Sea-ice ridges constitute a large fraction of the total Arctic sea-ice volume (up to 40%); nevertheless, they are the least studied part of the Arctic ice pack. Here we investigate sea-ice melt rates using rare underwater multibeam data that cover a period of one month during the advanced melt stage in the Arctic summer. We show that the degree of bottom melt increases with ice draft for first-year and second-year level ice, and a first-year ice ridge keel, with an average of 0.45 m, 0.55 m, and 0.95 m of total snow and ice melt in the observation period, respectively. While bottom melt rates of ridge keels are 3-4 times higher than first-year level ice, surface melt rates are almost identical. Our estimate attributes 57% of the ridge keel melt variability to keel draft (36%), slope (32%), and width (27%).

Melt ponds have a strong impact on the Arctic surface energy balance and the ice-associated ecosystem because they transmit more solar radiation compared to bare ice. In the existing literature, melt ponds are considered as bright windows to the ocean, even during freeze-up in autumn. In the central Arctic during the summer-autumn transition in 2018, we encountered a situation where more snow accumulated on refrozen melt ponds compared to the adjacent bare ice, leading to a reduction in light transmittance of the ponds even below that of bare ice. Supporting results from a radiative transfer model suggest that melt ponds with a snow cover >0.04 m lead to lower light transmittance than adjacent bare ice. This scenario has not been described in the literature before, but has potentially strong implications for example on autumn ecosystem activity, oceanic heat budget and thermodynamic ice growth.

Sea ice pressure ridges have been recognized as important locations for both physical and biological processes. Thus, understanding the associated light-field is crucial, but their complex structure and internal geometry render them hard to study by field methods. To calculate the in- and under-ridge light field, we combined output from an ice mechanical model with a Monte-Carlo ray tracing simulation. This results in realistic light fields showing that light levels within the ridge itself are significantly higher than under the surrounding level ice. Light guided through ridge cavities and scattering in between ridge blocks also results in a more isotropic ridge-internal light field. While the true variability of light transmittance through a ridge can only be represented in ray tracing models, we show that simple parameterizations based on ice thickness and macro-porosity allow accurate estimation of mean light levels available for photosynthesis underneath ridges in field studies and large-scale models.

The formation of platelet ice is well known to occur under Antarctic sea ice, where sub-ice platelet layers form from supercooled ice shelf water. In the Arctic however, platelet ice formation has not been extensively observed and its formation and morphology currently remain enigmatic. Here, we present the first comprehensive, long-term in situ observations of a decimeter thick sub-ice platelet layer under free-drifting pack ice of the Central Arctic in winter. Observations carried out with a remotely operated underwater vehicle (ROV) during the midwinter leg of the MOSAiC drift expedition, provide clear evidence of the growth of platelet ice layers from supercooled water present in the ocean mixed layer. This platelet formation takes place under all ice types present during the surveys. Oceanographic data from autonomous observing platforms lead us to the conclusion that platelet ice formation is a widespread but yet overlooked feature of Arctic winter sea ice growth.

There are a limited number of studies covering the temporal evolution and spatial distribution of under-ice meltwater and false bottoms for Arctic sea ice. At the same time, they both have a significant effect on the desalinization of sea ice and the ice bottom melt rates. Additionally, these observations are an important part of the meltwater budget. The MOSAiC drifting expedition was aimed to collect field data of coupled processes between ice, ocean, and atmosphere. During the melt season ice cores were collected every week from the unponded first- (FYI) and second-year level ice (SYI) of the investigated ice floe. In addition, ice mass balance buoys were installed in the vicinity of two coring sites, but in ponded areas. This allowed for the comparison of snow, ice, melt pond, under-ice meltwater layer, and false bottom thickness evolution, as well as ice and water physical parameters. Despite the 130 m distance between unponded and ponded FYI sites, the thickness of both under-ice meltwater layer and false bottoms was almost identical. For the SYI, the thicker unponded area had a draft below the meltwater layer and experienced only an ice bottom temperature rise to -1.2°C, while for thinner ponded SYI under-ice meltwater layer was observed. The depth of the seawater and under-ice meltwater layer interface was similar for FYI and SYI. The temperature of under-ice meltwater was close to 0°C, above its freezing point with pronounced diurnal cycles. The under-ice meltwater layer formed three weeks earlier below SYI than below FYI. Due to presence of under-ice meltwater, the FYI bulk salinity decreased from both top and bottom to bulk values below typical for multiyear ice due to only top surface flushing. The thickness of under-ice meltwater layer was stable, around 47 cm for FYI and 26 cm for SYI, in contrast to gradually increasing water equivalent of melted snow and ice. This imbalance indicates a significant horizontal transfer of meltwater.