Equatorial superrotation on tidally locked exoplanets

Adam Showman, Lorenzo M. Polvani

Research output: Contribution to journalArticle

137 Citations (Scopus)

Abstract

The increasing richness of exoplanet observations has motivated a variety of three-dimensional (3D) 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. When the radiative and advection timescales are comparable, 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 subsequently 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 Rossby waves induced by the day-night thermal forcing. The strong longitudinal variations in radiative heating - namely intense dayside heating and nightside cooling - trigger the formation of standing, planetary-scale equatorial Rossby and Kelvin waves. 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.

Original languageEnglish (US)
Article number71
JournalAstrophysical Journal
Volume738
Issue number1
DOIs
StatePublished - Sep 1 2011

Fingerprint

superrotation
extrasolar planets
Rossby wave
planetary waves
Jupiter (planet)
Jupiter
planet
equators
planets
Kelvin waves
heating
equatorial wave
atmospheric circulation
terrestrial planets
radii
jet stream
Kelvin wave
standing wave
shallow water
longitude

Keywords

  • hydrodynamics
  • methods: analytical
  • methods: numerical
  • planets and satellites: atmospheres
  • planets and satellites: individual (HD 189733b)
  • waves

ASJC Scopus subject areas

  • Space and Planetary Science
  • Astronomy and Astrophysics

Cite this

Equatorial superrotation on tidally locked exoplanets. / Showman, Adam; Polvani, Lorenzo M.

In: Astrophysical Journal, Vol. 738, No. 1, 71, 01.09.2011.

Research output: Contribution to journalArticle

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