AGU eLightning 3-minutes poster talk
[intro]
Hi everyone. This poster presents a study about the temporal evolution
of under-ice meltwater and false bottoms. It is based on the field
investigations during May-July 2020 during the MOSAiC expedition. This
analysis use data from the ice coring and ROV multibeam sonar.
[left figure]
During the summer season, the meltwater is accumulating above, inside,
and underneath sea ice. The under-ice meltwater may form an additional
layer of ice at the interface with colder seawater, and this ice layer
is called a false bottom. Weekly coring program allowed us to collect
physical parameters of first-year ice, meltwater, and false bottoms. On
the left image you can see the temporal evolution of salinity and
thickness of those three layers. During the melt season the first-year
ice salinity decreased from 5 to 1, and the presence of under-ice
meltwater doubled loss of ice salt.
While sea ice was melting gradually, this under-ice meltwater appeared
instantly reaching its mean thickness of 50 cm. This indicates a
significant horizontal transfer of meltwater. The salinity of that
meltwater was changing between 4 and 10 with a strong input from the
brine flushing.
[central figure]
It is important to know how the presence of this meltwater affects the
ice melt. As you can see in the central figure, we had 3% lower ice
draft decrease for the ice with false bottoms. Additional 5 %
difference can be added from the final thickness of false bottom.
From the sonar measurements of the ice draft, we can see the strong
draft increase in the areas with false bottoms, shown here as a black
dashed line.
[right figure]
This pronounced draft change allowed us to estimate the areal coverage
of false bottoms, which you can see in the right figure as blue-shaded
areas. The estimated areal coverage of false bottoms was 21%, which is
larger than in most previous observations. We also confirmed an
importance of small ridges for keeping this meltwater under the ice.
Areas not surrounded with ridges, like the unponded ice in the left
bottom corner of the map, didn’t have any false bottoms.
[summary]
As a summary, we presented an overview of the temporal evolution of
under-ice meltwater and false bottoms. This meltwater appeared instantly
and had a stable thickness, defined by the draft of the surrounding
ridges. It also doubled rates of the ice desalinization and slightly
decreased ice melt. The areal fraction of false bottoms was defined as
21%.
Thank you.