When stable, the polar vortex remains closer to the North Pole, constrained by a strong polar jet stream which keeps the cold air from moving into the continents below. As Paul Ullrich
, an associate professor of regional climate modeling at UC Davis explains, the polar jet stream is sustained by two things: a temperature difference between the warmer mid-latitudes and the polar region, and the Earth’s rotation. When the temperature difference across the jet stream is large, it tends to be strong and keeps the polar vortex stable. But when the difference is small, the jet stream tends to be weaker and more susceptible to twisting and curving. When it becomes unstable, the polar vortex can break off, at times expanding as far south as Florida.