Observational Consequences of Hydrodynamic Flows on Hot Jupiters

We use a grid-based shallow water model to simulate the atmospheric dynamics of the transiting hot Jupiter HD 209458b. Under the usual assumption that the planet is in synchronous rotation with zero obliquity, a steady state is reached with a well-localized cold spot centered 76° east of the antistellar point. This represents a departure from predictions made by previous simulations in the literature that used the shallow water formalism; we find that the disagreement is explained by the factor of 30 shorter radiative timescale used in our model. We also examine the case that the planet is in Cassini state 2, in which the expected obliquity is ~90°. Under these circumstances, a periodic equilibrium is reached, with the temperature slightly leading the solar forcing. Using these temperature distributions, we calculate disk-integrated bolometric infrared light curves from the planet. The light curves for the two models are surprisingly similar, despite large differences in temperature patterns in the two cases. In the zero-obliquity case, the intensity at the minimum is 66% of the maximum intensity, with the minimum occurring 72° ahead of transit. In the high-obliquity case, the minimum occurs 54° ahead of transit, with an intensity of 58% of the maximum.