Why does the Great Red Spot spin counter-clockwise?

[Irfan Nafi]

Jupiter's spins the same way the Earth does, so the Coriolis effect should make the Great Red Spot spin clockwise, but it's an anti-cyclone. Why is that?

 

[Orodruin]

The clockwise spin (on the southern hemisphere) occurs around low pressure regions into which gravity will tend to pull gas.

In a high pressure region, gas will try to flow away, resulting in the opposite rotation. Anticyclones occur on Earth too.

The rotation is not only due to the Coriolis force, it is due to the interplay between Coriolis and gravity.

 

[Irfan Nafi]

Why does gas try to flow away in a high pressure region?

 

[Orodruin]

Because the pressure gradient gives a resulting force on any volume of gas. It is just the same reason air generally would flow into fill a vacuum if allowed.

 

[Orodruin]

This image summarises it quite neatly:

 

[rootone]

Why does gas try to flow away in a high pressure region?

Same as if you blow up a balloon.
The air inside the inflated balloon is at a higher pressure than the outside atmosphere, so if you then open the air inlet up, the air inside comes back out again.

 

[Irfan Nafi]

So because of the high pressure the surrounding is trying to find equilibrium?

 

[rootone]

So because of the high pressure the surrounding is trying to find equilibrium?

Yes, the air masses are attempting to reach equilibrium.