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 0.60 cm/yr. Since then, however, the height has been rising at 1.9 cm/yr. The mound now stands 30 cm above the ocean's mean equilibrium level. Estimating the volume of fresh water in the mound requires the water's density, which Giles and her colleagues obtained from measurements made by NASA's gravity-sensing GRACE satellite. They calculate that the mound contains 8000 km³ of fresh water, about 11% of the Arctic total. The researchers also found that the mound's growth is correlated with how strong and anticyclonic the wind field is. If the gyre completely reverses, as simulations suggest it could, the stored water will be released, possibly reducing the Arctic ice cap and disrupting shoals of Arctic-dwelling fish. (K. A. Giles et al., Nat. Geosci., in press.)—Charles Day