Equatorial superrotation on tidally locked exoplanets

TL;DR

This paper explains how equatorial superrotation on tidally locked exoplanets arises from wave-mean-flow interactions, providing an analytic theory supported by models, and clarifies the conditions for jet formation and their properties.

Contribution

It introduces a new analytic theory for the dynamical mechanism behind equatorial superrotation on tidally locked exoplanets, linking wave interactions to jet formation.

Findings

Rossby and Kelvin waves induce eastward momentum transfer.

The equatorial jet width is comparable to the Rossby deformation radius.

Jet speed depends on forcing amplitude and frictional effects.

Abstract

The increasing richness of exoplanet observations has motivated a variety of three-dimensional atmospheric circulation models of these planets. Under strongly irradiated conditions, models of tidally locked, short-period planets (both hot Jupiters and terrestrial planets) tend to exhibit a circulation dominated by a fast eastward, or "superrotating," jet stream at the equator. Under appropriate conditions, this phenomenon can cause the hottest regions to be displaced eastward from the substellar point by tens of degrees longitude. Such an offset has been observed on HD 189733b, supporting the possibility of equatorial jets on short-period exoplanets. Despite its relevance, however, the dynamical mechanisms responsible for generating the equatorial superrotation in such models have not been identified. Here, we show that the equatorial jet results from the interaction of the mean flow with standing, planetary-scale Rossby and Kelvin waves induced by the day-night thermal forcing. The Rossby waves develop phase tilts that pump eastward momentum from high latitudes to the equator, thereby inducing equatorial superrotation. We present an analytic theory demonstrating this mechanism and explore its properties in a hierarchy of one-layer (shallow-water) calculations and fully 3D models. The wave-mean-flow interaction produces an equatorial jet whose latitudinal width is comparable to that of the Rossby waves, namely the equatorial Rossby deformation radius modified by radiative and frictional effects. For conditions typical of synchronously rotating hot Jupiters, this length is comparable to a planetary radius, explaining the broad scale of the equatorial jet obtained in most hot Jupiter models. Our theory illuminates the dependence of the equatorial jet speed on forcing amplitude, strength of friction, and other parameters, as well as the conditions under which jets can form at all.