Effects of large-scale energy dissipation in geostrophic turbulence

We compare the distinct effects of frictional damping and radiative,or thermal, damping on the equilibration of two-dimensional geostrophic turbulence, focusing on how the damping influences cyclone-anticyclone asymmetry of coherent vortices as well as the acceleration of zonal mean flow by wave dissipation. The difference between frictional and thermal damping is particularly striking in simulations of forced shallow water turbulence on the sphere. While shallow-water models have been successful in reproducing the formation of robust, and fully turbulent, latitudinal jets similar to those observed on the giant planets, they have to date consistently failed to reproduce prograde (superrotating) equatorial winds. Here it is demonstrated that shallow water models not only can give rise to superrotating winds, but do so very robustly, provided that the physical process of large-scale energy dissipation by radiative relaxation (thermal damping) is taken into account. With appropriate choice of thermal damping rate, equatorial superrotation can be achieved at apparently any deformation radius.