Strong Superrotation at High CO₂ in an Idealized Terrestrial Aquaplanet

Equatorial superrotation in the upper troposphere is shown to strengthen with increasing carbon dioxide (CO₂) in an idealized global atmospheric model. The model is run in aquaplanet mode over a shallow slab ocean and includes a full hydrological cycle with latent heat release and clear-sky radiative transfer but no parameterized deep convection. The degree of superrotation is explained quantitatively by balancing 1) the acceleration of the equatorial westerlies by the component of the horizontal eddy angular momentum flux convergence associated with divergent flow with 2) deceleration due to the vertical transport of low angular momentum air from the surface in the intertropical convergence zone. Both the weakening of the equatorial upward motion and the strengthening of the horizontal flux convergence due to divergent eddies are important for the strengthening of superrotation with increasing CO₂. The control climate has no Madden–Julian oscillation (MJO), so the strengthening of the equatorial eddy momentum flux convergence cannot be described as due to the increasing amplitude of the MJO with warming. Rather, this acceleration is associated with the interaction between an equatorial Kelvin wave and extratropical Rossby waves. The degree of superrotation at high CO₂ decreases monotonically as the resolution of the spectral model is increased from T42 to T213, with a suggestion of convergence at the higher resolutions. Simulations that incorporate a convective parameterization frequently utilized in this type of idealized model show no superrotation.