Evolution of sea ice cover: Result of interplay between dynamics and thermodynamics

The state of the sea ice cover results from an interplay between thermodynamics and dynamics. Changes in the ice cover further affect the way in which the ice responds to forcing, both thermodynamic and dynamic. This thesis discusses several aspects of sea ice thermodynamics and dynamics, and their contribution to the evolution of ice pack, and particularly to changes in the Arctic sea ice cover. The main focus is on the ice dynamics in different types of ice zones and under different conditions, which also enables the examination of the impact of thermodynamic forcing on sea ice dynamics. Changes in the Arctic sea ice thickness distribution during the period 1975-2000 are studied in detail, and the contribution of thermodynamics and dynamics as driving forcing is discussed. The results show that the shape of the sea ice thickness distribution has changed: the peak of the distribution has generally narrowed and shifted towards thinner ice. A prevalent feature is the loss of thick, mostly deformed ice, which has had a significant role in the decrease in the mean and modal ice thickness. The results also show a decrease in the seasonal variability of the mean ice thickness, but with strong regional differences. Also, the regional variability of the sea ice thickness has decreased, since the thinning has been the most pronounced in regions which formerly had the thickest ice cover. The observed changes in the regional ice draft distributions cannot be explained by local warming of the atmosphere, but changes in the ice drift patterns have had an essential impact. These results emphasize the importance of the description of sea ice dynamics in the models. Sea ice dynamics, and especially deformation, strongly affect the evolution of ice volume and properties of ice cover. There has still been a need for better understanding of the highly local and intermittent deformation process, as well as its variability that rises from different types of conditions and regions. Several aspects of these questions are covered in this thesis. With coastal and ship radar images, the study of the length scale dependency of sea ice deformation rate is extended to smaller length scales (from 100 m to 10 km) and time scales (from 10 min to 24 h) than were previously possible. Sea ice deformation rate is shown to exhibit a power law with respect to both length scale and time scale at all the scales covered. Both the overall deformation rate and the length scale dependency of deformation rate are found to depend strongly on the time scale considered. Small scale deformation is studied in different type of ice regions (coastal boundary zone, compact Arctic ice pack and marginal ice zone), and under different weather conditions. One of the key findings is the connection between air temperature and deformation rate: during warm days deformation rates are generally higher than during cold days. The deformation rate is found to respond to changes in air temperature in a time scale of days, which is clearly faster than previously assumed. This response is most likely connected to the effectiveness of the healing process. However, despite of the most effective healing during the coldest winter, the previously damaged areas are found to remain the weak points in the ice cover. This confirms that the deformation history is an important factor in determining how the ice cover responds to dynamic forcing