Transport and Mixing in Planetary Polar Vortices with Annular and Monopolar Potential Vorticity Structures

Abstract Polar vortices are a common feature in the solar system, but their structure varies between planets. Although polar vortices are characterized by a coherent region of high potential vorticity (PV) in the polar regions, the meridional variation of PV ranges from strong monopolar to annular distributions. The reduced stability of an annular vortex compared to a monopolar vortex may lead to increased horizontal transport between the midlatitudes and the poles, but this has yet to be thoroughly investigated. Here we perform such an investigation by quantifying the horizontal mixing in polar vortices of varying PV structure, for both a two-dimensional shallow-water model and a three-dimensional general circulation model (GCM). Good agreement is found between the models, with both models indicating that both vortices maintain a horizontal mixing barrier that isolates the polar regions from the extratropics. The primary difference between the two vortex structures is on the interior of the vortex edge, where there is increased mixing inside the annular vortex but a clear reduction of mixing inside the monopolar vortex. In the three-dimensional GCM, a “subvortex” region is identified below the annular polar vortex that allows for more transport of material into the polar regions than the subvortex of the monopolar vortex. This suggests a potential pathway for transport into annular vortices that does not occur for monopolar vortices. Although we start from a Mars-like atmosphere for simplicity and flexibility, our results should describe other terrestrial planets/moons with broad polar vortices with coherent regions of high PV.