False bottom (sea ice)

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,[2] 15% during SHEBA expedition in 1998[3] and 20% during MOSAiC expedition in 2020.[4] Both physical modelling[5][6] and in situ observations[7] suggest that false bottoms may decrease sea ice melt up to 8%. Salinity and temperature of under-ice meltwater and false bottoms is controlled by both ice melt and desalination. Salinity of false bottoms was 1.0 during ARCTIC 91 expedition,[8] 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.[8]

Areal coverage of false bottoms (blue-shaded areas) during MOSAiC expedition

Temporal evolution of false bottom thickness and salinity during MOSAiC expedition

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 surface melt ponds, 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 in 1897.[9] 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.[10] 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 contantly 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.[11] The growth and melt of false bottoms is controlled by the physical parameters of ocean.[12] False bottoms are often observed in areas of thin ice covered by surface melt ponds and encircled by thicker pressure ridges, with ridge draft limiting depth of under-ice meltwater layers.[7]

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,[4] or remotely using underwater sonars.[7] Ground-based upward looking sonar cannot distinguish "normal" or parental sea ice from false bottoms. Similarly, drifting buoys measuring sea-ice temperature cannot accurately detect false bottoms, but are able to identify thicker under-ice meltwater layers.