Superrotation in Planetary Atmospheres

Abstract

Superrotation is a dynamical regime where the atmosphere circulates around the planet in the direction of planetary rotation with excess angular momentum in the equatorial region. Superrotation is known to exist in the atmospheres of Venus, Titan, Jupiter, and Saturn in the solar system. Some of the exoplanets also exhibit superrotation. Our understanding of superrotation in a framework of circulation regimes of the atmospheres of terrestrial planets is in progress thanks to the development of numerical models; a global instability involving planetary-scale waves seems to play a key role, and the dynamical state depends on the Rossby number, a measure of the relative importance of the inertial and Coriolis forces, and the thermal inertia of the atmosphere. Recent general circulation models of Venus's and Titan's atmospheres demonstrated the importance of horizontal waves in the angular momentum transport in these atmospheres and also an additional contribution of thermal tides in Venus's atmosphere. The atmospheres of Jupiter and Saturn also exhibit strong superrotation. Recent gravity data suggests that these superrotational flows extend deep into the planet, yet currently no single mechanism has been identified as driving this superrotation. Moreover, atmospheric circulation models of tidally locked, strongly Understanding the Diversity of Planetary Atmospheres Edited by François Forget,

Atmospheric circulation of brown dwarfs and directly imaged exoplanets driven by cloud radiative feedback: global and equatorial dynamics

“…Our analysis (not shown) suggests that horizontal eddy momentum transport by transient waves is responsible for this local superrotation. This might be somewhat analog to those proposed for superrotation in solarsystem bodies, such as Venus, Titan, tropospheres of Jupiter and Saturn (see a recent review by Imamura et al 2020). In the model with drag = 10 6 s, a broad equatorial westward jet and high-latitude eastward jets with speeds ∼ 400 m s −1 emerge.…”

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

“…This is different from the local superrotation index (the ratio of specific angular momentum to its equatorial value) in Read and Lebonnois (2018) and Imamura et al. (2020). The superrotation intensity in Equation is used to discuss a simple relationship between the local superrotation intensity and Rossby number (Equations ).…”

“…The measure of the local superrotation intensity in Equation 1 is defined as the ratio of atmospheric angular velocity to solid-body rotation rate, to simply determine if a planet's atmosphere locally rotates faster than the planet and to evaluate the intensity of the angular velocity of the local superrotation. This is different from the local superrotation index (the ratio of specific angular momentum to its equatorial value) in Read and Lebonnois (2018) and Imamura et al (2020). The superrotation intensity in Equation 1 is used to discuss a simple relationship between the local superrotation intensity and Rossby number (Equations 3-5).…”

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Zonal circulation faster than the planetary rotation (i.e., superrotation) has been observed in planetary atmospheres (Imamura et al, 2020;Read & Lebonnois, 2018). On Venus and Titan, fast zonal flows of >100 m s −1 and meridional circulation are driven by diabatic heating in aerosol and cloud layers in the middle atmosphere (Flasar & Achterberg, 2009;Read & Lebonnois, 2018; Sanchez-Lavega et al, 2017), which corresponds to the stratosphere and mesosphere.

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Evidence for multiple Ferrel-like cells on Jupiter

“…Extending the zonal wind along the direction of the spin axis thus separates the equatorial region (17 • S to 17 • N) from the truncated cells at midlatitudes (see SI). The superrotating wind at low latitudes requires a source of momentum (Imamura et al, 2020). Theories for such sources include meridional (Potter et al, 2014;Laraia and Schneider, 2015) and vertical (Aurnou and Olson, 2001;Busse, 2002;Christensen, 2002;Heimpel et al, 2005;Kaspi et al, 2009;Dietrich and Jones, 2018), propagation of waves.…”