North Atlantic Oscillation

The North Atlantic Oscillation is a fluctuation in the difference in sea-level atmospheric pressure between the Icelandic Low and the Azores High, a difference that governs the strength and path of the westerly winds and storm tracks across the North Atlantic. In its positive phase, strong westerlies carry moist Atlantic air into Europe, bringing cool summers, mild winters, and frequent rain to central Europe; in its negative phase, the westerlies weaken, allowing temperature extremes, with heat waves and deep freezes farther north and a shift of storms and rainfall south toward the Mediterranean. The oscillation accounts for much of the variability in North Atlantic weather, especially from November to April, affecting wind speeds, the distribution of temperature, and storm patterns across a wide swath of the region.