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Geometrical constraints on the formation and melt of ridged sea ice Amundrud, Trisha Lynne

Abstract

The Arctic ice pack consists of flat level ice, open water, and large ridge structures. During winter, ice thickens and is compacted into ridges, increasing the Arctic ice volume. In summer, ridging is accompanied by ice melt processes, which act to decrease ice volume. Current ice-atmosphere-ocean models cannot reproduce the evolution of the ridged ice fraction, suggesting that ridging or melt may be inappropriately parameterized. To increase our understanding of ridged ice evolution, this thesis investigates the factors that constrain the ridging and melt processes. A unique ice draft distribution model is developed to simulate ice evolution in the Beaufort Sea, allowing direct comparison with observations of ice draft by moored sonar. Conventional ridging algorithms used in a 24-day simulation were found to overestimate the amount of very thick ice. Observations of level ice reveals that 75% of all ice floes are too small to create ridges of maximum draft. In addition, observed ridges have cusp-shaped keels with concave flanks, containing less thick ice and increased amounts of thinner ice than the triangular shaped keels assumed by most models. Including the observed constraints into the redistribution model produces ridged ice in agreement with observations, confirming the importance of the geometrical constraints to the creation of ridged ice. During the melt season, simulations of ice ablation in the Beaufort Sea indicate that level ice melt processes cannot reproduce the observed enhanced melt rates of ridged ice. A semi-quantitative model for internal melt due to flow through the porous keel is developed and an enhanced internal melt rate estimated. The rate of melting within the porous structure of the ridge keel is up to an order of magnitude greater than the rate of melting at the surface of level ice floes. Including the internal melt within the ice draft distribution model can reproduce the enhanced melt of ridged ice and is thus essential for the accurate simulation of the evolution of ridged ice. Similar to the geometric constraints on ice ridging, the internal geometry of ridge keels plays a large role in the annual evolution of the thickest sea ice.

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