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Turbulent Forcing Scale and Intensity on the Dynamics of Giant Planet Polar Vortices

Presentation #212.03D in the session “Giant Planets, Exoplanets and Systems”. Cross-listed as presentation #409.05D.

Published onOct 03, 2021
Turbulent Forcing Scale and Intensity on the Dynamics of Giant Planet Polar Vortices

We present numerical simulations to systematically explore the differences in the formation and maintenance of polar vortices under Saturnian (“S-Regime”) and Ice Giant (“I-Regime”) dynamical conditions. The wide variation of polar vortices observed on the giant planets was recently captured in simulations by Brueshaber et al. (2019; 2021) building on O’Neill et al. (2015; 2016). These papers demonstrated that the dynamical regimes of giant planet polar vortices are controlled primarily by the planetary Burger number, 𝐵𝑢 = (𝐿𝑑0 ∕𝑎)2 , where 𝐿𝑑0 is the first-baroclinic deformation length at the pole, and 𝑎 is the planetary radius. Small 𝐵𝑢, matching estimates for Jupiter produce a Jupiter-like regime of multiple circumpolar cyclones (“J-Regime”). Larger 𝐵𝑢, matching estimates of 𝐵𝑢 for Saturn and the Ice Giants, both produce a single cyclone over each pole; the resulting polar vortex has a larger diameter in the I-Regime than in the S-Regime. Turbulent forcing scale and intensity also has an impact on the size of the resulting polar vortices.

We employ a shallow-water model forced by mass pulses that represent thunderstorms in which positive(negative) mass pulses geostrophically adjust to form anticyclones(cyclones). Our four parameter experimental design systematically tests the role of (1) storm size, and (2) storm wind speed. Additionally, we (3) investigate the role of the storm polarity fraction (fraction of small-scale anticyclones to cyclones), using (4) 𝐵𝑢 values that sample the S- and I-Regimes. Our results provide new key insights into the dynamics of solitary polar cyclones that emerge on giant planets as a result of moist-convective forcing. We find that the wind speed of the polar cyclones in the S-and I-Regimes is substantially influenced by storm size, storm wind, and storm polarity fraction. The radius of a polar cyclone is influenced by the storm polarity fraction, but, is not influenced by the storm size or storm wind speed.

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