The neutral form drag coefficient is an important parameter when estimating surface turbulent fluxes over Arctic sea ice. The form drag caused by surface features (𝑪𝒅𝒏,𝒇𝒓) dominates the total drag in the winter, but long-term pan-Arctic records of 𝑪𝒅𝒏,𝒇𝒓 are still lacking for Arctic sea ice. In this study, we first developed an improved surface feature detection algorithm and characterized the surface features (including height and spacing) over Arctic sea ice during the late winter of 2009-2019 using the full-scan laser altimeter data obtained in the Operation IceBridge mission. 𝑪𝒅𝒏,𝒇𝒓 was then estimated using an existing parameterization scheme. This was followed by applying a satellite-derived backscatter coefficient (𝝈𝒐𝒗𝒗 ) to 𝑪𝒅𝒏,𝒇𝒓 regression model to extrapolate, for the first time, 𝑪𝒅𝒏,𝒇𝒓 to the pan-Arctic scale for the entire winter season over two decades (from 1999 to 2021). We found that the surface features have a larger height and smaller spacing over multi-year ice (1.15 ± 0.21 m and 142 ± 49 m) than over first-year ice (0.90 ± 0.16 m and 241 ± 129 m). The monthly mean 𝑪𝒅𝒏,𝒇𝒓 increases through the winter, from 0.2 × 10 −3 in November to 0.4-0.5 × 10 −3 in April. The central Arctic has the largest 𝑪𝒅𝒏,𝒇𝒓 (up to 2 × 10 −3), but experienced a drop of ~50% in the period from 2001/2002 to 2008/2009. The interannual fluctuations in 𝑪𝒅𝒏,𝒇𝒓 are strongly linked to the variability of sea ice thickness and deformation, and the latter has become increasingly important for 𝑪𝒅𝒏,𝒇𝒓 since 2009.

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%).

The amount of snow on Arctic sea ice impacts the ice mass budget. Wind redistribution of snow into open water in leads is hypothesized to cause significant wintertime snow loss. However, there are no direct measurements of snow loss into Arctic leads. We measured the snow lost in four leads in the Central Arctic in winter 2020. We find, contrary to the general consensus, that under typical winter conditions, minimal snow was lost into leads. However, during a cyclone that delivered warm air temperatures, high winds, and snowfall, 35.0 ± 1.1 cm snow water equivalent (SWE) was lost into a lead (per unit lead area). This corresponded to a removal of 0.7–1.1 cm SWE from the entire surface—∼6–10% of this site’s annual snow precipitation. Warm air temperatures, which increase the length of time that wintertime leads remain unfrozen, may be an underappreciated factor in snow loss into leads.

The Arctic Ocean receives a large supply of dissolved organic matter (DOM) from its catchment and shelf sediments, which can be traced across much of the basin’s upper waters. This signature can potentially be used as a tracer. On the shelf, the combination of river discharge and sea-ice formation, modifies water densities and mixing considerably. These waters are a source of the halocline layer that covers much of the Arctic Ocean, but also contain elevated levels of DOM. Here we demonstrate how this can be used as a supplementary tracer and contribute to evaluating ocean circulation in the Arctic. A fraction of the organic compounds that DOM consists of fluoresce and can be measured using in-situ fluorometers. When deployed on autonomous platforms these provide high temporal and spatial resolution measurements over long periods. The results of an analysis of data derived from several Ice Tethered Profilers (ITPs) offer a unique spatial coverage of the distribution of DOM in the surface 800m below Arctic sea-ice. Water mass analysis using temperature, salinity and DOM fluorescence, can clearly distinguish between the contribution of Siberian terrestrial DOM and marine DOM from the Chukchi shelf to the waters of the halocline. The findings offer a new approach to trace the distribution of Pacific waters and its export from the Arctic Ocean. Our results indicate the potential to extend the approach to separate freshwater contributions from, sea-ice melt, riverine discharge and the Pacific Ocean.

During the Norwegian young sea ICE (N-ICE2015) campaign, which took place in the first half of 2015 north of Svalbard, a deep winter snow pack (50 cm) on sea ice was observed, that was 50% thicker than earlier climatological studies suggested for this region. Moreover, a significant fraction of snow contributed to the total ice mass in second-year ice (SYI) (9% snow by mass), while very little snow was present in first-year ice (FYI) (3% snow by mass). We use a 1-D snow/ice thermodynamic model forced with reanalyses data in autumn and winter 2014/15. We show that snow-ice would form on SYI even with an initial ice thickness of 2 m in autumn. By the end of winter snow-ice can contribute up to 24-44% of the total thickness of SYI, if the ice is thin in autumn (0.6 m). This is important, especially in the absence of any bottom thermodynamic growth due to the thick insulating snow cover. We also show that growth of FYI north of Svalbard is controlled by the timing of growth onset relative to snow precipitation events and cold spells. These usually short-lived conditions are largely determined by the frequency of storms entering the Arctic from the Atlantic Ocean. In our case, a later freeze onset was favorable for FYI growth, due to less snow accumulation in early autumn. This limits snow accumulation on FYI but promotes bottom thermodynamic growth. We show our findings are related to regionally higher precipitation in the Atlantic sector of the Arctic, where frequent storms bring lot of precipitation in autumn and winter, and also affect the duration of cold temperatures required for ice growth in winter. We discuss the implications and the importance of snow-ice in the future Arctic, formerly believed to be non-existent in the central Arctic, due to thick perennial ice and little snow precipitation. The combination of sea ice thinning and high precipitation in the “Transpolar Drift region” highlights the need to understand the regionality of these processes across the Arctic.

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.