Western Arctic Ocean freshwater storage increased by wind-driven spin-up of the Beaufort Gyre

The Arctic Ocean's freshwater budget comprises contributions from river runoff, precipitation, evaporation, sea-ice and exchanges with the North Pacific and Atlantic(1). More than 70,000 km(3) of freshwater(2) are stored in the upper layer of the Arctic Ocean, leading to low salinities in upper-layer Arctic sea water, separated by a strong halocline from warm, saline water beneath. Spatially and temporally limited observations show that the Arctic Ocean's freshwater content has increased over the past few decades, predominantly in the west(3-5). Models suggest that wind-driven convergence drives freshwater accumulation(6). Here we use continuous satellite measurements between 1995 and 2010 to show that the dome in sea surface height associated with the western Arctic Beaufort Gyre has been steepening, indicating spin-up of the gyre. We find that the trend in wind field curl-a measure of spatial gradients in the wind that lead to water convergence or divergence-exhibits a corresponding spatial pattern, suggesting that wind-driven convergence controls freshwater variability. We estimate an increase in freshwater storage of 8,000 +/- 2,000 km(3) in the western Arctic Ocean, in line with hydrographic observations(4,5), and conclude that a reversal in the wind field could lead to a spin-down of the Beaufort Gyre, and release of this freshwater to the Arctic Ocean.