Discovering Uncharted Interior Pathways

If you Google the phrase “Great Ocean Conveyor,” one of the first images that will likely pop up on your computer screen is a map of the North Atlantic Ocean with color-coded arrows showing a looping, conveyor belt-like path that deep ocean currents follow as they flow north from the equator and south from the polar seas.

It’s a model that’s been used for more than 20 years to explain how the oceans distribute heat and influence our climate, and—more recently—to shape scientists’ hypotheses about the amount and fate of carbon dioxide that oceans sequester from our atmosphere.

In the conveyor belt paradigm, currents warmed by the Gulf Stream move northward and release their heat into the atmosphere, leaving the waters themselves colder and denser. At their northern terminus, the dense, cold waters sink beneath the polar seas and flow back southward along a discrete, well-defined path called the Deep Western Boundary Current that hugs the continental shelf between Canada and the equator. To replace this sinking water, warm surface waters from the tropics are pulled northward again, creating a continuous loop of climate-moderating currents.  

It’s a nice, neat, tidy system.

Oceanographers, however, have long known that this paradigm for describing deep ocean circulation is an oversimplification—a useful enough depiction of the general principle, but missing key pieces of the puzzle.  

“We’ve hypothesized, based on studies using indirect evidence like ocean salinities and temperatures, that there are re-circulations, which cause alternative pathways for these deep waters,” says Susan Lozier, professor of physical oceanography and chair of the Division of Earth and Ocean Sciences at the Nicholas School.

“If this hypothesis were true,” she explains, “it would significantly affect how scientists measure climate signals in the deep ocean. But the hitch was, we lacked the direct evidence to prove it.”

Now, a major study led by Lozier and longtime research collaborator Amy Bower, a senior scientist in the Department of Physical Oceanography at Woods Hole Oceanographic Institution (WHOI), has provided that critical evidence.  

The study, published in the May 14 issue of the journal Nature, used data from computer models and an armada of sophisticated Range and Fixing of Sound (RAFOS) floats, deployed during research cruises in the North Atlantic over the course of three years, to show that most of the southward flow of cold water from the Labrador Sea moved not along the Deep Western Boundary Current, but instead followed previously uncharted “interior pathways” in the deep ocean.  Groups of six RAFOS floats were released into the Labrador Sea every three months from 2003 through 2005 and were left in the water to collect data for two years.

Only 8 percent of the floats followed the conveyor belt of the Deep Western Boundary Current, Lozier and Bower’s study found. About 75 percent of them escaped that pathway and drifted into the open ocean before reaching the Grand Banks.

“Eight percent is a remarkably low number in light of expectations that the Deep Western Boundary Current is the dominant pathway for Labrador Sea water,” Lozier says. “This shows that the concept of the deep flow

operating like a conveyor belt doesn’t hold anymore. The pathways are more diffuse. They spread out much farther into the eddy-filled deep ocean, so it’s going to be more difficult for scientists to measure climate change signals.”

Lozier and Bower first conceived of their ambitious project eight years ago, in response to earlier studies, including a widely cited paper Lozier published in Science in 1997 that strongly suggested unknown interior pathways played an important role in deep circulation of the North Atlantic. A study of floats in the Labrador Sea in the late 1990s by scientists at the Scripps Institute of Oceanography and Woods Hole seemingly confirmed Lozier’s hypothesis, but results from this study were not convincing, in part because the submersible floats used to collect the data had to return repeatedly to the surface to report their positions and observations to satellite receivers. This meant the floats’ data could have been biased by upper ocean currents during the floats periodic ascents.

“The challenge for Amy and me,” Lozier recalls, “was finding a way to collect direct evidence, free of possible bias, that would test our hypothesis and either prove or disprove it.”

With funding from the National Science Foundation and technical support from the staff at Woods Hole, Lozier and Bower devised an elaborate plan they hoped would surmount that challenge.  

Bower and her colleagues built 76 specially designed RAFOS floats configured to submerge to a depth of 700 to 1,500 meters below the ocean’s surface—within the layer of water where a major portion of the cold, south-flowing current of Labrador Sea water flows.

A RAFOS float weighs about 22 pounds (10 kilograms) and can be dropped over the side of a small boat by one person, although they are most commonly deployed from large oceanographic research vessels. The float’s electronics are housed in a thin, six-and-a-half foot (two meter) glass tube that vaguely resembles a giant glass thermometer or overhead fluorescent strip light.  

