Apr 23 2020
When compared to faster-moving storms, hurricanes traveling slowly across an area are said to inflict more damage. This is because when a storm persists for a longer time, it gets more time to cause damage to a region with storm winds and triggers large volumes of rain that collectively result in flooding.
Storms like Harvey (2017), Florence (2018), and Dorian (2019) had caused enormous damage. These events prompted Gan Zhang from Princeton University to speculate whether these slow-moving storms are more commonly caused by global climate change.
Zhang is a postdoctoral research associate in atmospheric and oceanic sciences and he decided to address this question by utilizing a huge ensemble of climate replications. He collaborated with an international research team from the Meteorological Research Institute in Tsukuba, Japan and the Geophysical Fluid Dynamics Laboratory on the Forrestal campus at Princeton University. The study results were published in the Science Advances journal on April 22nd, 2020.
Along with his collaborators, Zhang chose six prospective warming patterns for the global climate and subsequently ran 15 different and potential initial conditions on each of these six warming patterns. This led to an ensemble of 90 potential futures.
In all the 90 potential simulations, the computers were instructed to believe that carbon dioxide levels around the world have increased by fourfold and that the average temperature of the planet has increased by around 4 °C—a warming level that experts believe could be reached before the end of the century if action is not 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.
Gan Zhang Postdoctoral Research Associate, Atmospheric and Oceanic Sciences, Princeton University
Zhang’s team found out that the forward motion of the storms would slow down by around 2 miles per hour—that is, approximately 10% to 20% of today’s usual speeds—at latitudes close to New York City and Japan.
“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 added.
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.
Suzana Camargo, Marie Tharp Lamont Research Professor, Lamont-Doherty Earth Observatory, Columbia University
Camargo was not part of this study.
Camargo continued, “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.”
But why do the storms reduce their speed? The scientists discovered that 4 ºC of warming would drive the westerlies to travel toward the poles. Westerlies are powerful currents that blow through the mid-latitudes, while weaker mid-latitude weather perturbations also accompany that change of direction.
Such changes can reduce the movement of the storms close to the populated regions on the U.S. eastern seaboard and in Asia (where these kinds of storms are known as cyclones or typhoons, and not hurricanes).
When individuals talk about the speed of a hurricane, they are usually referring to the winds that move quickly around the eye of the storm. Such wind speeds are the factors that determine the strength of a storm—for instance, a Category 5 hurricane has sustained winds that travel at a speed of over 157 miles per hour.
On the other hand, Zhang and his collaborators are analyzing the “translational motion,” also known as the “forward speed” of a storm, the pace at which a hurricane travels along its path. (The name originates from geometry, in which a figure is “translated” upon moving from one section of a graph to another.) Regardless of the hurricane’s wind speed, a storm is believed to be “slow-moving” if it has a low translational speed.
The Dorian hurricane, which wrecked the Grand Bahama Island from September 1st to 3rd, 2019, was a Category 5 hurricane with wind speeds reaching as high as 220 miles per hour; however, its translational speed was only 1.3 mph, rendering it as one of the slowest hurricanes to be ever recorded, to date.
Are Storms Already Slowing Down?
A few scientists have proposed that since 1900, the translation speeds of tropical storms have slowed across land areas in the United States. Zhang and his collaborators utilized their climate models to observe if warming caused by humans account for the perceived slowdown, but they were not able to find any convincing link, at least on the basis of trends since 1950 in their climate simulations.
The researchers also observed that the perceived slowdown of translational speeds reported in the latest studies could have emerged mainly from natural variability instead of human-induced climate changes. The metaphor of dieting was used by Zhang to describe the vagueness 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. 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.
Gan Zhang Postdoctoral Research Associate, Atmospheric and Oceanic Sciences, Princeton University
In the same way, the perceived slowdown trend in tropical storms or hurricanes over the past 100 years could have been caused by small-scale local variations or could simply be a random trend, Zhang added.
“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 further elaborated.
“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,” Zhang concluded.
The study titled, “Tropical Cyclone Motion in a Changing Climate,” was written by Gan Zhang, Hiroyuki Murakami, Thomas Knutson, Ryo Mizuta, and Kohei Yoshida. The study was financially supported by Princeton University’s Cooperative Institute for Modeling the Earth System via the Predictability and Explaining Extremes Initiative.
Dorian Timelapse
Video Credit: Princeton University.