Hurricanes May Be Reshaping Big Parts of the Ocean

The storm formerly known as Hurricane Lorenzo is blasting Ireland and the United Kingdom today with winds up to 80 miles per hour, heavy rain, and flooding. Lorenzo meandered in a comma-shaped path from the Azores to the British Isles as it grew to Category 4 strength before weakening slightly on Wednesday (and losing its official designation as a hurricane).

Lorenzo does not pack the deadly punch that Hurricane Dorian did just a few weeks ago when it stalled over the Bahamas, but it is causing problems over a huge area. Scientists are watching storms like Lorenzo not only for their effects on residents of islands and coastal cities, but also on the ocean itself.

Back in late August, Tal Ezer’s neighborhood in Norfolk, Virginia, was feeling the effects of Dorian, which was churning more than 800 miles south. That’s too far away to feel the storm surge pushed by hurricane-force winds, but there was still minor flooding that clogged traffic and ruined his daily jog.

Ezer, a physical oceanographer, says Dorian actually slowed down the Gulf Stream current, which flows northward from Florida along the coast up to the North Atlantic, by almost 50 percent. He knows that because he took data from a giant underwater cable that stretches between Florida and the Bahamas that measures the speed of the current.

Normally the Gulf Stream flows so strongly (3 to 5 mph) along the Eastern seaboard that it pulls water away from the coast. Imagine a warm, fast moving current in the middle of a still, cooler river. When hurricane winds temporarily plug up the Gulf Stream flow, that water sloshes back against the coastline, Ezer and other researchers have found, including flooding his neighborhood.

“We started seeing flooding when Dorian was stuck near the Bahamas,” says Ezer, an earth scientist at Old Dominion University. The flooding intensified as Dorian swept past Virginia, as people expected. But it’s the long-distance changes wrought by hurricanes, on both coastlines and the deep ocean, that scientists are only now starting to tease apart and understand.

This slowing of the Gulf Stream, which carries more water than all the planet’s rivers, is one of those effects. And as hurricanes become stronger and more intense under climate change, scientists say it’s likely these massive storms will impose other changes on the environment hundreds of miles away.

In September, Ezer published his calculations of how a different storm, 2016’s Hurricane Matthew, slowed down the Gulf Stream in the journal Ocean Dynamics. Now he’s putting together the numbers from Dorian. When Dorian parked itself over the Bahamas, the Gulf Stream current dropped 47 percent, to 4.5 billion gallons per second. Its normal velocity returned a few days later, when the storm had passed.

Oceanographers measure the speed of the Gulf Stream current by looking at subsea telephone cables, which generate a magnetic field. Researchers can assess speed by checking the voltage that arises when salt ions in the water move through the field. Ezer also used data from orbiting satellites that scan the surface of the Gulf Stream as it flows past Cape Hatteras, North Carolina.

It’s not just the Gulf Stream feeling the long-distance effects of hurricanes. Another group of scientists recently found big hurricanes also can stir up the deep ocean by dumping tons of nutrient-rich plankton on the sterile bottom.

Researchers looked at what happened to the ocean when Category 3 Hurricane Nicole blew past Bermuda in 2016. It turns out the storm traveled right by a special underwater monitoring station that collects “marine snow”—a combination of dead plankton and aerosol particles from the atmosphere that accumulates on the surface and sinks to the bottom. In a study published last month, scientists at the Marine Biological Laboratory in Woods Hole, Massachusetts, and the Bermuda Institute of Ocean Sciences found that hurricanes accelerate a process called the “biological pump,” in which phytoplankton drifting near the ocean’s surface capture carbon from the atmosphere and transfer it to deeper ocean layers and finally the seafloor.

Hurricane Nicole’s high-velocity winds cooled the ocean surface and created intense currents and underwater waves, some of which lasted more than two weeks. This churning pushed nutrients into the surface layer, triggering an algae bloom. Once the bloom died, the material sank to the deep ocean and provided a big boost of food for marine life in the deep ocean, where light doesn’t reach. The study found that the amount of food raining down was tripled at about 5,000 feet deep, and was up to eight times greater at 10,500 feet.

Critters that have evolved to scavenge for food find themselves living in a temporary smorgasbord of tasty planktonic treats, explains Rut Pedrosa Pàmies, a coauthor on the new study.

While that may seem like a good thing in the short term, a long-term shift in the biological pump—which helps to keep carbon sequestered in the deep ocean and out of the atmosphere—might cause problems for our warming planet. Pedrosa says the next step is to investigate how the deep-dwelling marine life is changed by hurricanes swirling above. “I would like to have eyes on the deep ocean when hurricanes come by, that would be great to know,” she says. Some fish, for example, evacuate their shallow habitat for the safety of deeper water, according to a recent study by NOAA researchers who tracked triggerfish as they fled Hurricanes Jose and Maria in 2017.

Oceanographers Pedrosa and Ezer are waiting for the next big storm to provide them with data that can help answer outstanding questions. Pedrosa says she doesn’t know yet whether the disturbance she found in the deep ocean by hurricanes are a minor blip in the biological pump or something bigger that could tip the balance toward a more troublesome future.

The Atlantic hurricane season is more than half over, but it doesn’t officially end until November 30. After Lorenzo, the next named storm would be Melissa.