This paper describes a sectorially averaged seasonal model developed for simulating the long-term reponse of the climate system to the astronomical forcing. The model domain covers the northern hemisphere. The atmospheric dynamics is represented by an improved zonal averaged quasi-geostrophic model. It includes a new parameterization of the meridonal transport of quasgeostrophic potential vorticity and a parameterization of the Hadley sensible heat transport. The atmosphere interacts with the other components of the climate system (ocean, sea ice, and land surface covered or not by snow and ice) through vertical fluxes of momentum, heat and water vapor. The model explicitly incorparates detailed radiative transfer, surface energy balances, and snow and sea ice budgets. The vertical profile of the upper ocean temperature is computed by an integral mixed-layer model which take into account meridional convergence of heat. Sea ice is represented by a thermodynamic model including leads and a new parameterization for lateral accretion. This paper presents the model climate for present conditions and results of sensitivity experiments obtained by modifying some internal parameters or by deactivating certain parameterizations in the model. Simulation of the present climate shows that the model is able to reproduce the main characteristics of the general circulation and, in particular, the surface wind field. The seasonal cycles of oceanic mixed layer, sea ice, and snow cover are also well reproduced. Sensitivity experiments show the importance of the meridional sensible heat transport by the Hadley circulation in the tropics, the seasonal cycle of the oceanic mixed-layer depth and sea ice formation in latitude bands where the average water temperature is above the freezing point. In a forthcoming paper, this mdoel will be coupled to an ice sheet model and applied to the simulation of the last glacial cycle in the northern hemisphere. ¿American Geophysical Union 1991