Winter ocean-ice interactions under thin sea ice observed by IAOOS platforms during N-ICE2015: salty surface mixed layer and active basal melt

IAOOS (Ice Atmosphere Arctic Ocean Observing System) platforms, measuring physical parameters at the atmosphere‐snow‐ice‐ocean interface deployed as part of the N‐ICE2015 campaign, provide new insights on winter conditions North of Svalbard. The three regions crossed during the drifts, the Nansen Basin, the Sofia Deep, and the Svalbard northern continental slope featured distinct hydrographic properties and ice‐ocean exchanges. In the Nansen Basin, the quiescent warm layer was capped by a stepped halocline (60 and 110 m) and a deep thermocline (110 m). Ice was forming and the winter mixed layer salinity was larger by ∼0.1 g/kg than previously observed. Over the Svalbard continental slope, the Atlantic Water (AW) was very shallow (20 m from the surface) and extended offshore from the 500 m isobath by a distance of about 70 km, sank along the slope (40 m from the surface) and probably shed eddies into the Sofia Deep. In the Sofia Deep, relatively warm waters of Atlantic origin extended from 90 m downward. Resulting from different pathways, these waters had a wide range of hydrographic characteristics. Sea‐ice melt was widespread over the Svalbard continental slope and ocean‐to‐ice heat fluxes reached values of 400 W m⁻² (mean of ∼150 W m⁻² over the continental slope). Sea‐ice melt events were associated with near 12 h fluctuations in the mixed‐layer temperature and salinity corresponding to the periodicity of tides and near‐inertial waves potentially generated by winter storms, large barotropic tides over steep topography, and/or geostrophic adjustments.