It is now apparent that there is a statistically critical link between the strength of the winter polar vortex, Siberian snow cover, and winter temperatures across a great part of North America as well as Europe and northern Asia.
By using a snow cover as a forecaster of the power of the winter polar vortex, this relationship is used to precisely predict winter temperatures. Other seasonal forecasts overlook the snow-polar vortex relationship and tend to focus just on ENSO to forecast large scale temperature patterns.
How the Polar Vortex Affects Regional Temperatures
The polar vortex is a quick-flowing stream of air that circles the North Pole in the winter months in the upper atmosphere, referred to as the stratosphere. Typically, when the polar vortex is strong, temperatures are mild in the mid-latitudes over the Eastern US and Northern Eurasia. When the vortex is weak, temperatures tend to be cold in the Eastern US as well as northern Asia and Europe.
Strong Polar Vortex
Strong is the usual state of the polar vortex. When the polar vortex is strong, this produces strong low pressure in the Arctic region. Since the pressure differs between the Arctic and mid-latitudes, air flows into low pressure and this confines the cold air to high latitudes close to the Arctic.
Therefore, it is frequently mild across Europe, the Eastern US, and East Asia during wintertime when the polar vortex is strong. During strong polar vortex, the airflow is rapid and in a direction from west to east.
Weak Polar Vortex
When the polar vortex is weak, the airflow is fainter and travels north and south instead of west to east. This permits a redistribution of air masses where cold air from the Arctic goes into the mid-latitudes and warm air from the subtropics is transported into the Arctic. This combining of air masses also favors more snow and storms in the mid-latitudes.