Evolution of first‐year and second‐year snow properties on sea ice in the Weddell Sea during spring‐summer transition

Observations of snow properties, superimposed ice, and atmospheric heat fluxes have been performed on first‐year and second‐year sea ice in the western Weddell Sea, Antarctica. Snow in this region is particular as it does usually survive summer ablation. Measurements were performed during Ice Station Polarstern (ISPOL), a 5‐week drift station of the German icebreaker RV Polarstern . Net heat flux to the snowpack was 8 W m −2 , causing only 0.1 to 0.2 m of thinning of both snow cover types, thinner first‐year and thicker second‐year snow. Snow thinning was dominated by compaction and evaporation, whereas melt was of minor importance and occurred only internally at or close to the surface. Characteristic differences between snow on first‐year and second‐year ice were found in snow thickness, temperature, and stratigraphy. Snow on second‐year ice was thicker, colder, denser, and more layered than on first‐year ice. Metamorphism and ablation, and thus mass balance, were similar between both regimes, because they depend more on surface heat fluxes and less on underground properties. Ice freeboard was mostly negative, but flooding occurred mainly on first‐year ice. Snow and ice interface temperature did not reach the melting point during the observation period. Nevertheless, formation of discontinuous superimposed ice was observed. Color tracer experiments suggest considerable meltwater percolation within the snow, despite below‐melting temperatures of lower layers. Strong meridional gradients of snow and sea‐ice properties were found in this region. They suggest similar gradients in atmospheric and oceanographic conditions and implicate their importance for melt processes and the location of the summer ice edge.