| Summary: | The Nucleus for European Modelling of the Ocean (NEMO) Ocean General Circulation Model (OGCM) is shown for the first time to conserve Boussinesq mass at the limit of uncertainty from computer precision when using output data offline, which is sufficient for thermodynamic calculations to be reliable. Long term variability and seasonal cycles of freshwater surface and boundary fluxes show different response rates to perturbations of freshwater flux: a slow baroclinic response leading to freshwater storage and a fast barotropic response redistributing summer river inflow. Two thirds of Arctic Ocean surface heat loss occurs in the Barents and Kara Seas, a region only 14% of its total area. The seasonal thermodynamic storage cycle shows that the limited observations of the Arctic Ocean happen to be made when minimum storage occurs, implying that observations are unbiased by storage. The Arctic Ocean has an important role in global ocean circulation, sea ice and climate. It is poorly observed, so OGCMs complement observations but these must conserve mass for heat and freshwater estimates to be reliable and independent of any reference value. Atlantic Water is a layer of Atlantic Ocean origin water that enters the Barents Sea, providing a major source of Arctic Ocean heat. Its varying strength has implications for Arctic sea ice and freshwater. The monthly, annual and seasonal patterns of heat and freshwater storage in a modelled Arctic Ocean area are studied from 1981-2007, showing that surface fluxes have a strong seasonal variability and boundary fluxes show a diluted version of this variability. The water circulating through the Barents Sea drastically decreases in temperature as heat is lost to the atmosphere from cooling and formation of ice and denser water. A large surface heat loss also occurs near the Arctic Ocean in the Nordic Seas, from recirculating water from Fram Strait.
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