Higher Risk of Cyclones Near, East of Date Line
Media Release 21 September 2006
Higher Risk of Tropical Cyclones Near and East of the Date Line
Weak to moderate El Niño conditions are likely to increase the chances of tropical cyclone activity for several tropical South Pacific countries over coming months.
“For the coming tropical cyclone season, from November 2006 – May 2007, we are likely to see above average numbers of tropical cyclones in several parts of the South Pacific near and east of the Date Line. Countries with increased risk over this period include Fiji, Wallis and Futuna, Tonga, Niue, and the southern Cook Islands,” said NIWA climate scientist Dr Jim Salinger. “Islands west of the Date Line are still likely to experience tropical cyclones, with a normal rate of occurrence.”
Tropical sea surface temperatures, which play an important role in the development of tropical cyclones, are presently above average along the equator across the entire Pacific Basin. “Climate forecasting organisations in the Pacific are in general agreement that we are seeing the development of a weak to moderate El Niño, although the situation is still evolving. This brings above normal risk of tropical cyclones near and east of the Date Line,” said Dr Salinger. “There is a good chance that the first tropical cyclone of the coming season in the South Pacific region may occur before the end of November, about a month earlier than is normal in either a neutral or La Niña seasons.” About ten tropical cyclones on average can be expected over the entire Southwest Pacific region during a weak El Niño season.
“In the Southwest Pacific, tropical cyclones usually develop in the wet season, from November through April, but occasionally occur in May. Peak cyclone occurrence is usually from January to March. In seasons similar to the present, several tropical cyclones usually occur in the region between Vanuatu and Niue with some affecting other areas. About half of the tropical cyclones that develop reach hurricane force with mean wind speeds at least 64 knots (118 km/h).”
For New Zealand, the predicted weak El Niño conditions will not have much effect on the likelihood of experiencing an ex-tropical cyclone. There is an 80% chance of an ex-tropical cyclone passing within 500 km of the country sometime between November and May, with the highest risk districts being Northland and Gisborne. By the time such systems reach New Zealand they are no longer classified as tropical cyclones, but can still cause strong winds and heavy rainfall. The most common months for ex-tropical cyclones affecting New Zealand are January to March.
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. Last season (2005/06) in the South Pacific, Cyclones Larry and Monica were particularly severe, both reaching class 5 in strength. These were especially destructive to coastal and some populated environments in Queensland and the Northern Territory of Australia. Three class 4 tropical cyclones occurred further east, but these missed populated areas.
This tropical cyclone information has been prepared as a collaborative effort between NIWA and Meteorological Services around the Pacific. It has been prepared based on contributions and climate information received from the Meteorological Services of Australia (Bureau of Meteorology), Cook Islands, Fiji, French Polynesia, Kiribati, New Caledonia, Niue, Papua New Guinea, Samoa, Solomon Islands Tonga, Tuvalu, Vanuatu and in the United States National Oceanic and Atmospheric Administration (NOAA) and the International Research Institute for Climate and Society (IRI).