1 Introduction

The discovery of extrasolar planets has led to a growing list of work devoted to modeling their atmospheres (Burrows et al. 1997; Seager & Sasselov 1998, 2000; Goukenleuque et al. 2000; Barman et al. 2001). While no spectra of these objects have yet been measured, one might be encouraged by the successes obtained in the case of the similar brown dwarfs, for which theoretical models now reproduce the observations well, even in the case of low-temperature objects ( $T_{\rm eff}\sim 1000\,$K or less) (e.g. Marley et al. 1996; Allard et al. 1997; Liebert et al. 2000; Geballe et al. 2001; Schweitzer et al. 2001 to cite only a few). However, an important feature of extrasolar planets is their proximity to a star: the irradiation that they endure can make their atmospheres significantly different than those of isolated brown dwarfs with the same effective temperature. This has been shown to profoundly alter the atmospheric vertical temperature profile (Seager & Sasselov 1998; Goukenleuque et al. 2000; Barman et al. 2001). We will show that most importantly, it also affects the horizontal temperature distribution and atmospheric chemistry so that the models calculated thus far may fail to provide an adequate description of the atmospheres of the most intensely irradiated planets. Advection has never previously been considered, but it can play a major role for the composition, temperature, spectral appearance and evolution of extrasolar planets.

We will focus on the extrasolar planets for which irradiation is the most important: 51Pegb-like planets, which we henceforth dub "Pegasi planets'' and define as gas giants orbiting solar-type stars at less than 0.1AU. Their importance is demonstrated by the fact that they orbit nearly 1% of stars surveyed so far and constitute 27% of currently-known extrasolar giant planets. They are also more easily characterized by the transit method than are other planets, as indicated by the discovery of the transiting gas giant HD 209458b (Charbonneau et al. 2000; Henry et al. 2000). In the preceeding paper (Guillot & Showman 2002, hereafter Paper I), we showed how the atmospheric boundary condition governs the evolution of Pegasi planets. We also advocated that an additional source of energy is needed to explain the radius HD 209458b, and that this would most likely be provided by the downward transport and subsequent dissipation of kinetic energy with a flux of $\sim$1% of the absorbed stellar energy. In this paper, we use the temperature profiles obtained in Paper I to evaluate the dynamical state of Pegasi-planet$\,\,$atmospheres, and discuss how dynamics may influence the cloud abundance, chemical composition, and thermal state (all of which will be amenable to observation in the near future). We then present preliminary numerical simulations of the circulation that indicate plausible circulation patterns and show how downward propagation of kinetic energy from the atmosphere to the interior can occur.

After reviewing the expected interior structure (Sect. 2), we begin in Sect. 3 with the problem of tides: Pegasi planets$\,\,$have been predicted to rotate synchronously (Guillot et al. 1996), implying that they always present the same face toward the star. We argue however that dynamical torques may maintain the interior in a non-synchronous rotation state, which has important implications for understanding atmospheric processes. In Sect. 4, we discuss the probable wind speeds, day-night temperature differences, and flow geometries, including both order-of-magnitude arguments and our numerical simulations. A summary of the results is provided in Sect. 5.