Why cold air plunges south
Greenhouse gas emissions from human activities have warmed the globe by about 1.8°F (1°C) over the past 50 years. However, the Arctic has
warmed more than twice as much.
Amplified Arctic warming is due mainly to dramatic melting of ice and snow in recent decades, which exposes darker ocean and land surfaces that absorb a lot more of the sun's heat.
Because of rapid Arctic warming, the north/south temperature difference has diminished. This reduces pressure differences between the Arctic and mid-latitudes, weakening jet stream winds. And just as slow-moving rivers typically take a winding route, a slower-flowing jet stream tends to meander.
Large north/south undulations in the jet stream generate wave energy in the atmosphere. If they are wavy and persistent enough, the energy can travel upward and disrupt the stratospheric polar vortex.
Sometimes this upper vortex becomes so distorted that it splits into two or more swirling eddies.
These "daughter" vortices tend to wander southward, bringing their very cold air with them and leaving behind a warmer-than-normal Arctic. One of these eddies will sit over North America this week, delivering bone-chilling temperatures to much of the nation.