The Arctic gyre spins up to store fresh water. Centered at 79° N, 159° W, some 900 km north of Barrow, Alaska, and spread over an area twice the size of Texas, the Beaufort Gyre is a slowly circulating system of ice and seawater. When, as is the case now, the gyre spins in a clockwise, anticyclonic direction, surface winds and the Coriolis force push the water toward the gyre’s center to create a vast, low mound of water. As Katharine Giles of University College London and her colleagues note in a new study, Arctic surface waters are unusually fresh, thanks to melting ice and falling rain and snow. An anticyclonic gyre therefore acts as a huge store of fresh water. To determine how huge, Giles and her colleagues used satellite altimetry data going back 15 years. Between 1996 and 2002, the mound’s height shrank at a rate of about...

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