Effects of Polar Indirect Circulation on Superrotation and Multiple Equilibrium in Long-Term AGCM Experiments With an Idealized Venus-Like Forcing: Sensitivity to Horizontal Resolution and Initial Condition

A simplified model setup has been used in atmospheric general circulation models (AGCMs), to clarify the fluid dynamical process of terrestrial planets. In the present work, the research aim is to ascertain the dynamical effects of polar indirect circulation on superrotation and multiple equilibrium states in Venus-like planets. The model setup previously used for Venus AGCM intercomparison is applied to the Model for Interdisciplinary Research On Climate AGCM, and the horizontal resolution and initial conditions are altered in the long-term experiments. The structures of general circulation and planetary-scale waves in the T42 (Truncation wave number 42) experiment are similar to those in the T63 experiment. In the presence of the polar indirect circulation, the superrotational flow weakens in the cloud layer and its momentum is transported toward the lower atmosphere at high latitudes. In contrast, in the T21 experiment, because the polar indirect circulation is not fully resolved, the vertical momentum transport due to the indirect circulation is ineffective in the lower atmosphere, and thus, the cloud top superrotational flow becomes greater than those in the higher-resolution experiments. The multiple equilibrium states caused by different initial zonal flows appear in the T21 experiments, although they are not seen in the experiments of T42 and higher. Thus, the polar indirect circulation in the Gierasch-Rossow-Williams mechanism weakens the superrotational flow in the cloud layer and breaks the steady state multiplicity of the general circulation.