Slow hurricanes, like Dorian, become dangerous and hard to predict
Hurricane Dorian has been a slow, rough beast of a storm. Starting as a category 5 monster as it blew into the northern Bahamas, it stalled there for more than 24 hours. All the while, this near record-breaking cyclone pummeled the islands with wind, rain and surging seas. When Dorian finally got moving again on September 3, it slouched northward toward the U.S. coast as a category 2 hurricane. At that point, it still had sustained winds of about 177 kilometers per hour (110 miles per hour).
The second-strongest Atlantic hurricane on record — and strongest outside the tropics — Dorian made landfall in the Bahamas on September 1. East of southern Florida, this nation consists of some 700 islands and more than 2,000 coral reefs (known as cays). At landfall, Dorian blew with sustained winds of about 298 kilometers per hour (185 miles per hour). Its fury was dangerous. But exaggerating its impact was the fact that this storm stalled over the northern Bahamas for more than a day. It shifted a mere 40 kilometers (around 25 miles). That’s the second slowest trek for an Atlantic hurricane (after 1965’s Betsy, a cat 4 hurricane). Dorian’s snail’s pace challenged forecasters as they tried to predict the storm’s likely trek west toward the United States.
Dorian is just the latest of several recent strong but sluggish cyclones. The trend included 2017’s Hurricane Harvey, 2018’s Hurricane Florence and Cyclone Idai, which struck Mozambique in March. In fact, over the past 70 years, cyclones around the globe have been slowing down, notes James Kossin. He’s a climate scientist based in Madison, Wisc., who works for the National Oceanic and Atmospheric Administration.
Stalled cyclones mean more extreme rainfall. And that poses a major threat to coastal communities in a storm’s path. That was true for both Harvey and Florence, according to Kossin and climate scientist Timothy Hall of the NASA Goddard Institute for Space Studies in New York City. They had a paper on such storms in the June issue of Climate and Atmospheric Science.
Some parts of the Bahamas saw more than 61 centimeters (2 feet) of rain from Dorian, according to NASA’s satellite-based estimates. They were released September 3.
Hall talked to Science News about Dorian’s stall and what scientists can say about its link to climate change.
SN: What does it mean for a storm to stall?
Hall: “Hurricanes are like corks floating on a stream,” he says. “Their paths are determined by the large-scale wind fields in which they sit. When those wind fields collapse temporarily — like they did for Dorian and did for Harvey in 2017 — the hurricane doesn’t have any guidance. It just stalls until a new set of wind fields is in place.”
The National Hurricane Center posts storm characteristics every six hours. In his and Kossin’s June paper, he notes, “We defined meandering as an abrupt change in direction from one six-hour time step to the next. If the center of the storm spends at least 48 hours within a 200-kilometer [124-mile] radius, we called it a stall. We just did an unofficial analysis of Dorian and it was clearly a stall.”
In fact, he says of Dorian over the Bahamas, “It basically came to a complete standstill.”
SN: Is there a link between stalling and climate change?
Hall: From what climate scientists know about climate change and hurricanes, “Some things are virtually certain. Rising sea levels are leading to more coastal flooding, for sure. And increased rainfall is a pretty robust projection from changing climate. There’s a strong consensus now in the [climate-science] community that the intensity of tropical cyclones is getting stronger.”
Other things are less well-known, but still important. Among them, he says, is how climate change might affect a hurricane’s path — “which would include the propensity to stall.” In a warmer climate, he notes, “the large-scale wind patterns in the atmosphere may slow down.”
Currently, he says, it’s still hard to tease out that signal from direct observations. “It’s really at the edge of what we understand.”
SN: You found that stalling hurricanes mean more heavy rains.
Hall: Harvey certainly stalled, he says. But aside from that, he notes that “there was so much rainfall that was driven by waters in the Gulf of Mexico that were extremely warm. Something like between 10 and 25 percent of the rain that fell on Houston could be attributed to human-induced warming.”
As for Dorian, “the waters were very warm in the region where it stalled as well.” They are a couple of degrees Celsius above the average for this time of year. But the gold standard for linking weather events to climate change are what’s known as attribution science studies. And it’s too soon to have one of those for Dorian.
SN: What’s the takeaway for now?
