Feature article

Tropical cyclone update

By Jim Salinger, Jim Renwick, Stuart Burgess, and Ashmita Gosai, NIWA

'Yolanda' developed over southern Tuvalu on 1 December, and then tracked south, passing east of Fiji on the 4th, and then southeast over southern Tonga on the 5th. Maximum sustained winds reached 74 km/h, with heavy rainfall. Fortunately no damage was caused by this event. 'Zoe' formed on 24th December from a tropical depression near Tuvalu and Fiji’s Rotuma Island, intensifying to tropical cyclone intensity as it tracked west on the 26th. The system became very intense over the Santa Cruz Islands of Tikopia and Anuta in the Solomon Islands, from the 28th through 30th generating huge seas, with estimated maximum sustained wind speeds of 287 km/h (Category 5, i.e. the strongest in hurricane definitions), and gusts to 350 km/h reported. This event was very destructive to these islands, ripping foliage off most of the vegetation, and wiping out the coconut and other plantations. The islanders were fortunate that they could take shelter in caves, however it will be some time before life returns to normal. The cyclone then tracked southeast to pass between Vanuatu and Fiji on the 30th and 31st, losing intensity.

The tropical cyclone season has yet to peak, and some Pacific Island countries east of the dateline are likely to experience a higher risk of tropical cyclone occurrence than is usual. It should be noted that tropical cyclones are still very likely about and west of the dateline, as has occurred (but with a lower than normal frequency of occurrence there). This eastward elongation of the normal pattern is expected because of the mature moderate El Niño conditions affecting the tropical Pacific region. The Southern Oscillation Index remains negative, and is expected to remain so throughout the peak period of the cyclone season. Countries with increased risk over the January to May 2003 period (see Table 1) are: Wallis and Futuna, Samoa, Tokelau, Niue, the northern and Southern Cook Islands, and Society and Austral Islands of French Polynesia.

In seasons similar to the present during January to May, on average eight tropical cyclones usually occur over the whole of the South West Pacific, but there can be more or less. The peak period of cyclone occurrence is during January, February, and March. On average, the highest numbers occur in the region around Vanuatu and the adjacent Coral Sea.

The below table and figure show the average number of tropical cyclones passing near the main island groups of the Southwest Pacific over the January through May period during moderate El Niño conditions.

Tropical Cyclone Occurrence, Moderate El Niño January–May periods, 1970–2001 (Average number of tropical cyclones)