Wind-induced equatorial bulge in Venus and Titan general circulation models: Implication for the simulation of superrotation

The centrifugal force associated with the superrotation in Venus' and Titan's middle atmosphere reduces the effective gravity and thereby modifies the shape of the geopotential surface, which manifests itself as an equatorial bulge. General circulation models (GCMs) based on the hydrostatic primitive equations cannot correctly represent this dynamics since the vertical component of the centrifugal force does not appear in the hydrostatic equation. Consequently, they are likely to underestimate the poleward pressure gradient force and superrotation in gradient wind balance. This effect can be accounted for in nonhydrostatic GCMs or in quasi-hydrostatic GCMs, in which the hydrostatic equation is supplemented by the vertical component of the centrifugal and Coriolis force and which do not make the shallow-atmosphere approximation. A quasi-hydrostatic GCM is shown to predict faster superrotation than a hydrostatic GCM run under otherwise identical conditions.