Going with the flow
Researchers brave Drake Passage to map world's largest ocean current
Posted November 21, 2008
Most people who cross Drake Passage by ship are eager for the two-day journey to be over as quickly as possible. The ocean passage splitting the tips of South America and the Antarctic Peninsula is infamously rough, turning even hardened seafarers green around the gills.
But a team of oceanographers will spend more than three weeks in the Drake in November and December to learn more about the world’s largest ocean current, the Antarctic Circumpolar Current (ACC). Drake Passage, the chokepoint where the current narrows, is the ideal place to study the ACC, according to Teresa Chereskin, a principal investigator for the project and chief scientist for the cruise aboard the RVIB Nathaniel B. Palmer.
“We’re there long enough so that we see all kinds of weather,” said Chereskin, a researcher with Scripps Institution of Oceanography at the University of California in San Diego. “Last year, we were really blessed in the first few days because it was ‘Drake Lake,’ and you don’t get that very much.”
A year ago at about the same time, Chereskin and her team, including principal investigators Kathleen Donohue and Randolph Watts, both from the University of Rhode Island (URI), deployed an array of instruments in the passage to take data on a number of ocean properties — from acoustic travel times in the water column, which can be related to density and temperature, to ocean current and pressure. This year they will collect their first data via telemetry from the instruments.
“Our observations will help us understand why the current is there, what forces it, and what controls its variability,” explained Donohue, URI associate research professor.
The ACC is a set of sharp density fronts and their associated currents that spirals around Antarctica from west to east, linking the world’s three major bodies of water — the Atlantic, Pacific and Indian oceans. “In that sense, you can think of it as a major conduit between the [ocean] basins. That’s one important reason to study it,” Donohue noted.
The ACC formed about 41 million years ago when Drake Passage between South America and Antarctica opened. Some scientists believe the birth of the ACC eventually led to the formation of major ice sheets on the world’s southernmost continent, though the theory is still widely debated.
However, oceans and climate are indisputably linked. For example, the ocean current that brushes across Western Europe helps keep temperatures more temperate there than its high latitude would otherwise suggest. Similarly, the ACC helps insulate Antarctica.
But climate change has come to Antarctica, particularly West Antarctica, where glaciers are moving faster and discharging more ice into the ocean and ice shelves are collapsing. The changes are also whipping up the west winds that drive the ACC around the continent.
“We anticipate that the Antarctic Circumpolar Current may be highly sensitive to climate change,” Donohue said. “It is driven by winds, and since the winds appear to be changing as a result of climate change, the current may change as well.”
But to observe those changes — and provide data to model ocean processes for predictive purposes — the oceanographers need some basic information about the ACC. Last year aboard the Palmer, the ship’s crew and the scientists deployed 38 instruments called current and pressure measuring inverted echo sounders (CPIES) in a line across the passage, as well as in a more densely packed array to map circulation and eddy patterns.
“In the array, we think we have the mapping skill to do two-dimensional maps, analogous to synoptic weather maps,” Chereskin said.
The devices use sound to determine water temperature. Sound travels faster in warmer water than colder water, so by sending a short sound through the water and listening for how long it travels, the researchers can infer the average temperature of the water.
The inverted echo sounder and pressure sensor part of the instrument is anchored to the bottom of the ocean by a round steel frame and two 50-pound weights. Inside a white plastic case is a glass sphere with all the electronics, a computer and batteries. Tethered about 50 meters above that is another device that measures the water current directly.
Chereskin explained that the devices were strategically placed based on Doppler sonar data gleaned from the ARSV Laurence M. Gould, which travels across the Drake numerous times each year to transport people and material between Punta Arenas, Chile, and the U.S. Antarctic Program’s Palmer Station.
“We have a decade-long time series of ocean currents that are just taken from the Gould when it transits across Drake Passage to supply Palmer Station,” she said. “The idea of having the line is to give us good enough resolution that we can resolve the transfer [of properties] in the individual jets that comprise the ACC. It’s not just a single broad current.”
In Drake Passage, the scientists will actually study three distinct jets or fronts of the ACC. The Subantarctic Front is the strongest of the trio and meanders farthest north as it hugs the Patagonian continental shelf. The middle one is the Polar Front and the most southerly is the Southern ACC Front.
The oceanographers are also interested in how properties such as heat get transferred across the ACC from north to south and vice versa. “Is the current a barrier or a blender?” Chereskin said.
The scientists have five years to answer that and many other questions for the International Polar Year project. International partners include the French and the British. This is the second year of the experiment.
“This is going to be the first time to revisit the instruments,” Chereskin said. “We’re bringing some spares in case any need to be replaced, and we’re getting the first year of data via telemetry. … It’s a hard place to keep equipment going.”
The instruments are fairly easy to deploy, Chereskin said, even in rough weather. But to calibrate the instruments, the ship must first conduct a conductivity, temperature and depth (CTD) measurement for each site. That requires steadier seas.
“We were really lucky last year. The weather held out really well for us … it does change really fast in Drake Passage,” Donohue said. “It was a great experience for us, and the captain and crew of the Palmer, as well as Raytheon technical team, were crucial to the success of the deployment cruise.”
The team will retrieve the instruments during the last year of the experiment. “The other thing we’re hoping to do at the end of this experiment is to recommend some strategies for the long-term monitoring of the current,” Donohue added.
NSF-funded research in this story: Teresa Chereskin, Scripps Institution of Oceanography, Award Nos. 0636493 and 0338103; and Kathleen Donohue and Randy Watts, University of Rhode Island, Award No. 0635437
About the Sun