The architecture for a new large-scale anisotropic constitutive law intended for use in a sea ice dynamics model is presented here. This architecture accounts directly for refrozen lead systems in the pack ice strength (with an anisotropic failure surface) and in the ice thickness distribution (with an oriented thickness distribution). The lower limit (5 km) of the model resolution is controlled by the fracture spacing of old, thicker ice and the maximum lead width. The upper limit of the model resolution (100 km) is controlled by curvature in the lead directions and variations in the lead width. These, in turn, are controlled by the variations in internal ice stress due to driving forces (winds and currents), which set the time resolution. This architecture features abrupt changes in the failure surface and the associated flow rule that cannot be averaged over a time step. In addition, the principal stress normal to a new lead must be zero as it opens. This model has subscale simulations that allow for the inclusion of phenomena such as ridging, rafting, buckling, and fracture on the behavior of the ice. With this new ice constitutive law, it is possible to test directly the ice failure strength, plastic flow rule, and ice thickness distribution. The data most useful for this testing come from ice stress and position buoys together with synthetic aperture radar deformation data. Some data comparisons have already been made. ¿ 1998 American Geophysical Union