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 irradiated exoplanets have long predicted the existence of equatorial superrotation in their atmospheres, which has been attributed to the result of the strong day-night thermal forcing. As predicted, recent Doppler observations and infrared phase curves of hot Jupiters appear to confirm the presence of superrotation on these objects.
Highlights
Superrotation is a dynamical regime where the atmosphere moves around the planet in the direction of planetary rotation, and is usually defined as the atmospheric angular velocity in the equatorial region exceeding that of the solid planet
The IPSL Venus general circulation models (GCMs) (Lebonnois et al 2010, 2016) and Atmospheric GCM For the Earth Simulator (AFES) VGCM (Sugimoto et al 2019a) have been able to build superrotation from a motionless state, demonstrating that the origin of the superrotation is the transport of angular momentum by the mean meridional circulation, that pumps angular momentum through unbalanced surface exchanges and accelerates the atmosphere until it is balanced by eddy transport terms and the surface exchanges balance between high and low latitudes
As described in the previous sections, superrotation is ubiquitous among planetary atmospheres, and the planetary atmospheres of the solar system show this phenomenon occurs over a wide range of planetary characteristics including fast and slow planetary rotation
Summary
Superrotation is a dynamical regime where the atmosphere moves around the planet in the direction of planetary rotation, and is usually defined as the atmospheric angular velocity in the equatorial region exceeding that of the solid planet. Superrotation permanently exists in the atmospheres of Venus, Titan, Jupiter and Saturn (Read and Lebonnois 2018) (Fig. 1). According to remote and direct measurements, the atmosphere at heights of 60–70 km above the surface travels around the planet in 4–5 Earth days, corresponding to speeds of ∼100 m s−1. This corresponds to an angular velocity about the rotation axis that is more than 50 times faster than the rotation rate of the solid planet. Jupiter and Saturn exhibit strong superrotation in the equatorial region at the level of the visible cloud tops. Showman and Guillot (2002) predicted that strong superrotation occurring on close-in, strongly irradiated giant exoplanets—the “hot Jupiters”—would
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
