  Three-dimensional numerical simulations show that large-scale latent heating
resulting from condensation of water vapor can produce multiple zonal jets
similar to those on the gas giants (Jupiter and Saturn) and ice giants (Uranus
and Neptune). For plausible water abundances (3-5 times solar on Jupiter/Saturn
and 30 times solar on Uranus/Neptune), our simulations produce ~20 zonal jets
for Jupiter and Saturn and 3 zonal jets on Uranus and Neptune, similar to the
number of jets observed on these planets. Moreover, these Jupiter/Saturn cases
produce equatorial superrotation whereas the Uranus/Neptune cases produce
equatorial subrotation,consistent with the observed equatorial jet direction on
these planets.Sensitivity tests show that water abundance, planetary rotation
rate,and planetary radius are all controlling factors, with water playing the
most important role; modest water abundances, large planetary radii, and fast
rotation rates favor equatorial superrotation, whereas large water abundances
favor equatorial subrotation regardless of the planetary radius and rotation
rate. Given the larger radii, faster rotation rates, and probable lower water
abundances of Jupiter and Saturn relative to Uranus and Neptune, our
simulations therefore provide a possible mechanism for the existence of
equatorial superrotation on Jupiter and Saturn and the lack of superrotation on
Uranus and Neptune. Nevertheless, Saturn poses a possible difficulty, as our
simulations were unable to explain the unusually high speed (~400m/s) of that
planet's superrotating jet. The zonal jets in our simulations exhibit modest
violations of the barotropic and Charney-Stern stability criteria. Overall, our
simulations, while idealized, support the idea that latent heating plays an
important role in generating the jets on the giant planets.
