Tropical cyclones are getting more sluggish | Science News for Students

Tropical cyclones are getting more sluggish

How speedily the storms move across the landscape has been slowing, which could increase flooding
Jul 16, 2018 — 6:45 am EST
a satellite image of Hurricane Harvey making landfall in 2017

Tropical cyclones, such as 2017’s Hurricane Harvey (shown making landfall), have slowed down. That could make them more dangerous.

NOAA

Tropical cyclones aren’t as speedy as they used to be. They still spin rapidly. But they often travel at a more leisurely path down across the landscape, a new study finds.

Cyclones are fierce, swirling storms that form over oceans. Those in the Atlantic Ocean are called hurricanes. Today these storms move 10 percent slower, on average, than nearly 70 years ago. And that matters because as they trek across the landscape, these slowpokes can dump more rain — causing more flooding — and spend more time lashing these areas with their winds.

James Kossin works at the National Centers for Environmental Information in Madison, Wisc. It’s part of the National Oceanic and Atmospheric Administration. This scientist looked at how quickly tropical cyclones have been moving across the planet. He focused on changes between 1949 and 2016.

Story continues below image.

a map showing slowdowns in tropical cyclone movment by global region
Tropical cyclones are getting more sluggish almost everywhere in the world. The largest drops in tropical cyclone travel time are in the western North Pacific Ocean region.
J.P. KOSSIN ET AL/NATURE 2018, NOAA

Storms have been traveling more slowly almost everywhere, he found. But the slowdown differed by region. He found the biggest slowing in the Northern Hemisphere.

Over the same nearly 7-decade period that he studied, climate change has been warming our world. Earth’s average surface temperature has climbed by about half a degree Celsius (0.9 degree Fahrenheit).

Scientists think tropical cyclones that form over warmer waters will, on average, spin faster. That translates to speedier winds. The new study suggests that climate change also is slowing the speed at which these storms cross the landscape.

Kossin shared his findings June 7 in Nature.

Trek slows most after landfall

The cyclone slowdown was especially obvious as storms moved over land. Those that started in the western North Pacific, such as near Japan, slowed by 30 percent. Those coming from the North Atlantic are moving 20 percent more slowly over land, says Kossin.

Tropical storms get pushed around by winds and air currents across the globe. Climate warming has been slowing this air movement in tropical areas. Kossin now finds that climate change also seems to be slowing down cyclones.

What also concerns scientists: Global warming could increase how much cloud water the atmosphere can hold. That means storms could collect more moisture before dumping it as rain.

Christina Patricola is an atmospheric scientist at Lawrence Berkeley National Laboratory in California. The new study makes an important link between the pace of tropical cyclones and the slowing winds around the planet (due to global warming), she says in a commentary about the new findings. Kossin correctly guessed, she notes, that if climate change is generally slowing down air movement, tropical cyclones also should slow.

It’s not clear whether storms will keep slowing, she notes. Right now, scientists can’t predict how cyclone slowing might vary across the planet.

One recent slowpoke was 2017’s Hurricane Harvey. It hovered over southern Texas for almost 5 full days. During this time it dumped an average of 76 centimeters (30 inches) of water across a region in which some six million people live. If you had stacked all of that water over New York City (a far smaller area), it would have reached a height of 68 meters (224 feet)! At the height of Harvey’s fury, this storm also spawned more than 30 tornadoes in Texas.

Some scientists now ask: Could such a mega-storm be a sign of things to come?

Power Words

(for more about Power Words, click here)

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.

average     (in science) A term for the arithmetic mean, which is the sum of a group of numbers that is then divided by the size of the group.

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.

commentary     (in science) An opinion piece, often written to accompany — and add perspective to — a paper by others, which describes new research findings.

cyclone     A strong, rotating vortex, usually made of wind. Notable examples include a tornado or hurricane.

global warming     The gradual increase in the overall temperature of Earth’s atmosphere due to the greenhouse effect. This effect is caused by increased levels of carbon dioxide, chlorofluorocarbons and other gases in the air, many of them released by human activity.

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.

moisture     Small amounts of water present in the air, as vapor. It can also be present as a liquid, such as water droplets condensed on the inside of a window, or dampness present in clothing or soil.

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.

Pacific     The largest of the world’s five oceans. It separates Asia and Australia to the west from North and South America to the east.

planet     A celestial object that orbits a star, is big enough for gravity to have squashed it into a roundish ball and has cleared other objects out of the way in its orbital neighborhood.

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.

Citation

Journal: J.P. Kossin. A global slowdown of tropical-cyclone translation speedNature. Vol. 558, June 7, 2018, p. 104. doi:10.1038/s41586-018-0158-3.

Journal: C.M. Patricola. Tropical cyclones are becoming sluggishNature. Vol. 558, June 7, 2018, p. 36. doi:10.1038/d41586-018-05303-w.