We are now discovering that there is a statistically significant link between Siberian snow cover, the strength of the winter polar vortex and winter temperatures across much of North America and northern Europe and Asia.
By using Siberian snow cover during the previous October as a predictor of the strength of the winter polar vortex, AER seasonal forecasts have exploited this relationship to more accurately predict winter temperatures across North America and Northern Eurasia. Other seasonal forecasts neglect the snow-polar vortex relationship and tend to focus solely on ENSO to predict large scale temperature patterns.
How the Polar Vortex Affects Regional Temperatures
The polar vortex is a fast flowing stream of air that circles the North Pole during the winter months in the upper atmosphere, known as the stratosphere. Often when the polar vortex is strong, temperatures are mild in the mid-latitudes across the Eastern US and Northern Eurasia; and when the vortex is weak, temperatures tend to be cold across the Eastern US and northern Europe and Asia.
(a) Weak or “Perturbed” Polar Vortex: Jan-Feb 2010
(b) Strong Polar Vortex Example: Dec-Jan 1988/89
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.
During strong polar vortex, the air flow is fast and in a direction from west to east.
Low pressure in the Arctic region is referred to as the positive phase of the Arctic Oscillation (AO), which is also know as the North Atlantic Oscillation (NAO).
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.
During a weak polar vortex, high pressure occurs in the Arctic region and is referred to as the negative phase of the Arctic Oscillation (AO). 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.