摘要：Context. General circulation models of the atmosphere of hot Jupiters
have shown the existence of a supersonic eastward equatorial jet. These results have been
obtained using numerical schemes that filter out vertically propagating sound waves and
assume vertical hydrostatic equilibrium, or were acquired with fully compressive codes
that use large dissipative coefficients.
Aims. We remove these two limitations and investigate the effects of
compressibility on the atmospheric dynamics by solving the standard Euler equations.
Methods. This was done by means of a series of simulations performed in
the framework of the equatorial β-plane approximation using the finite-volume
shock-capturing code RAMSES.
Results. At low resolution, we recover the classical results described
in the literature: we find a strong and steady supersonic equatorial jet of a few
km s-1 that
displays no signature of shocks. We next show that the jet zonal velocity depends
significantly on the grid meridional resolution. When this resolution is fine enough to
properly resolve the jet, the latter is subject to a Kelvin-Helmholtz instability. The jet
zonal mean velocity displays regular oscillations with a typical timescale of a few days
and a significant amplitude of about 15% of the jet velocity. We also find compelling evidence for the
development of a vertical shear instability at pressure levels of a few bars. It seems to
be responsible for an increased downward kinetic energy flux that significantly affects
the temperature of the deep atmosphere and appears to act as a form of drag on the
equatorial jet. This instability also creates velocity fluctuations that propagate upward
and steepen into weak shocks at pressure levels of a few mbars.
Conclusions. We conclude that hot-Jupiter equatorial jets are
potentially unstable to both a barotropic Kelvin-Helmholtz instability and a vertical
shear instability. Upon confirmation using more realistic models, these two instabilities
could result in significant time variability of the atmospheric winds, may provide a
small-scale dissipation mechanism in the flow, and might have consequences for the
internal evolution of hot Jupiters.
关键词：hydrodynamics;instabilities;shock waves;methods: numerical;planets and satellites: atmospheres
