The Role of Water Vapor in the Response of the Extratropical Circulation of Earth‐Like Planets to Obliquity Changes. Issue 16 (24th August 2022) - Lobo, Ana H.
Abstract: 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. Plain Language Summary: We explore how global wind patterns and atmospheric energy transport respond to a change in radiation distribution, using a simple general circulation model of an Earth‐like planet covered by a thin ocean. We examine changes in both the incoming stellar radiation, through changes in obliquity, and the planet's outgoing thermal radiation, by varying the atmosphere's optical thickness. We focus on the high‐latitude effects, where increased obliquity leads to a counterintuitive reduction in eddies. We show that on high‐obliquity planets with strong seasonal cycles, water vapor plays a dominant role in the energy budget and serves as a mechanism for storing energy. The energy released from condensation in early winter helps keep the poles warm for several extra weeks, reducing the temperature differences between the high and low latitudes. This leads to smaller values of potential energy available to fuel eddies, which in turn leads to an absence of the Ferrel cell and storm tracks. For planets with a thicker ocean, energy becomes increasingly stored in the ocean surface, rather than in the atmospheric column. These results suggest that, regardless of ocean surface properties, water‐covered high‐obliquity planets would not experience significant high‐latitude storm activity. Key Points: Atmospheric moisture can mitigate extreme seasonal excursions, reducing summer temperature maxima on high‐obliquity planets Latent heat release slows polar cooling, keeping the winter pole warm for longer, and weakening baroclinic eddies and storm tracks Combined effects of atmospheric moisture storage and surface heat capacity suggest that high‐obliquity planets tend to have weak baroclinicity