Effects of forcing scale and intensity on the emergence and maintenance of polar vortices on Saturn and Ice Giants

Abstract

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 gas giants Jupiter and Saturn by the Juno and Cassini spacecraft, respectively, and on the ice giants Uranus and Neptune by ground- and space-based telescopes, was recently captured in simulations by Brueshaber et al. (2019) (hereafter, ‘B19’) using the EPIC shallow-water numerical model. B19, expanding on a prior finding by O’Neill et al. (2015), reconfirmed this prior discovery 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. However, B19 found that the effect of Bu alone was not sufficient to explain the differences in the polar vortices in the S- and I-Regimes. B19 speculated that the turbulent forcing scale and intensity had an impact on the size of the resulting polar vortices, which motivate the current study.Here, like in B19, 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. We develop a new four-parameter experimental design to systematically test the role of (1) storm size, and (2) storm wind speed. In addition, we (3) investigate the role of the storm polarity fraction (the 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 within S- and I-Regimes is substantially influenced by storm size, storm wind speed, and storm polarity fraction. The radius of the polar cyclone is also influenced by the storm polarity fraction, but, is not influenced by the storm size or storm wind speed. Our new results clarify the role of storm forcing on the intensity and size of S- and I-Regime polar cyclones. Lastly, we resolve the conundrum found in B19 regarding polar cyclone circulations, providing a more dynamically consistent set of results of classifying polar cyclones into dynamical regimes based on Bu.