Source: Earth Sciences New Zealand
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, 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.”
The latest climate projections for New Zealand, available on the Ministry for the Environment’s website, provide detailed information for every 5-kilometre square across the country. They are based on refining the projections from six global climate models in the most recent report by the Intergovernmental Panel on Climate Change.
Development of these updated, high-resolution climate projections involved huge supercomputing power. The dataset is now helping New Zealanders – including councils, companies and communities – to better understand their specific climate-related risks. In turn, this will improve decision-making for climate adaptation and support people to build resilience to extreme weather events
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.
Development of these updated, high-resolution climate projections involved huge supercomputing power. The dataset is now helping New Zealanders – including councils, companies and communities – to better understand their specific climate-related risks. In turn, this will improve decision-making for climate adaptation and support people to build resilience to extreme weather events
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.
