Annual cycle of the global mean energy balance in a mechanistic middle atmosphere GCM

A new mechanistic climate model from the surface to the lower thermosphere is presented. The model is based on a standard spectral dynamical core and includes an idealized radiation scheme with continuous computation of energy fluxes between the surface and the top of the atmosphere (TOA). The surface energy budget is fully taken into account by means of a swamp ocean with prescribed lateral oceanic heat fluxes. The moisture budget is based on a new transport scheme and simple parameterizations of condensation and convection. Subgrid-scale parameterizations include advanced gravity-wave (GW) and diffusion schemes. Since all parameterized processes are formulated in an energy conserving fashion, the climatological energy budgets at the surface and at the TOA are equilibrated with an error of less than 0.2 W/m^2. A simulation with doubled CO2-concentration yields the usual pattern of a colder middle atmosphere, except for a slight warming in the vicinity of the polar summer mesopause, leading to an upward shift of the GW drag which opposes the radiation signal. Again, the energy budgets are equilibrated. The model shows a pronounced annual cycle of the imbalance at the TOA of several W/m^2, with the minimum occurring in late NH winter. This variation is synchronous with the imbalance at the surface on timescales of a few weeks, i.e., there is basically no storage of energy in the atmosphere on the seasonal time scale. The annual cycle of the imbalance at the TOA results from the hemispheric differences in the distribution of land and ocean surfaces, which are characterized by different heat capacities and albedos. This leads to a maximum of the absorbed solar radiation (ASR) and a minimum of the global-mean surface temperature during late NH winter. The latter implies a minimum in the outgoing long-wave radiation (OLR) which gives the major contribution to the imbalance at the TOA. These mechanistic model results are supported by existing observational analyses. A possible implication is that the imbalances of the surface energy budget cause imbalances at the TOA, and not vice versa. We will discuss whether this holds also for oceanic perturbations of the surface energy budget that occur on longer time scales and are much weaker compared to those associated with the annual cycle.