Wobbly

The extreme cold in North America this winter (as opposed to the warm weather in Europe and the deadly heat in Australia) has been caused by a "wobble" in the polar vortex.  Why the wobble?

Start with the fact that historically the atmosphere is not as tall over the north pole as it is over the equator.  

Simple:  cold things contract, warm things expand.   

As the atmosphere warms up due to the combustion of fossil fuels, it expands.  Among other things, it is reaching up to those cell phone and GPS satellites and the friction will slow them down until they fall out of the sky.

The atmosphere is expanding everywhere, but mostly over the poles.  White snow reflects sun warmth.  When the snow melts the dark rock absorb sun warmth.  So the poles are heating up faster.  That means that the difference in atmospheric height between equator and pole is decreasing.

The lessened difference in atmospheric height is changing the northern jet stream - slowing it down and moving it north.

Two effects:  the wobble in the jet stream that causes polar air to move south in some places while warmer air moves north in others, and a slowing down of weather patterns, so that the hot / cold / wet / dry weather tends to stick around longer and cause extreme flooding or drought or heat or cold.