Other
Scientific paper
Oct 2010
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2010dps....42.5608s&link_type=abstract
American Astronomical Society, DPS meeting #42, #56.08; Bulletin of the American Astronomical Society, Vol. 42, p.1079
Other
Scientific paper
The increasing richness of hot Jupiter observations has motivated a variety of three-dimensional atmospheric circulation models of these planets. Under the tidally locked, strongly irradiated conditions relevant to hot Jupiters, such models tend to exhibit a circulation dominated by a fast eastward, or "superrotating," jet stream at the equator. As first predicted by Showman and Guillot in 2002, 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 real hot Jupiters. Despite its relevance, however, the dynamical mechanisms responsible for generating the equatorial superrotation in hot Jupiter models have not been identified. Here, we show that the equatorial jet results from an interaction of the mean flow with standing Rossby waves induced by the day-night 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 demonstrate this mechanism in a sequence of one-layer (shallow-water) calculations and fully 3D models. This 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 has implications for the dependence of the equatorial jet speed on forcing amplitude, strength of friction, and other parameters, as well as for the conditions under which jets can form at all.
Polvani Lorenzo M.
Showman Adam P.
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