A variety of observations provide evidence for vigorous motion in the atmospheres of brown dwarfs and directly imaged extrasolar giant planets; these observations include spectral evidence for clouds, disequilibrium chemistry, lightcurve variability, and maps of surface patchiness. These observations raise major questions about the nature of the atmospheric circulation on these exotic worlds, which resemble high-heat-flux, high-gravity, rapidly rotating versions of Jupiter. In this talk, I will survey the dynamical regime of these objects with an eye toward explaining these observations. Brown dwarfs rotate rapidly, and for plausible wind speeds, the flow at large scales will be rotationally dominated. The interaction of convection with the overlying, stably stratified atmosphere will generate a wealth of atmospheric waves, and I argue that, just as in the stratospheres of planets in the solar system, the interaction of these waves with the mean flow will lead to a significant atmospheric circulation at
regional to global scales. One-layer and 3D global simulations of the atmospheric circulation suggest that, under relevant conditions, brown dwarfs can develop large-scale atmospheric circulations comprising banded flow patterns, zonal jets, turbulence, and in some cases stable vortices. These large-scale coherent structures will be accompanied by a vertical (overturning) circulation that will modulate the condensate cycle and lead to cloud patchiness analogous to that inferred from the observations. The simulations provide a foundation for understanding lightcurves and surface maps of brown dwarfs, and moreover illuminate the continuum of dynamical processes between brown dwarfs and Jupiter itself.
Talk Host: Michael McDonald