A River Runs Through It

Chronicling the Currents of the North Atlantic

By Mike Carlowicz

Source: Woods Hole Currents

In the northwestern Atlantic Ocean, along the edge of the U.S. continental shelf, some of the most important currents in the world are flowing. You can't always see them from the deck of a ship and you don't really notice the flow, except for the occasional mat of Sargassum drifting by.

But WHOI Senior Scientist John Toole can see the rivers of water flowing through the ocean. What Toole and his colleagues observe from these vast and deep currents may confirm or change what we know about how the oceans influence global climate.

Their vision is made keen by an instrument Toole has been developing in his mind, his laboratory, and at sea for more than a decade: the moored vertical profiler. The profiler crawls up and down a steel cable anchored to the seafloor and held vertically by floats. It rises to just below the surface and slides back down to the depths (see illustration to the right). Sensors on the profiler continuously measure the temperature, salt content (salinity), and velocity of water, and then store the data in computer memory.

Ocean scientists have traditionally collected water profiles from research ships, lowering and raising instruments and sampling bottles from the deck. But ocean expeditions to any one spot in the sea are typically rare and short, preventing scientists from gathering anything more than snapshots of what is happening at a few fleeting moments in the ocean. With the advent of the autonomous moored profiler, Toole and other physical oceanographers can repeatedly sample conditions in the same vertical slice of the ocean day after day, for months to years at a time.

Collaborating with European investigators and colleagues in the WHOI Department of Physical Oceanography, Toole has launched a program to use moored profilers and other instruments to gather data on the circulation system of the northwestern Atlantic. They have named the project "Station W" in memory of Valentine Worthington, a pioneering WHOI physical oceanographer and a student of the deepwater currents of the North Atlantic.

The Station W program began in October 2001 with the placement of a mooring in 3000 meters (9800 feet) of water, 320 kilometers (200 miles) southeast of Cape Cod at 69â°W longitude. It was the first element of an array of five moorings that will stretch 150 km (93 miles) toward Bermuda. The line of moorings will span the continental slope north of the Gulf Stream, the powerful surface current that brings warm, salty waters all the way to Greenland and Northern Europe. It will straddle the other great current system of the western Atlantic, the Deep Western Boundary Current (DWBC), which acts as a pipeline transporting cold Arctic and North Atlantic waters southward These currents are crucial to the circulation of the Atlantic Ocean, which plays a key role in regulating global climate.

"It is difficult to imagine many sites of greater oceanographic and climatic significance," said Robert Dickson, head of Deep Sea Physical Oceanography at the Centre for Environment, Fisheries, and Aquaculture Science in the United Kingdom.

As Goes the Gulf Stream, So Goes the Climate
In 20,000 Leagues Under the Sea, Jules Verne wrote: "We then went with the current of the sea's greatest river, which has its own banks, fish, and temperature. I mean the Gulf Streamâ...we must pray that this steadiness continues becauseâ...if its speed and direction were to change, the climates of Europe would undergo disturbances whose consequences are incalculable."

Verne and scientists of the day didn't have much data, but they were aware of just how much the Gulf Stream meant to nations bordering the North Atlantic. What they guessed then, we can prove now. The Gulf Stream brings warm, salty water from the tropics to the high-latitude North Atlantic. As that water moves north, it gives up heat and some moisture to the atmosphere, making climates in Europe much warmer and moister than one would expect for a landmass at such high latitude.

The process leaves behind cool, salty water that is denser than surface waters. In the seas that ring the northern Atlantic—the Labrador, Irminger, and Greenland seas—this dense water sinks to the depths and flows southward. These conduits of cold, deep waters converge into the Deep Western Boundary Current, which flows adjacent to and sometimes beneath the Gulf Stream. This sinking and southward flow draws more warm water north to replace it, and contributes to a worldwide circulation pattern known as the Global Thermohaline Circulation, or the "Great Ocean Conveyor."

As the world debates global warming and what to do about it, oceanographers are looking for telltale signs that Earth is already changing. Most projections of climate change anticipate a weakening of the circulation in the North Atlantic. Essentially, as greenhouse gases warm the atmosphere, they will cap the Arctic and subarctic seas with additional fresh water from melting ice and increased precipitation. Dickson and others theorize that a blanket of fresh water on top of the high-latitude ocean could slow the sinking of dense, salty waters, thereby slowing the ocean conveyor and altering climate patterns.

"The North Atlantic ocean circulation is a critical component influencing the climate of the Northern Hemisphere," Toole said. "But the ocean's precise role in climate variability remains unclear partially because of the lack of long-term records."

Station W will provide a day-by-day, season-by-season chronicle of the characteristics and intensity of currents flowing through the northwestern Atlantic. Integrating Station W data with that of other ocean observing programs will enable oceanographers to infer changes in the strength of the Gulf Stream and in the intensity and structure of the Boundary Current. It also will advance understanding of how changes in the waters of the Arctic are communicated to lower latitudes. The hope, Toole said, is that these two current systems could provide critical information for analyzing how the ocean ultimately adjusts to climate change.

With signals of global warming emerging from many scientific studies, oceanographers are looking closely to see if the cold, salty waters in the DWBC are growing fresher or warmer. "The supposition is that changes in the Arctic will be transferred south in the ocean circulation," said Dickson, who has been collaborating with Toole and WHOI Research Specialist Ruth Curry. "Station W sits at the first point at which a wide range of upstream influences come together." "If there is a change in the ocean conveyor," Curry said, "we would see it here."

