Richard G. Cosentino

Meandering Shallow Atmospheric Jet as a Model of Saturn's North-polar Hexagon

The Voyager flybys of Saturn in 1980–1981 revealed a circumpolar Hexagon at ~78° north planetographic latitude that has persisted for over 30 Earth years, more than one Saturn year, and has been observed by ground-based telescopes, Hubble Space Telescope and multiple instruments on board the Cassini orbiter. Its average phase speed is very slow with respect to the System III rotation rate, defined by the primary periodicity in the Saturn Kilometric Radiation during the Voyager era. Cloud tracking wind measurements reveal the presence of a prograde jet-stream whose path traces the Hexagons shape. Previous numerical models have produced large-amplitude, n = 6, wavy structures with westward intrinsic phase propagation (relative to the jet). However, the observed net phase speed has proven to be more difficult to achieve. Here we present numerical simulations showing that instabilities in shallow jets can equilibrate as meanders closely resembling the observed morphology and phase speed of Saturns northern Hexagon. We also find that the winds at the bottom of the model are as important as the winds at the cloud level in matching the observed Hexagons characteristics.

Disruption of Saturn’s quasi-periodic equatorial oscillation by the great northern storm

The equatorial middle atmospheres of the Earth1, Jupiter2 and Saturn3,4 all exhibit a remarkably similar phenomenon—a vertical, cyclic pattern of alternating temperatures and zonal (east–west) wind regimes that propagate slowly downwards with a well-defined multi-year period. Earth’s quasi-biennial oscillation (QBO) (observed in the lower stratospheric winds with an average period of 28 months) is one of the most regular, repeatable cycles exhibited by our climate system1,5,6, and yet recent work has shown that this regularity can be disrupted by events occurring far away from the equatorial region, an example of a phenomenon known as atmospheric teleconnection7,8. Here, we reveal that Saturn’s equatorial quasi-periodic oscillation (QPO) (with an ~15-year period3,9) can also be dramatically perturbed. An intense springtime storm erupted at Saturn’s northern mid-latitudes in December 201010–12, spawning a gigantic hot vortex in the stratosphere at 40° N that persisted for three years13. Far from the storm, the Cassini temperature measurements showed a dramatic ~10 K cooling in the 0.5–5 mbar range across the entire equatorial region, disrupting the regular QPO pattern and significantly altering the middle-atmospheric wind structure, suggesting an injection of westward momentum into the equatorial wind system from waves generated by the northern storm. Hence, as on Earth, meteorological activity at mid-latitudes can have a profound effect on the regular atmospheric cycles in Saturn’s tropics, demonstrating that waves can provide horizontal teleconnections between the phenomena shaping the middle atmospheres of giant planets.The 2010–2011 storm that appeared at Saturn’s northern mid-latitudes significantly altered the wind structure and atmospheric temperature even far away from the storm, by disrupting the quasi-periodic atmospheric oscillations at the equator for more than 3 years.

A New, Long-lived, Jupiter Mesoscale Wave Observed at Visible Wavelengths

Small-scale waves were observed along the boundary between Jupiters North Equatorial Belt and North Tropical Zone, ~16.5° N planetographic latitude in Hubble Space Telescope data in 2012 and throughout 2015 to 2018, observable at all wavelengths from the UV to the near IR. At peak visibility, the waves have sufficient contrast (~10%) to be observed from ground-based telescopes. They have a typical wavelength of about 1.2° (1400 km), variable-length wave trains, and westward phase speeds of a few m/s or less. New analysis of Voyager 2 data shows similar wave trains over at least 300 hours. Some waves appear curved when over cyclones and anticyclones, but most are straight, but tilted, shifting in latitude as they pass vortices. Based on their wavelengths, phase speeds, and faint appearance at high-altitude sensitive passbands, the observed NEB waves are consistent with inertia-gravity waves at the 500-mbar pressure level, though formation altitude is not well constrained. Preliminary General Circulation Model simulations generate inertia-gravity waves from vortices interacting with the environment and can reproduce the observed wavelengths and orientations. Several mechanisms can generate these waves, and all may contribute: geostrophic adjustment of cyclones; cyclone/anticyclone interactions; wind interactions with obstructions or heat pulses from convection; or changing vertical wind shear. However, observations also show that the presence of vortices and/or regions of convection are not sufficient by themselves for wave formation, implying that a change in vertical structure may affect their stability, or that changes in haze properties may affect their visibility.

New Observations and Modeling of Jupiter's Quasi-Quadrennial Oscillation: NEW QQO OBSERVATIONS AND MODEL

All of the HST, TEXES, and EPIC data are included as supporting information in a single folder with separate directories for each set. Additional information on how to access the data using the open source programming language Python is also included.