Jetstream Causes MET
A complicated question. Firstly, to get air moving initially, you have to have a pressure difference. For wind, which moves sideways, this is a horizontal pressure difference (also known as pressure gradient).
However, for upper winds the cause of this pressure difference is not usually the usual fluctuations of pressure at the surface. Rather, it is a difference in temperature. At the surface, this temperature difference (normally at a front) has no effect on the pressure, but the further up you go, the temperature difference means the air either side of the front expands at a different rate, causing huge differences in pressure by the time you reach the area of the tropopause. It is therefore this difference in temperature which is the fundamental cause of upper winds.