Just days before Hurricane Irma made landfall in Florida this past September, a team of marine scientists were in Miami on a project unrelated to the looming storm. They were deploying an ocean glider to monitor the strong offshore current known as the Gulf Stream. Later, however, they learned, from data collected by the robotic vehicle, that two other Atlantic hurricanes around the same time—Jose and Maria—acted as surprisingly powerful brakes on that major current.
Winds blowing against the Gulf Stream tend to slow it down, said Robert Todd, an oceanographer at the Woods Hole Oceanographic Institution in Massachusetts who is leading this glider research, but “seeing the Gulf Stream temporarily slow down by 25–40% was not expected.”
The glider’s measurements also revealed that the Gulf Stream carried an unusually large volume of fresh water after Hurricane Irma pummeled Florida.
These new observations show that short-lived events like hurricanes can have a measurable impact on the flow of the Gulf Stream, said Christopher Meinen, an oceanographer at the National Oceanic and Atmospheric Administration in Miami, Fla., who was not involved in the research. What’s more, he continued, they “illustrate how innovative new technologies can be used to help scientists observe the ocean safely at times when the weather would make sending a research ship full of scientists out unsafe.”
A Current That Drives Climate
The Gulf Stream, which originates off the Florida coast and winds up the East Coast of the United States before veering into the mid-Atlantic, ranks among the world’s largest ocean currents. It transports warm water thousands of kilometers, bringing heat from the tropics to the northern Atlantic. “The Gulf Steam is an important part of the climate system,” said Todd. However, its subsurface flow is monitored on a regular basis only in selected places like the strait between Florida and the Bahamas. Along 1,500 kilometers of the stream, we haven’t been measuring in any routine, sustained way, said Todd. Now “gliders are filling in that gap.”
An Unexpected Slowdown
Todd and his team launched a 2-meter-long glider from Miami on 7 September 2017. Propelled by the Gulf Stream, the vessel traveled at a speed of about half a knot—about 1 kilometer per hour—during its roughly 3-month-long journey north toward Martha’s Vineyard, Mass. Along the way, the glider dove as deep as 1,000 meters and measured water temperature, salinity, the current’s speed, sediments, and microorganisms like phytoplankton and zooplankton in the water column.
The researchers found that the Gulf Stream slowed down significantly for a few weeks as Hurricanes Jose and Maria passed over in mid- to late September when the glider was off the coast of South Carolina. They attribute this trend to the hurricanes’ winds blowing opposite to the direction of the current.
This slowdown didn’t occur just at the ocean surface, the data showed. From the glider’s repeated measurements at different depths, the team found that the slowdown persisted throughout the water column. “The velocity was all reduced,” said Todd, who is scheduled to report on his group’s results Thursday at the 2018 Ocean Sciences Meeting in Portland, Ore.
Less Salty, Too
Unlike Hurricanes Maria and Jose, Hurricane Irma traveled up Florida’s west coast and therefore did not significantly contribute to the slowing of the Gulf Stream. Nonetheless, it caused a different, measurable effect. The storm’s rainfall, paired with runoff from the land, added a lot of fresh water to the ocean, which the glider picked up as a signal of unusually fresh water in the Gulf Stream. “Irma was the source of that fresh water,” said Todd.
Todd and his collaborators continue to monitor the Gulf Stream by deploying a glider roughly every 2 months from Miami. These hurricane-related results are just part of the team’s findings based on their ongoing work. “We just happened to have one deployment that was timed well,” said Todd.