risk
Samoa 1.3 1.4 Average risk
Tuvalu 1.0 1.1 Average risk
Northern New Zealand 0.7 1.0 Average risk
Southern Papua New Guinea 0.5 0.6 Average risk
Tokelau 0.5 0.7 Average risk
Society Islands 0.6 0.8 Average risk
Austral Islands 0.5 0.8 Average risk
Tuamotu Islands 0.2 0.4 Average risk
Pitcairn Island 0.1 0.3 Average risk
Solomon Islands 0.9 1.4 Cyclones still likely
Northern Cook Islands 0.3 0.8 Cyclones still possible
Marquesas Islands >0.1 0.1 Cyclones unlikely