Arctic instrument to be adapted, slimmed down for PIG
Posted July 10, 2009
The instruments the scientists will use to study the interaction between the ocean and the ice shelf that slows the flow of Pine Island Glacier into the Amundsen Sea are based on technologies that the Ocean Turbulence Laboratory at the Naval Postgraduate School first developed for the Arctic.
Tim Stanton, who leads the team, explained he wanted to develop a more compact, affordable instrument package after a successful, yearlong $20 million ice camp project in the Arctic about 10 years ago convinced him of the need to develop unmanned turbulence observation systems that could be widely deployed for multi-year periods in polar regions.
“As a result of that experiment, I realized that we had to develop cheaper, autonomous systems that didn’t require that magnitude of field effort if we were really to understand the Arctic system in this way,” he said.
The result was the autonomous ocean flux buoy , which includes a surface buoy that sits on the ice and an instrument package suspended into the water by a series of poles from the bottom of the surface buoy. The surface buoy contains an internal processor, GPS, batteries and Iridium phone technology to send data directly back to Stanton’s lab. The sensors on the instrument package use little power as they take complex measurements of turbulent ocean mixing under way.
“We’ve been able to demonstrate that we can make this relatively sophisticated measurement robustly and remotely,” Stanton said. “Now we can look at the influence of the ocean as it interacts with the ice, and it’s that balance that really determines the ice cover that we are seeing changing up in the Arctic regions.”
In the Antarctic, Stanton’s team has developed the small-hole ocean flux profiler , with a similar surface buoy but a much skinnier instrument package that must fit through a hole in the ice only about 20 centimeters wide but hundreds of meters deep.
This profiler will move vertically along a cable spanning a vast ocean cavity once a day to measure the water column. Its instruments include a high-precision acoustic altimeter on the top of the profiler. It can tell the profiler where to “park” each day, just a few meters below the bottom of the ice shelf, so the scientists can measure ice melt rates.
They believe winds are drawing warm, deep ocean water near the surface, which becomes part of the Antarctic Circumpolar Current (ACC) that chugs around the continent and hugs the seafloor particularly close to the coast of West Antarctica. That warm water apparently does serious melt damage where the base of the glacier comes afloat, called the grounding line.
The researchers also know that some of that water, now mixed with fresh glacial melt, works its way up the underbelly of the ice shelf. But how warm is it? How intense? How fast is it moving? What happens at the glacial boundary between the ice and the ocean when this turbulent exchange of heat and salt occurs?
“As that fluid rises, it’s going to entrain more and more warm water in from the circumpolar current under the glacier, to do more melting, which will accelerate and accelerate,” Stanton said. “The turbulence is very important in generating the high melt rates.”
Of course, there’s more than one way to punish an ice shelf. David Holland , a co-principal investigator on the project, noted that huge melt pools on the surface of the Larsen B Ice Shelf along the Antarctic Peninsula caused it to disintegrate in spectacular fashion in 2002. He said other ice shelves in Greenland, where more data are available, show deterioration similar to Pine Island.
“It’s still to be determined if each region has its own personality,” he said.
Robert Bindschadler said it was unlikely that the ice shelf for PIG would collapse like Larsen B or the Wilkins Ice Shelf, though huge chunks do calve off every several years as part of a regular cycle over the last 30 years of satellite observations. However, the Pine Island ice shelf is hundreds of meters thicker, particularly at the grounding line, and sheltered by a bay.
“The real key is underneath the ice,” said Bindschadler, the project’s principal investigator.
And that’s where they’re going.