Climate Change on Jupiter

In April 2004 Philip S. Marcus (Department of Mechanical Engineering, University of California, Berkeley, California, USA) published a paper about a possible climate cycle on planet Jupiter: “Prediction of a global climate change on Jupiter“, Nature, Vol.428.

Marcus was able to explain the behaviour of weather systems (anticyclones and cyclones) in the atmosphere of Jupiter, a so called gas giant. A gas giant is a type of planet mostly composed of gases (primarily hydrogen and helium, but also water, carbon dioxide, methane, ammonia and other (organic) molecules) partial in a high compressed state, with only a small rocky core in proportion to size. Only the outermost layers of the deep atmosphere are observable, where all the weather is happening: at the bottom a layer with water and ice clouds, above a layer with ammonia ice and ammonia hydrosulfide. There are cumulus clouds, storms and thunderstorms with hail, rain and snow similar to Earth but a solid surface doesn`t exist. Weather on Jupiter is mainly driven by internal heat but ony in the second place from the Sun leading to features like 12 zonal jet streams and a lot of vortices (cyclones and anticyclones) via convection and Coriolis forces (due to fast rotation of the planet). In contrast to Earth vortices on Jupiter are very long-lived due to lack of friction by a solid rough surface.

The Great Red Spot moving at a low line of latitude on the Southern Hemisphere is the largest and longest-lived of the vortices, a oval-shaped high pressure system (anticyclone) of 15000-25000 km (!) lasting more than 300 years yet. In the 1930s three White Ovals, also anticyclones but smaller ones, occured farther south. In the period something unexpected happened. The White Ovals begun to merge step-by-step leaving over only one larger White Oval at the end.


The merger of the White Ovals Source: Hubble Space Telescope, NASA

Marcus managed to simulate this merger event in a computer modell fitting to observations. He concluded that the merger of the White Ovals  was a part of a recurring ~ 70 year-climate cycle on Jupiter.

It goes like this:

On Jupiter weather systems are mostly organized in vortex streets separated by jetstreams. A vortex street is only stable if anticyclones and cyclones are alternating. Two neighboring anticyclones or cyclones are merging to a bigger one but cyclone and anticyclone are repelling each other. Vortices degrade and diminish in size over time through turbulent erosion. Finally they become small enough to be trapped by troughs of propagating jetstream waves (Rossby waves). That’s possible because these troughs have a cyclonic edge (like on Earth) repelling anticyclones. Thus more and more triplets are trapped. If two cyclones or anticyclones are trapped together they merge immediately.  But also triplets are trapped, say two anticyclones with a cyclone in between. Through interactions with free vortices on vortex street triplets are deranged: Two of the three vortices change places, the two anticyclones become adjacent and merge. The process of trapping and merging is repetitive.

Usually the latitudinal gradient in temperature on Jupiter is low. Global internal heat sources are more important for temperature than the faraway Sun and the entirety of vortices are effectively mixing the air masses. But with the loss of these air-mixing vortices by merging the latitudinal gradient in temperature increases As a result jetstreams turn into a more active and wavy pattern. Troughs and crests of the Rossby waves grow, break and roll-up in new vortices, new cyclones and anticyclones.
Also the loss of vortices means a loss of reflecting clouds so that more sunlight is warming the bottom of the atmospheric weather layer (troposphere). A higher vertical gradient in temperature supporting convection may help to feed the new vortices with latent heat.
Now there are vortex streets with a lot of air-mixing vortices again decreasing latitudinal gradient in temperature. A new climate cycle has begun.
The Great Red Spot (GRS) is moving alone in his vortex street. That´s because at such a low latitude Coriolis force is too weak to form cyclones. Without cyclones separating the anticyclones in vortex street the Great Red Spot GRS was able to feed all the other anticyclones reaching it’s huge size.
Jens Christian Heuer