What causes the jet stream to move farther south?

The dynamics of jet streams are actually quite complicated, so this is a very simplified version of what creates jets.  The basic idea that drives jet formation is this:  a strong horizontal temperature contrast, like the one between the North Pole and the equator, causes a dramatic increase in horizontal wind speed with height.  Therefore, a jet stream forms directly over the center of the strongest area of horizontal temperature difference, or the front.  As a general rule, a strong front has a jet stream directly above it that is parallel to it.  Figure B shows that jet streams are positioned just below the tropopause (the red lines) and above the fronts, in this case, the boundaries between two circulation cells carrying air of different temperatures.

i'm pretty sure its b i might be wrong though!