Southwest Pacific Tropical Cyclone Outlook
The NIWA and MetService assessment of named tropical cyclone (TC) activity indicates 8 to 10 named TCs could occur in the Southwest Pacific basin between November 2020 and April 2021. This seasonal outlook is for normal to below normal activity in terms of overall named cyclone systems in the region.
Tropical cyclones have a significant impact across the Southwest Pacific, with the season officially starting in November and lasting until the end of April. For the coming season, important differences are expected between the western and eastern halves of the Southwest Pacific basin and also for early and late season activity.
Elevated TC presence is expected in and around the Coral Sea and north Tasman Sea, especially during the late season between February and April.
Risk of a TC interaction is expected to be higher across the maritime regions around New Caledonia, Norfolk Island and to the northwest of New Zealand. Reduced TC activity is expected east of the International Dateline.
Vanuatu and New Caledonia typically experience the greatest TC activity, with an average of about two or three named cyclones passing close to those islands each year.
For this season, elevated activity is expected for New Caledonia. Near normal activity is expected for Tokelau, Fiji, Vanuatu, the Solomon Islands and Papua New Guinea. Normal or slightly reduced activity is expected for Tonga, Wallis & Futuna, and Samoa. Most other islands to the east of the International Date Line are expected to have reduced TC risk for the season.
Despite the risk reduction in some places, cyclones are still expected for countries that typically experience one or more named cyclones per year. At least three severe cyclones reaching category 3 or higher might occur anywhere across the region, so all communities should remain prepared.
On average, at least one ex-tropical cyclone passes within 550km of New Zealand each year. For the coming season, the risk for an ex-tropical cyclone affecting New Zealand is considered above normal. If an ex-tropical cyclone comes close to the country, there is a near-equal probability of it tracking to either the east or west of the North Island, and landfall of a degrading ex-tropical cyclone is possible.
Significant rainfall, extreme winds, hazardous marine conditions and coastal damage are all possible leading up to and during these events.
At present, sea surface temperature anomalies across the eastern equatorial Pacific Ocean are cool (negative) while the central equatorial Pacific Ocean has recently been trending toward cool conditions.
Atmospheric circulation patterns over French Polynesia and northern Australia indicate ENSO (El Niño-Southern Oscillation) conditions are indicative of the emergence of La Niña. The Australia Bureau of Meteorology monitoring of the Niño3.4 region (central-western equatorial Pacific Ocean) shows sea surface temperature anomalies are below 0.8°C. Oceanic and atmospheric forecasts for ENSO indicate moderate-to-strong La Niña conditions for the TC season are very likely.
Tropical cyclones are categorised in strength from 1 to 5, with 5 being the most intense. Past seasons with conditions similar to present suggest several cyclones that develop could intensify to at least category 3 strength. For the coming season, at least 3 cyclones are anticipated to reach at least category 3 strength, with mean wind speeds of at least 118 km/h winds. Category 5 strength cyclones, where sustained winds exceed 199 km/h, have occurred in some years (known as ‘analogue’ seasons) with similar conditions like what exists ahead of the 2020/21 season. Therefore, all communities should remain alert and well-prepared for severe TC events.
New Zealand should also remain vigilant as the season unfolds. Most historic seasons used in the preparation of this outlook showed multiple ex-tropical cyclones passing within 550 km of the country. Significant wind, waves and rainfall are possible from ex-tropical cyclones.
The effects of ex-tropical cyclones can be spread over a large area, particularly if the decaying ex-tropical cyclone interacts with mid-to-high latitude weather systems.
All communities, regardless of changes in TC risk, should still remain vigilant and be aware if the regional climate situation (including ENSO) changes. As with most years, TC activity is expected to increase during the second half of the season from February-April. Early season TC activity is expected to be largely reduced, except near Fiji, and a potential start to cyclone activity may also occur close to or after the New Year.
NIWA, MetService, MeteoFrance, BoM, NOAA and
Pacific Island National Meteorological Services will all
continue to track the progression of ENSO and TC activity,
with an update to this guidance in January 2021 if
It does not take a direct hit or a severe cyclone to cause significant damage or life-threatening weather. When dangerous weather is forecast, please heed the advice of your local meteorological service, civil defence, or disaster management offices.
New Zealand’s National Institute of Water & Atmospheric Research (NIWA) and Meteorological Service of New Zealand (MetService) formulated this seasonal tropical cyclone outlook, along with contributions from the University of Newcastle and meteorological forecasting organizations from the Southwest Pacific, the Australian Bureau of Meteorology, MeteoFrance and the Pacific Island National Meteorological Services.