Climates of High Obliquity Terrestrial Planets

Ongoing discoveries of terrestrial exoplanets suggest that these planets might span a wide range of climatic regimes and atmospheric circulations, which will strongly impact their habitability. Here, we use an idealized moist general circulation model in aquaplanet configuration to characterize the atmospheric circulations of terrestrial planets with high obliquities. We increase the obliquity from earth-like values of 23.5 degrees up to 60 degrees. This not only provides for a systematic exploration of the impact of obliquity on a planet’s climate, but also coincides with the range of obliquities experienced by Mars’ orbit in the past.

Because obliquity impacts the seasonal distribution of insolation, our simulations have a seasonally varying insolation forcing. While changes in surface temperature with obliquity (in the planetary mean, maxima, and equator to pole gradients) are relatively predictable, changes in seasonal precipitation patterns are less so. In traditional low obliquity cases net precipitation (the difference between precipitation and evaporation) is primarily balanced by mean moisture flux convergence in the tropics and eddy moisture flux convergence in the extratropics; for high obliquity cases, however, storage effects become increasingly important in the polar regions because of rapid temperature changes. More specifically, as polar temperatures drop quickly from their maximum values around the summer solstice, the water vapor in the atmospheric column rapidly condenses out, producing large amount of precipitation there. At lower latitudes, precipitation is primarily associated with seasonally migrating convergence zones within the Hadley circulation. With higher obliquity, however, solstice-season cross-equatorial Hadley cells become broader, with peak precipitation being progressively more separated from their poleward boundary and widely used diagnostics, such as the lower-level moist static energy maximum and the energy flux equator. This emphasizes the need for more robust theories that are broadly applicable to climates more exotic than Earth’s.