Hurricanes that move slowly over a region can inflict more damage than storms that move more quickly because they have more time to batter an area with strong winds and dump copious amounts of rain, which can cause floods.
The extraordinary damage caused by storms like Dorian (2019), Florence (2018) and Harvey (2017) prompted Princeton’s Gan Zhang to wonder whether global climate change will make these slow-moving storms more common.
A vast ensemble of climate simulations was used by Zhang, a postdoctoral research associate in atmospheric and oceanic sciences, to answer the query. He worked with an international team of researchers from the Geophysical Fluid Dynamics Laboratory on Princeton University’s Forrestal campus and the Meteorological Research Institute in Tsukuba, Japan. The results of this work appear in the April 22 issue of Science Advances.
An ensemble of 90 potential futures was produced by Zhang and his colleagues after they chose six warming patterns for the global climate and applied 15 distinct initial circumstances to each of the six patterns.
They instructed the computers to assume in all 90 simulations that the planet’s average temperature has increased by about 4 degrees Celsius and that global carbon dioxide levels have quadrupled, a level of warming that experts predict could be reached before the turn of the century if no action is taken to reduce the use of fossil fuels.
“Our simulations suggest that future anthropogenic warming could lead to a significant slowing of hurricane motion, particularly in some populated mid-latitude regions,” Zhang said. His research discovered that at latitudes close to Japan and New York City, storm forward momentum will slow by around 2 miles per hour, or 10% to 20% of the present normal rates.
“This is the first study we are aware of that combines physical interpretation and robust modeling evidence to show that future anthropogenic warming could lead to a significant slowing of hurricane motion,” he said.
“Since the occurrence of Hurricane Harvey, there has been a huge interest in the possibility that anthropogenic climate change has been contributing to a slow down in the movement of hurricanes,” said Suzana Camargo, the Marie Tharp Lamont Research Professor at Columbia University’s Lamont-Doherty Earth Observatory, who was not involved in this research.
Our simulations suggest that future anthropogenic warming could lead to a significant slowing of hurricane motion, particularly in some populated mid-latitude regions. This is the first study we are aware of that combines physical interpretation and robust modeling evidence to show that future anthropogenic warming could lead to a significant slowing of hurricane motion.
Gan Zhang
“In a new paper, Gan Zhang and collaborators examined the occurrence of a slowdown of tropical cyclones in climate model simulations. They showed that in this model, there is a robust slowdown of tropical cyclone motion, but this occurs mainly in the mid-latitudes, not in the tropics.”
Why would the storms slow down?
The westerlies, which are strong currents that blow through the midlatitudes, were shown to push toward the poles with a 4 degree warming, according to the study. Additionally, weaker mid-latitude weather perturbations coincide with that shift.
These modifications may slow down storms that approach populous regions in Asia (where these storms are referred categorized as typhoons or cyclones rather than hurricanes) and along the east coast of the United States. Typically, when people discuss hurricane speeds, they are referring to the winds that are swirling around the storm’s eye.
The intensity of a storm is determined by such wind speeds. For instance, a Category 5 hurricane has sustained winds of more than 157 mph. Zhang and his coworkers are examining the “translational motion,” often known as the “forward speed” of a storm, which is the rate at which a cyclone travels along its course.
(The term comes from geometry, where a figure is “translated” when it slides from one part of a graph to another.)
A storm is referred to as “slow-moving” if its translational speed is modest, regardless of how quickly its winds are blowing. One of the slowest storms ever recorded, Hurricane Dorian, which tore through Grand Bahama Island from September 1 to 3, 2019, was a Category 5 hurricane with wind gusts of up to 220 mph. However, its translational speed was only 1.3 mph.
Are storms already slowing down?
According to some studies, since 1900, tropical storm translation speeds have slowed over land areas of the United States. To determine if the observed slowdown was driven by human-induced warming, Zhang and his colleagues utilized their climate models, but they were unable to establish a strong connection at least based on trends since 1950 in their simulations.
Additionally, they pointed out that rather than due to anthropogenic climate change, the observed lowering translational speeds reported in previous studies may have their origins in natural variability.
Zhang used the metaphor of dieting to explain the ambiguity of hurricane observations.
“If I go to the gym and eat fewer sweets,” he said, “I would expect to lose weight. But if I’m only using a bathroom scale to weigh myself, I’m not going to get convincing data very soon, for many reasons including that my bathroom scale isn’t the most accurate,” he continued.
“Assume after two weeks, I see some weak trend,” he said. “I still can’t tell whether it’s due to exercise, diet or just randomness.”
In a similar vein, he noted, the reported deceleration trend in hurricanes or tropical storms over the past century may simply be a result of chance.
“In the debate between ‘Everything is caused by climate change’ and ‘Nothing is caused by climate change’ what we are doing here is trying to offer that maybe not everything can be immediately attributed to climate change, but the opposite is not right, either,” Zhang said.
“We do offer some evidence that there could be a slowdown of translational motion in response to a future warming on the order of 4 degrees Celsius. Our findings are backed by physics, as captured by our climate models, so that’s a new perspective that offers more confidence than we had before.”
