Tides control flow of Antarctic ice streams
“My observations from a few years ago were that Ice Stream D in the West Antarctic was slowing to about half average speed and then speeding up,” says Dr. Sridhar Anandakrishnan, associate professor of geoscience, Penn State. “I thought that the speeding up and slowing down was tied to rising and falling of the ocean tides.”
The ice streams in West Antarctica move large amounts of ice downward from the center of the glacier toward the ocean. Most of the glacier rests upon bedrock and/or rubble on land, but part of the glacier floats above the ocean. The grounding line, the line where the glacier stops being grounded and floats, is quite a distance back from the leading edge of the glacier.
Some ice streams are moving rapidly, some are slowing down and others have completely stopped moving. Researchers have looked at a number of ice streams and recently, they discovered that Whillans Ice Stream exhibits the most bizarre behavior because it actually stops dead and then slips for a short time, moving large distances, before it stops again.
“We were astonished that a one meter tide variation can bring the ice stream to a halt in such a short period of time and that it can accelerate to full throttle in about one minute,” says Robert Bindschadler, lead author of the study and a glaciologist and senior fellow at NASA Goddard Space Flight Center. “It underscores the sensitivity of the system to extremely modest forcing.”
The researchers report in todays (Aug. 22) issue of Science, that there is a clear association between this stick-slip phenomenon and the ocean tide.
Anandakrishnan and Bindschadler working with Richard B. Alley, Evan Pugh professor of geoscience, Penn State; Matt A. King, University of Newcastle, Newcastle Upon Tyne, UK, and Laurence Padman, Earth and Space Research, Seattle, combined data from various ice streams and produced a model of how the tides control the slip stick of ice stream motion. They note that “If there were no tides at all, slip events would be predicted to occur approximately every 12 hours.”
However, the movement of the ice streams occurs every 18 and then 6 hours. That is, the stream remains still for 18 hours and then slips for 10 to 30 minutes and halts. Then 6 hours later, the stream slips again and halts. The first slip after 18 hours corresponds to just short of high tide and the second slip is when the tide is falling, but is not low.
“The up stream portion of the ice stream keeps moving all the time,” says Anandakrishan. “The tide rises and puts pressure upward on the ice stream. Somewhere in the middle, the ice stream sticks.”
Eventually the pressure being exerted on the ice stream bed from above is enough to overcome the sticking point and the stream slips and then halts. The tide continues to rise and then recede still putting pressure on the ice stream until once again the ice slips.
“The motion of the ice streams is not as regular during neap tide because the sea rise is not as high,” says Anandakrishnan.
Each day the ocean by the West Antarctic has only one high tide and one low tide separated by 12 hours. The levels of the tides vary on a 28-day cycle creating spring tides of up to 5 feet and neap tides of 16- to 20-inches separated by 14 days.
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