Hall: “I’m glad that people are talking about stalling as an additional feature in the hazards of tropical cyclones. It just highlights how, yes, category [storm intensity] is important. But it’s not the be-all, end-all of a storm’s hazard. There’s the physical size of the storm, how it moves, the angle at which it impacts the coastline — all of which are going to have an effect on storm surge and flooding.
angle The space (usually measured in degrees) between two intersecting lines or surfaces at or close to the point where they meet.
Atlantic One of the world’s five oceans, it is second in size only to the Pacific. It separates Europe and Africa to the east from North and South America to the west.
atmosphere The envelope of gases surrounding Earth or another planet.
attribution science A field of research, largely used in climate studies. It seeks to test whether — and by how much — climate change may be responsible for certain extreme weather events, such as droughts, extreme flooding, hurricanes, excessive heat or odd storm trajectories.
Caribbean The name of a sea that runs from the Atlantic Ocean in the East to Mexico and Central American nations in the West, and from the southern coasts of Cuba, the Dominican Republic and Puerto Rico down to the northern coasts of Venezuela and Brazil. The term is also used to refer to the culture of nations that border on or are islands in the sea.
climate The weather conditions that typically exist in one area, in general, or over a long period.
climate change Long-term, significant change in the climate of Earth. It can happen naturally or in response to human activities, including the burning of fossil fuels and clearing of forests.
consensus An opinion or conclusion shared by most, if not all, of a specific group.
coral Marine animals that often produce a hard and stony exoskeleton and tend to live on reefs (the exoskeletons of dead ancestor corals).
cyclone A strong, rotating vortex, usually made of wind. Notable examples include a tornado or hurricane.
gold standard A common term used to mean the premier currently most reliable standard for judging the quality or authenticity of something.
hurricane A tropical cyclone that occurs in the Atlantic Ocean and has winds of 119 kilometers (74 miles) per hour or greater. When such a storm occurs in the Pacific Ocean, people refer to it as a typhoon.
link A connection between two people or things.
NASA Short for the National Aeronautics and Space Administration. Created in 1958, this U.S. agency has become a leader in space research and in stimulating public interest in space exploration. It was through NASA that the United States sent people into orbit and ultimately to the moon. It also has sent research craft to study planets and other celestial objects in our solar system.
National Oceanic and Atmospheric Administration (or NOAA) A science agency of the U.S. Department of Commerce. Initially established in 1807 under another name (The Survey of the Coast), this agency focuses on understanding and preserving ocean resources, including fisheries, protecting marine mammals (from seals to whales), studying the seafloor and probing the upper atmosphere.
physical (adj.) A term for things that exist in the real world, as opposed to in memories or the imagination.
projection Some feature that extends out (or projects) from the body of a structure.
radius A straight line from the center to the circumference of a circle or sphere.
reef A ridge of rock, coral or sand. It rises up from the seafloor and may come to just above or just under the water’s surface.
satellite A moon orbiting a planet or a vehicle or other manufactured object that orbits some celestial body in space.
sea level The overall level of the ocean over the entire globe when all tides and other short-term changes are averaged out.
storm surge A storm-generated rise in water above normal tidal level. In most cases, the largest cause of storm surge is strong onshore winds in a hurricane or tropical storm.
tropical cyclone A strong, rotating storm. These usually form over tropical areas around the equator where the water is warm. Tropical cyclones have strong winds of more than 119 kilometers (74 miles) per hour and usually have heavy rain. Large ones in the Atlantic are known as hurricanes. Those in the Pacific are termed typhoons.
tropics The region near Earth’s equator. Temperatures here are generally warm to hot, year-round.
weather Conditions in the atmosphere at a localized place and a particular time. It is usually described in terms of particular features, such as air pressure, humidity, moisture, any precipitation (rain, snow or ice), temperature and wind speed. Weather constitutes the actual conditions that occur at any time and place. It’s different from climate, which is a description of the conditions that tend to occur in some general region during a particular month or season.
Journal: T.M. Hall and J.P. Kossin. Hurricane stalling along the North American coast and implications for rainfall. Climate and Atmospheric Science. Published online June 3, 2019.
Journal: J.P. Kossin. A global slowdown of tropical-cyclone translation speed. Nature. Vol. 558, June 7, 2018, p. 104. doi:10.1038/s41586-018-0158-3.
Journal: C.M. Patricola. Tropical cyclones are becoming sluggish. Nature. Vol. 558, June 7, 2018, p. 36. doi:10.1038/d41586-018-05303-w.