This jet stream—the one that forms at the polar front—affects weather in the United States. ... This shift occurs because the entire hemisphere is cooler, so the boundary between the cold polar air and the warm southern air shifts south. As a result, the jet stream shifts toward the south in winter.
The fast moving air currents that circulate above the surface of earth is called as jet streams. These winds travel between the troposphere and stratosphere and hence are much stronger during the winters in northern & southern hemisphere due to higher temperature differences. However, the winters in United States are more extreme which causes unstable polar vortex which send cold arctic air into the jet stream and make winter colder in southern pole.
