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