Once deployed, the floats drifted underwater with the currents for two years, recording location information as well as temperature and pressure measurements once a day. After two years, they returned to the surface and transmitted their treasure trove of stored data to scientists back in the lab through the ARGOS satellite-based data retrieval system.

To communicate with the floats and to track their positions while they were still submerged, the researchers deployed anchored low-amplitude sound beacons in the general area of the experiment. The beacons were set to “ping” automatically every day, enabling the scientists to determine the distance between the floats and beacons, based on the time delay between when the ping went off and when it was detected by the RAFOS floats’ onboard hydrophones.

The ambitious program would have been prohibitively expensive, Lozier notes, had it not been for a collaboration with Eugene Colbourne of the Northwest Atlantic Fisheries Center in St. Johns, Newfoundland. Colbourne regularly conducts hydrographic surveys around the Grand Banks and agreed to deploy the researchers’ floats during his cruises.  

Since the RAFOS float paths only could be tracked for two years, Lozier worked with Nicholas School PhD student Stefan Gary and German oceanographer Claus Böning of the Leibniz Institute of Marine Sciences—both listed as co-authors on the Nature paper—to run computer models that simulated the launch and dispersal of more than 7,000 “e-floats” from the same starting point.

Subjecting the e-floats to the same underwater dynamics as the real ones, Lozier, Gary and Böning traced their pathways and found that the spread of the e-floats was “very similar” to that of the actual RAFOS float trajectories after two years.

The combined observations from the real and simulated experiments provided clear evidence that southward interior pathways in the deep ocean are more important than previously shown for the transport of Labrador Sea water to the subtropics, says Peter B. Rhines, professor of oceanography and atmospheric sciences at the University of Washington.

“Drs. Bower and Lozier have brought the remarkable technology of neutrally buoyant deep, drifting buoys to bear on a matter of great importance to global climate. The global ocean circulation which ventilates the great depths of the seas is often portrayed as a ‘conveyor belt.’ While this is a useful analogy, their work establishes conclusively that ocean eddies—swirling water masses, much like the rotating storms of the atmosphere—stir the deep ocean. In doing so, the eddies spread the ‘conveyor’ over a vast region of the North Atlantic,” Rhines says.    

Since the southward flow of cold Labrador Sea water is a major component of the waters that flow toward the equator as part of the global overturning circulation, this finding will significantly change how oceanographers observe and monitor the deep ocean.

“We will need to make more measurements in the deep ocean interior, not just close to the coast where we previously thought the cold water was confined,” Bower says.  

The Labrador Sea is an area of special focus for climatologists, she explains, because the effects of climate change are magnified at higher altitudes. Surface waters there absorb heat-trapping carbon dioxide from the atmosphere, and much of that CO2 is taken to depth within the sinking waters in this region, where it is no longer available to warm Earth’s climate.

“We know that a good fraction of the human-caused carbon dioxide released since the Industrial Revolution is stored in the ocean,” says Lozier. “The question is, how much is stored at depth? And for how long?

“To answer these questions, we need to learn more about where these deep, cold currents flow, how they act as sinks for heat and carbon dioxide, and their ultimate fate in the ocean,” she explains.

Toward this end, Bower and Lozier plan to expand their research in coming years to study the southward flow of cold water originating even farther north in the remote waters of the Greenland Sea.  

Additionally, Lozier hopes to make use of a new generation of high-tech underwater submersibles to speed and smooth the data-collection process. In the past five years, she explains, researchers have developed programmable, unmanned battery-operated units that can glide through the deep ocean, collect real-time data at pre-set depths and then surface and transmit the data back to scientists in the lab via satellite, avoiding the long time delays associated with RAFOS floats or the potential data bias of the profiling floats used in the 1990s.

“The idea of being able to program gliders to go where you want, collect what you need, transmit it back to you in real time, and then follow new instructions about where to collect data next—it’s an oceanographer’s dream,” she says.

“I sometimes envy those scientists who can collect a sample in the morning and then go into their lab to do the research that afternoon,” she laughs. “Observational oceanography is many things: fascinating, important and rewarding. But no one ever said it was simple.”

Tim Lucas is the Nicholas School’s national media relations and marketing specialist.  

Monte Basgall, senior writer at Duke News and Communications, and communications officers at Woods Hole Oceanographic Institution contributed to this article.