Baroclinic instability on hot extrasolar planets

Abstract

We investigate baroclinic instability in flow conditions relevant to hot extrasolar planets. The instability is important for transporting and mixing heat, as well as for influencing largescale variability on the planets. Both linear normal mode analysis and non-linear initial-value calculations are carried out -focusing on the freely-evolving, adiabatic situation. Using a highresolution general circulation model (GCM) which solves the traditional primitive equations, we show that large-scale jets similar to those observed in current GCM simulations of hot extrasolar giant planets are likely to be baroclinically unstable, on a timescale of a few to a few tens of planetary rotations, generating cyclones and anticyclones that drive weather systems. The growth rate and scale of the most unstable mode obtained in the linear analysis are in qualitative, good agreement with the full non-linear calculations. In general, unstable jets evolve differently depending on their signs (eastward or westward), due to the change in sign of the jet curvature. For jets located at or near the equator, instability is strong at the flanks -but not at the core. Crucially, the instability is either poorly or not at all captured in simulations with low resolution and/or high artificial viscosity. Hence, the instability has not been observed or emphasized in past circulation studies of hot extrasolar planets.

Shear-driven instabilities and shocks in the atmospheres of hot Jupiters

“…This is even more so for finite-volume codes such as RAMSES that have problems handling hydrostatic equilibria, and we found that problem to be very helpful in assessing the reliability of our setup. In this appendix, we thus qualitatively reproduce one of the models presented by Polichtchouk & Cho (2012), namely their equatorial jet case, since the base flow is close to the jet configuration we studied here. As noted by Polichtchouk et al (2014), the detailed evolution of the atmosphere is very sensitive to the exact structure of the jet and the initial perturbation, so that our goal here is not to reproduce the results of Polichtchouk & Cho (2012) quantitatively, but instead to show that we obtain the same qualitative evolution of the flow.…”

“…In this appendix, we thus qualitatively reproduce one of the models presented by Polichtchouk & Cho (2012), namely their equatorial jet case, since the base flow is close to the jet configuration we studied here. As noted by Polichtchouk et al (2014), the detailed evolution of the atmosphere is very sensitive to the exact structure of the jet and the initial perturbation, so that our goal here is not to reproduce the results of Polichtchouk & Cho (2012) quantitatively, but instead to show that we obtain the same qualitative evolution of the flow. Indeed, since we neither use the same equations (Euler vs. primitive) nor the same geometry (Cartesian vs. spherical), a one-to-one quantitative comparison is not possible.…”

“…The latter is computed using the following relation, adapted to the equatorial β-plane geometry from Polichtchouk & Cho (2012):…”

Effects of Bulk Composition on the Atmospheric Dynamics on Close-in Exoplanets

“…Previous studies (e.g., Thrastarson & Cho 2010;Polichtchouk & Cho 2012;Mayne et al 2014;Cho et al 2015) demonstrated that the simulation results might be sensitive to specifications such as forcing and dissipation setup, initial condition, and numerical schemes used in different dynamical cores. In this study, we adopted the MITgcm (Adcroft et al 2004) dynamical core that solves the primitive equations using the finite volume method in a cubesphere grid.…”

Three-Dimensional Atmospheric Circulation of Warm and Hot Jupiters: Effects of Orbital Distance, Rotation Period, and Nonsynchronous Rotation

“…6). Although such instability on Earth and Mars is enhanced by the existence of entropy gradients on the lower surface, several studies have shown that baroclinic instabilities are possible even on gas giants like Jupiter that lack such surfaces (Conrath et al 1981;Read 1988;Williams 2003;Lian & Showman 2008;O'Gorman & Schneider 2008;Polichtchouk & Cho 2012). Typically, such instabilities occur most readily at mid-to high latitudes, where meridional temperature gradients are large and isentropes slope steeply.…”