| Résumé | This paper targets three, diverse audiences: persons calculating ice loads on structures, persons navigating ice-covered waters, and persons using remotely sensed data to estimate ice thickness. The most extensive set of drilling measurements to have been made on thick multi-year ice since the 1980s is presented based upon 22 floes sampled in some of the most remote regions in the world from years 2007 to 2011. Thickness and freeboard measurements from more than 600 drill-holes indicated that mean thicknesses and standard deviations of floes in Nares Strait, the Canadian Arctic Archipelago (CAA) and the Beaufort Sea ranged from 3.4 ± 1.3 m to 14.7 ± 3.8 m. Approximately 23% of the floes had an average thickness of 10 m or more, making them comparable to the 8 to 12 m thick hummock that caused the highest loads ever measured on the offshore structure Molikpaq. The continued presence of thick multi-year ice shows that, although the modal thickness of thermodynamically grown old ice has decreased, driving forces continue to create deformed multi-year ice. The overall thickness distribution of thermodynamically grown and mechanically deformed multi-year ice is lognormally distributed in all of the examined geographic regions. Probability of exceedance curves were used to address underestimated probabilities in the tail. Assessment of first order autocorrelation coefficients showed that drill hole measurements were statistically independent for ~85% of the 100 to 200 m long transects. Linear regressions of freeboard versus thickness had less predictive skill at the point-scale (R² of 0.57, 10 m spacing) than averages at the transect-scale (R² of 0.78, ~100 m) and floe-scale (R² of 0.88, up to ~1 km). The anomalously high freeboard of some drill holes, transects and floes has a number of plausible explanations including loose ice blocks on the underside of relatively level floes, ice blocks in the keel of deformed floes, elevated blocks on the top surface of relatively level ice without a keel, and/or the low density of the elevated and submerged portions of multi-year ice. |
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