Contribution of Convection-Induced Heat Flux to Winter Ice Decay in the Western Nansen Basin

Plain Language Summary Gradually decaying Arctic sea ice changes the boundary conditions at the surface, separating ocean and atmosphere. In recent years, substantial reductions in sea ice during winter have been observed in the Atlantic sector of the Arctic Ocean, which forms the gateway for warm water inflow from the midlatitudes. In this study, we used routine output from the Mercator Ocean global operational system (MOGOS) to assess the efficiency of winter thermohaline convection transporting heat from deep layers to the ocean surface along the Atlantic origin water (AW) pathway, between Svalbard and Franz Joseph Land in the Nansen Basin. Positive temperature extremes in the AW layer in midwinter promote favorable prerequisite conditions for deep-reaching thermohaline convection, with explicit signs captured by the MOGOS. Balance equations with several assumptions for the compact region around the position (81.30 degrees N, 31 degrees E) of the long-term (2004-2010) mooring demonstrated that winter heat loss at the ocean surface is mainly compensated by convective heat flux from the AW layer. Heat and salt fluxes, associated with horizontal advection, are compatible with convective fluxes, while contribution of ice formation/melt is substantially smaller. Conclusion about the dominant role of vertical convection in shaping thermohaline structure and reducing sea ice in winter is supported by correlation analysis of the MOGOS output and mooring-based measurements. Unfavorable background conditions (thick and consolidated sea ice in combination with specific directions of ice drift) may significantly alter convection development, as demonstrated for two sequential years with substantially different external forcing. Over the last two decades Arctic sea ice has continued to decrease in extent and volume. Recent satellite observations point out that in the Atlantic sector of the Arctic Ocean substantial reduction of sea ice is happening not only in summer but in winter as well. We argue that the reason behind winter ice decay might be the heat impact from the deep ocean via thermohaline convectionopposite directed vertical motion of water under gravitational forcing. Warm water inflow from midlatitudes shapes the vertical structure of the water column in the Arctic Ocean in the way that in the upper few hundred meter temperature and salinity increase with depth. Due to cooling in winter, water at the ocean surface becomes heavier than deep water, thus providing favorable conditions for vertical convection. Recent shift to thinner Arctic sea ice provided better conditions for intensive convection development in winter along the pathways of warm water inflow in the Atlantic sector of the Arctic Ocean. Model-based results, used in this study, support our work hypothesis on the decisive influence of oceanic heat, released upward via convection, in shaping winter sea ice conditions in the Western Nansen Basin of the Arctic Ocean.