Morphology of deformed first-year sea ice features in the Southern Ocean

A total of 42 deformed sea-ice features were identified in almost 19 km of drill profile data from the Ross, Amundsen, Bellingshausen and Weddell Seas. The features were classified according to the shapes of their keels underwater and the presence of a sail on the surface. Coupled ridges have triangular keels and sails and, with mean maximum keel depth of 3.61 m, they are the most massive of the deformed ice features. Widowed ridges have a triangular keel similar to those of coupled ridges but do not have a prominent ice sail. With a mean ridge width of 15 m, they are the narrowest of the deformed ice features. Rubble piles are the widest, but they contribute least to the total ice mass. Ignoring the ice mass of deformed ice features and approximating their thickness by surrounding level ice underestimates the total ice mass by 44–55%. First-year ridges in the Antarctic are flatter and less massive than those in the Arctic. We propose that the differences in morphology are due to differences in sampling strategies, parent-ice characteristics and the magnitude and duration of driving forces. For the purpose of estimating ice thickness from surface observations, we believe hydrostatic equilibrium is a fair assumption on large scales, but statistical relationships between variables that can be observed from the surface and variables that are hidden from the surface are not sufficiently strong and consistent for the estimation of ice thickness. In all cases, the fact that not all keels are associated with a sail and not all sails are associated with a keel should be addressed when estimating ice thickness from surface observations.