  We investigate baroclinic instability in flow conditions relevant to hot
extrasolar planets. The instability is important for transporting and mixing
heat, as well as for influencing large-scale variability on the planets. Both
linear normal mode analysis and non-linear initial value calculations are
carried out -- focusing on the freely-evolving, adiabatic situation. Using a
high-resolution general circulation model (GCM) which solves the traditional
primitive equations, we show that large-scale jets similar to those observed in
current GCM simulations of hot extrasolar giant planets are likely to be
baroclinically unstable on a timescale of few to few tens of planetary
rotations, generating cyclones and anticyclones that drive weather systems. The
growth rate and scale of the most unstable mode obtained in the linear analysis
are in qualitative, good agreement with the full non-linear calculations. In
general, unstable jets evolve differently depending on their signs (eastward or
westward), due to the change in sign of the jet curvature. For jets located at
or near the equator, instability is strong at the flanks -- but not at the
core. Crucially, the instability is either poorly or not at all captured in
simulations with low resolution and/or high artificial viscosity. Hence, the
instability has not been observed or emphasized in past circulation studies of
hot extrasolar planets.
