Weak Jets and Strong Cyclones: Shallow-Water Modeling of Giant Planet Polar Caps
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Author(s)
O'Neill, Morgan E
Emanuel, Kerry Andrew
Flierl, Glenn Richard
Date Issued
April 2016
Journal
Journal of the Atmospheric Sciences
Publisher
American Meteorological Society
Citation
O’Neill, Morgan E. et al. “Weak Jets and Strong Cyclones: Shallow-Water Modeling of Giant Planet Polar Caps.” Journal of the Atmospheric Sciences 73, 4 (April 2016): 1841–1855 © 2016 American Meteorological Society
Version
Final published version
Abstract
Giant planet tropospheres lack a solid, frictional bottom boundary. The troposphere instead smoothly transitions to a denser fluid interior below. However, Saturn exhibits a hot, symmetric cyclone centered directly on each pole, bearing many similarities to terrestrial hurricanes. Transient cyclonic features are observed at Neptune’s South Pole as well. The wind-induced surface heat exchange mechanism for tropical cyclones on Earth requires energy flux from a surface, so another mechanism must be responsible for the polar accumulation of cyclonic vorticity on giant planets. Here it is argued that the vortical hot tower mechanism, claimed by Montgomery et al. and others to be essential for tropical cyclone formation, is the key ingredient responsible for Saturn’s polar vortices. A 2.5-layer polar shallow-water model, introduced by O’Neill et al., is employed and described in detail. The authors first explore freely evolving behavior and then forced-dissipative behavior. It is demonstrated that local, intense vertical mass fluxes, representing baroclinic moist convective thunderstorms, can become vertically aligned and accumulate cyclonic vorticity at the pole. A scaling is found for the energy density of the model as a function of control parameters. Here it is shown that, for a fixed planetary radius and deformation radius, total energy density is the primary predictor of whether a strong polar vortex forms. Further, multiple very weak jets are formed in simulations that are not conducive to polar cyclones. Keywords: Circulation/ Dynamics; Convective-scale processes; Cyclogenesis/cyclolysis; Dynamics; Hurricanes; Planetary atmospheres; Shallow-water equations
MIT Department
Massachusetts Institute of Technology. Department of Earth, Atmospheric, and Planetary Sciences
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Article is made available in accordance with the publisher's policy and may be subject to US copyright law. Please refer to the publisher's site for terms of use.
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DOI of Published Version
https://doi.org/10.1175/JAS-D-15-0314.1
