Each year, around 10 hurricanes form over the western Atlantic Ocean, but only two on average make landfall over the eastern US. Whether or not they do is decided by large-scale wind patterns around each storm. Models of how winds would change under various climate change scenarios have produced mixed results, and the limited record of actual storms making landfall shows no obvious trends of changes with warming so far.
Karthik Balaguru at Pacific Northwest National Laboratory in Washington state and his colleagues have now used the latest climate models to simulate hurricane paths with warming under a high-emissions scenario.
They project that the number of hurricanes making landfall over the eastern US would increase by around 37 per cent by the end of the century, with the greatest increase over the Gulf of Mexico and the US east coast south of Virginia. They found there would be a small decrease in hurricanes making landfall further north.
Warming Atlantic waters are known to increase the intensity of storms. The researchers found that storms could also become more powerful due to a projected decrease in wind shear – the difference between wind direction and speed at the surface and higher up – which can break up storms.
But the jury is still out on whether this will actually happen, says Jhordanne Jones at Purdue University in Indiana. “A lot of their work here is still very theoretical,” she says.
For one, the researchers assumed that the number of hurricanes that form each year would remain the same with climate change, but climate models differ on this, says Thomas Knutson at the National Oceanic and Atmospheric Administration.
The researchers also found that warming in the eastern Pacific is the primary mechanism behind the wind pattern changes projected by the models. The warmer water amplifies an atmospheric wave of high and low pressure, changing the circulation over the Gulf of Mexico such that wind more often blows towards the southern coast of the US.
But climate models also differ on what will happen in the eastern Pacific, and have largely failed to match observations in recent decades, says Jones. Projections are complicated by challenges in modelling ocean dynamics and fine-grained phenomena such as changes in cloud cover, she says. “These signals, these oscillations, these cycles – we don’t quite know how they will respond.”
Depending on which model the team used to project the future of the eastern Pacific, the number of hurricanes making landfall increased between 7 and 67 per cent, says Balaguru. “That adds to the level of uncertainty,” he says.
