37th DPS Meeting, 4-9 September 2005
Session 20 Outer Planets I
Oral, Tuesday, September 6, 2005, 9:00-10:30am, Law LG19

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[20.09] Deep Jets from Shallow Forcing on Gas Giant Planets

A.P. Showman, Y. Lian (Univ. Arizona), P.J. Gierasch (Cornell Univ.)

We present numerical simulations of the formation, vertical structure, and stability of zonal jets produced from latitudinal contrasts in solar heating on a giant planet. In particular, we test the assumption -- which is common in the literature -- that jets induced by solar heating would remain confined to shallow layers of the atmosphere. We solve the full nonlinear primitive equations in spherical geometry using the MITgcm, which is a state-of-the-art circulation model developed at MIT within the past few years. The simulations contain a stratosphere and stably stratified troposphere overlying a deep adiabatic region representing Jupiter's interior. Thermal forcing (radiative heating and cooling) is applied at pressures less than 3 bars, but not in deeper layers where the radiative-heating rates are expected to be low. In our simulations, the flow experiences baroclinic instabilities in the thermally forced layers that generate multiple zonal jets in the upper troposphere. Interestingly, these multiple jets gradually develop a deep barotropic component extending to the bottom of the model at 100 bars (far below the level of forcing). Long-time integrations show that the deep wind component can achieve magnitudes comparable to the wind speeds in the upper troposphere. These results disprove the common-sense notion that shallow forcing only produces shallow jets. An implication is that the deep winds measured by the Galileo probe to pressures of 22 bars could just as easily result from shallow forcing as from deep (e.g., convective) forcing (contrary to the claims of many publications). Linear calculations performed off-line suggest that these deep jets result from Coriolis accelerations acting on deep meridional circulations induced by the upper-level forcing. Interestingly, some of our simulations develop a superrotating (eastward) equatorial jet, which may be relevant in explaining the superrotating equatorial jets on Jupiter and Saturn.

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Bulletin of the American Astronomical Society, 37 #3
© 2004. The American Astronomical Soceity.