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I recently read that the Great Red Spot arises from the shear instability of zonal flow from zonal jets travelling in opposite directions.

Is there any reason why such zonal jets travel in opposite directions anyway? I thought it would be more intuitive if such zonal jets, over their billions of years of existence, would be overcome by the greatest group of zonal jets that collectively travel in a single/similar direction, and eventually all join together to form one in one single direction.

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""such zonal jets, over their billions of years of existence"" who watched those zonal jets for that billions of years? –  Georg Oct 8 '11 at 17:24
    
I don't know, haha. I mean, since the atmosphere of Jupiter (as we know it today) was formed, shouldn't the "rogue" jets be "conquered" by those of greater number and magnitude? –  Soyuz Oct 9 '11 at 10:37
    
All we know is pictures from the uppermost atmosphere layers, and that only for sme hundred years. Theoreticising about "weather" on Jupiter billions of years ago is pure speculation. –  Georg Oct 9 '11 at 11:02
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@George: The OP is asking how you can have a complex steady state, and the age of Jupiter makes it certain that Jupiter has been the same for billions of years. It does not require direct observation, much as concluding that the dark side of the moon wasn't a tropical jungle 3000 years ago doesn't require direct observation. –  Ron Maimon Oct 9 '11 at 17:25
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An old question but it seems to still be unanswered and I can add my two cents. I think that the OP asks why the bands, or zonal jets, do not all travel in one direction now, after billions of years where a 'dominant' zone traveling in one direction would have time to 'overpower' other zonal winds.

Short answer: There are mechanisms which act continuously on Jupiter's atmosphere, forcing the winds in different bands to rotate in different directions.

Long answer: The reason that they don't form one homologous band is that there are driving forces causing the winds in each band to rotate in opposite directions. The mechanisms which form Jupiter's bands are not certain (as the wiki article explains). One possible cause is something similar to Hadley cells on Earth. On Earth we have similar bands of wind to Jupiter we just can't see them as clearly. On the Earth, these bands are formed due to uneven solar heating over the Earth's surface, which causes the air to rise and fall, and the rotation of the Earth which sets the direction of the wind in each band due to the Coriolis effect. The same mechanisms may form the bands on Jupiter although internal heating could also play a part.

There are other theories - such as the effect of tidal forces imparted by Jupiter's many moons on its atmosphere - but whatever the cause the fact is that these mechanisms act continuously on Jupiter's atmosphere forcing the bands to rotate in different directions .

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