Field observations of the temporal evolution of meltwater and false bottoms for underformed level ice during MOSAiC expedition
There is a limited number of studies covering the temporal evolution and spatial distribution of the under-ice meltwater and false bottoms. 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 total mass balance of the melt water.
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 the ponded areas. This allowed to compare snow, ice, melt pond, bottom meltwater, and false bottom thickness evolution, as well as ice and water physical parameters.
Despite the 130 m between unponded and ponded FYI sites, the thickness of both under-ice meltwater and false bottom was almost identical. For the SYI, the thicker unponded area had a draft below the meltwater layer and experienced only a warming at the ice bottom up to -1.2°C, while for thinner ponded SYI under-ice meltwater was observed. The depth of the seawater and under-ice meltwater interface was similar for FYI and SYI. The temperature of under-ice meltwater was close to 0, above its freezing point with pronounced diurnal cycles. The under-ice meltwater formed under SYI three weeks earlier than under FYI. Due to presence of under-ice meltwater, the FYI bulk salinity decreased from both direction until values below typical for multiyear ice due to 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.
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