Posted on August 14, 2023
Could climate change cause the collapse of a critical ocean current that influences everything from sea levels in South Florida to monsoons in the Pacific and temperatures in Europe?
On Tuesday morning, University of Miami scientists boarded a ship bound for the Bahamas to help answer that question. The team, led by UM oceanography professor Lisa Beal, will monitor conditions in the Florida Current, a piece of the Gulf Stream that feeds into another key current that helps distribute heat throughout the Atlantic Ocean.
This conveyor belt of ocean heat may be on the verge of collapse, according to a hotly debated study published in the prestigious research journal Nature last month, which grabbed global headlines by predicting that this disaster would chaotically reshape weather patterns around the world.
Monitoring the strength of the Florida Current will help scientists figure out whether or not that conveyor belt, known as the Atlantic Meridional Overturning Circulation, is actually slowing down.
“It will give us another clue as to whether we think changes going on in the Gulf Stream are consistent with a weakening of the overturning circulation,” said Beal.
One of the potential consequences of a weakening Florida Current could be higher sea levels and more sunny-day flooding in Miami, along with the rest of the U.S. east coast and the Bahamas, Beal said. But she said scientists can’t be sure until they can measure what’s going on within the current.
“What I’m trying to do is add some more information that we can give to people in the city so that their expectations of when there’s going to be a flooding event, for example, will be a little more accurate,” Beal said in a video released by the University of Miami.
A never-before-seen look at the Florida Current
Over the next week, Beal’s team will place a series of nine monitoring devices along a roughly 60-mile stretch of seafloor between South Florida and the Bahamas. These stations will measure how fast the current is flowing, as well as the sea level and the temperature and salinity of the water, every 10 minutes for the next five years.
This torrent of data will give scientists an unprecedented level of detail about conditions in the Florida Current.
Up until now, scientists have been relying on a single abandoned phone cable strung between Florida and the Bahamas to monitor the current. By measuring the voltage of electricity that crosses the phone line, researchers can infer how quickly the water must be passing over it.
That gives scientists at least some data dating back to 1982, which they’ve used to determine that the Florida Current might have slowed down over the past four decades.
But once Beal’s instruments are in the water, she said, she’ll be able to monitor the current much more precisely, allowing her team to see how ocean temperatures affect its flow and trace the source of individual pockets of water based on their temperature and salinity.
She’ll even be able to pinpoint the ocean conditions that lead to specific King Tide floods in Miami over the next five years.
“We’ll be able to say, ‘Oh, look, we have more water mass from the South Atlantic,’ or ‘We had increased recirculation,’ or ‘The structure of the current was closer to shore or further offshore, and that affected the sea level,’” she said.
A glimpse at future sea levels
Pinpointing ocean conditions isn’t just an academic exercise. The shape of the Florida Current directly impacts daily life in South Florida, although many people don’t realize it.
That’s because the ocean isn’t a flat surface. Prevailing winds and currents pile water up higher in some parts of the sea than others. The Florida Current, for instance, pushes sea levels more than 2 feet higher on the Bahamian side of the current than on the Florida coast, according to the National Oceanic and Atmospheric Administration.
If the current weakens, the difference in sea level between Florida and the Bahamas could start to even out, raising sea levels in Miami — and beyond.
“The Florida Current is a very large part of the oceanic conditions in this area, so the impacts will not just be concentrated on Florida,” said Paloma Cartwright, a Ph.D. student in Beal’s lab who is working on the project. “You’ll see them up the east coast of North America and in the Bahamas.”
A fifth of the land in Miami-Dade County sits 2 feet or less above sea level, according to county elevation data, meaning even small changes in ocean currents that alter local sea levels could have an outsized impact on large swaths of Miami.
Measuring data at the bottom of the sea
Part of the reason Beal’s study is so unique is that it’s not easy to put scientific equipment in the Florida Strait.
“The current is so strong that everything just gets blown over,” said Beal.
That rules out anchoring long ropes to the bottom of the sea and floating instruments at various depths to measure temperature, water speed and salinity — the current would topple the line over. Instead, Beal and her team will anchor monitoring devices onto the seafloor at nine spots and measure conditions from there.
Each monitoring device will send out a high-frequency chirp every 10 minutes. The sound waves pass through the water, bounce off the ocean surface and eventually make their way back down to the seafloor, where the monitoring devices measure how long it takes the echo to come back. That allows the scientists to figure out the water temperature, because the speed of sound in water changes depending on the temperature.
Each device also has a pressure gauge, which measures the weight of the water pressing down on it. That tells the scientists how much water is above each device, which allows them to gauge the sea level. Finally, the devices also have instruments that measure how fast water is moving along the seafloor.
By combining all three of these measurements and a whole lot of math, the scientists can also estimate the salinity of the water and model what the entire water column looks like above each sensor. That will give them more of the data they need to figure out what’s happening in the Florida Current and whether it signals that the Atlantic Meridional Overturning Circulation is on track to collapse.
“Even if (the Florida Current) is weakening, is it weakening in a way that affects the AMOC?” Beal said. “We don’t know unless we can measure how the heat content is changing, because that’s related to the water masses, that’s related to whether the changes are in the upper water column or the lower water column, and ultimately whether we understand that they’re part of the AMOC or not.”
A yearlong wait for data
Although the sensors are going in the water this week, Beal won’t get data back from them for about a year. That’s when she and her team plan to pull up the sensors, download their data and replace them on the seafloor.
“We won’t have real-time data, which is a shame,” Beal said. But that’s because of the physical limitations of doing research in the deep ocean. All of Beal’s sensors will be sitting under several hundred feet of water, putting them out of range of most communication signals.
“We’re stuck in this very old world of Morse code,” Beal said. “We use acoustics to communicate with our instruments, literally dot dot dot, dash dash … so imagine how long it takes to get information that way. It’s really inefficient.”
It’s even more difficult to listen for the dots and dashes from a ship that’s running its engines to hold its position in a moving current. So Beal opted instead to collect the sensors every nine to 12 months and catch up on all the data she’s missed.
“We’ll be able to go in and see the details, but after the fact,” she said.
This climate report is funded by Florida International University and the David and Christina Martin Family Foundation in partnership with Journalism Funding Partners. The Miami Herald retains editorial control of all content.