Temporal evolution of under-ice meltwater layers and false bottoms and their impact on summer Arctic sea ice mass balance

Abstract

Low-salinity meltwater from Arctic sea ice and its snow cover accumulates and creates under-ice meltwater layers below sea ice. These meltwater layers can result in the formation of new ice layers, or false bottoms, at the interface of this low-salinity meltwater and colder seawater. As part of the Multidisciplinary drifting Observatory for the Study of the Arctic Climate (MOSAiC), we used a combination of sea ice coring, temperature profiles from thermistor strings and underwater multibeam sonar surveys with a remotely operated vehicle (ROV) to study the areal coverage and temporal evolution of under-ice meltwater layers and false bottoms during the summer melt season from mid-June until late July. ROV surveys indicated that the areal coverage of false bottoms for a part of the MOSAiC Central Observatory (350 by 200 m2) was 21%. Presence of false bottoms reduced bottom ice melt by 7–8% due to the local decrease in the ocean heat flux, which can be described by a thermodynamic model. Under-ice meltwater layer thickness was larger below first-year ice and thinner below thicker second-year ice. We also found that thick ice and ridge keels confined the areas in which under-ice meltwater accumulated, preventing its mixing with underlying seawater. While a thermodynamic model could reproduce false bottom growth and melt, it could not describe the observed bottom melt rates of the ice above false bottoms. We also show that the evolution of under-ice meltwater-layer salinity below first-year ice is linked to brine flushing from the above sea ice and accumulating in the meltwater layer above the false bottom. The results of this study aid in estimating the contribution of under-ice meltwater layers and false bottoms to the mass balance and salt budget for Arctic summer sea ice.

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“…Freshening just under the ice with the formation of under-ice meltwater layers was observed by the IMB (SIMBA, Table 1) temperature profiles, at L1 and L2 on July 31 and June 26, respectively, but at L3 already on June 16 (Lei et al, 2022a). Also, in the Central Observatory the earliest record of under-ice meltwater layers was on June 16 Salganik et al, 2023). The difference in timing of under-ice meltwater layers may be related to the thinner ice present at L3 and part of the Central Observatory.…”

“…We can clearly see the near-freezing Ta after melt onset (Light et al, 2022) on May 25 (Figure 17d), indicative of a melting ice and snow surface (Figure 17) from excess surface energy flux (Persson et al, 2012) and eventually leading to increasing radiation into the upper ocean after about June 4 (Figure 18c). As shown in Figure 17a-c the cold interior of the sea ice warmed gradually (see also Lei et al, 2022a;Salganik et al, 2023) from May to June 2020, through warming from both above and below. The increased sea ice temperature suggests that the volume fraction of brine was gradually increasing and gradually enhanced its permeability (Golden et al, 1998).…”

“…In addition, a seasonal IMB (SIMB3, Table 1) deployed at L2 observed the bottom ice melt at this site beginning June 11, 2020 (Perovich et al, 2023). At the Central Observatory the earliest basal melt was observed on May 27 (Salganik et al, 2023), and melt ponds started to form at the same time (Webster et al, 2022). Freshening just under the ice with the formation of under-ice meltwater layers was observed by the IMB (SIMBA, Table 1) temperature profiles, at L1 and L2 on July 31 and June 26, respectively, but at L3 already on June 16 (Lei et al, 2022a).…”

“…The difference in timing of under-ice meltwater layers may be related to the thinner ice present at L3 and part of the Central Observatory. This thinner ice could have allowed earlier meltwater drainage, lower draft or higher occurrence of ridges at these sites, each of which could have controlled the accumulation of meltwater below the ice (Salganik et al, 2023).…”

The role of local-ice meltwater in the triggering of an under-ice phytoplankton bloom in an Arctic fjord

“…The regional and short-term response to meltwater input during the transition period from ice-covered to ice-free conditions is less addressed and remains poorly understood. One of the main hindrances to this is the fact that melting begins while the water column is still covered by ice (Boone et al, 2017;Salganik et al, 2023). This imposes a physical barrier that hinders sampling of the water column with in situ measurements and obstructs remote sensing observations.…”