Every 1-degree rise in ocean temperatures produces huge increase in rainfall, Irish study finds

Rising ocean temperatures are producing huge increases in rainfall associated with North Atlantic hurricanes – with a 140 per cent overall increase overland associated with every 1-degree rise above normal temperatures.

The trend, which is exacerbating major freshwater flooding events, is the main finding of a study led by a researcher at Maynooth University and has the greatest implications for the US and Caribbean, especially as ocean temperatures continue to warm due to climate change.

However, it also has consequences for Ireland because of likely increased rainfall and associated flooding due to ex-tropical cyclones crossing the Atlantic.

The research at the Icarus Climate Research Centre shows both a 40 per cent rise in hurricane rainfall rate (based on millimetres per hour), and a 140 per cent increase in total rainfall over land when ocean temperatures in the North Atlantic are 1 degree warmer than normal.

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Researchers led by Dr Samantha Hallam, who is based at Icarus and the National Oceanography Centre in the UK, analysed the hurricane rainfall rate and total rainfall increase for a 1-degree rise in ocean temperatures in the main area for hurricane development in the Atlantic over the 1997-2017 period.

In more recent years, there have been increasing financial losses from freshwater flooding associated with hurricanes making landfall in the US with more intense rainfall which caused flash flooding.

While risks from wind and storm surge were well known, rainfall from Atlantic hurricanes is less studied, though it has caused 37 major freshwater flooding events since 2017.

Their motive was to investigate the freshwater flood threat while looking at anomalies over a 21-year period, Dr Hallam said. Some years the ocean was warmer, in others it was cooler, and they focused on what happened when it was 1-degree above normal.

It was not an attribution study, where the effects of climate change were considered, she explained, though overall ocean temperatures have been increasing since the 1980s due to global warming.

This study, published in Environmental Research Communications on Thursday, looks at impacts across nine US States. In 2017, hurricane Harvey caused more than $100 billion (€94 billion) of mostly flood-related damage in the US and hurricane Ian last year caused flood losses estimated at $8 billion-$18 billion, of which some 50 per cent were uninsured.

Dr Hallam said in addition to increased rainfall intensity due to “a thermodynamic response”, caused by the higher temperature, it coincided with increased wind speeds making hurricanes travel faster over land and more such weather events.

“The 40 per cent rise in tropical cyclone rainfall rate over land contrasts with the 6 per cent increase in the rainfall rate observed over the Atlantic Ocean, and appears to indicate both a thermodynamic and a dynamic response to anomalously warm ocean temperatures, meaning hurricanes are making landfall with higher wind speeds when ocean temperatures are warmer,” she added.

Overall, their research provides useful insights for seasonal flood prediction from Atlantic tropical cyclones (hurricanes) and the associated flood risk over the next 20 or 30 years, which would be helpful for homeowners, policymakers and insurers.

For Ireland, more serious ex-tropical cyclones appeared to be hitting the country with greater frequency, she confirmed, as evidenced by hurricane Ophelia in 2017 and hurricane Lorenzo in 2019.

The study was supported by the Marine Institute and funded by the Government, and involved scientists from the University of Reading; University of Southampton; and the UK’s National Oceanography Centre.