August 14, 2013
After yesterday’s recovery of mooring A, we steamed north to a sampling station in the late hours of the night. At 4:00 am on the return trip back to the mooring station, Roy Spracklin, the quartermaster at the wheel, called out in his characteristically calm voice, ‘three polar bears up ahead’.
A mother bear and two large cubs ran along a ridge of thick ice, just a few hundred yards ahead of us. I’ve seen many photos of polar bears curiously stopping to examine a ship. These three did not. The ship’s cacophony of engines churning against the quiet backdrop of sea must have alarmed them. They kept their backs to us, running until we were well ahead of them. Later, when zooming in on the photograph, Zoe noticed the mother bear had a collar around her neck. Likely her last encounter with humans was tranquilizing, if not downright unpleasant.
In dispatch seven I wrote that with each mooring recovery, scientists here gather more data to weave into stories about the physics, chemistry, and biology of the Beaufort Sea. Yesterday I sat down with Rick Krishfield to learn more about the physics.
In 2003, Andrey Proshutinsky and Rick Krishfield started the Beaufort Gyre exploration project to understand why the Beaufort Sea holds more freshwater than any other sea in the Arctic Ocean. Andrey hypothesized that a mechanism called Ekman pumping is responsible for the freshwater buildup in the Beaufort Gyre. You may have noticed Ekman pumping in a cup of loose-leaf tea. If the tea leaves are fine enough and you get the water spinning by stirring it with your spoon, you’ll notice that the leaves pool together in the center. Stir counter-clockwise and they might rise a little in the center. Stir clockwise and they’ll be pressed to the bottom center of the cup.
In the Arctic, atmospheric winds drive a clockwise rotation of the Beaufort Gyre. This rotation causes convergence of surrounding waters and down-welling in the center of the gyre. Freshwater entering the Beaufort Sea from rivers, rain, snow, and melted sea ice becomes entrained in the Beaufort Gyre and pools in the center. Relatively fresh Pacific Ocean waters also contribute to this mélange. When Arctic winds are stronger over the Beaufort Sea, the gyre spins up and retains more freshwater. When the winds are weaker, the gyre slows, releasing its grip on the freshwater.
The strength of the gyre oscillates not only with the seasons (stronger in the winter, weaker in the summer), but also on decadal time scales driven by changes in atmospheric and ocean conditions known as the Arctic Ocean Oscillation. For the past 14 years the Arctic Ocean oscillation hasn’t changed its sign and freshwater has continued to accumulate in the Beaufort Gyre. Rick and colleagues are eager to see whether the Arctic Oscillation will swing back this year, causing some of the freshwater to discharge. If not, it may be entering a new regime.