Contrasted Summer Processes in the Sea Ice for Two Neighboring Floes North of 84°N: Surface and Basal Melt and False Bottom Formation

We report continuous observations in the high Arctic (north of 84 degrees N) over the full 2013 summer season at two nearby sites with distinct initial snow depth, ice thickness, and altitude with respect to the local ice topography. The two sites, subject to similar atmospheric conditions that did not favor strong ice melt, showed contrasting evolutions. One site, with initially thin sea ice (1.40 m) at a relatively low location of the floe, witnessed the formation of a spectacular 1.20-m-deep melt pond, a pond-enhanced erosion of the ice surface, and a sudden pond drainage into the ocean. Then, the outpoured fresh water rapidly froze, heated the old ice from below, and also acted as a temporary shield from the ocean heat flux while it was progressively ablated through dissolution. Eventually, the site almost recovered its initial ice thickness. In contrast, the other site, with initially thicker sea ice (1.75 m) at a relatively high location on its floe, did not support any significant meltwater and underwent over 0.5 m of continuous basal ablation. The two sites experienced formation of superimposed and interposed ice. Sea ice survived summer melt at the two sites, which entered the refreezing season with similar snow and ice thicknesses. For the first time, processes associated with the formation of a deep melt pond and subsequent false bottom evolution are continuously documented with ice mass balance instruments. Plain Language Summary Summer processes in the sea ice in a changing Arctic are documented at two nearby sites in the high Arctic (north of 84 degrees N) in summer 2013. We report the first continuous observations of the formation of a melt pond more than 1.2 m deep, and the evolution of the fresh water after it outpoured to the ocean through a drainage hole at one site located in a topographic low. A nearby site, located on a topographic high, experienced very different evolution with no meltwater retention at the surface and continuous basal melting.