Cyclone Gabrielle has been described as “one of the worst storms” to hit New Zealand in living history.
Between February 12 and 14, parts of the country recorded rainfall amounts of 300-400 millimetres, wind gusts of 130-140 kilometres per hour and waves recorded as high as 11 metres, MetService recordsshowed.
Napier Airport recorded its wettest February day - and second-wettest of all time since records began in 1950 - after 175.8mm fell between 9am on February 13 and 9am on February 14.
Napier, along with Auckland and Whangārei, has had more than five times its average rainfall for the first 46 days of the year.
Redclyffe Bridge. Photo / Warren Buckland
“Gabrielle is one of the worst storms to hit Aotearoa New Zealand in living history,” MetService head of weather communications Lisa Murray said.
“Like Cyclone Bola in 1988, Giselle that caused the Wahine disaster in 1968 and the unnamed cyclone of 1936, Gabrielle caused shocking impacts to the North Island.”
How did it form?
The cyclone originated as a small low north of Fiji early this month.
Although initially weak, it moved slowly westwards during Waitangi weekend, into the Coral Sea south of the Solomon Islands, where conditions were very favourable for the low to intensify into a tropical cyclone.
The three key ingredients that helped the formation were:
Very warm sea surface temperatures in the Coral Sea of around 30 degrees Celcius
Abundant moisture
Light winds higher up in the atmosphere.
At this stage in its development, there were indications that it might track towards the upper North Island after becoming a tropical cyclone, although the path was uncertain.
Why was it so impactful?
The difference between Gabrielle and others is the track it took, along with warmer sea surface temperatures and an atmospheric environment that encouraged tropical cyclone growth.
Tropical cyclones only form over sea surface temperatures higher than 26C.
Gabrielle was named on February 8 in the Coral Sea and at that time the ocean sea surface temperature was warmer than normal along the corridor from the Coral Sea to New Zealand.
This was likely to have contributed to Gabrielle maintaining its intensity longer than normal, and likely contributed to the system holding onto slightly more moisture than normal, MetService said.
Extensive flood damage at a house in Omahu, Hastings. Photo / Paul Taylor
Travelling through this warmer environment helped the tropical cyclone gather moisture, which fuelled more rain.
Days ahead of this event, the weather models aligned their projections of the forecast track of Gabrielle towards New Zealand, giving meteorologists confidence it would impact the country.
The key to how impactful Gabrielle was going to be was the curve of the track just north of New Zealand.
At this point, higher up in the atmosphere a passing trough lined up to give extra energy to the cyclone and helped steer it towards the top of the Coromandel Peninsula.
On this more southern direction, the centre of the cyclone moved closer to Aotea/Great Barrier Island, where it sat at midnight on February 13, driving wind and rain into Northland, Auckland and the Coromandel Peninsula.
After this, the cyclone’s track changed to southeasterly and headed towards the East Cape above Tairāwhiti/Gisborne, driving strong winds and torrential rain into the east coast of the North Island, before moving east on a track far north of the Chatham Islands.
This particular track, combined with how expansive the cyclone was and how deep the central pressure of the cyclone was, is why Gabrielle was so destructive.
As Cyclone Gabrielle passed New Zealand it had an extremely low central pressure of 966.6hPa as recorded at Great Mercury Island.
The lower the pressure, the more intense the winds and therefore the greater the impacts of the storm.
Climate scientists believe that the number of ex-tropical cyclones impacting New Zealand will remain about the same or could decline, but it’s expected they will become more severe as the planet warms.
That will mean tropical cyclones will be of a higher category, bringing stronger winds, heavier rain, and more problems from wind damage, flooding and storms.
The Southern Hemisphere season for tropical cyclones is normally November 1 to April 30.
Cyclone Gabrielle hit Esk Valley hard, including this Hukarere school.
Photo / Warren Buckland
The South Pacific can expect on average about nine tropical cyclones a year, but there are exceptions. Of this, on average usually only one affects New Zealand.
There have been years when we have seen more than that, as has happened in the last two months with ex-cyclone Hale and now Gabrielle.
New Zealand is currently in a La Niña phase, which is conducive to tropical cyclones forming in and around the Coral Sea near Vanuatu and New Caledonia - a breeding ground for many of the cyclones that have historically impacted New Zealand.
There is a likely regime change from La Niña to a neutral climate state over the next month, MetService said - but there won’t be a tangible change to our weather maps through the rest of this summer whilst the atmosphere catches up.
Details per region
Northland: 250-400mm of rain was recorded across the region, with the largest amount at Glenbervie Forest (408.7mm). Whangārei Airport received 320.4mm. Wind gusts over 130km/h were recorded in the most exposed stations with 141km/h recorded at Cape Rēinga. Whangārei Airport recorded a gust of 102km/h.
Auckland: Over 200mm of rain was recorded around the region with Waitākere recording the most (248mm). Wind gusts of up to 130-150km/h were recorded at the most exposed sites. The Auckland Harbour Bridge recorded 115km/h and the airport recorded 78km/h.
Coromandel Peninsula: The Pinnacles station in the ranges recorded 447mm of rain and other elevated stations received 220-270mm. Whitianga recorded 170.6mm. Golden Valley (near Waihī) experienced wind gusts of up to 109km/h.
Bay of Plenty: The highest rainfall recorded in the region was at Queenshead (163.5mm). Tauranga recorded 122.7mm, Rotorua 86.1mm, Whakatāne 82.8mm, and Te Puke 78.6mm. Wind gusts recorded in the area were between 80-90km/h.
Tairāwhiti/Gisborne: The largest rainfall accumulation was recorded in Hikuwai, where 488mm of rain fell. Several other stations (mainly in the ranges) recorded over 400mm. Gisborne Airport received 185.3mm (although due to power cuts, it stopped recording after 2am Tuesday). Wind gusts were recorded across the region up to 93km/h.
Dartmoor Road residents David and Mary Wesley's house after it was inundated by raging floodwaters after Hawke's Bay was smashed by Cyclone Gabrielle. Photo / Mike Scott
Hawke’s Bay: Elevated stations in the ranges recorded the most rain with Pukeorapa receiving 337.5mm. Napier Airport recorded 203.8mm and Hastings recorded 143.8mm. Wind gusts were recorded up to 90km/h with exposed stations near the coast reaching higher such as Cape Kidnappers (131km/h).
Wairarapa: The most rainfall was recorded at elevated stations in the ranges, between 180-280mm. Masterton Airport received 62mm. Exposed stations near the coast recorded high wind gusts - Cape Turnagain recorded 146km/h and Castlepoint recorded 113km/h.
Taranaki: Rainfall was moderate with 30-45mm recorded across the region, but the wind was very strong - New Plymouth Airport recorded gusts of up to 128km/h.
Whanganui: Rainfall of 60-90mm was recorded around the region and gusts of 98km/h were experienced at Whanganui Airport.
Wellington: The highest accumulations were recorded in northern and eastern parts of the Hutt Valley (100-160mm). Mt Kaukau (an elevated site) recorded the highest wind gust of 146km/h, while around the city gusts of up to 100km/h were experienced.
Marlborough: Snowflake Ridge (a very elevated site) recorded 230mm of rain. Other stations around the region recorded up to 100-130mm. Kaikōura received 59.8 mm and Picton received 59.5mm. Cape Campbell recorded wind gusts of up to 93km/hr and Kaikōura recorded 65km/h.
