Hurricane Dorian, which was considered one of the most powerful Atlantic hurricanes on record, battered the Bahamas, Florida, and the Carolinas in 2019. The record-breaking storm pummeled these regions with 185-mph winds and up to 30 inches of rain, killing at least 50 people and leaving 76,000 in need of urgent support. Bahamas officials reported that ab0ut 2,500 people went missing due to the hurricane.
While the hurricane was categorized as a Category 4 storm, it was extremely dangerous. As Time, an American weekly news magazine, previously reported: “a Category 4 hurricane will blow out most windows on high-rise buildings, uproot most trees and will likely down many power lines. Power outages can last for weeks or even months after storms of this level. Water shortages are also common in the aftermath of Category 4 hurricanes, potentially making the affected area uninhabitable for weeks or months.”
Previous reports show that Hurricane Dorian moved as slowly as 1 or 2 mph, which was a huge concern. A slow-moving storm not only makes the flooding worse but also causes significant coastal erosion. Recent studies are now showing that hurricanes are getting wetter and more sluggish. A 2018 research revealed that the speed of hurricanes and tropical storms has slowed about 10% on average over the past 70 years or so. Over land in the North Atlantic and Western North Pacific specifically, storms are moving 20% to 30% more slowly.
Why Slow-Moving Hurricanes Are Dangerous
A perfect example of a slow-moving hurricane is Hurricane Harvey. In 2017, it weakened to a tropical storm after it made landfall, then stalled for days. Scientist Tom Di Liberto described it as the “storm that refused to leave.” According to Business Insider, a fast-growing business site with deep financial, media, tech, and other industry verticals, the storm dumped unprecedented amounts of rain on the Houston area, killing more than 100 people and causing $125 billion in damages.
In a Facebook post, climate scientist Michael Mann wrote that Hurricane Harvey was more intense than it would have been in the absence of human-caused warming, which means stronger winds, more wind damage, and a larger storm surge. Stronger rains are expected in slow-moving hurricanes. The slower the speed, the greater the rainfall amounts can become, which could harm many lives.
Flooding from Tropical Storm Alberto in 1994, for instance, killed 31 people. It moved slowly across the western Florida Panhandle, Alabama, and Georgia, receiving 21 inches of rain in just 24 hours. In June 2001, Tropical Storm Allison unleashed more than 40 inches of rain, resulting in massive flooding and 23 deaths in Texas alone. Between 1963 to 2012, about 27% of all US hurricane deaths were from rainfall flooding. Strong winds, meanwhile, contributed to 8% of the deaths during that time.
Scientists have already warned that the trend for slower-moving tropical cyclones (TCs) results in more catastrophic rainfall. “Stalling TCs have the potential for depositing damaging amounts of rain. This trajectory-induced increase in rainfall is exacerbated by the climate-warming impact on the hydrologic cycle. Increased atmospheric moisture enhances the likelihood of extreme rainfall events of all types,” the authors said.
Slow-moving hurricanes can also result in long-lasting winds. According to Weather.com, an online site that provides local and national weather forecasts, persistent winds combined with the soaked soil can make trees topple easier at lower wind speeds than otherwise with drier soil. This can lead to widespread tree damage and power outages. With slow-moving hurricanes, the persistence of winds will not allow the water to recede from inlets, bays, and rivers as it would normally with a storm that moves inland at a more progressive pace.
Expect More Slow-Moving Hurricanes Due to Climate Change
While scientists said that the reasons for the trend are not fully understood, they have created some theories. For instance, they theorized that the weakening of general atmospheric circulation patterns, including those of the tropics, resulted in slower global wind speeds. Other studies have suggested that the disproportionate warming of the planet at its northern extreme increased levels of greenhouse gases, playing a huge role in reducing airspeeds around the world.
A recent study published in Science Advances provided an explanation as to why human-caused warming will cause more slow-moving hurricanes in the future. The researchers provided evidence that there could be a slowdown of translational motion in response to future warming on the order of 4 degrees Celsius. They found that there would be a marked slowdown as the world warms due to a poleward shift of the mid-latitude westerly winds.
According to Phys.org, an internet news portal that provides the latest news on science, the researchers selected six potential warming patterns for the global climate and ran 15 different possible initial conditions on each of the six patterns, resulting in an ensemble of 90 possible futures. They conditioned the computers to assume that global carbon dioxide levels have quadrupled and the planet's average temperature has risen by about 4 degrees Celsius.
Gan Zhang, a postdoctoral research associate in atmospheric and oceanic sciences, said that this is the first study that combined physical interpretation and robust modeling evidence, which showed that future anthropogenic warming could lead to a significant slowing of hurricane motion. The results revealed that future anthropogenic warming could lead to a significant slowing of hurricane motion, particularly in some populated mid-latitude regions.
The simulations also suggested that the storms' forward motion would slow by about 2 miles per hour at latitudes near Japan and New York City—about 10 to 20% of the current typical speeds. "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. 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," Suzana Camargo, the Marie Tharp Lamont Research Professor at Columbia University's Lamont-Doherty Earth Observatory, who was not involved in this research, said.