Characterizing Regimes of Atmospheric Circulation in Terms of Their Global Superrotation

Abstract

The global super-rotation index S compares the integrated axial angular momentum of the atmosphere to that of a state of solid-body co-rotation with the underlying planet. S is similar to a zonal Rossby number, which suggests it may be a useful indicator of the circulation regime occupied by a planetary atmosphere. We investigate the utility of S for characterising regimes of atmospheric circulation by running idealised Earth-like general circulation model experiments over a wide range of rotation rates Ω, 8Ω _E to Ω _E/512, where Ω _E is the Earth’s rotation rate, in both an axisymmetric and three-dimensional configuration. We compute S for each simulated circulation, and study the dependence of S on Ω. For all rotation rates considered, S is of the same order of magnitude in the 3D and axisymmetric experiments. For high rotation rates, S ≪ 1 and (S ∝ Ω ⁻², while at low rotation rates S ≈ 1/2 = constant. By considering the limiting behaviour of theoretical models for S, we show how the value of S and its local dependence on Ω can be related to the circulation regime occupied by a planetary atmosphere. S ≪ 1 and S ∝ Ω ⁻² defines a regime dominated by geostrophic thermal wind balance, and S ≈ 1/2 = constant defines a regime where the dynamics are characterised by conservation of angular momentum within a planetary-scale Hadley circulation. S ≫ 1 and S ∝ Ω ⁻¹ defines an additional regime dominated by cyclostrophic balance and strong equatorial super-rotation that is not realised in our simulations. The global super-rotation index S compares the integrated axial angular momentum of the atmosphere to that of a state of solid-body co-rotation with the underlying planet. S is similar to a zonal Rossby number, which suggests it may be a useful indicator of the circulation regime occupied by a planetary atmosphere. We investigate the utility of S for characterising regimes of atmospheric circulation by running idealised Earth-like general circulation model experiments over a wide range of rotation rates Ω, 8Ω _E to Ω _E/512, where Ω _E is the Earth’s rotation rate, in both an axisymmetric and three-dimensional configuration. We compute S for each simulated circulation, and study the dependence of S on Ω. For all rotation rates considered, S is of the same order of magnitude in the 3D and axisymmetric experiments. For high rotation rates, S ≪ 1 and (S ∝ Ω ⁻², while at low rotation rates S ≈ 1/2 = constant. By considering the limiting behaviour of theoretical models for S, we show how the value of S and its local dependence on Ω can be related to the circulation regime occupied by a planetary atmosphere. S ≪ 1 and S ∝ Ω ⁻² defines a regime dominated by geostrophic thermal wind balance, and S ≈ 1/2 = constant defines a regime where the dynamics are characterised by conservation of angular momentum within a planetary-scale Hadley circulation. S ≫ 1 and S ∝ Ω ⁻¹ defines an additional regime dominated by cyclostrophic balance and strong equatorial super-rotation that is not realised in our simulations.