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Extending previous work with a dry model, this study investigates the sensitivity of superrotation to the location/strength of baroclinic eddies in an idealized moist aquaplanet GCM with terrestrial rotation rate and planetary radius. A suite of fixed-SST experiments is performed in which the extratropical SST gradient is flattened poleward of some specified latitude. Consistent with the dry simulations, transition to superrotation is found as this reference latitude moves near the subtropics. The superrotation is dependent on the equatorial acceleration due to interactions between equatorial Kelvin waves and subtropical Rossby waves, but is strongly enhanced by a reduction in drag by the baroclinic eddies on the subtropical upper troposphere. The reduction in the extratropical drag and the strength of superrotation depend on the strength and structure of the Hadley cell, and hence on convective closure. The transition to strong superrotation is aided by a positive feedback that cannot occur when a strong Hadley cell drag limits the equatorial vertical shear and upper-troposphere equatorial westerlies.
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This content will become publicly available on September 15, 2026
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.
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- Award ID(s):
- 2246700
- PAR ID:
- 10642461
- Publisher / Repository:
- American Meteorological Society
- Date Published:
- Journal Name:
- Journal of Climate
- Volume:
- 38
- Issue:
- 18
- ISSN:
- 0894-8755
- Page Range / eLocation ID:
- 4789 to 4805
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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