How do the North Atlantic Oscillation and East Atlantic Pattern shape our weather?
In a recent study, researchers at the National Centre for Atmospheric Science found that El Niño and La Niña years led to more accurate winter weather forecasts in Europe.
Two key atmospheric patterns that shape winter weather in Europe are the North Atlantic Oscillation and the East Atlantic Pattern. These are the patterns that researchers were looking at in their study of forecasts.
We asked our atmospheric scientists to explain the North Atlantic Oscillation and East Atlantic Pattern, and what they mean for our weather.
What is the North Atlantic Oscillation and why is it important?
The North Atlantic Oscillation, also known as the NAO, is the main source of atmospheric variability in the North Atlantic region.
The NAO is the difference between the atmospheric pressure at the surface of the planet between a northern location and a southern location – scientists often compare the pressure between Iceland and the Azores.
When the pressure difference is large, the north-south pressure gradient is strong, and the North Atlantic jet stream is strong. We experience a positive NAO. This leads to typically milder, wetter and more stormy winters in the UK and Northwest Europe.
When the NAO is negative, the pressure difference is small, the north-south pressure gradient is weak, and the North Atlantic jet stream is weak. This means that the UK and Northwest Europe typically experience calmer, drier conditions, and potentially very cold winters.
Tracking changes in the NAO provides essential insights for winter weather forecasts.
What is the East Atlantic Pattern and why is it important?
The East Atlantic Pattern, often referred to as EA, is another major driver of North Atlantic atmospheric variability.
A positive EA is associated with low pressure just to the west of the UK. Together with the NAO, the EA affects the position and strength of the North Atlantic jet stream. The combination of NAO and EA determines where the biggest changes in rain and snow and cold temperatures are felt across Europe during wintertime.
How have the North Atlantic Oscillation and East Atlantic Pattern changed, and what were the impacts?
The NAO and EA vary from year to year, but in the past they have gone through periods of being more positive or more negative for around a decade or so.
When the NAO and EA are more extreme, the more severe the weather can be for Europe during winter. In positive NAO winters, we might experience more wind storms. While more persistent cold spells are linked to negative NAO winters.
In recent years, there has also been an increase in overall variability of the North Atlantic jet stream – with more extreme years occurring than in earlier decades. But it is not yet known whether these changes are due to climate change, or to decadal variability.