Causes of decadal changes of the freshwater content in the Arctic Ocean

Decadal changes of the liquid freshwater content in the Arctic Ocean are studied with a suite of forward and adjoint model simulations. Adjoint sensitivities show that freshwater volume changes in the Norwegian Atlantic Current north of the Lofoten basin and a salinity maximum in the Fram Strait and in the Canadian Archipelago lead to an enhanced northward transport of freshwater. The dynamical sensitivities indicate that stronger freshwater export from the Arctic is related to an enhanced cyclonic circulation around Greenland, with an enhanced export through the Canadian Archipelago and a stronger circulation within the Fram Strait. Associated with this circulation around Greenland is a large-scale cyclonic circulation in the Arctic. Cyclonic wind stress anomalies in the Arctic Ocean as well as over the Nordic seas and parts of the subpolar Atlantic are optimal to force the freshwater transport changes.Results from a simulation over the period 1948-2010 corroborate the result that Arctic freshwater content changes are mainly related to the strength of the circulation around Greenland. Volume transport changes are more important than salinity changes. Freshwater content changes can be explained by wind stress-driven transport variability, with larger export for cyclonic atmospheric forcing. By redistributing freshwater within the Arctic, cyclonic wind stress leads to high sea level in the periphery of the Arctic, and the stronger gradient from the Arctic to the North Atlantic enhances the export through the passages. A second mechanism is the wind-driven Sverdrup circulation, which can be described by Godfrey's (1989) island rule including friction. For this, wind stress in the Arctic is not important