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Event attribution: Cyclone Gabrielle

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Top image: NASA/Cyclone Gabrielle  Aotearoa New Zealand March 2023

New high-resolution modelling predicts that rainfall from tropical cyclones will significantly increase under global warming. Earth Sciences New Zealand and the University of Waikato found that, under a high emissions scenario, extreme rainfall from cyclones could rise by up to 35% across the southwest Pacific, including for ex-tropical cyclones impacting Aotearoa New Zealand.  
 
Researchers used the latest climate change projections released last year by Earth Sciences NZ (then NIWA) with the Ministry for the Environment.  Study lead and climate scientist Dr Peter Gibson says they simulated more than 1,800 years of data from different climate models and levels of greenhouse gas emissions.   
“We wanted to assess how the frequency and severity of tropical cyclones in the southwest Pacific may change by the end of the century. Under the full range of climate scenarios examined there was an increase in tropical cyclone rainfall. This was proportional to the rate of emissions, with the most pronounced rise in extreme rainfall occurring under the highest emissions.

“If that scenario plays out, we can expect around 30 – 35% more extreme rainfall during tropical cyclones than we experience today. Even under the best-case scenario, we will still see an increase proportional to the warming experienced. Extreme windspeeds are also likely to increase, but not to the same extent.”

The more greenhouse gases that are emitted, the higher temperatures get, with a high emissions scenario representing around a 3°C increase by the end of the century. A warmer atmosphere holds more water, so when storms develop, they have access to more water vapour that can be released as heavier rainfall.

And while the model simulations didn’t show an overall increase in the total number of cyclones by the end of the century, there is compelling evidence that the strongest (category 4 and above) will become more frequent.

“One or two ex-tropical cyclones typically effect New Zealand each year. However, it’s the really big, much rarer events that cause the most damage. And the difference between a river flooding or not can often be small, so any increase in rainfall is significant and potentially very impactful.

The February 2023 Cyclone Gabrielle [top image], which at its peak reached category 3, and the Auckland Anniversary weekend event, resulted in total estimated costs of $14.5 billion, with the total insured losses for both events totalling $3.8 billion, with $1.8 billion from Cyclone Gabrielle claims, and $2 billion from the Auckland anniversary weekend event. Most the claims were because of widespread flooding.
 
Until now, most projections of tropical cyclones have relied on global climate models, which have a relatively low spatial resolution. This work refines those models to a regional scale that can zoom in closer and get a much more accurate picture. 

“The eye of a cyclone is fairly compact, at only tens of kilometres across. Global climate models are coarse, with each pixel going down to around 100km, meaning they miss much of the crucial detail and small-scale physics of a storm. Our refined regional model goes down to around a 12km resolution, making it a state-of-the-art dataset for assessing ex-tropical cyclones that impact New Zealand.”

These findings, published in the Journal of Geophysical Research: Atmospheres, are consistent with other studies around the world, highlighting the potential for more intense tropical cyclones and heavier rainfall in the future, said Dr Gibson. This indicates increasing risks for vulnerable communities across the southwest Pacific and New Zealand. 

“It’s clear that if we reduce emissions, we reduce warming, and we mitigate the worst impacts of severe weather events. As computing capabilities improve, newer, high-resolution models will become available to create even more accurate and reliable long term climate projections of cyclones and their impacts. More research is being undertaken by Earth Sciences NZ into specific weather and climate hazards in the projections, to provide clarity on the future climate New Zealanders are likely to face in the decades to come.”

NIWA: New Zealand Scientists Unlock Antarctic Ice Melt Mystery

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Top image: NIWA
 
A groundbreaking Antarctic research expedition, led by New Zealand scientists, has for the first time directly observed water flowing out from beneath the West Antarctic Ice Sheet—revealing how massive, hidden floods beneath the ice are contributing to the melting of the continent’s largest ice shelves.
 
The findings, recently published in Nature Geoscience, offer dramatic new insights into how meltwater beneath Antarctica flows and affects sea level rise, says expedition lead Victoria University of Wellington Associate Professor Huw Horgan:
 

“Researchers have long suspected that water, from geothermal heat and other sources, flows from under the ice sheets. Satellite observations from elsewhere in West and East Antarctica have revealed active lakes under the ice caps. Our quest was not just to discover the network of hidden lakes and streams that can’t be seen from the surface, but to complete the first close-up observation of these watercourses meeting the ocean in cavities under the ice shelf. These sub-glacial watercourses play a central role in the melting of the ice shelf, so the findings will enable us to predict sea level rise more accurately.”

Supported by New Zealand’s Antarctic Science Platform, the scientists from NIWA, Victoria University, GNS Science and the University of Otago, as well as researchers in the UK and the US including Cornell University, undertook the fieldwork in the autumn of 2021. It took two weeks for the international team to travel from Scott Base 1,200 kilometres towards the Kamb Ice Stream across the Ross Ice Shelf, a floating ice sheet about the area of France.

“The first challenge was just to get there,” says Associate Professor Horgan, who also works in Switzerland for ETH Zurich’s Laboratory of Hydraulics, Hydrology and Glaciology.

“Fortunately, Antarctica New Zealand has developed long-range ice traverse capabilities to safely move people and gear over vast distances. The second challenge was drilling half a kilometre into the Kamb Ice Stream, a major glacier feeding into the Ross Ice Shelf. Vital to the expedition’s success were the skills of the drilling team from the Antarctic Research Centre at the Victoria University of Wellington who, using high-pressure hot water at 80°C, drilled a 500-metre-deep borehole through the Kamb Ice Stream.”
Drilling team from the Antarctic Research Centre at the Victoria University of Wellington. Image: NIWA
Once the team melted through the ice down into the ocean, they were able to film the under-ice watercourse and collect unique data on its temperature, salinity, sediment, and flow behaviour using a range of NIWA’s specialist ocean equipment—as well as Cornell University’s IceFin remotely operated vehicle. 
Camera being lowered down through the ice. Image: NIWA

“With the help of our camera, we even discovered a school of lobster-like (amphipods) creatures, 400 kilometres from the open ocean.”

The measurements provided many surprises for the team, says NIWA and University of Auckland physical oceanographer Professor Craig Stevens:

“Our first discovery was the unique shape of the under-ice cavity where the meltwater was flowing. The size of the watercourse—about 250 metres deep and 150 metres wide—was larger than expected and mostly filled with ocean water, with only a small portion being freshwater from beneath the ice sheet.”

Using sediment cores the team concluded that the water comes from sub-glacial lakes upstream, which fill and empty the lakes in cycles – when they empty, a flood of water rushes towards the sea.

“Understanding these hidden systems is crucial to improving climate models and predicting sea level rise more accurately. The Ross Ice Shelf and others like it play a critical role in slowing the flow of inland glaciers to the sea. Their decline will significantly accelerate global sea level rise.”

“Antarctica may be remote, but what happens there affects us all,” says Professor Stevens. “We need to understand this distant changing environment, so we can prepare for future challenges here at home.”