An Introduction to the Stratosphere

About every third post on the weather page I write something about a stratospheric circulation pattern but have yet to go into full detail about what it is that I am taking about. I have mentioned the ramifications but not the underlying processes. Well, we're about to dive into "stratosphere 101".

The Polar Vortex
The polar vortex is a persistent large-scale cyclonic circulation pattern in the middle and upper troposphere and the stratosphere, centered around the polar regions. It strengthens when the temperature gradient when the stratosphere over the mid-latitude is warm and the stratosphere over the polar regions is cold. The explanation from the Atmospheric and Environmental Research Center (AER) is the best one I can find.

"Strong Polar Vortex
 
Strong is the more common state of the polar vortex. When the polar vortex is strong, this creates strong low pressure in the Arctic region.  Because of the pressure difference between the Arctic and mid-latitudes, air flows into low pressure and this confines the cold air to high latitudes closer to the Arctic. Therefore it is often mild across the Eastern US, Europe and East Asia during winters when the polar vortex is strong.

Weak Polar Vortex

When the polar vortex is weak or “perturbed”, the flow of air is weaker and meanders north and south (rather than west to east). This allows a redistribution of air masses where cold air from the Arctic spills into the mid-latitudes and warm air from the subtropics is carried into the Arctic. This mixing of air masses also favors more storms and snow in the mid-latitudes. Air flows away from the high pressure Arctic. The north to south direction of the polar vortex carries cold Arctic air into the mid-latitudes of Eastern US, Europe and East Asia. Therefore it is cold across the Eastern US, Europe and East Asia during winters when the polar vortex is weak."