National Institute of Water and Atmospheric Research (NIWA)

Southwest Pacific Tropical Cyclone Outlook: Strong El Niño expected to produce above average activity and severe tropical storm impacts


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Meteorological and climate analysis centres across the Southwest Pacific are indicating above average numbers of tropical cyclones (TC) for the 2015–16 season (November 2015 to April 2016). 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[1]. The outlook indicates that 11 to 13 named TCs are expected for the coming season. TC activity is elevated for a majority of the Pacific Island countries, especially those situated close to or east of the International Date Line (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 east of the Dateline. Note that the TC activity outlook for islands like New Caledonia 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 six severe TCs (Category 3 or higher[2]) 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 slightly higher than normal. If an ex-tropical cyclone comes close to the country, the current background climate conditions suggest it has an equal probability of passing east or west of Auckland city. Significant rainfall, damaging winds and coastal impacts can occur leading up to and during these episodic events.
Outlook analysis

Conditions associated with a strong El Niño are indicated by sea surface temperature anomalies across the central and eastern Equatorial Pacific Ocean, and the atmospheric circulation patterns that exist over French Polynesia and northern Australia. There is an expectation amongst a number of international forecast centres that the present El Niño event will be one of the strongest in the last 60 years. The current event still has the potential of surpassing the 1997/98 event. Taking this climate scenario into account, elevated TC activity can be expected for many islands in the Southwest Pacific during the 2015–2016 season, with 11 to 13 named TCs forming across the region during the November 2015–April 2016 period.

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 and there are indications that activity may be above average this season for many islands, including Vanuatu, Fiji, and Wallis & Futuna. The outlook indicates elevated TC activity for the 2015–16 season for many islands east of the International Date Line that is typically associated with El Niño with reduced risk for Papua New Guinea. Elevated risk is forecast for countries east of the International Date Line include Tokelau, Samoa, Tuvalu and further east in the Northern Cook Islands, the Southern Cook Islands, and more broadly across French Polynesia.

On average, New Zealand usually experiences at least one interaction per season with an ex-tropical cyclone during El Niño conditions. Most of the analog seasons identified for this forecast (1972/73; 1982/83; 1987/88; 1991/92; 1997/98 ) 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 forecast, Figure 3) indicate that TCs can affect all parts of French Polynesia (including the Society Islands, the Austral Islands, the Tuamotu Archipelago, and the Marquesas).

All Pacific Islands should remain vigilant in case conditions in the equatorial Pacific change during the TC season. Past El Niño seasons have seen TC tracks with increased sinuosity (irregular or looping motions rather than a curvilinear trajectory), which means they have potential to impact a large area.

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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