An Examination of the Sea Ice Rheology for Seasonal Ice Zones Based on Ice Drift and Thickness Observations

The validity of the sea ice rheological model formulated by Hibler (1979), which is widely used in present numerical sea ice models, is examined for the Sea of Okhotsk as an example of the seasonal ice zone (SIZ), based on satellite-derived sea ice velocity, concentration and thickness. Our focus was the formulation of the yield curve, the shape of which can be estimated from ice drift pattern based on the energy equation of deformation, while the strength of the ice cover that determines its magnitude was evaluated using ice concentration and thickness data. Ice drift was obtained with a grid spacing of 37.5 km from the AMSR-E 89 GHz brightness temperature using a maximum cross-correlation method. The ice thickness was obtained with a spatial resolution of 100 m from a regression of the PALSAR backscatter coefficients with ice thickness. To assess scale dependence, the ice drift data derived from a coastal radar covering a 70 km range in the southernmost Sea of Okhotsk were similarly analyzed. The results obtained were mostly consistent with Hibler's formulation that was based on the Arctic Ocean on both scales with no dependence on a time scale, and justify the treatment of sea ice as a plastic material, with an elliptical shaped yield curve to some extent. However, it also highlights the difficulty in parameterizing sub-grid scale ridging in the model because grid scale ice velocities reduce the deformation magnitude by half due to the large variation of the deformation field in the SIZ. Plain Language Summary Sea ice plays an important role in shaping the polar climate. Therefore, it is quite important to accurately simulate its behavior in climate models. This study focuses on the treatment of the dynamical processes of sea ice in numerical models, especially the rheology relating ice stress to deformation. In these models, sea ice rheology is closely related to ice thickness. In many present models, the viscous-plastic rheology formulated by Hibler has been used, irrespective of region modeled and grid scales. In this study, the validity of this formulation was examined for the Sea of Okhotsk ice, a typical seasonal ice zone (SIZ), based on AMSR-derived ice drift and PALSAR-derived ice thickness, in comparison with the Beaufort Sea in the Arctic. The nearby coastal radar data were also used to assess scale dependence. Our observational results support the Hibler's formulation to some extent, suggesting its validity and applicability to the SIZ. However, it was also noted that the variation of the deformation field in the Sea of Okhotsk is so large that the deformation magnitude obtained on a grid scale is reduced by half, indicating the difficulty in parameterizing subgrid scale ridging in the model