What caused record-breaking river of rain over top of south?

Thursday, 25 August 2022

Prime Minister Jacinda Ardern visits Nelson to meet residents affected by the August floods and land slips. First published August 22.

Exceptional. Extreme. Most unusual.

That’s just a sprinkling of the words scientists used to describe the atmospheric river of rain – the strongest recorded in August – that bore down on Te Tauihu (the top of the south).

For those who lived through it, it was all of the above. But, it was also something else: devastating.

So what caused the steady stream of rain that started on Tuesday, August 16, and didn’t break until the Saturday morning?

Flooding around north Nelson and Glenduan in August.

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Before the rain started, forecasters knew this atmospheric river was going to be different.

Dr Daniel Kingston, senior lecturer in geography, University of Otago, predicted last Tuesday the rain would be “notable because of its duration”, as the rain would become stuck over the country.

This Japan Meteorological Agency August 16 satellite image, was posted on Twitter by MetService, which said the image nicely showed the strong blocking high to the east, low pressure to the west, and very warm airmass extending from the tropics sandwiched in between

“The event might also be more damaging than otherwise because it comes on top of an already exceptionally wet winter, meaning that the ground is already very wet with limited capacity to absorb further rain.”

Essentially, weather conditions caused by La Nina helped to create a much wetter weather system that was carrying “much more water than normal”, which then ran into a “blocking system” that stopped it from being able to pass over the country.

A week later, he said it had “played out pretty much as it was forecast”.

“What happened was we had very warm, wet air approaching New Zealand from the north – very warm and very wet – and that water system got stuck in place for a number of days, and because we had this warm, moist air being funnelled southwards from the tropics it just deposited huge amounts of water.”

Part of the Dry River bridge in Golden Bay collapsed in the deluge. Tākaka received a third of its annual rainfall in three days.

So how much rain was there?

By Saturday morning, the MetService reported that weather stations across the top of the south had recorded more than a metre of rain.

At Nelson Airport the monthly average for all of August is 80mm. On Saturday morning the monthly total was already sitting at 296mm, with 89.2mm falling on Wednesday alone.

Niwa meteorologists said on Thursday that the atmospheric river was a record-breaker – the strongest in August since records began in 1959, and the second-strongest recorded in winter as a whole (between June and August).

Niwa meteorologist Ben Noll said it was “pipped to the post” by a previous atmospheric river in July 1998.

The flooded Maitai River at the Riverside footbridge last week.

The rainfall in Nelson was a one-in-120-year event, and hit Tākaka with a third of its annual rainfall in three days.

A one-in-120-year event is one that is expected, on average, to be equalled or exceeded once every 120 years. Over 1000 years, you would expect ten one-in-100 year events, and about eight one-in-120 year events.

Noll said atmospheric rivers were measured using a value called the Integrated Water Vapour Transport (IVT) value, which incorporates both the amount and flow of moisture in the atmosphere.

Noll said IVT values above 1250kg per metre per second were considered “exceptional”, and last week’s river “greatly exceeded that threshold”.

The top IVT value in New Zealand last week was 1749kg per metre per second.

“Atmospheric rivers [AR] are a normal feature of New Zealand’s climate. However … the flood-inducing AR was extremely unusual for the time of year, being both long-lasting and very moisture-laden,” he said.

Usually the most moist atmospheric rivers were in summer, not winter, and August usually had the annual minimum moisture measures.

Noll said there were several contributing factors in the significance of the atmospheric river including a La Niña weather system, warmer-than-average temperatures in the Southwest Pacific sea, a “blocking” high pressure system to the east of New Zealand, and other factors which “fostered a long-lived atmospheric river”.

Climate scientist Peter Gibson said climate change could have a big influence on the future strength of atmospheric rivers and their resulting weather impacts.

Lorraine Neumann attempts to find a rose planted as a memorial to her late daughter, after a landslide at her Brook St, Nelson covered her property in knee-deep mud from a landslide.

“Extreme precipitation from atmospheric rivers is already being enhanced by climate change in New Zealand.”

He said research at Niwa was ongoing to understand how conditions may change in the future.

“A warmer atmosphere can hold more water vapour, so it’s likely that New Zealand will see more intense atmospheric rivers as climate change continues,” he said.

“The challenging part will be figuring out precisely which locations will see the biggest increases.”

Now, as people across Nelson and Marlborough pick through the debris and wait to know the fate of their homes, the question becomes – will this happen again?

This one came on the back of the wettest July on record nationally, which meant the rainfall was falling on already soaked earth, increasing the risk of slips.

Dr Luke Harrington, senior lecturer in climate change at the University of Waikato, said the “big unknown” was whether the high pressure system that allowed the atmospheric river to stall was something we would see more frequently in winter.

“I’m not sure we have clear answer to that question yet.”

Kingston said it was clear this event was characterised by the sheer volume of water – and a warmer planet led to more water in the atmosphere.

“As the atmosphere warms it can hold more moisture, increasing the likelihood for extreme heavy rainfall events such as this. Sea surface temperatures around New Zealand are also warmer than average right now, which can further amplify these sorts of events.”

While it was hard to say if atmospheric rivers would become more frequent it was a “warning bell” for the effects of climate change, he said.

Heavy rainfalls in Ashburton and Canterbury in May 2021, the July 2021 storms in the top of the south, and the February atmospheric river in Buller were just some of the extreme weather events to hit the country in the past 18 months.

“There are alarm bells ringing. We have been given this really clear picture of happens when you get repeated heavy rainfall events,” Kingston said.

“We can’t say in the future that we haven’t been warned.”

It was clear that the impact of rain events like this would need to be taken into account for flood plain and infrastructure planning.