As part of an ongoing effort to obtain a best possible, time-evolvinganalysis of most available ocean and sea ice data, a dynamic andthermodynamic sea-ice model has been coupled to the MassachusettsInstitute of Technology general circulation model (MITgcm). Icemechanics follow a viscous-plastic rheology and the ice momentumequations are solved numerically using eitherline-successive-over-relaxation (LSOR) or elastic-viscous-plastic(EVP) dynamic models. Ice thermodynamics are represented using eithera zero-heat-capacity formulation or a two-layer formulation thatconserves enthalpy. The model includes prognostic variables for snowand for sea-ice salinity. The above sea ice model components wereborrowed from current-generation climate models but they werereformulated on an Arakawa C grid in order to match the MITgcm oceanicgrid and they were modified in many ways to permit efficient andaccurate automatic differentiation. This paper describes the MITgcmsea ice model; it presents example Arctic and Antarctic results from arealistic, eddy-permitting, global ocean and sea-ice configuration; itcompares B-grid and C-grid dynamic solvers and the effects of othernumerical details of the parameterized dynamics and thermodynamics ina regional Arctic configuration; and it presents example results fromcoupled ocean and sea-ice adjoint-model integrations.