Formation of Jets and Equatorial Superrotation on Jupiter

Abstract

The zonal flow in Jupiter's upper troposphere is organized into alternating retrograde and prograde jets, with a prograde (superrotating) 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. 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, 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.Comment: 23 pages, 10 figure

Weak Jets and Strong Cyclones: Shallow-Water Modeling of Giant Planet Polar Caps

“…Scott and Polvani (2007) use a forced-dissipative model with full spherical geometry and find a polar cyclone that swims around the pole, constrained by the poleward-most jet. Schneider and Liu (2009) and Liu and Schneider (2010) also find a broad polar cyclone that precesses around the pole in a spherical shell that extends in pressure to 3 bars. The model used in the present work is the first to force a polar domain with small thermal perturbations that mimic observed moist convection in size, strength, and duration, motivated by abundant observations of intense moist convection on the gas giants (Little et al 1999;Gierasch et al 2000;Li et al 2004).…”

“…For example, hypoviscosity, with no clear physical interpretation, has been used by Scott and Polvani (2007); they also consider linear Rayleigh drag. Schneider (2010, 2011) and Schneider and Liu (2009) also use Rayleigh drag. In those papers, the stated motivation for linear drag is the cumulative effect of magnetohydrodynamic drag that occurs much deeper.…”

Saturn Atmospheric Structure and Dynamics

“…When Uranus/Neptune cases were explored with 30 times solar water, equatorial subrotation occurred, reminiscent of the observed equatorial subrotation on Uranus and Neptune. Schneider and Liu (2009) performed 3D weather-layer simulations of Jupiter using the dry primitive equations. Their simulations extended to ~3 bars pressure and contained a linear drag poleward of 33 at the base of the model to represent Ohmic dissipation in the deep interior (Liu et al 2008).…”

“…Their approach is generally similar to that of Lian and Showman (2009); however, they adopted a simple grey radiative-transfer scheme, imposed an intrinsic flux at the base of the model, and included a convective parameterization that transported thermal energy whenever conditions approached neutrally stable. Schneider and Liu's (2009) simulations developed multiple east-west midlatitude jets (whose speed depends on the strength of the imposed drag) and a Jupiter-like equatorial superrotation. They suggest that the superrotation seen in their simulations results from wave/mean-flow interactions associated with Rossby waves generated by convective events near the equator.…”

“…Three independent shallow-atmosphere models now exist that, under Jupiter-and Saturn-like conditions, spontaneously produce multiple midlatitude jets and a strong superrotating equatorial jet similar to those on Jupiter or Saturn -the one-layer shallow-water model of Scott and Polvani (2008) and the 3D primitiveequation models of Lian and Showman (2009) and Schneider and Liu (2009). In contrast to some previous shallowatmosphere studies, these models produce equatorial superrotation naturally, without ad-hoc forcing functions.…”