EQUATORIAL SUPERROTATION ON TIDALLY LOCKED EXOPLANETS

By A. Showman, L. Polvani. 2011

266 citations recorded by the source corpus.

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.

Subfields: Astronomy and Astrophysics

View Paper: https://www.semanticscholar.org/paper/4f33ca1aa9d8b59f2953e658ab6f1b60926db9be

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In Astronomy and Astrophysics

1. N. Madhusudhan
2. Jon Fortney
3. A. Showman
4. D. Deming
5. H. Knutson
6. D. Charbonneau
7. Sara Seager
8. J. D'esert

Citations in the corpus

• ROTATION AND WINDS OF EXOPLANET HD 189733 b MEASURED WITH HIGH-DISPERSION TRANSMISSION SPECTROSCOPY (2015, background): “One common outcome of both numerical (e.g., Showman et al. 2009; Rauscher & Menou 2010; Heng et al. 2011) and analytical (Showman & Polvani 2011) simulations is that relatively deep in the planet atmosphere (at pressures of ⇠1 bar) giant planets develop equatorial jet winds stream- ar X iv :1 51 2.”
• The Habitability of Planets Orbiting M-dwarf Stars (2016, background): “Planetary rotation rate has been shown to affect atmospheric circulation (Joshi et al., 1997; Merlis & Schneider, 2010; Showman et al., 2011; Showman & Polvani, 2011; Showman et al., 2013; Kaspi & Showman, 2015), as more slowly rotating planets exhibit a weaker Coriolis force, and longer periods of…”
• ATMOSPHERIC HEAT REDISTRIBUTION ON HOT JUPITERS (2013, unknown): “…(8) therefore scales asUH/L. 10 Perez-Becker & Showman model about a state of rest, whereτadv→ ∞ (this linearization is carried out in detail in Showman & Polvani 2011).9 This strongly suggests that in this region of theτrad, τdrag plane, the 9 To derive equation (15) from the the shallow…”
• 3.6 AND 4.5 μm PHASE CURVES AND EVIDENCE FOR NON-EQUILIBRIUM CHEMISTRY IN THE ATMOSPHERE OF EXTRASOLAR PLANET HD 189733b (2012, background): “…planets should develop a broad superrotating (eastward) equatorial jet that circulates energy between the day and night sides (e.g., Showman & Guillot 2002; Showman et al. 2009; Showman & Polvani 2011; Langton & Laughlin 2008; Dobbs-Dixon et al. 2010; Heng et al. 2011; Rauscher & Menou 2012b).”
• 3D mixing in hot Jupiters atmospheres. I. Application to the day/night cold trap in HD 209458b (2013, background): “3.1) and has been theoretically understood (Showman & Polvani 2011).”
• 3D climate modeling of close-in land planets: Circulation patterns, climate moist bistability and habitability (2013, background): “Nevertheless, these simulations seem to confirm that zonal wind speeds increase with forcing amplitude (Showman & Polvani 2011).”
• INFERENCE OF INHOMOGENEOUS CLOUDS IN AN EXOPLANET ATMOSPHERE (2013, result): “Combining these results with the analytical theory of Showman & Polvani (2011) suggests that thermal phasecurve eastward shifts are robust outcomes of the hotJupiter circulation regime.”
• Exoplanetary Atmospheres: Key Insights, Challenges, and Prospects (2019, background): “Left: Theoretical predictions from a GCM of HD 189733b showing an eastward jet and shifting of the hot spot away from the substellar point (241).”
• STABILIZING CLOUD FEEDBACK DRAMATICALLY EXPANDS THE HABITABLE ZONE OF TIDALLY LOCKED PLANETS (2013, background): “Equatorial superrotation is a generic feature of tidally locked planets arising from strong day–night forcing (Showman & Polvani 2011).”
• The habitability of Proxima Centauri b II. Possible climates and Observability (2016, background): “…two cold traps persist in all the tidally locked simulations (dry, wet, down to 1 bar thin or up to 20 bars thick atmospheres, etc.) explored in this work, but their position can slightly vary, as a result of planetary-scale equatorial Kelvin and Rossby wave interactions (Showman & Polvani 2011).”