Formation of Jets and Equatorial Superrotation on Jupiter

Journal of the Atmospheric Sciences (in press) Formation of Jets and Equatorial Superrotation on Jupiter TAPIO SCHNEIDER∗AND JUNJUN LIU California Institute of Technology, Pasadena, California (Submitted 28 March 2008; in final form 24 September 2008) ABSTRACT The zonal flow in Jupiter’s upper troposphere is organized into alternating retrograde and prograde jets, with a prograde (super- rotating) jet at the equator. Existing models posit as the driver of the flow either differential radiative heating of the atmosphere or intrinsic heat fluxes emanating from the deep interior; however, they do not reproduce all large-scale features of Jupiter’s jets and thermal structure. Here it is shown that the difficulties in accounting for Jupiter’s jets and thermal structure resolve if the effects of differential radiative heating and intrinsic heat fluxes are considered together, and if upper-tropospheric dynamics are linked to a magnetohydrodynamic (MHD) drag that acts deep in the atmosphere and affects the zonal flow away from but not near the equator. Baroclinic eddies generated by differential radiative heating can account for the off-equatorial jets; meridionally propagating equatorial Rossby waves generated by intrinsic convective heat fluxes can account for the equatorial superrotation. The zonal flow extends deeply into the atmosphere, with its speed changing with depth, away from the equator up to depths at which the MHD drag acts. The theory is supported by simulations with an energetically consistent general circulation model of Jupiter’s outer atmosphere. A simulation that incorporates differential radiative heating and intrinsic heat fluxes reproduces Jupiter’s observed jets and thermal structure and makes testable predictions about as-yet unobserved aspects thereof. A control simulation that incorporates only differential radiative heating but not intrinsic heat fluxes produces off-equatorial jets but no equatorial superrotation; another control simulation that incorporates only intrinsic heat fluxes but not differential radiative heating produces equatorial superrotation but no off-equatorial jets. The proposed mechanisms for the formation of jets and equatorial superrotation likely act in the atmospheres of all giant planets. 1. Introduction The zonal flow in Jupiter’s upper troposphere has been inferred by tracking cloud features, which move with the horizontal flow in the layer between about 0.5 and 1 bar atmospheric pressure (Ingersoll et al. 2004; West et al. 2004; Vasavada and Showman 2005). In this layer, the zonal flow is organized into a strong prograde (su- perrotating) equatorial jet and an alternating sequence of retrograde and prograde off-equatorial jets (Fig. 1a). This flow pattern has been stable at least between the observations by the Voyager and Cassini spacecrafts in 1979 and 2000, with some variations in jet speeds, for example, a slowing of the prograde jet at 21◦N plane- tocentric latitude by ∼40 m s−1 (Porco et al. 2003; In- gersoll et al. 2004). The zonal flow in layers above the clouds has been inferred from the thermal structure of the atmosphere, using the thermal wind relation between meridional temperature gradients and vertical shears of the zonal flow. Meridional temperature gradients and thus vertical shears between 0.1 and 0.5 bar are gener- ally small: meridional temperature contrasts along iso- bars do not exceed ∼10 K (Conrath et al. 1998; Simon- ∗Corresponding author address: Tapio Schneider, California Insti- tute of Technology, 1200 E. California Blvd., Pasadena, CA 91125- 2300. E-mail: tapio@caltech.edu Miller et al. 2006). The thermal stratification in the same layer is statically stable (Simon-Miller et al. 2006; Read et al. 2006). About the zonal flow in lower layers, it is only known that at one site at 6.4◦N planetocentric lat- itude, where the Galileo probe descended into Jupiter’s atmosphere, it is prograde and increases with depth from ∼90 m s−1 at 0.7 bar to ∼170 m s−1 at 4 bar; beneath, it is relatively constant up to at least ∼20 bar (Atkin- son et al. 1998). At the same site, the thermal stratifi- cation is statically stable but approaches neutrality with increasing depth between 0.5 and 1.7 bar; beneath, it is statically nearly neutral or neutral up to at least ∼20 bar (Magalhães et al. 2002). These are the large-scale fea- tures (if the Galileo probe data are representative of large scales) that a minimal model of Jupiter’s general circu- lation should be able to reproduce. There are two plausible energy sources for Jupiter’s general circulation. First, ∼8 W m−2 of solar radia- tion are absorbed in Jupiter’s atmosphere (Hanel et al. 1981), where the time-mean insolation at the top of the atmosphere, given Jupiter’s small obliquity of 3◦, varies approximately with the cosine of latitude. Sec- ond, ∼6 W m−2 of intrinsic heat fluxes emanate from Jupiter’s deep interior (Ingersoll et al. 2004; Guillot et al. 2004; Guillot 2005); observations of convective storms (Gierasch e

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