Southwest Pacific Tropical Cyclone Outlook

Media Release 14 October 2016

Southwest Pacific Tropical Cyclone Outlook: Moderate La Niña or neutral tropical conditions expected to produce near average activity across most islands

Meteorological and climate analysis centres across the Southwest Pacific are indicating near average numbers of tropical cyclones (TC) for the 2016–17 season (November 2016 to April 2017). The 30-year average number of all Southwest Pacific tropical storms that formed between 1981-2010 is 12.4 for the November and April TC season. The average number of storms that developed into named TCs (Category 1 or stronger) during the same interval is 10.4 for the SW Pacific basin [between 135°E (mid-Gulf of Carpentaria) and 120°W (French Polynesia)]. The outlook indicates that 8 to 10 named TCs are expected for the coming season. TC activity is elevated for the Pacific Island countries to the north of the Coral Sea and close to the International Date Line near Tonga and Niue (see Figure 1).

It should be recognised that the six-month outlook reflects an expectation of overall elevated activity during both the early season (November to January) and the late season (February to April) particularly west of the Dateline. Note that the TC activity outlook for islands like New Caledonia, Fiji, Vanuatu and Tonga indicates two or more cyclones could interact with each of those countries during the season despite subtle projected differences from normal. At least 5-6 severe TCs (Category 3 or higher [See http://www.bom.gov.au/cyclone/about/intensity.shtml]) are expected to occur anywhere across the Southwest Pacific during the season. All communities should remain vigilant and follow forecast information provided by their national meteorological service.

On average, New Zealand experiences at least one ex-tropical cyclone passing within 550km of the country every year. For the coming TC season, the risk for New Zealand is near normal. If an ex-tropical cyclone comes close to the country, the current background climate conditions suggest it has a greater probability of passing east of Auckland and the North Island. Significant rainfall, damaging winds and coastal impacts can occur leading up to and during these episodic events.

Outlook analysis

Ocean and atmosphere forecasts for ENSO indicate weak-to-moderate La Niña or neutral conditions are most likely for summer. Presently, sea surface temperature anomalies across the central and eastern Equatorial Pacific Ocean and the atmospheric circulation patterns over French Polynesia and northern Australia indicate conditions are close to neutral but leaning toward La Niña. Taking this climate outlook scenario into account, normal TC activity can be expected for most islands in the Southwest Pacific, with 8 to 10 named TCs forming across the region during the November 2016–April 2017 period. Despite the outlook for mostly normal conditions for this season, southwest Pacific islands fringing the north Coral Sea, including Papua New Guinea and the Solomon Islands and those nations situated adjacent to and east of the International Date Line including Tonga and Niue (and to the south of those nations) may experience slightly elevated activity. Reduced risk is expected for Tuvalu.

Southwest Pacific TCs are grouped into classes ranging from 1 to 5, with 5 being the most dangerous. For the coming TC season, at least six storms are anticipated to reach at least Category 3, with mean wind speeds of at least 64 knots or 118 km/h (so-called ‘hurricane force’ winds). Of those systems, four storms may reach at least Category 4 strength, with mean wind speeds of at least 86 knots or 159 km/h. In addition, Category 5 strength TCs (winds greater than 106 knots or 196 km/h) are known to occur during seasons like the current one. Therefore, all communities should remain alert and well prepared for severe events.

Tropical cyclones have a significant impact across the Southwest Pacific from year to year. Vanuatu and New Caledonia typically experience the greatest activity, with an average of 2 or 3 TCs passing close to land each year. On average, New Zealand usually experiences at least one interaction per season with an ex-tropical cyclone. Some of the analog seasons identified for this outlook show multiple ex-tropical cyclones coming close (within 550 km) to the country. Significant wind, waves and rainfall are possible from these systems. Their effects can be spread over a larger area when the ex-tropical cyclone interacts with separate weather systems.

Even though TC activity is expected to be relatively low for some countries, historical cyclone tracks (see supporting information for this outlook, Figure 3) indicate that TCs can affect all parts of the SW Pacific region. As with most years, activity is expected to increase during the late part of the TC season from February-April.

All Pacific Islands should remain vigilant in case conditions in the equatorial Pacific change during the TC season. Past analog seasons that started similar to the present have seen intensification to well-coupled La Niña conditions and increased activity west of the Dateline in the late season. NIWA will continue to track the progression of ENSO and update the guidance in January if needed.

New Zealand’s National Institute of Water & Atmospheric Research (NIWA) and Meteorological Service of New Zealand (MetService) along with meteorological forecasting organizations from the Southwest Pacific, including the Australian Bureau of Meteorology, MeteoFrance and the Pacific Island National Meteorological Services have prepared this tropical cyclone outlook.

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