Super-rotating jets in the atmospheres of terrestrial planets

Introduction: Super-rotation is a ubiquitous phenomenon in the four substantial atmospheres possessed by solid bodies in the solar system. The slowly rotating planet, Venus, and moon, Titan, are both well-known to have atmospheres that rotate substantially more quickly than does the solid surface underneath. The more rapidly rotating planets, Mars and Earth, exhibit less spectacular global super-rotation, but both provide examples of prograde jets near the equator which rotate more rapidly than the equatorial surface. It is also worth noting in passing that atmospheric super-rotation is not restricted to planets with solid surfaces and shallow atmospheres, but that Jupiter and Saturn both possess rapid prograde equatorial jets [e.g. 1] and hence exhibit local super-rotation. In each case the detailed mechanism, or combination of mechanisms, which produces the super-rotating jets might vary, but all require longitudinally asymmetric motions, waves or eddies, to transport angular momentum up-gradient into the jets. This paper will review recent global circulation model results and, in the case of Mars and Earth, atmospheric reanalyses by data assimilation, in order to diagnose possible mechanisms in each case. Measures of super-rotation: For comparative purposes it is useful to introduce measures of local and global super-rotation [2, 3]. Local super-rotation, s, is defined as the ratio of the angular momentum of the atmosphere about the rotation axis of the planet to the angular momentum of air at rest relative to the surface at the equator minus one. Global super-rotation, S, is the ratio of the total atmospheric angular momentum of the atmosphere about the rotation axis of the planet to the total angular momentum of an equivalent atmosphere in solid-body rotation with the planet, again minus one. In both cases, positive values indicate superrotation compared to that which might be achieved by rearranging an atmosphere initially at rest only by axisymmetric processes [4]. Mars and Earth: The Earth exhibits a small positive global super-rotation with S = 1–2% [2], and relatively weak local super-rotation can also be observed, particularly in the westerly phase of the Quasi-Biennial Oscillation (QBO); a prograde wind u = 20 m/s at the equator on Earth is equivalent to a local super-rotation of s = 4%. Modelling studies [3] using the UK (spectral dynamics with angular momentum conservation enforced) version of the LMD-UK Mars Global Circulation Model (MGCM) [5, 6], as well as results from earlier studies [7], demonstrate that Mars might exhibit stronger super-rotation, with S = 4–6% and s = 8–16% depending on the dust loading and time of year [3]. See also Fig. 1 here.