  A variety of observational evidence demonstrates that brown dwarfs exhibit
active atmospheric circulations. In this study we use a shallow-water model to
investigate the global atmospheric dynamics in the stratified layer overlying
the convective zone on these rapidly rotating objects. We show that the
existence and properties of the atmospheric circulation crucially depend on key
parameters including the energy injection rate and radiative timescale. Under
conditions of strong internal heat flux and weak radiative dissipation, a
banded flow pattern comprising east-west jet streams spontaneously emerges from
the interaction of atmospheric turbulence with the planetary rotation. In
contrast, when the internal heat flux is weak and/or radiative dissipation is
strong, turbulence injected into the atmosphere damps before it can
self-organize into jets, leading to a flow dominated by transient eddies and
isotropic turbulence instead. The simulation results are not very sensitive to
the form of the forcing. Based on the location of the transition between
jet-dominated and eddy-dominated regimes, we suggest that many brown dwarfs may
exhibit atmospheric circulations dominated by eddies and turbulence (rather
than jets) due to the strong radiative damping on these worlds, but a jet
structure is also possible under some realistic conditions. Our simulated light
curves capture important features from observed infrared lightcurves of brown
dwarfs, including amplitude variations of a few percent and shapes that
fluctuate between single-peak and multi-peak structures. More broadly, our work
shows that the shallow-water system provides a useful tool to illuminate
fundamental aspects of the dynamics on these worlds.
