Atmospheric super-rotation

Atmospheric super-rotation is a phenomenon where a planet's atmosphere rotates faster than the planet itself. This behavior is observed in the atmospheres of Venus, Titan, Jupiter, and Saturn. Venus exhibits the most extreme super-rotation, with its atmosphere circling the planet in four Earth days, much faster than the planet's own rotation of 243 Earth days. The phenomenon of atmospheric super-rotation can influence a planet's climate and atmospheric dynamics.

In understanding super-rotation, the role of atmospheric waves and instabilities is crucial. These dynamics, including Rossby waves and Kelvin waves, are integral in transferring momentum and energy within atmospheres, contributing to the maintenance of super-rotation. For instance, on Venus, the interaction of thermal tides with planetary-scale Rossby waves is thought to contribute significantly to its rapid super-rotational winds. Similarly, in Earth's atmosphere, Kelvin waves generate eastward along the equator, playing a vital role in phenomena like the El Niño-Southern Oscillation, demonstrating the broader implications of these dynamics in atmospheric science.[1][2]

The atmosphere of Venus is a prominent case of extreme super-rotation; the Venusian atmosphere circles the planet in just four Earth days, much faster than Venus' sidereal day of 243 Earth days.^[1] The initial observations of Venus' super rotation were Earth-based. Modern GCM models and observations are often enhanced by looking at past ancient climates. In a model where Venus is assumed to have an atmospheric mass similar to Earth, SS-AS circulation could have dominated over superrotation in an ancient thinner atmosphere.^[2]

Superrotation present in the stratosphere of Titan has been inferred by Voyager IRIS, Cassini CIRIS, stellar occultation and temperature observations, and Doppler shifts of the Huygens probe’s radio signal.^[3] Latitudinal pressure gradients established from measurements taken by Voyager IRIS were sufficient to produce superrotation of the atmosphere.^[4] Stratospheric zonal winds on Titan were observed on the order of 100-200 m s⁻¹,^[5] faster than the highest zonal winds on Earth at ~60-70 m s⁻¹. Questions on the effect of obliquity in superrotation on Titan is often compared to Venus, as they share similar centrifugal accelerations to achieve dynamic balance. Any seasonal variations effected by obliquity between Titan and Venus is much different, as the small obliquity of Venus at 2.7° negates any strong seasonal effects. Titans obliquity at 26.7° is high enough to cause seasonal variations within the stratospheric spin.^[4] Attempts to model superrotation on the gas giants, including Titan, has been abundant. The first observations of Titan in the 1980's revealed little information about circulation within the atmosphere due to the low contrast photochemical haze covering the moon. The first general circulation model (GCMs) in the 1990s provided insight into the stratospheric properties that should be expected on Titan with further observation, and predicted superrotation with winds up to 200 m/s.^[6] Superrotation was supported by the first 3D Titan GCM created by the Laboratoire de Météorologie Dynamique (LMD), in which they used an atmosphere similar to the observations of Voyager and recently Cassini.

The most recent GCM that is able to simulate superrotation in the stratosphere successfully is TitanWRF. Modeled after the PlanetWRF, which was designed to be a global weather, research, and forecasting (WRF) model, TitanWRF added planetary physics and generalized parameters to produce a successful superrotation model. Work done with TitanWRF v2 was able to simulate gradients in latitudinal temperature, zonal wind jets and superrotation in the stratosphere.^[3] Comparing TitanWRF v2 simulations with constant solar forcing (seasonal cycle removed) models,^[7] showed that in the latter, a rapid buildup in rotation, attaining > 100m/s, happened in a few Titan years. The parameters in these older forcing models differ greatly in the mechanisms involved in generating the initial superrotation compared to the more realistic TitanWRF models. After initial spin up, similarities evolve between the different models when a steady state is produced,^[3] but differ again in the final states of the model. The initial mechanism producing spin up to superrotation is still an on going question, as correlations between models differ greatly within this regime.

The visible cloud tops of Jupiter and Saturn provides further evidence on its deep atmospheric circulation demonstrating the presence of atmospheric super-rotation.^[8] Jupiter's auroras, in particular, highlight the planet's rapid atmospheric movements through their ethereal glow and varying cloud depths.

On Earth, there is a phenomenon that its thermosphere has a slight net super-rotation, exceeding the surface rotational velocity. The size of this phenomenon varies widely across different models.^[9][10][11] Some models suggest that global warming is likely to cause an increase in super-rotation in the future, including possible change in surface winds patterns.^[12][13] In simplified GCM models, equatorial superrotation emerges without obliquity and the addition of tropical heating anomalies.^[5] At present, a counter balance between the easterly Coriolis torque and the westerly torque maintains subrotation in the upper tropical troposphere. This leads to the prospect that with warmer and tropical wave sources in past ancient climates, Earths atmosphere might have superrotated.^[14]

Super-rotation in planetary atmospheres extends to the study of exoplanets, particularly, hot Jupiters. These distant worlds, orbiting close to their stars, often exhibit extreme atmospheric conditions, including super-rotation, which influences their thermal structures and potential habitability. Observations from telescopes like the Hubble Space Telescope have unveiled super-rotational wind speeds of thousands of kilometers per hour on some hot Jupiters. Moreover, the phenomenon shows how hot Jupiters is tidally locked, where one side continuously faces the star. This suggests a mechanism for heat distribution in planets, a factor in understanding their climatic conditions and patterns.^[15][16]