NMSU Astronomy

Colloquium: Shawn Brueshaber (Host: Ali Hyder)

October 16, 2020 @ 3:00 pm – 4:00 pm

Dynamical Regimes of Giant Planet Polar Vortices

Shawn Brueshaber,Western Michigan University

We present a numerical model that reveals a mechanism governing the polar atmospheric dynamics of
Jupiter, Saturn, Uranus and Neptune. Exploration of the polar regions of the gas giants has produced
surprisingly diverse results and these discoveries raise questions about the mechanism that
differentiates these polar atmospheric dynamics regimes. To help determine what physical mechanisms
control these differences, we use the Explicit Planetary Isentropic Coordinate (EPIC) model to carry
out forced-turbulence shallow-water simulations in a gamma-plane configuration using two sets of the
experiments. The first investigates the effects of three parameters, the planetary Burger number, Bu =
(Ld / a)2 (Ld is the Rossby deformation radius, a is the planetary radius), input storm strength, s, and the
storm polarity fraction (i.e., fraction of cyclonic and anticyclonic storms), α. The second set of
experiments focuses on the detailed circulation of the polar vortices by investigating the role of storm
size, storm intensity, and storm polarity fraction. The model is forced by small-scale stochastic mass
pulses that parametrically represent cumulus storms, widely thought to be an important mechanisms in
maintaining the vigorous circulation on giant planets.

Bu emerges to be the most important of the tested parameters, able to distinguish between four distinct
dynamical regimes, matching those of the giant planets, which from large to small Bu, are: i) a large
cyclonic polar vortex (i.e., Ice-Giant-Regime), ii) a compact intense cyclonic polar vortex (“Saturn-
Regime”), iii) two large vortices or one vortex offset from the pole (Transitional-Regime), and iv)
meandering jets with no centrally dominant vortex, or with multiple circumpolar cyclones (Jupiter-
Regime). The boundaries of these regimes are found to be only slightly modulated by the values of s
and α. By applying this correlation with respect to Bu in reverse, an observation of a particular polar
regime could in principle be used to constrain Ld.

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
Saturn- and Ice-Giant-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 the storm wind speed. Our new results clarify the role of storm
forcing on the intensity and size of Saturn- and Ice-Giant-Regime polar cyclones.


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