The ability to do so under ice, developed by Lee's group, is important in a place such as Davis Strait where scientists want to measure how much fresh water flows through the strait and at what times of year so they have a baseline for comparison in coming years.
Early development of the university's seagliders was paid for by the Office of Naval Research. NSF funded work to add an under-ice capability to the glider to expand its capabilties and improve its ability to take samples in hostile Arctic waters.
"This cutting-edge technology has the potential to make year-round measurements over broad areas where access by other means is severely limited, due to the presence of sea ice for part or all of the year," according to Martin Jeffries, NSF's AON program director.
Moorings--strings of instruments tethered to the seafloor--are also monitoring water in the Davis Strait but are not ideal for detecting plumes of fresh water, Lee says. For one thing, the freshest water is often found in a thin layer about 50 meters (164 feet) thick just under the sea ice. Tethering an instrument atop a mooring so it reaches that thin layer puts the instrument at risk of being ruined if an especially thick, low-hanging piece of ice comes along and strikes it.
Seagliders pass through that upper 50 meters as they dive from the top to the bottom of the strait and so can supply data in places that instruments on the mooring can't, Lee says.
A seaglider's first trip under the ice, in December 2006, last only 14 days.
In the latest deployment, two APL seagliders went into the water on Sept. 5, 2008. They relied on five sound sources in Davis Strait to figure out where they were and navigate once under the ice.
One operated for 25 weeks, spending 51 days and traveling more than 724 kilometers (450 miles) under the ice, before being collected Feb. 26 by the Danish Navy. During under-ice operations, the glider periodically sought small openings in the ice cover and succeeded in surfacing 10 times to transmit data. It made two round trips under the ice of about 370 kilometers (230 miles) each. Its journey was not as direct as desired on some legs because of weak signals from the navigation beacon and a now-known bug in the glider's navigation system, Lee says. Still it collected an unprecedented record of fresh water moving through the strait.
The second glider operated as if it were in the open ocean because it dipped under the ice just before operators activated its "under-ice" mode. It therefore operated as if there were no ice overhead, trying to surface and, once it found a hole in the ice, stubbornly transmitting all of its data. To avoid freezing into the ice, gliders operating in "under-ice" mode stay only a brief time at the surface before diving back into the ocean. This glider tarried too long became frozen in the ice and was likely subsequently crushed as it was carried with the ice in the Baffin Island Current.
View a video of the seaglider and an interview with Craig Lee of the University of Washington.