Polar Vortices on Terrestrial Planets

The term polar vortex is commonly used to refer to a strong, planetary-scale zonal atmospheric flow that encircles a planet's pole in middle or high latitudes. Such polar vortices are a near ubiquitous feature of planets in our solar system, and are the site of unique chemical and microphysical processes. Although common, there are many differences in the structure and temporal variability of observed polar vortices. The cause of these differences often not known, and there is a need for a better understanding of the fundamental fluid dynamics of polar vortices, for understanding the climate of planets in our solar system as well as other solar systems. Here we examine the existence, structure and evolution of polar vortices in idealized atmospheric model simulations with a seasonal cycle, for a wide range of orbital, planetary, and atmospheric parameters. For all simulations there exists a polar vortex, but with varying seasonality: For rapid rotation or low obliquity the vortex is strongest in winter (and weak or non-existent in summer), as observed on Earth, Mars, and Titan, but for slow rotation together with high obliquity the vortex occurs in the summer and is at the ascending (and not descending) branch of the Hadley Cell. Further, even within the winter vortex regime, there are variations in the meridional extent, strength, and potential vorticity (PV) of the vortex, with the strength and PV structure varying non-monotonically with both rotation rate and obliquity. This non-monotonic variation is related to a transition from an eddy-mediated flow to a more axisymmetric, angular momentum conserving flow for slower rotation or higher obliquity.