Spontaneous Superrotation and the Role of Kelvin Waves in an Idealized Dry GCM

Abstract

The nondimensional parameter space of an idealized dry primitive equation model is explored to find superrotating climate states. The model has no convective parameterization and is forced using a simple thermal relaxation to a prescribed radiative equilibrium temperature. It is demonstrated that, of four nondimensional parameters that determine the model's state, only the thermal Rossby number has a significant effect on superrotation. The mode that drives the transition to superrotation in an intermediate-thermalRossby-number atmosphere is shown to behave like a Kelvin wave in the tropics.

Superrotation in Terrestrial Atmospheres

“…The mechanism for generation and maintenance of superrotation at large thermal Rossby numbers is posited to be the development of a global-scale disturbance, involving Kelvin waves (Mitchell and Vallis 2010;Pinto and Mitchell 2014;Potter et al 2014). By contrast, equatorial Rossby waves are involved in the generation and maintenance of superrotation in our theory and simulations, and they have substantial amplitude primarily within an equatorial Rossby radius, as seen in Fig.…”

“…Table 1 shows the different parameter values used to create a range of different climate scenarios. We chose to vary these parameters because, based on previous work (e.g., Del Genio et al 1993;Del Genio and Zhou 1996;Liu and Schneider 2011;Mitchell and Vallis 2010;Potter et al 2014;Pinto and Mitchell 2014), they seemed to be pertinent to the generation of superrotation. Because rescaling the planetary rotation rate in a hydrostatic model like ours is dynamically equivalent to rescaling both the planetary radius and diabatic heating rates (e.g., Kuang et al 2005;Garner et al 2007), we did not consider separate variations of the planetary radius.…”

“…For example, Mitchell and Vallis (2010) and Potter et al (2014) argued that terrestrial atmospheres transition from subrotation to superrotation when the thermal Rossby number…”

“…Among other differences between our general circulation models used to test the theories, the main distinction is that we use one in which the radiative equilibrium is statically unstable and that employs a convection scheme to provide vertical heat transport. Mitchell and Vallis (2010) and Potter et al (2014) use a model with a statically stable radiative equilibrium. However, given that our arguments merely require the presence of divergence fluctuations to generate Rossby waves, irrespective of how the divergence fluctuations are created, this distinction between the models may not be essential: even the model with a statically stable radiative equilibrium has preferential tropospheric heating and a divergent circulation in low latitudes.…”

Geophysical fluid dynamics: whence, whither and why?

“…This motion implies that angular momentum has a local maximum in the interior of the fluid, which in turn implies that the motion is eddying. Rossby waves are the obvious way to generate an upgradient transfer, and in a slowly rotating regime (such as Titan) they seem to be excited in conjunction with Kelvin waves, with a Doppler shift allowing phase locking, but by no means is the mechanism transparent [100,101]. Detailed GCMs still have trouble reproducing the zonal wind on Venus [102], and the mechanism of superrotation on the gas giants almost certainly differs again (e.g.…”