North Atlantic Oscillation
Consists of opposing variations of barometric pressure near Iceland and near the Azores.
The pressure difference between Iceland and the Azores fluctuates on time scales of days to decades, and can be reversed at times.
On average, a westerly current, between the Icelandic low pressure area and the Azores high pressure area, carries cyclones with their associated frontal systems towards Europe.
This is the dominant mode of winter climate variability in the North Atlantic region, ranging from central North America to Europe.