How warming affects rare northern storms like Henri

By Chelsea Harvey | 08/23/2021 06:40 AM EDT

Hurricane Henri is a remarkable event in a summer already marked by extraordinary weather.

Tropical Storm Henri approaches Rhode Island.

A man photographs waves as Tropical Storm Henri approaches Rhode Island yesterday morning. Scientists say rare northern storms like Henri are becoming more intense because of climate change. AP Photo/Stew Milne

Tropical Storm Henri struck Rhode Island yesterday afternoon with 60-mph winds, making it the first named storm to hit the state in 30 years.

That was a rare occurrence in the Northeast, where most hurricane-force storms weaken over cooler waters or get pushed into the Atlantic by easterly winds. Those that do strike Northern states are almost never major storms by the time they hit.

Henri brought heavy rainfall and power outages from New York and New Jersey through New England. New York City received 1.94 inches of rain between 10 and 11 p.m. Saturday night, marking its wettest single hour in recorded history.


Those impacts came as Henri traversed an unusual path over the weekend. It gained strength as it moved north toward the New England coastline on Saturday and briefly achieved hurricane status. It was downgraded back to a tropical storm yesterday morning before plunging ashore.

That makes Henri a remarkable storm in a summer marked by extraordinary weather events, from record-shattering heat waves in the western U.S. to catastrophic floods in Europe, China and India. Scientists say many of these extreme events have been influenced by climate change, and that they offer glimpses into the future of a warming world.

Is that what drove Henri’s bizarre behavior?

Yes and no.

Scientists have found that tropical cyclones are migrating closer to the poles over time. In parts of the Northern Hemisphere, that means storms are wandering farther north as the oceans warm.

But the story gets more complicated the closer scientists look — the effects aren’t the same everywhere, or for every storm. Scientists observe a clear poleward shift of hurricanes in the western North Pacific, for instance. But in the Atlantic, the trend is much smaller.

That’s probably because of complicating factors in the Atlantic Ocean basin, according to James Kossin, a senior scientist with the analytics firm the Climate Service and a researcher at the University of Wisconsin, Madison, who has studied the poleward migration of hurricanes.

During much of the 20th century, ocean temperatures in the Atlantic were strongly influenced by heavy air pollution from Europe and North America. Pollution can have a cooling effect on the local climate, and scientists believe it helped mask some of the effects of global warming in the region. Hurricane activity was likely dampened for decades.

Toward the end of the last century, new air quality regulations went into place and pollution began to decline. The ocean warmed and hurricane activity began to pick back up.

This sequence of events means some long-term hurricane patterns in the Atlantic are less clear than they are in other ocean basins. It’s not certain, for now, whether to expect more wandering Atlantic hurricanes in the coming decades compared to other parts of the world.

In Henri’s case, an unusual set of conditions happened to line up at just the right time. A high-pressure system on one side of the storm, coupled with a low-pressure system on the other side, combined forces to drive Henri north up the coast.

Under normal circumstances, the storm would have likely veered back out to sea.

In other words, Henri’s weird track is something of a fluke.

Similar circumstances have allowed for occasional Northeastern landfalls in the past, including Hurricanes Bob, which struck southern New England in 1991, and Gloria, which hit Long Island in 1985.

Superstorm Sandy, which devastated New York and New Jersey in 2012, was also the product of unusual conditions. Sandy collided with a swirling low-pressure system as it approached the East Coast. That sent it careening suddenly westward into the New Jersey shoreline.

But even if these Northeastern landfalls remain relatively rare in the future, climate change is still turning them into bigger threats over time.

For one thing, sea levels are rising all along the East Coast. No matter where a hurricane strikes, the impact of storm surge and the risk of damaging floods is growing over time.

Warmer ocean waters also juice hurricanes as they move across the sea from Africa, increasing the odds that they spin up into major storms. Research suggests that hurricanes are already growing more intense, on average, as the climate warms.

In the cooler Northeast, warmer waters could mean that stray storms like Henri have a higher chance of maintaining hurricane strength as they move north. Even if their odds of making landfall remain relatively small, the few that do hit land may pose bigger risks.

Warm waters probably helped Henri along, Kossin pointed out. Sea surface temperatures off the coast of New York and New England were several degrees warmer than average as Henri chugged toward the shore.

“So it’s maybe not so clear that climate change is playing a role in Henri’s unusual track, but it may play a role in Henri’s intensity at higher latitudes,” Kossin said in an email to E&E News.

According to NOAA records, ocean waters off the Northeast coast are heating up faster than the global average. In fact, they’re some of the fastest-warming waters around the U.S.

“[I]t’s a definitive statement that warmer ocean temperatures at higher latitudes along the coast will allow storms to survive longer as they move northward, all other things equal,” Kossin added.