The reason Jetstreams form is because of the tropopause. The tropopause is at its lowest at the poles and at its highest at the equator. If i remember its around 25,000feet at the poles 60,000+ at the equator...well anyway between the poles and the equator as the tropopause altitude changes, there are areas where the tropopause may dip steeply, for example at 40 deg. N it may be at 39,000 feet while at 39 deg. N it may be at 31,000 feet, then at 38 deg. N it will be at 41,000.....try to picture this one in your head :-) Now those altitudes...say they are like this running west to east....they act like a river( the middle of the river being a low altitude while its sides being at a very high altitude........this funnels the wind and the coriolis does the rest.
As to why the jetstream changes season to season, im not sure but i always remember SSW....Strong South Winter, thats how the jetstream is in the winter.
Alto Adige