Terrestrial Superrotation: A Bifurcation of the General Circulation

In rapidly rotating planetary atmospheres that are heated from below, equatorial superrotation can occur through convective generation of equatorial Rossby waves. If the heating from below is sufficiently strong that convection penetrates into the upper troposphere, then the convection generates equatorial Rossby waves, which can induce the equatorward angular momentum transport necessary for superrotation. This paper investigates the conditions under which the convective generation of equatorial Rossby waves and their angular momentum transport lead to superrotation. It also addresses how the strength and width of superrotating equatorial jets are controlled.In simulations with an idealized general circulation model (GCM), the relative roles of baroclinicity, heating from below, and bottom drag are explored systematically. Equatorial superrotation generally occurs when the heating from below is sufficiently strong. However, the threshold heating at which the transition to superrotation occurs increases as the baroclinicity or the bottom drag increases. The greater the baroclinicity is, the stronger the angular momentum transport out of low latitudes by baroclinic eddies of extratropical origin. This competes with the angular momentum transport toward the equator by convectively generated Rossby waves and thus can inhibit a transition to superrotation. Equatorial bottom drag damps both the mean zonal flow and convectively generated Rossby waves, weakening the equatorward angular momentum transport as the drag increases; this can also inhibit a transition to superrotation. The strength of superrotating equatorial jets scales approximately with the square of their width. When they are sufficiently strong, their width, in turn, scales with the equatorial Rossby radius and thus depends on the thermal stratification of the equatorial atmosphere.The results have broad implications for planetary atmospheres, particularly for how superrotation can be generated in giant planet atmospheres and in terrestrial atmospheres in warm climates.

Section: Introductionsupporting

confidence: 74%

Section: Conclusion Discussion and Open Questionsmentioning

confidence: 84%

mentioning

confidence: 99%

See 1 more Smart Citation

Convective Generation of Equatorial Superrotation in Planetary Atmospheres

Liu

Schneider

2011

“…These strong westerlies show that the upper troposphere is in a state of superrotation. The superrotation is common in aquaplanet simulations (Suarez and Duffy 1992;Saravanan 1993;Battisti and Ovens 1995).…”

Section: B Effect Of Zonal Sst Anomaliesmentioning

confidence: 99%

Some Atmospheric Processes Governing the Large-Scale Tropical Circulation in Idealized Aquaplanet Simulations

Gastineau

Treut

2011

The large-scale tropical atmospheric circulation is analyzed in idealized aquaplanet simulations using an atmospheric general circulation model. Idealized sea surface temperatures (SSTs) are used as lowerboundary conditions to provoke modifications of the atmospheric general circulation. Results show that 1) an increase in the meridional SST gradients of the tropical region drastically strengthens the Hadley circulation intensity, 2) the presence of equatorial zonal SST anomalies weakens the Hadley cells and reinforces the Walker circulation, and 3) a uniform SST warming causes small and nonsystematic changes of the Hadley and Walker circulations. In all simulations, the jet streams strengthen and move equatorward as the Hadley cells strengthen and become narrower.Some relevant mechanisms are then proposed to interpret the large range of behaviors obtained from the simulations. First, the zonal momentum transport by transient and stationary eddies is shown to modulate the eddy-driven jets, which causes the poleward displacements of the jet streams. Second, it is found that the Hadley circulation adjusts to the changes of the poleward moist static energy flux and gross moist static stability, associated with the geographical distribution of convection and midlatitude eddies. The Walker circulation intensity corresponds to the zonal moist static energy transport induced by the zonal anomalies of the turbulent fluxes and radiative cooling. These experiments provide some hints to understand a few robust changes of the atmospheric circulation simulated by ocean-atmosphere coupled models for future and past climates.

“…A stationary heat source near the equator, for example, leads to the generation of stationary Rossby waves, which can dissipate away from the equator and so transport angular momentum toward the equator. This leads to superrotation when the heat source is strong enough (Suarez and Duffy 1992;Saravanan 1993;Kraucunas and Hartmann 2005;Arnold et al 2012). The stationary wave mechanism is responsible for superrotation in simulations of tidally locked planets, in which stellar heating is radially symmetric around an equatorial focal point (Joshi et al 1997;Merlis and Schneider 2010;Pierrehumbert 2011).…”

Section: Introductionmentioning

confidence: 99%

Superrotation in Terrestrial Atmospheres

Laraia

Schneider

2015

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.