On the effects of horizontal resolution and diffusion in a two-layer general circulation model with a zonally symmetric forcing

The general circulation of a two-layer spectral model with a simple symmetrical forcing is determined from long-term integrations. It is found that the zonal as well as the meridional mean flow is clearly influenced by the horizontal resolution of the model. Related to that, it is also found that the meridional eddy transport of momentum is increased significantly by an increase of the horizontal resolution. As a main result of the present study, it is demonstrated, that the response of the model to an increase of the horizontal resolution can also be obtained to a considerable extent by using a suitable, strongly scale-dependent, linear diffusion in the model. Considering especially the spectral distribution of kinetic energy, it is found that an increase of the horizontal resolution as well as the use of a selective horizontal diffusion on the smaller scales results in a significant increase of the eddy kinetic energy on the large scales connected with an enhancement of the baroclinicity of the zonal mean flow. As to be expected, the effect of increasing horizontal resolution or the scale selective diffusion is also to decrease the kinetic energy on the smaller scales, with the result that the energy spectrum is approaching a – 3-power law for a wider range of wavenumbers.