A couple struggle against strong winds from ex-tropical cyclone Dovi on the Auckland waterfront on February 13. Another one to two systems are expected this season. Photo / Alex Burton
A couple struggle against strong winds from ex-tropical cyclone Dovi on the Auckland waterfront on February 13. Another one to two systems are expected this season. Photo / Alex Burton
New Zealand faces a slightly elevated risk from ex-tropical cyclones this season – partly thanks to our third La Niña in as many years.
A just-issued outlook put New Zealand's risk at "normal or elevated" for the November-to-April season, with Niwa flagging the potential for one to two of these destructive systems swinging our way.
Each season - usually around late summer - at least one ex-tropical cyclone veers within 550km of the country, packing gale-force winds and enough moisture to drive torrential rain.
"We're looking at La Niña for the third consecutive year – and it'll again have its fingerprints of regional tropical cyclone patterns," Niwa meteorologist Ben Noll said.
"This tends to enhance activity in the western part of the southwest Pacific and toward Vanuatu, New Caledonia and the Coral Sea.
"Because of that, New Zealand has an elevated risk from these cyclones as they track southward."
Noll added that, unlike in typical seasons where most cyclone activity occurs over February to April, the risk this time was higher over November to January.
Large parts of the country were lashed with heavy rain and high winds from two systems last season – Ruby and Dovi – amid several moisture-packed "atmospheric rivers" stretching back to the subtropics.
Further back in 2017-18, New Zealand was hit by ex-tropical cyclones Fehi and Gita, which struck within weeks of each other in February and caused tens of millions of damage to the South Island's West Coast.
Cyclone Hola, which formed a month later, nearly reached Category 5 strength as it bore down on Vanuatu and caused widespread havoc, killing one.
What was left of it later brought heavy rain and wind to northern North Island as it swung by our coast.
"We've certainly had our fair share of extreme weather events over the past few years, so at some level, we almost expect one to come our way at some point," Noll said.
"This outlook is simply reminding us that cyclone season is almost upon us, and that it's time to prepare and put ourselves in that mindset."
New Zealand faces an elevated risk from ex-tropical cyclones over the next six months, with Niwa flagging the potential for one to two of these destructive systems swinging our way. Photo / JMA
Warmer-than-average sea temperatures currently being recorded around New Zealand and across the Pacific – a pattern only expected to strengthen over coming weeks – were likely to have their own influence.
Noll described current sea temperatures around the Coral Sea – an important genesis region for cyclones – as "akin to a bathtub".
"As [cyclones] track southward, they can more easily maintain their integrity if waters in places like the northern Tasman Sea are warmer than average, or are seeing marine heatwave conditions," he said.
"Warmth in our local seas can also translate to even more energy and moisture for these systems, should they approach New Zealand."
Noll has warned that marine heatwave conditions this summer could possibly grow as severe as the two largest events we've ever recorded here.
As disastrously demonstrated by Ian in Florida and Fiona in Nova Scotia, even a small number of cyclone systems could make for a memorable season if they wrought enough destruction.
Around 50 years of observations in the Pacific have indicated a long-term trend toward fewer tropical cyclones – but stronger ones.
Across the wider southwest Pacific, the outlook predicted six to 10 systems big enough to be named, with at least three to four anticipated to reach Category 3 strength – or wind speeds of at least 119km/h.
However, an analysis of previous "analogue" years with conditions similar to this one didn't indicate any particular risk of the very worst cyclones: Category 5 events, where sustained wind speeds exceeded 199km/h.
The risk was considered elevated for Vanuatu, normal or elevated for Papua New Guinea and New Caledonia, and reduced for Tonga, Niue, Tokelau, Samoa, American Samoa, and the Society Islands.
