Differential summer melt rates of ridges, first- and second-year ice in the central Arctic Ocean during the MOSAiC expedition

Abstract

<p>During the melt season, sea ice melts from the surface and bottom. The melt rates substantially vary for sea ice ridges and undeformed first- and second-year ice. Ridges generally melt faster than undeformed ice, while the melt of ridge keels is often accompanied by further summer growth of their consolidated layer. This summer consolidation is related to refreezing of less saline meltwater, originating from snowmelt and ridge keel melt. We examine the spatial variability of ice melt for different types of ice from <em>in situ</em> drilling, coring, and from multibeam sonar scans of remotely operated underwater vehicle (ROV). Seven ROV scans, performed from 24 June 2020 to 28 July 2020 during the Multidisciplinary drifting Observatory for the Study of the Arctic Climate (MOSAiC) expedition were analyzed. The area investigated by the ROV (400 by 200 m) consisted of several ice ridges, surrounded by first- and second-year ice. Seven ice drilling transects were additionally performed to validate ROV measurements. The maximum keel depth of the ridge investigated by ice drilling was 6.5 m. We show a substantial difference in melt rates of first-year ice, second-year ice, and sea ice ridge keels. We also show how ridge keels decay depending on keel depth, width, steepness, and orientation relative to the ice drift direction. These results are important for quantifying ocean heat fluxes for different types of ice during advanced melt, and for estimation of the ridge contribution to the total ice mass and summer meltwater balances of the Arctic Ocean.</p>

Different mechanisms of Arctic first-year sea-ice ridge consolidation observed during the MOSAiC expedition

“…These observations give insight into the process of ridge keel melt, explaining the faster total melt similar to level ice melt rates for each ice-water interface. Such high internal keel melt at a ridge flank agrees with melt estimates for another ice ridge investigated during the melt period of MOSAiC (Salganik et al, 2022), where melt rates at both ridge flanks and central parts of the ridge were measured. Based on this comparison we assume that at Alli's Ridge the middle parts were also melting more slowly than the flanks (Salganik et al,Art.…”

“…From February 5 to April 22 the estimated keel melt at Fort Ridge from T60 heating measurements with a keel depth of 8.5 m was 0.12 m, while for the same period the estimated keel melt at Alli's Ridge from the hydrostatic equilibrium was only 0.04 m. This difference indicates that Alli's Ridge had lower keel melt than some other ridges, including Fort Ridge and Jaridge, investigated during MOSAiC using underwater sonar (Salganik et al, 2022). This lower melt may be related to the relatively shallow and wide keel of Alli's Ridge and the potential sheltering by nearby ridges, located in the direction of the ice drift from Alli's Ridge (Figure 1a).…”

Modeling the Winter Heat Conduction Through the Sea Ice System During MOSAiC

“…Previously, we followed the typical model assumptions and represented deformed ice as fully consolidated. In reality, the bottom portion of sea ice ridges features water‐filled voids (Leppäranta et al., 1995; Salganik et al., 2023). Hence, no heat conduction occurs within nonconsolidated deformed ice but only in the fully frozen part, which exhibits a thickness ratio to the surrounding level ice of 1.4–1.8 (Høyland, 2002).…”