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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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Constraints from Invariant Subtropical Vertical Velocities on the Scalings of Hadley Cell Strength and Downdraft Width with Rotation Rate
Abstract Weak-temperature-gradient influences from the tropics and quasigeostrophic influences from the extratropics plausibly constrain the subtropical-mean static stability in terrestrial atmospheres. Because mean descent acting on this static stability is a leading-order term in the thermodynamic balance, a state-invariant static stability would impose constraints on the Hadley cells, which this paper explores in simulations of varying planetary rotation rate. If downdraft-averaged effective heating (the sum of diabatic heating and eddy heat flux convergence) too is invariant, so must be vertical velocity—an “omega governor.” In that case, the Hadley circulation overturning strength and downdraft width must scale identically—the cell can strengthen only by widening or weaken only by narrowing. Semiempirical scalings demonstrate that subtropical eddy heat flux convergence weakens with rotation rate (scales positively) while diabatic heating strengthens (scales negatively), compensating one another if they are of similar magnitude. Simulations in two idealized, dry GCMs with a wide range of planetary rotation rates exhibit nearly unchanging downdraft-averaged static stability, effective heating, and vertical velocity, as well as nearly identical scalings of the Hadley cell downdraft width and strength. In one, eddy stresses set this scaling directly (the Rossby number remains small); in the other, eddy stress and bulk Rossby number changes compensate to yield the same, ~Ω−1/3scaling. The consistency of this power law for cell width and strength variations may indicate a common driver, and we speculate that Ekman pumping could be the mechanism responsible for this behavior. Diabatic heating in an idealized aquaplanet GCM is an order of magnitude larger than in dry GCMs and reanalyses, and while the subtropical static stability is insensitive to rotation rate, the effective heating and vertical velocity are not.
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- Award ID(s):
- 1912673
- PAR ID:
- 10474853
- Publisher / Repository:
- American Meteorological Society
- Date Published:
- Journal Name:
- Journal of the Atmospheric Sciences
- Volume:
- 78
- Issue:
- 5
- ISSN:
- 0022-4928
- Format(s):
- Medium: X Size: p. 1445-1463
- Size(s):
- p. 1445-1463
- Sponsoring Org:
- National Science Foundation
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