Nick Carne
North Atlantic hurricanes are weakening more slowly than they used to when they hit land, according to a new study in the journal Nature, and it’s linked to climate change.
Japanese researchers say those that develop over warmer oceans carry more moisture and therefore stay stronger for longer. There is thus a real risk than in the future they will reach communities further inland and be more destructive.
Previous studies have shown that climate change can intensify hurricanes over the open ocean, but this is the first, the researchers say, to establish a link between a warming climate and the smaller subset of hurricanes that have made landfall.
“We know that coastal areas need to ready themselves for more intense hurricanes, but inland communities, who may not have the know-how or infrastructure to cope with such intense winds or heavy rainfall, also need to be prepared,” says Pinaki Chakraborty, from the Okinawa Institute of Science and Technology Graduate University.
Chakraborty and colleague Lin Li analysed data for storms that made landfall over North America between 1967 and 2018, then used the decrease in storm intensity to define a timescale of decay. They report that while hurricanes were likely to decay by 75% within a day of landfall half a century ago, today it is just 50%.
“When we plotted the data, we could clearly see that the amount of time it took for a hurricane to weaken was increasing with the years,” says Li. “But it wasn’t a straight line – it was undulating – and we found that these ups and downs matched the same ups and downs seen in sea surface temperature.”
The pair created computer simulations of four different hurricanes, setting different temperatures for the surface of the sea. Once each virtual hurricane reached Category 4 strength, they simulated landfall by cutting off the supply of moisture from beneath.
They found that even though each simulated hurricane made landfall at the same intensity, the ones that developed over warmer waters took more time to weaken. Using additional simulations, they found that “stored moisture” was the missing link.
When hurricanes make landfall, they carry a stock of moisture that slowly depletes. When Chakraborty and Li created virtual hurricanes that lacked stored moisture after hitting land, they found that the sea surface temperature no longer had any impact on the rate of decay.
“This shows that stored moisture is the key factor that gives each hurricane in the simulation its own unique identity,” says Li. “Hurricanes that develop over warmer oceans can take up and store more moisture, which sustains them for longer and prevents them from weakening as quickly.”
More stored moisture also makes hurricanes “wetter”, perhaps explaining why some recent hurricanes have unleashed high volumes of rainfall.
“Current models of hurricane decay don’t consider moisture: they just view hurricanes that have made landfall as a dry vortex that rubs against the land and is slowed down by friction,” says Li. “Our work shows these models are incomplete, which is why this clear signature of climate change wasn’t previously captured.”
In a related commentary in Nature, Dan Chavas and Jie Chen from Purdue University, US, note that the extent of damage occurring inland depends on both the rate of storm decay and the speed of storm motion at landfall.
“Hence, a slower decay could also lead to increases in damage farther inland, although changes in the speed of motion remain a point of contention,” they write. “Longer-lived storms might also increase the chances of interaction with the jet stream, which can sometimes produce hazardous weather that can extend much farther inland.”