The Role of Water Vapor in the Response of the Extratropical Circulation of Earth-Like Planets to Obliquity Changes

t In this study, we use idealized aquaplanet simulations of Earth-like planets to explore the large-scale atmospheric circulation’s response to changes in shortwave radiation, through changes in the planet obliquity, and in longwave radiation, through changes in the prescribed optical depth. We primarily focus on the extratropical circulation and the counterintuitive weakening of extratropical eddy activity, and associated precipitating storm tracks, with increased obliquity. We show that on high-obliquity planets with a small surface thermal inertia, net energy deficit during the winter months is primarily balanced by the latent energy component of the atmospheric heat capacity, which buffers any significant atmospheric cooling. As temperatures finally start to decrease, condensational latent heat release of atmospheric moisture further slows down the cooling and keeps the winter pole warmer than the midlatitudes until after the winter solstice. This prevents vigorous baroclinic eddy activity, which lives off the potential energy stored in the sloping isopycnals, as well as the development of a Ferrel cell and storm track. For planets with larger surface heat capacity, we see a similar suppression, with the energy storage in the ocean surface, rather than atmospheric latent energy storage, primarily balancing the net energy loss during the winter months. These results suggest that, regardless of surface thermal inertia, water-covered high-obliquity Earth-like planets would not experience significant extratropical storm activity and highlight the need for a proper characterization of polar properties in planetary modeling and observational investigations.