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I have been playing around with this nice visualisation and noticed that in the mid-stratosphere (P=10hPa), the airflow around the equator and south of it is in the opposite direction to the earth's rotation [airflow relative to a fixed point on the surface], as one would expect.

In the northern hemisphere though (centered around Svalbard) the air flow is in the same direction as the rotation of the ground below and is at high velocity. This seems somewhat counter-intuitive - what is the reason for it?

enter image description here

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Hrm, no idea how I didn't see this question earlier! What is the timescale for the data in the image? I assume it is time-averaged, but how long is the average? – tpg2114 Feb 7 '15 at 16:46
It's a simulation, I'm not sure what the timescale is. Feel free to play with it on the link in the question :) Options are in the bottom left (click EARTH) – Mark K Cowan Feb 7 '15 at 17:07
If air moves toward the east at all, it is moving faster than the earth, if the reference is the earth's surface. But I agree, it is surprising. There's a lot about that simulation I don't understand. It seems to disagree with what I thought was basic circulation of air on the earth. – Mike Dunlavey Feb 7 '15 at 17:39
@MikeDunlavey There's a bunch of things happening there. But as I posted in my answer, moving east is expected just from a kinematics perspective. But what you see there is also acceleration due to the contraction caused by the blockage of that vortex centered in the view. And so there can be much more complicated structures and patterns than "wind goes X direction at Y lattitude" when you are looking at small time-averages like this. When you look at climatological scales (decade+), you'll see patterns that look more like the image in my answer. – tpg2114 Feb 7 '15 at 17:53
@tpg2114: I appreciate what you're saying. I looked at the simulation, and the entire southern hemisphere has a broad easterly flow, and the equator is all westerly. This makes me doubt the simulation. – Mike Dunlavey Feb 7 '15 at 21:07
up vote 4 down vote accepted

There's all kinds of awesome and complicated stuff going on in that image... so let's look at a much simpler one to see what you mean (source):

enter image description here

At the equator, the predominant wind direction is easterly, or opposite the direction of Earth rotation. Away from the equator, north of 30 degrees, the winds are primarily westerly, with the direction of rotation, which means the atmosphere is moving faster there than the Earth.

If we stopped the Earth from rotating, then the air would flow from the equator to the poles just based on temperature gradients. However, because the Earth is rotation, there is a Coriolis force that acts on the air as it moves. This causes northward motion to turn towards the east (generating westerly winds).

This is what causes the wind in the northern hemisphere to accelerate to the east. If you are closer to the equator, the wind will be coming from a more south-westerly direction because the Coriolis effect is weaker. If you are closer to the north pole, the wind will be coming from almost due west.

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My question had been open for a year with no responses and you just answered it completely with physics so basic that I'm embarrassed I hadn't figured it out myself in that time! Thanks! – Mark K Cowan Feb 7 '15 at 23:21
@MarkKCowan A year! No wonder I missed it! When I saw the date, I completely read that as like... 20 days ago. I really need to pay closer attention. I hope you didn't need an answer way sooner :) – tpg2114 Feb 7 '15 at 23:33

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