  Shallow-water numerical simulations show that the atmospheric circulation of
the close-in extrasolar giant planet (EGP) HD 209458b is characterized by
moving circumpolar vortices and few bands/jets (in contrast with ~10 bands/jets
and absence of polar vortices on cloud-top Jupiter and Saturn). The large
spatial scales of moving circulation structures on HD 209458b may generate
detectable variability of the planet's atmospheric signatures. In this Letter,
we generalize these results to other close-in EGPs, by noting that
shallow-water dynamics is essentially specified by the values of the Rossby
(Ro) and Burger (Bu) dimensionless numbers. The range of likely values of Ro (~
0.01 - 10) and Bu (~ 1 - 200) for the atmospheric flow of known close-in EGPs
indicates that their circulation should be qualitatively similar to that of HD
209458b. This results mostly from the slow rotation of these
tidally-synchronized planets.
