Field observations of the temporal evolution of meltwater and false bottoms for level ice during MOSAiC expedition

There are a limited number of studies covering the temporal evolution and spatial distribution of under-ice meltwater and false bottoms for Arctic sea ice. At the same time, they both have a significant effect on the desalinization of sea ice and the ice bottom melt rates. Additionally, these observations are an important part of the meltwater budget. The MOSAiC drifting expedition was aimed to collect field data of coupled processes between ice, ocean, and atmosphere. During the melt season ice cores were collected every week from the unponded first- (FYI) and second-year level ice (SYI) of the investigated ice floe. In addition, ice mass balance buoys were installed in the vicinity of two coring sites, but in ponded areas. This allowed for the comparison of snow, ice, melt pond, under-ice meltwater layer, and false bottom thickness evolution, as well as ice and water physical parameters. Despite the 130 m distance between unponded and ponded FYI sites, the thickness of both under-ice meltwater layer and false bottoms was almost identical. For the SYI, the thicker unponded area had a draft below the meltwater layer and experienced only an ice bottom temperature rise to -1.2°C, while for thinner ponded SYI under-ice meltwater layer was observed. The depth of the seawater and under-ice meltwater layer interface was similar for FYI and SYI. The temperature of under-ice meltwater was close to 0°C, above its freezing point with pronounced diurnal cycles. The under-ice meltwater layer formed three weeks earlier below SYI than below FYI. Due to presence of under-ice meltwater, the FYI bulk salinity decreased from both top and bottom to bulk values below typical for multiyear ice due to only top surface flushing. The thickness of under-ice meltwater layer was stable, around 47 cm for FYI and 26 cm for SYI, in contrast to gradually increasing water equivalent of melted snow and ice. This imbalance indicates a significant horizontal transfer of meltwater.