False Bottom (sea Ice)
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False bottom is a form of
sea ice Sea ice arises as seawater freezes. Because ice is less density, dense than water, it floats on the ocean's surface (as does fresh water ice). Sea ice covers about 7% of the Earth's surface and about 12% of the world's oceans. Much of the world' ...
that forms at the interface between
meltwater Meltwater (or melt water) is water released by the melting of snow or ice, including glaciers, glacial ice, tabular icebergs and ice shelf, ice shelves over oceans. Meltwater is often found during early spring (season), spring when snow packs a ...
and
seawater Seawater, or sea water, is water from a sea or ocean. On average, seawater in the world's oceans has a salinity of about 3.5% (35 g/L, 35 ppt, 600 mM). This means that every kilogram (roughly one liter by volume) of seawater has approximat ...
via the process of double-diffusive convection of heat and salt.


Characteristics

False bottoms have been observed under drifting
Arctic The Arctic (; . ) is the polar regions of Earth, polar region of Earth that surrounds the North Pole, lying within the Arctic Circle. The Arctic region, from the IERS Reference Meridian travelling east, consists of parts of northern Norway ( ...
sea ice, under land-fast ice in
Greenland Greenland is an autonomous territory in the Danish Realm, Kingdom of Denmark. It is by far the largest geographically of three constituent parts of the kingdom; the other two are metropolitan Denmark and the Faroe Islands. Citizens of Greenlan ...
, and at Ward Hunt Ice Shelf. Being located under ice, false bottoms are not easy to investigate, and the current observations are quite variable. For example, the areal coverage of false bottoms was 50% at the drifting station Charlie in 1959, 15% during SHEBA expedition in 1998 and 20% during
MOSAiC expedition The Multidisciplinary drifting Observatory for the Study of Arctic Climate (MOSAiC, ) expedition was a one-year-long expedition into the Central Arctic (September 2019 - October 2020). For the first time a modern research icebreaker was able to op ...
in 2020. Both physical modelling and
in situ is a Latin phrase meaning 'in place' or 'on site', derived from ' ('in') and ' ( ablative of ''situs'', ). The term typically refers to the examination or occurrence of a process within its original context, without relocation. The term is use ...
observations suggest that false bottoms may decrease sea ice melt up to 8%. Meanwhile, measurements from manual ice thickness gauges in Fram Strait in the summer of 2020 showed a nearly 50% reduction in bottom ice melt due to false bottoms. The salinity and temperature of under-ice meltwater and false bottoms are controlled by both ice melt and desalination. The salinity of false bottoms was 1.0 during the ARCTIC 91 expedition, 0.4 during SHEBA and 2.3 during MOSAiC. The average thickness of false bottoms was 20 cm during the ARCTIC 91 expedition, 15 cm during SHEBA, and 8 cm during MOSAiC. The presence of false bottoms can increase the rates of sea ice desalination.


Formation

During Arctic summer, snow and ice melt results in the accumulation of low-salinity meltwater. Most of this meltwater is transferred to the ocean, while some of it migrates to the surface
melt ponds Melt ponds are pools of open water that form on sea ice in the warmer months of spring and summer. The ponds are also found on glacial ice and ice shelves. Ponds of melted water can also develop under the ice, which may lead to the formation of ...
, the sea ice matrix, and under-ice meltwater layers. False bottoms form due to a substantial difference in freezing temperatures of water with different salinities. Their formation in summer was first documented by
Fridtjof Nansen Fridtjof Wedel-Jarlsberg Nansen (; 10 October 1861 – 13 May 1930) was a Norwegian polymath and Nobel Peace Prize laureate. He gained prominence at various points in his life as an explorer, scientist, diplomat, humanitarian and co-founded the ...
in 1897. During MOSAiC expedition, false bottoms occurred in areas of thin and ponded sea ice encircled by thicker sea ice ridges and were formed at the same time when surface melt ponds drained. False bottoms are formed at the upper part of the interface of meltwater and seawater. The ice crystals initially grow downwards towards seawater, and further grow horizontally until a formation of a horizontal ice layer. After the formation of this horizontal layer, false bottoms constantly migrate upwards due to conductive heat flux, supported by the temperature difference between meltwater and seawater, and the rate of such migration is mostly defined by its thickness. The growth and melt of false bottoms are controlled by the physical parameters of the ocean. False bottoms are often observed in areas of thin ice covered by surface
melt pond Melt ponds are pools of open water that form on sea ice in the warmer months of spring and summer. The ponds are also found on glacial ice and ice shelves. Ponds of melted water can also develop under the ice, which may lead to the formation of ...
s and encircled by thicker pressure ridges, with ridge draft limiting the depth of under-ice meltwater layers.


Under-ice meltwater layer

The false bottom formation is directly linked to the appearance of under-ice meltwater layers. The appearance of such meltwater layers often happens after surface melt pond drainage during the melt season. The depth of under-ice meltwater layers is usually limited by the draft of thicker and usually deformed ice, surrounding thinner ice with under-ice meltwater. The salinity of under-ice meltwater depends on the sources of meltwater including snow and ice, on the desalination of the ice above under-ice meltwater layers, and on the presence of false bottoms. During the MOSAiC expedition in
Fram Strait The Fram Strait is the passage between Greenland and Svalbard, located roughly between 77th parallel north, 77°N and 81st parallel north, 81°N latitudes and centered on the prime meridian. The Greenland Sea, Greenland and Norwegian Seas lie sou ...
, the average thickness of meltwater layers was 0.46 m under first-year ice and 0.26 m under second-year ice. The thickness of meltwater layers under multiyear ice during the SHEBA expedition in the
Beaufort Sea The Beaufort Sea ( ; ) is a marginal sea of the Arctic Ocean, located north of the Northwest Territories, Yukon, and Alaska, and west of the Canadian Arctic Archipelago. The sea is named after Sir Francis Beaufort, a Hydrography, hydrographer. T ...
was 0.35–0.47 m. Observations for
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multiyear ice in the
Wandel Sea The Wandel Sea (; also known as McKinley Sea) is a body of water in the Greenland Sea, stretching from northeast of Greenland to Svalbard. It is obstructed by ice most of the year. This sea is named after Danish polar explorer and hydrographer, ...
in North Greenland showed under-ice meltwater layers with 1.1–1.2 m thickness, later transformed into thick platelet ice layer with 0.01 m thick false bottoms under it.


Observation techniques

False bottoms may create errors in estimates of sea ice thickness from its draft measurements. They can be investigated manually using ice coring and
drilling Drilling is a cutting process where a drill bit is spun to cut a hole of circular cross section (geometry), cross-section in solid materials. The drill bit is usually a rotary Cutting tool (machining), cutting tool, often multi-point. The bit i ...
, hotwire thickness gauges or remotely using underwater
sonar Sonar (sound navigation and ranging or sonic navigation and ranging) is a technique that uses sound propagation (usually underwater, as in submarine navigation) to navigate, measure distances ( ranging), communicate with or detect objects o ...
s. Ground-based upward-looking sonar cannot distinguish "normal" or parental sea ice from false bottoms. Similarly, drifting buoys measuring sea-ice temperature ( ice mass balance buoys) cannot accurately detect false bottoms but can identify thicker under-ice meltwater layers.


References

{{reflist Sea ice Cryosphere