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If and how the sea ice cycle drives the methane cycle in the high Arctic is an open question and crucial to improving source/sink balances. This study presents new insights into the effects of strong and fast freezing on the physical-chemical properties of ice and offers implications for methane fluxes into and out of newly formed lead ice. During the 2019-2020 transpolar drift of the Multidisciplinary Drifting Observatory for the Study of Arctic Climate (MOSAiC), we took weekly samples of growing lead ice and underlying seawater at the same site between January and March 2020. We analyzed concentrations and stable carbon isotopic signatures (513C-CH4) of methane and calculated methane solubility capacities (MSC) and saturation levels in both environments. During the first month, intense cooling resulted in the growth of two-thirds of the final ice thickness. In the second month, ice growth speed decreased by 50%. Both growth phases, disentangled, exposed different freeze impacts on methane pathways. The fast freeze caused strong brine entrapment, keeping the newly formed lead ice permeable for 2 weeks. These physical conditions activated a methane pump. An increased MSC induced methane uptake at the air-ice interface, and the still-open brine channels provided top-down transport to the ocean interface with brine drainage. When the subsurface layer became impermeable, the top-down pumping stopped, but the ongoing uptake induced a methane excess on top. During the second growth phase, methane exchange exclusively continued at the ice-ocean interface. The shift in the relative abundance of the 12C and 13C isotopes between lead ice and seawater toward a 13C-enrichment in seawater reveals brine drainage as the main pathway releasing methane from aging lead ice. We conclude that in winter, refrozen leads temporarily function as active sinks for atmospheric methane and postulate that the relevance of this process may even increase when the Arctic fully transitions into a seasonally ice-covered ocean when leads may be more abundant. To highlight the relevance of methane in-gassing at the air-ice interface as a potential but still unconsidered pathway, we include estimates of the occurrence and frequency of young lead ice from satellite observations of leads during MOSAiC. [ABSTRACT FROM AUTHOR]

Automated sea ice charting from synthetic aperture radar (SAR) has been researched for more than a decade, and we are still not close to unlocking the full potential of automated solutions in terms of resolution and accuracy. The central complications arise from ground truth data not being readily available in the polar regions. In this paper, we build a data set from 20 near-coincident x-band SAR acquisitions and as many airborne laser scanner (ALS) measurements from the Multidisciplinary drifting Observatory for the Study of Arctic Climate (MOSAiC), between October and May. This data set is then used to assess the accuracy and robustness of five machine-learning-based approaches by deriving classes from the freeboard, surface roughness (standard deviation at 0.5 m correlation length) and reflectance. It is shown that there is only a weak correlation between the radar backscatter and the sea ice topography. Accuracies between 44 % and 66 % and robustness between 71 % and 83 % give a realistic insight into the performance of modern convolutional neural network architectures across a range of ice conditions over 8 months. It also marks the first time algorithms have been trained entirely with labels from coincident measurements, allowing for a probabilistic class retrieval. The results show that segmentation models able to learn from the class distribution perform significantly better than pixel-wise classification approaches by nearly 20 % accuracy on average. [ABSTRACT FROM AUTHOR]

Leads and fractures in sea ice play a crucial role in the heat and gas exchange between the ocean and atmosphere, impacting atmospheric, ecological, and oceanic processes. We estimated lead fractions from high-resolution divergence obtained from satellite synthetic aperture radar (SAR) data and evaluated them against existing lead products. We derived two new lead fraction products from divergence with a spatial resolution of 700 m calculated from daily Sentinel-1 images. For the first lead product, we advected and accumulated the lead fractions of individual time instances. With those accumulated divergence-derived lead fractions, we comprehensively described the presence of up to 10 d old leads and analyzed their deformation history. For the second lead product, we used only divergence pixels that were identified as part of linear kinematic features (LKFs). Both new lead products accurately captured the formation of new leads with widths of up to a few hundred meters. We presented a Lagrangian time series of the divergence-based lead fractions along the drift of the Multidisciplinary drifting Observatory for the Study of Arctic Climate (MOSAiC) expedition in the central Arctic Ocean during winter 2019–2020. Lead activity was high in fall and spring, consistent with wind forcing and ice pack consolidation. At larger scales of 50–150 km around the MOSAiC expedition, lead activity on all scales was similar, but differences emerged at smaller scales (10 km). We compared our lead products with six others from satellite and airborne sources, including classified SAR, thermal infrared, microwave radiometer, and altimeter data. We found that the mean lead fractions varied by 1 order of magnitude across different lead products due to different physical lead and sea ice properties observed by the sensors and methodological factors such as spatial resolution. Thus, the choice of lead product should align with the specific application. [ABSTRACT FROM AUTHOR]

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