Page 2/2 - Posted November 21, 2008
Instruments to monitor ocean passage for five-year project
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.”
Photo Credit: Wikipedia Commons
Map showing Drake Passage between South America and the Antarctic Peninsula.
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 National Science Foundation -funded 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 .Back 1 2