Atmospheric circulation on slowly rotating planets in the Solar System, and beyond

Thesis

Atmospheric circulation on Venus and Titan is characterised by prograde superrotating jets at the equator, and meridional overturning circulations that extend a significant distance towards the poles. Circulation of this type is characteristic of planets that are slowly rotating. By contrast, the atmospheres of the Earth and Mars, which are more rapidly rotating, feature zonal jets in mid-latitudes, and overturning circulation that is confined to the tropics. However, while planetary rotation is likely the main determinant of a given planet’s atmospheric circulation, other factors also have an important influence. This has been illustrated by numerical experiments using models of slowly rotating Earth- like planets, which do not automatically generate Venus or Titan-like circulations.

The aim of this thesis is to improve our understanding of the different circulation regimes that may be obtained on slowly rotating planets. This is achieved by developing a theoretical framework within which the circulation of different slowly rotating planets can be easily compared. Additionally, for the case of Titan, this thesis presents an in- depth investigation of the specific atmospheric phenomena that differentiate it from other slow rotators.

Recently, the array of possible circulation regimes has expanded significantly, due to the discovery of tidally locked planets orbiting stars other than the Sun. If we restrict our attention to temperate planets, then theoretical considerations suggest most will be slowly rotating. Tidally locked planets have a permanent day side and night side, which strongly influences their atmospheric circulation. Consequently, these planets occupy circulation regimes that are different from those exhibited by the slowly rotating planets in the Solar System.

To characterise the atmospheres of slowly rotating tidally locked planets, this thesis proposes a new analysis technique that divides the total circulation into physically meaningful constituent components, in much the same way as the circulation of the Solar System planets is divided into zonal jets and meridional overturning. It is also shown that the proposed decomposition of the circulation can be used to aid the observational characterisation of these planets, by showing how different parts of the decomposed circulation contribute distinct features to a planet’s thermal emission.

Authors: Lewis, NT

• Type of award: DPhil
• Level of award: Doctoral
• Awarding institution: University of Oxford
• Language: English
• Keywords: Solar System planets; exoplanets; atmospheric circulation