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

Equatorial superrotation on tidally locked exoplanets