Convective Generation of Equatorial Superrotation in Planetary Atmospheres

Giant planet tropospheres lack a solid, frictional bottom boundary. The troposphere instead smoothly transitions to a denser fluid interior below. However, Saturn exhibits a hot, symmetric cyclone centered directly on each pole, bearing many similarities to terrestrial hurricanes. Transient cyclonic features are observed at Neptune's South Pole as well. The wind-induced surface heat exchange mechanism for tropical cyclones on Earth requires energy flux from a surface, so another mechanism must be responsible for the polar accumulation of cyclonic vorticity on giant planets. Here it is argued that the vortical hot tower mechanism, claimed by Montgomery et al. and others to be essential for tropical cyclone formation, is the key ingredient responsible for Saturn's polar vortices. A 2.5-layer polar shallow-water model, introduced by O'Neill et al., is employed and described in detail. The authors first explore freely evolving behavior and then forceddissipative behavior. It is demonstrated that local, intense vertical mass fluxes, representing baroclinic moist convective thunderstorms, can become vertically aligned and accumulate cyclonic vorticity at the pole. A scaling is found for the energy density of the model as a function of control parameters. Here it is shown that, for a fixed planetary radius and deformation radius, total energy density is the primary predictor of whether a strong polar vortex forms. Further, multiple very weak jets are formed in simulations that are not conducive to polar cyclones.

Section: Beta Drift On Earth and Giant Planetsmentioning

confidence: 92%

Weak Jets and Strong Cyclones: Shallow-Water Modeling of Giant Planet Polar Caps

O’Neill

Emanuel

Flierl

2016

“…11, increasing S r beyond S r ; 1 by decreasing D h actually weakens the superrotation. Scaling arguments relating the strength of superrotating jets to their widths and to the static stability in the equatorial region were provided and tested in Liu and Schneider (2010) and Liu and Schneider (2011).…”

Section: B Relation To Prior Workmentioning

confidence: 99%

“…This is the case in Earth's troposphere in the annual mean, and it may be the case on Uranus and Neptune, which are subrotating (Liu and Schneider 2010). Only when the baroclinically unstable region is moved into low latitudes by artificially increasing radiative heating gradients near the equator and reducing them in higher latitudes can baroclinic instability promote the onset of superrotation (Williams 2003).…”

Section: Introductionmentioning

confidence: 98%

“…The net result is preferential generation of Rossby waves near the equator by convective heating fluctuations. If some of these convectively generated Rossby waves dissipate at higher latitudes-for example, through interaction with the mean-flow shear-they will transport angular momentum toward the equator and thus can generate superrotation (Schneider and Liu 2009;Liu and Schneider 2010).…”

Section: Introductionmentioning

confidence: 99%

See 1 more Smart Citation

Superrotation in Terrestrial Atmospheres

Laraia

Schneider

2015

Self Cite

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.

“…Specifically, the equatorial Rossby wave-Kelvin wave structure of the Gill response to a heating generates equatorward momentum fluxes that are likely the cause of superrotation seen in models with stationary wave-like equatorial heating (e.g., Suarez and Duffy 1992;Saravanan 1993;Kraucunas and Hartmann 2005). Other studies have found that equatorial Rossby waves (e.g., Liu and Schneider 2011;Arnold et al 2012) or Kelvin waves (e.g., Iga and Matsuda 2005) play a crucial role in the generation of superrotation. In this study, we aim partly to demonstrate the need for thinking of equatorial westerly jet dynamics as distinct from the dynamics of eddy-driven midlatitude jets.…”

Section: Introductionmentioning

confidence: 98%

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

Potter

Vallis

Mitchell

2014

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.