Heidar Thor Thrastarson and James Y-K. Cho
Astronomy Unit, School of Mathematical Sciences
Queen Mary University of London

We use a general circulation model (GCM) to study the three-dimensional atmospheric flow and temperature structures of tidally synchronized extrasolar planets. Our model solves the full primitive equations for the atmospheric region from about 1 mbar to 100 bar, with thermal forcing represented by simple Newtonian relaxation. We have performed an extensive exploration of the physical and numerical parameter space relevant for tidally synchronized giant planets, in idealized scenarios, using HD209458b as a reference planet. The radius, mass, and orbital period (hence rotation rate) are derived from observations.

In our simulations, robust features include a small number of jets and large-scale coherent vortices (often in the form of a pair of modons, vortex dipoles). The temperature distribution is strongly associated with the flow structures, and is far from a simple hot-day/cold-night scenario---despite the strong thermal forcing on the dayside. The large vortices generally exhibit variability in time, translating or oscillating in longitude with corresponding variability in the position of relative hot and cold regions. In addition, although robust features can be identified in general, we have found a significant sensitivity to the initial flow state, which is presently unknown for extrasolar planets. The latter result highlights the unsuitability of using GCMs for making quantitative “predictions”, as suggested or assumed in many extrasolar planet circulation studies found in the literature.