Weather: Climate change a factor in extreme autumn weather

By Hamish Cardwell for RNZ

Climate change was a factor in autumn being the second-equal warmest on record, but last season also brought extreme flooding, high winds, tornados, and drought - all while a marine heatwave hurt the aquaculture industry.

The season also saw a new hourly rainfall record, while it was the third hottest month of May.

Niwa said temperatures were above average or well above average in every region and one factor for this was global climate change.

In May dozens of locations set record or near-record temperatures.

It was especially warm in Castlepoint in Wairarapa, where the mean temperature of 16.2 degrees Celsius was 3.5C higher than average.

This meant the mean temperature there was closer to that expected in December (16.4C) compared to May (12.7C).

In Middlemarch, Otago the average daily maximum temperature was 16.6C, which is 4.3C above normal.

Numerous inland South Island places recorded average daily maximum temperatures at least 3C higher than usual for the time of year.

Parts of Southland, Otago, inland Canterbury and the West Coast had mean temperatures more than 2C higher than average.

The very warm air temperatures were a result of more frequent northerly airflows, ongoing warmer than average sea surface temperatures around the country, and climate change.

NZ King Salmon has been forced to close farms due to warmer water temperatures.
NZ King Salmon has been forced to close farms due to warmer water temperatures. Photo credit: Supplied / NZKS

Meanwhile, a prolonged ocean heatwave continues with record-high temperatures for April - some places up to 5 degrees hotter than normal.

NZ King Salmon has been forced to close farms and will let more than 100 staff go due to warmer water temperatures brought on by climate change.

The company reported a net loss of $55.7 million in the 2022 financial year.

For the first time ever there has been a mass bleaching of native sea sponges in Aotearoa.

The sea temperatures are almost certainly to blame, with scientists fearful for what it could mean for the entire Fiordland ecosystem.

Autumn had it all - tornadoes, record rainfall, droughts and floods

This March was the eighth warmest on record, April was the ninth warmest and May was the third warmest.

Middlemarch had its driest ever autumn - with records extending back to 1896.

Invercargill had a dry spell from 14-31 March, contributing to a meteorological drought in Southland.

But it was the second-wettest autumn on record in Wairoa, driven by two extreme rainfall events in March and April.

On 21 March rain fell in rates leading to the second wettest hour on record in the Auckland region - North Shore recorded 76.8mm of rain in an hour between 8am and 9am.

At Maungatapere near Whangārei, 103mm of rain was recorded from 4am to 5am making it the new national hourly rainfall record for a low elevation station (less than 500 metres above sea level).

Whangārei also observed its wettest hour on record (64.4mm) since at least January 1979.

On 23 March a state of emergency was declared in Tairāwhiti as river levels rose rapidly, causing evacuations in a number of townships, and destroying roads and bridges and cutting off some communities.

There was another storm on 13 April that caused flooding again in Gisborne and in Hawke's Bay.

On 20 May a woman died after being hit a by a tree brought down by wind in Cambridge, while a likely tornado hit Levin, damaging dozens of homes while a hailstorm in nearby Ōhau caused considerable damage to property and crops.

Flooding in Tolaga Bay this year.
Flooding in Tolaga Bay this year. Photo credit: RNZ

From 8 to 9 May, heavy rainfall on the West Coast caused several large slips and surface flooding

There were at least two other likely tornados in Waikanae on the Kāpiti Coast earlier this week, with rooves ripped off at least four properties.

Meanwhile, according to Niwa's New Zealand Drought Monitor, meteorological drought conditions were present in southern parts of Southland and Stewart Island throughout the second half of March.

Rainfall was below normal or well below normal in parts of Northland, Auckland, Gisborne, Hawke's Bay, Wairarapa, Canterbury (south of Christchurch), Otago and southwestern Southland.

Akaroa and Matamata were New Zealand's driest locations relative to normal, with just 24 percent of normal May rainfall recorded, respectively.

In Akaroa, it was the second-driest May since records began in 1977.

Winter to be warmer and wetter than average

New Zealanders should expect this winter to be warmer and wetter than average.

Niwa's seasonal climate outlook forecasts warmer than average temperatures across the country this winter, alongside increased rainfall in many regions.

Niwa meteorologist Ben Noll said warm seas and subtropic winds would keep the country warm throughout most of winter.

Cold spells may prevent this winter from being the warmest on record, but temperatures were still trending upwards, he said.

Niwa also suggests increased rainfall and moisture could indicate more floods.