Hurricanes are getting a boost from climate change and taking longer after making landfall to slow down and disperse. These changes are likely to mean that hurricanes in the future will affect communities farther inland.

A new study showcases how climate change is making hurricanes more dangerous and farther-reaching. Hurricanes that form above warmer waters in higher atmospheric temperatures can carry more moisture, the team explains, which allows them to keep raging stronger and for longer after reaching dry land. The problem is only going to get worse if climate change continues unabated and mean temperatures keep increasing.

Stormy futures

“The implications are very important, especially when considering policies that are put in place to cope with global warming,” said Professor Pinaki Chakraborty, senior author of the study and head of the Fluid Mechanics Unit at the Okinawa Institute of Science and Technology Graduate University (OIST).

“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.”

The link between climate change and more powerful hurricanes is already well documented, with previous research showing that they’re becoming more intense over the open ocean. This is the first study to look at how climate change makes these storms — also known as typhoons — behave after they reach dry land.

The team looked at hurricanes in the North Atlantic that made landfall throughout the last five decades. On the first day after reaching dry land, the storms weaken roughly twice as slowly today as they did 50 years ago, the team explains.

They further explored the mechanisms behind this behavior in a series of computer simulations of four hurricanes in different sea surface temperature contexts. Once these simulated hurricanes reached Category 4 strength, the team simulated their making landfall by turning off any upwelling moisture.

“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. 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,” said Lin Li, first author and PhD student in the OIST Fluid Mechanics Unit.

Li adds that hurricanes are “heat engines, just like engines in cars”, where the fuel is moisture taken up from the surface of the ocean. The heat energy it carries intensifies and sustains the storm by powering winds. Once a hurricane reaches dry land, its fuel supply is cut, meaning it will eventually decay.

Although each hurricane in the simulation made landfall at the same intensity, those that formed over warmer oceans took more time to dampen down, the team explains. All in all, they write that “warmer oceans significantly impact the rate that hurricanes decay, even when their connection with the ocean’s surface is severed”.

Additional simulations showed that the moisture stored in each hurricane explained this inertia. They start weakening as this stored moisture starts depleting. Simulated hurricanes that weren’t allowed to store moisture showed no changes in their rate of decay relative to the surface temperatures of the water they formed over.

“This shows that stored moisture is the key factor that gives each hurricane in the simulation its own unique identity,” said 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”, meaning they release more rainfall over the areas they reach.

The authors explain that our current models don’t take into account hurricanes’ stored humidity, making them incomplete — which is why we haven’t yet understood the relationship between sea surface temperatures and the behavior of hurricanes over dry land.

The team is now working on studying hurricanes from other areas of the world to see whether climate change is impacting hurricane decay rates across the globe.

“Overall, the implications of this work are stark. If we don’t curb global warming, landfalling hurricanes will continue to weaken more slowly,” Prof. Chakraborty concludes. “Their destruction will no longer be confined to coastal areas, causing higher levels of economic damage and costing more lives.”

The paper “Slower decay of landfalling hurricanes in a warming world” has been published in the journal Nature.