Superrotation in Terrestrial Atmospheres

Abstract

Atmospheric superrotation with prograde equatorial winds and an equatorial angular momentum maximum is ubiquitous in planetary atmospheres. It is clear that eddy fluxes of angular momentum toward the equator are necessary to generate it. But under what conditions superrotation arises has remained unclear. This paper presents simulations and a scaling theory that establish conditions under which superrotation occurs in terrestrial atmospheres. Whether superrotation arises depends on the relative importance of factors that favor or disfavor superrotation. Convection preferentially generates Rossby waves near the equator, where the Rossby number is O(1). Since the Rossby waves transport angular momentum toward their source regions, this favors superrotation. Meridional temperature gradients preferentially lead to baroclinic instability and wave generation away from the equator. Eddy transport of angular momentum toward the baroclinic source region implies transport out of low latitudes, which disfavors superrotation. Simulations with an idealized GCM show that superrotation tends to arise when the equatorial convective generation of wave activity and its associated eddy angular momentum flux convergence exceed the baroclinic eddy angular momentum flux divergence. Convective and baroclinic wave activity generation is related through scaling arguments to mean-flow properties, such as planetary rotation rates and meridional temperature gradients. The scaling arguments show, for example, that superrotation is favored when the off-equatorial baroclinicity and planetary rotation rates are low, as they are, for example, on Venus. Similarly, superrotation is favored when the convective heating strengthens, which may account for the superrotation seen in extreme global warming simulations.

Rossby Number Dependence of Venus/Titan‐Type Superrotation and Its Related Intermittency

“…Since the pioneering work of Williams and Holloway (1982), the sensitivity of the general circulation to planetary rotation and size has been widely investigated using Earth‐like troposphere GCMs (Del Genio et al., 1993; Dias Pinto & Mitchell, 2014, 2016; Laraia & Schneider, 2015; Lu & Yamamoto, 2020; Mitchell & Vallis, 2010; Potter et al., 2014; Read et al., 2018; Y. Wang et al., 2018; Williams & Holloway, 1982). It is straightforward to introduce nondimensional parameters in idealized GCMs (Dias Pinto & Mitchell, 2014; Mitchell & Vallis, 2010) with simplified heating and friction processes.…”

The Role of the Divergent Circulation for Large-Scale Eddy Momentum Transport in the Tropics. Part I: Observations

“…While early paradigms of tropical momentum transport envisioned a nearly inviscid Hadley cell decelerated by Rossby waves of extratropical origin, Lee (1999) showed that in fact, the westerly eddy forcing compensates the deceleration by the off-equatorial Hadley cell in the deep tropics. As this eddy forcing plays a key role in theories of equatorial superrotation (Hide 1969), the maintenance of the tropical momentum budget has received much attention in idealized studies of planetary atmospheres (Showman and Polvani 2011;Pinto and Mitchell 2014;Laraia and Schneider 2015). It has also been suggested that the terrestrial atmosphere might transition to a superrotating state in a much warmer climate (Caballero and Huber 2010).…”

The Impact of Planetary Rotation Rate on the Reflectance and Thermal Emission Spectrum of Terrestrial Exoplanets around Sunlike Stars

“…Indeed, the flow in the stratosphere (particularly the middle and upper stratosphere) switches between easterly in the 1x simulation to westerly and superrotating even in the 4x simulation. Laraia and Schneider [2015] noted that equatorial upper tropospheric flow became westerly when Earth's rotation rate was slowed by a factor of 2 (i.e., a "2x" day length simulation). The vertical and latitudinal extent of the superrotating jet stream expands through the 32x simulation ( Figure 6 ), the same point at which the tropospheric westerly jet streams at mid-latitudes are entirely absent.…”