Past observations of sea ice thickness in the Eastern Canadian Arctic (ECA) have generally been restricted to drill-hole measurements at a few local sites on landfast ice. Here we use data from the laser altimeter ICESat and the radar altimeter Cryosat-2 to present a 14-year record (2003-2016) of highresolution and spatially extensive ice thickness observations for the ECA and identify 12 sub-regions with distinct patterns. The mean sea ice growth rate within the seasonally ice-covered ECA from November to April is 23 cm mo-1 (565 km 3 mo-1), with the fastest increase in thickness occurring through strong ice convergence and deformation in eastern Hudson Bay and Foxe Basin. Our results demonstrate characteristically asymmetrical distributions of sea ice thickness in both Hudson Bay and Baffin Bay, but in opposing directions. In Hudson Bay the spring ice cover is 40 cm thicker in the eastern region compared to the northwestern region, whereas in Baffin Bay the ice is 20 cm thicker in the western half of the bay compared to the eastern half. In Hudson Bay we find that years with strong and positive ice drift vorticity (i.e. cyclonic and convergent conditions) correlate with increasingly asymmetrical sea ice covers, with the level of west-east asymmetry varying from 2 to 11 cm per 100 km. However, in Baffin Bay the ice drift vorticity is typically negative (i.e. anticyclonic and divergent) with no obvious link to the asymmetry of the spring ice cover. Finally, we estimate that large interannual variations in spring sea ice volume within the ECA lead to ±15% variations in the volume of freshwater available at the ocean surface during summer.
The last decade has witnessed the nine lowest Arctic September sea ice extents in the observational record. It also forms the most recent third of the long‐term trend in that record, which reached −13.4% decade−1 in 2015. While hemispheric analyses paint a compelling picture of sea ice loss across the Arctic, the situation with multiyear ice in the Beaufort Sea is particularly dire. This study was undertaken in light of substantial changes that have occurred in the extent of summer multiyear sea ice in the Arctic inferred from the passive microwave record. To better elucidate these changes at a sub‐regional scale, we use data from the Canadian Ice Service archive, the most direct observations of sea ice stage‐of‐development available. We also build upon the only previous sea ice climatological analysis for Canada's western Arctic region by sea ice stage‐of‐development that ended in 2004. The annual evolution of sea ice by stage of development in Canada's western Arctic changed dramatically between 1983 and 2014. The rate of these changes and their spatial prevalence were most prominent in the last decade. In summer, total sea ice loss occurred via reductions in old and first‐year sea ice over increasingly large areas and over more months per year. Resultant delay of thermodynamic freeze up has increased the annual open water duration in the study region. The winter sea ice cover was increasingly composed of first‐year sea ice at the expense of old ice. Breakup timing has not significantly changed in the region.
During September 2016 an ice-free Beaufort Sea was observed for only the second time. Like previous regional sea ice minima (1998, 2008, and 2012), seasonal preconditioning of the ice pack toward younger, thinner ice types contributed to premature breakup that accelerated the ice-albedo feedback and enhanced summer melt. In 2016, anomalously high sea ice export and ice pack divergence during February and April promoted the unusual widespread formation of new ice within the Beaufort. Thin ice types reached a peak regional concentration of 30% in March, when the ice cover is typically dominated by thick first-year and multiyear sea ice. Combined CryoSat-2 and Soil Moisture and Ocean Salinity (SMOS) data indicate that the regional ice volume plateaued from December to March as export offset ice growth and ultimately culminated in a −30% volume anomaly in April 2016. This atypically thin ice cover broke up 7 weeks earlier than average, with open water forming not only within coastal flaw leads but also within the offshore pack ice. By July 2016, vast areas of open water within the highly fractured ice cover accelerated the ice-albedo feedback and led to rapid melt. Though maintaining a partial ice cover during summer throughout the observational record, significant negative trends in September sea ice area within the Beaufort are now punctuated by two recent ice-free Septembers (2012 and 2016). As the Beaufort transitions toward a seasonally ice-free sea, we examine the role of winter preconditioning through sea ice transport and its growing importance within an increasingly seasonal and mobile Arctic ice cover. Plain Language SummaryDuring September 2016 the Beaufort Sea became ice-free for the second time in 5 years after it had historically maintained a partial ice cover throughout the observational record. September sea ice area in the Beaufort has significantly declined, and while the region continues to completely freeze over during winter, the composition of this ice cover is changing. The Beaufort is transitioning from an ice cover dominated by multiyear sea ice to one characterized by thinner first year ice types. This thinner ice cover is less resilient to summer melt and is also mechanically weaker and therefore more mobile. As a result, the ice cover is more prone to large fracture events that lead to new ice formation during winter and precondition the end of winter ice cover for enhanced summer melt. Within this paper, we focus on a series of events during winter 2016 that broke up the ice cover and preconditioned it toward younger and thinner ice types. Ultimately, this promoted early breakup, which enhanced summer melt and ultimately rendered the region ice-free by September 2016. Building on this, we find a statistical relationship between winter ice export and summer ice loss.
Arctic sea ice is diminishing with climate warming 1 at a rate unmatched for at least 1000 years 2 . As the receding ice pack raises commercial interest in the Arctic 3 , it has become more variable and mobile 4 which increases safety risks to maritime users 5 . Satellite observations of sea ice thickness are currently unavailable during the crucial melt period from May to September, when they would be most valuable for applications such as seasonal forecasting 7 , owing to major challenges in the processing of altimetry data 8 . Here we use deep learning and numerical simulations of the CryoSat-2 radar altimeter response to overcome these challenges and generate the first pan-Arctic sea ice thickness dataset during the Arctic melt period. CryoSat-2 observations capture spatial and temporal patterns of ice melting rates recorded by independent sensors and match the time series of sea ice volume modelled by the Pan-Arctic Ice Ocean Modeling and Assimilation System (PIOMAS) reanalysis 9 . Between 2011 and 2020, Arctic sea ice thickness was 1.87 ± 0.10 m at the start of the melting season in May and 0.82 ± 0.11 m by the end in August. Our year-round sea ice thickness record unlocks new opportunities for understanding Arctic climate feedbacks on different timescales. For instance, sea ice volume observations from the early-summer may extend the lead time of skilful August-October sea ice forecasts by several months, at the peak of the Arctic shipping season.
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