Dickson and Curry have seen some intriguing and ominous signs of such a change. In 40 years of data gathering, scientists have observed a decrease in the salinity of the waters in the sub-polar North Atlantic. "The freshening of the whole water column in this great storage basin has been one of the largest changes in the modern instrumented oceanographic record," Dickson said, "and its influence has been tracked south almost as far as the Equator."

With programs such as Station W, physical oceanographers from Europe and North America are starting to build a network of floating and moored observatories that can document variations in the circulation of the Atlantic from the Arctic Circle to the Equator.

British scientists are leading an eight-year, $36-million effort known as the RAPID Climate Change Programme, which will deploy arrays of deep moorings northeast of the Station W line and along another line stretching from Florida to North Africa at 26â°N latitude. A joint European and U.S. program called the Arctic and Subarctic Ocean Flux (ASOF) Study will measure currents and water properties in the seas that connect the North Atlantic with the Arctic Ocean. The Research Council of Norway is set to fund a ten-year study of the processes that drive and transform Arctic waters. And the Canadian Department of Fisheries and Oceans will make measurements of flow through the main passageways of the Canadian Arctic Archipelago.

For their own part, Toole and colleagues were recently awarded $3.7 million from the U.S. National Science Foundation to build and deploy the four additional moorings planned for Station W. From 2004 to 2008, WHOI scientists will make two cruises per year to service the moorings, retrieve data, and make a few traditional hydrographic measurements between Woods Hole and the Sargasso Sea. The additional moorings will enable scientists to test theories on a wide range of issues relating to ocean circulation and climate. "We have a wealth of theories, every piece approved by models," Dickson said. "The need is to resolve theory, and it is precisely to meet that need that Station W was designed."

Try and Try Again
"Sometimes, bad things happen for a reason," said John Toole a few months after his Station W mooring system broke into pieces in the North Atlantic in the fall of 2002. An ambitious research program had run into a snafu, but it was trouble that Toole would eventually appreciate.

In October 2001, Toole, Senior Engineering Assistant Scott Worrilow, and a WHOI mooring team installed the first piece of the Station W line. It went off without a hitch. When Toole and Worrilow returned in October 2002 to recover the profiler and its precious data, they found what they hoped for: the instrument had operated for ten months before its batteries ran out. Data from more than 300 transects through the water column were collected at depths between 90 meters and 2950 meters (300 and 9700 feet). The profiler had traveled about 750 kilometers (465 miles) up and down the mooring cable-about the distance from Boston to Washington, DC.

On the same cruise, Toole and Worrilow deployed a replacement mooring, a prototype "telemetering" profiler that could relay data back to shore once a day via satellite. On October 18, 2002, the new mooring was lowered from R/V Oceanus into the Atlantic, its surface buoy bobbing and flashing its beacon at the crew.

By Friday, November 9, Worrilow began receiving bizarre readings from the ARGOS satellite system used to track the subsurface buoy. It suggested that the buoy had surfaced. But ARGOS has been known to give erroneous readings from time to time, so Worrilow and colleagues went home for the weekend expecting to find normal readings again on Monday. When Worrilow came in on November 12, the ARGOS signal persisted. Senior Research Specialist Dan Frye examined ARGOS data from the surface buoy and discovered that it, too, was driftingâ...and not in the same direction. The mooring had broken.

By November 14, Senior Engineering Assistant John Kemp had chartered a local fishing vessel—the Morue, operated by Matt Stommel, son of famed oceanographer Henry Stommel—to find and recover the mooring. In stormy seas, they searched through the night and recovered the surface buoy and tether, but nothing else. The next morning, Kemp located the remaining components (now lying on the seafloor), fired the acoustic releases, and waited for the backup flotation to rise to the surface. Watching on the Morue's echosounder, Kemp saw the glass ball floats rise to 800 feet depth, then stop. The mooring had apparently snagged on the bottom. Lacking proper equipment and cooperative seas, Kemp and Stommel headed home.

On November 20, Worrilow and Stommel went back to sea with trawling gear and other equipment. They used the echosounder to locate the remains of the mooring, which was still on station. They dragged the fishing gear through the water, and snagged their high-tech catch on the first pass. Nearly all the components were recovered in reusable condition, except for the subsurface mooring sphere and its ARGOS transmitter, which had drifted out of range.

Analyzing the recovered mooring pieces, the team found that a stainless steel tension rod connecting the subsurface components to the surface buoy had broken under the stress of swift Gulf Stream currents. That design flaw, however, turned out to be a savior for Toole. After the recovery, he discovered that the profiler itself was not working. The mooring break prevented what might have been a more heartbreaking outcome: recovering the mooring a year later to find that no data had been collected.

"The mooring break is a reminder of the inherent risks associated with ocean-observing systems," Toole said. "In our efforts to acquire continuous, high-quality observations, we push the limits of technology. The successful recovery of the major components is a testament to WHOI's rapid response capability and to the ingenuity of the engineers and crewmembers."

"Now we've got to do more engineering homework on how to make the subsurface-to-surface data connection work," said Worrilow, who has placed more than 500 moorings and 15 moored profilers in his career.

Until then, the Station W program will continue in its less sophisticated but more reliable configuration. A replacement, subsurface mooring with a new profiler was redeployed at Station W on June 3, 2003, from R/V Connecticut. Toole and Worrilow will next head to sea in April 2004 to deploy the full five-element array and to begin collecting the climate signals carried by the great currents of the North Atlantic. z

Station W was made possible through the support of The G. Unger Vetlesen Foundation, the WHOI Ocean and Climate Change Institute, and the National Science Foundation.