Sea-ice ridges are a major component of Arctic sea-ice export through the Fram Strait

Abstract. This study presents seven years (2012–2019) of Arctic sea-ice draft observations from upward-looking sonars combined with coincident observations of ice drift velocity from four moorings located across the Arctic outflow in the Fram Strait at 78.83° N. The data set covers in total about 150 000 km of drifting Arctic sea ice, at a 1 m spatial resolution, providing one of the most extensive spatially referenced sea-ice draft records in the Arctic available today. Level ice makes up about 40–50 % of the ice cover, with modal ice thickness varying between 1 m and 2.5 m, and thicker level ice westward towards the east Greenland shelf. Using local level-ice thickness and a variable-threshold ridge detection algorithm, we identify and quantify the sizes of sea-ice ridges, including shallow ridges with keel drafts less than 5 m deep, often overlooked by traditional methods using a fixed threshold. The study highlights ridges as a significant component of the sea-ice cover, with keels covering some 20–30 % of the ice bottom and contributing 28–55 % of the total sea-ice volume. The typical spatial density varies from 6 to 9 individual ridges per kilometer of sea-ice, with approximately 3000 to 5500 ridges per month at each site. A westward increase in ridge frequency and coverage was associated with the differences in the origin of sea ice arriving at the mooring locations. Further, we show that shallow ridges comprise up to 80 % of all ridges and 35–45 % of the ridged ice volume, and thus play an important role in the sea-ice volume budget. Thus, shallow ridges deserve greater attention, especially given the ongoing changes in the Arctic sea-ice cover.