Western Arctic Ocean freshwater storage increased by wind-driven spin-up of the Beaufort Gyre
Katharine A. Giles*, Seymour W. Laxon, Andy L. Ridout, Duncan J.Wingham and Sheldon Bacon
Abstract
The Arctic Ocean’s freshwater budget comprises contributions from river runoff, precipitation, evaporation, sea-ice and exchanges with the North Pacific and Atlantic1. More than 70,000 km3 of freshwater2 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 west3–5. Models suggest that wind-driven convergence drives freshwater accumulation6. 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 km3 in the western Arctic Ocean, in line with hydrographic observations4,5, 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.
Introduction
The Canada Basin contains the largest proportion of the Arctic Ocean's freshwater with the majority located in the Beaufort Gyre2 (Fig. 1a), a permanent anti-cyclonic circulation system. Comparisons between the Beaufort Gyre climatology, derived from winter data collected between 1950 and 1989, and an aerial hydrographic survey from March- April 2008 containing 64 station locations over 560,000 km2, indicate that the Beaufort Gyre freshwater content has increased by 8,500 km3 (ref. 4; the uncertainty was not estimated). A similar increase of 8,400 +/- 2,000 km3 was found over the whole Arctic Ocean from analysis of conductivity temperature depth (CTD) and Expendable CTD observations from ships, submarines and ice drifting stations between the 1990s and 2006- 2008 (ref. 5), with the results also 'hinting'5 at a shift and expansion of the Gyre. However, as sampling is biased towards summer months, only observations between July and September were used5. Simultaneously, the combined analysis of hydrographic data collected between 1990 and 2008 and a coupled sea- ice- ocean general circulation model indicates that freshwater export through Davis Strait reduced by 50%, comparable to the observed increase in storage (Holliday, unpublished data). To use these snapshots of freshwater change to understand its variability and governing physics, models must be employed to put them into context. The wind exerts a frictional force on the ocean surface and the ocean surface waters respond to balance this force with the Coriolis force. This motion is termed Ekman transport. Variations in the magnitude and direction of the wind cause spatial gradients in the Ekman transport, and water to accumulate or dissipate, changing the sea surface height (SSH) and depth of the halocline. The resulting vertical velocity of the SSH or halocline is termed Ekman pumping. Modelling experiments suggest that freshwater is accumulated in the Beaufort Gyre during anticyclonic regimes and forced to the Arctic Ocean margins during cyclonic regimes, where it may then be released to the North Atlantic6. Therefore, the storage of freshwater in the Beaufort Gyre is predicted to vary with the wind stress curl. This is supported by data collected between 2003 and 2007 at two moorings in the Beaufort Sea that show an increase in the freshwater content and a strong negative wind stress curl over the same period3.
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