Thickness of multi-year sea ice on the northern Canadian polar shelf: a second look after 40 years

Abstract

This paper presents a systematic record of multi-year sea-ice thickness on the northern Canadian polar shelf, measured during the autumn and early winter of 2009–2010. The data were acquired by submerged sonar moored in the Penny Strait where they measured floes drifting south from the notional “last ice area” until 10 December, when the ice stopped moving for the remainder of the winter. Old ice comprised about half of the 1669 km long survey. The average thickness of old ice within 25 km segments of the survey track was 3–4 m; maximum keels were 12–16 m deep. Floes with high average draft were of two types, one with interspersed low draft intervals and one without. The presence or absence of thin patches apparently distinguished large floes formed via the aggregation of smaller floes of various ages and deformation states from those of a more homogeneous age and deformation state. The former were larger and of somewhat lower mean thickness (1–5 km; 3.5–4.5 m) than the latter (400–600 m; 6.5–14 m). A calculated accretion of new ice onto the multi-year floes measured in the autumn of 2009 was used to seasonally adjust the observations to thicknesses expected by late winter, when comparative data were acquired in the 1970s. The adjusted mean thickness for all 25 km segments with 4/10 or more old ice was 3.8 m (sample deviation of 0.5 m), a value indistinguishable within sampling error from values measured in the up-drift area during the 1970s. The recently measured ice-draft distributions were also very similar to those from the 1970s. These results suggest that the dynamical processes that create very thick multi-year ice via ridging close to the study area have been less influenced by climate change than the thermodynamic processes behind the formation and decay of thinner level floes. The hazards posed by such ice therefore persist regionally, although the risk has decreased with the decrease in old-ice concentration during recent decades.