what causes it
I found in my researches that there was no clear explanation so I am forced to formulate one of my own ... undoubtedly the correct one ;o)
I found the same problem with the earth's ocean of air as I did with the earth's system of water (as given here). Geo-type scientists take no account of the earth's tidal currents. Apparently there is no communication between them and formal physicists else this laughable situation would not occur.
There is a net westward flow of tidal air compliments of the moon
This occurs as a simple consequence of gravitational dynamics between the earth and moon (and to a smaller extent, the sun). The moon causes a tidal bulge on the side of the earth facing the moon (due to gravity) and another on the opposite side due to centrifugal force because the earth-moon system rotates on an axis between the earth's axis of rotation and its surface.
As you can see, if the earth were not rotating, the tidal bulges would stay in the line connecting the center of mass of the earth-moon system. Because the earth's rotation is dragging on the bulge, it moves in that direction giving rise to a restoring vector from the moon pulling it back to that line. The moon is now pulling on the earth's asymmetric bulge which serves to slow down the earth's rotation and simultaneously the moon has a corresponding vector, accelerating it in its orbit around the earth. The moon goes faster and rises in its orbit which in turn slows it down. (Complicated, isn't it? ;o)
Well, angular momentum is thus conserved. This is the same thing that happens with the earth's oceans albeit in a much smaller weigh. (< that's a pun not a mispell)
Note the high pressure area in front of the bulges. The bulges want to go back to the line connecting the earth-moon but are prevented from doing so by friction with the earth's surface. This is what drives the jet stream. If we get a "leak" between the high and low pressure areas, air will flow through ... a big flow through a small opening, i.e. the jet stream. This serves to relieve the pressure difference on either side of the bulges. Such leaks occur where the convection cells meet and an easterly flow due to Coriolis forces is already present there making it a weak point to breach the bulge.
They say that the jet stream drives the weather but we can't say that for sure. It may be that the weather controls the sinuous track of the jet stream or bits of both control everything. It's like watching two men fighting and one "throws" the other off a building. Is it a case of murder or did the one falling commit suicide and the other guy was just trying to prevent the fall? Your call.
Proof of wind
Suppose there is a rotating planet that is as smooth as a baby's behind. And we have an atmosphere sitting on that butt which doesn't have much friction with it ... and it has a moon orbiting it ... and ... there is no sun shining so there is no convection current to skew the atmospheric flow. What will happen?
Well, the small friction will cause the tidal bulge to be offset as with the earth. Then the moon will pull on that bulge to restore it to the line connecting the center of mass of the planet-moon system. Over time, it will get the atmosphere to go faster and faster till it is at rest relative to the moon. That is, a two-body gravitational system will tend to stop rotating relative to one another and come to rest with both faces "locked". Our moon's face is already locked and one day the earth's rotation will also stop and they will show the same face to each other permanently.
In the above case, the atmosphere is easier to "lock up" because it can be moved easily by the moon. So it will lock up first followed by the rotating butt surface much later. Well, not exactly. Because there is some small friction with the butt surface, the atmosphere will continue to rotate relative to the moon but as it comes nearer to being locked to the moon there will be a corresponding very high surface wind which would serve to keep the atmosphere from being locked to the moon. Thus, if the atmosphere tries to stop relative to the moon and the surface of the earth doesn't ... we get a very high wind relative to that butt surface.
Because the actual earth is very rough, these super high surface winds cannot occur ... butt ... we do get some of that tidal air current just like we get in the ocean. This tidal air current is not acknowledged by the meteorological community just as (global) tidal water currents are not acknowledged by oceanographers.
I find this fact to be absolutely ...