Fresh water and its sources during the SHEBA drift in the Canada Basin of the Arctic Ocean

Abstract

Salinity and δ 18 O measurements were made along a shelf-basin section out to the initial SHEBA site in the Beaufort Gyre and along the SHEBA drift track to determine the sources and amounts of surface freshening observed in the Canada Basin. Abundant amounts of Mackenzie River runoff, up to 8 m of inventory in the top 40 m of the water column, were found in the interior ocean at the start of the SHEBA program. Toward the eastern edge of the Northwind Ridge, enhanced amounts of runoff were also observed, originating either from the Mackenzie River or from the Yukon River (Alaska Coastal Current). For sea-ice melt, the shelf-basin section revealed net ice formation (brine) from the continental margin out to 200 km , but net sea-ice melt under the permanent pack. Enhanced amounts of sea-ice melt were seen in the water column near the western edge of the Canada Basin, where a shallow (>20 m) warm-water core was capped by stratification, suggesting that basal ice in keels may have melted as ice passed over this region. The sea-ice melt inventory in the upper water column reflected the annual cycle of sea-ice formation and melting (∼1 m) . However, this seasonal variability was dwarfed by large spatial variation. The unusually fresh surface layer and thin ice observed recently in the Canada Basin interior appear to be manifestations of a complex interaction between wind fields, runoff and ice. During the period discussed here and in the Beaufort Sea, atmospheric conditions favored both the creation of large areas of open water and the rapid entry and accumulation of runoff in the interior ocean. The resulting stratification constrained the depth of the polar mixed layer in winter. Large areas of open water in summer became sites of enhanced first-year ice production in winter followed by transport of ice or its melt to the interior the following summer. These data suggest that the hydrological feedback loop may be as significant a component of arctic change as the albedo feedback loop.