Modeling the High-Frequency Component of Arctic Sea Ice Drift and Deformation

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Abstract
Buoy observations of sea ice drift show that sea ice motion and deformation contain substantial high-frequency variability at subdaily timescales. However, numerical simulations of the sea ice dynamics normally do not include processes on such short timescales. Instead, by applying traditional water-drag formulations for the ocean–sea ice boundary layer, the inertial frequencies tend to be omitted in the spectra of modeled sea ice motion and deformation due to fictitious ocean damping. Here a dynamic–thermodynamic sea ice model is presented that includes “inertial embedding” of the sea ice into the oceanic boundary layer to reproduce sea ice motion and deformation not only at daily and lower frequencies but also at subdaily frequencies up to 2 cycles per day and higher. Comparison with buoy data shows that the embedded model much more successfully simulates the observed sea ice motion in regard to the high-frequency component. The results demonstrate that both inertial embedding of the sea ice–ocean boundary layer and damping by internal ice stresses are required to accurately reproduce the observed sea ice motion. It is demonstrated that, by an energy cascade, the nonlinear dynamics of the sea ice model supplies almost all of the high-frequency energy. When physics is included that allows high-frequency motion to be simulated, the model yields both deformational variability and net sea ice divergence of the same magnitude as observed. Moreover, both sea ice drift and divergence are coherent with observations at low and high frequencies. This study shows it is necessary to include high-frequency deformation in order to fully describe the evolution of sea ice. The growth of sea ice in areas of open water is more than an order of magnitude larger than basal growth underneath thick ice, especially during winter. Hence, during times of positive ocean–atmosphere temperature gradient, repeated short-term opening and closing of the sea ice, due to inertial or tidal motion, significantly increase the sea ice growth rate as compared with basal-only ice growth. The ramifications of the modified sea ice mass budget are discussed. In the simulations described here, for example, inclusion of inertial embedding increases the winter ice mass by about 20%.