North Atlantic Oscillation (NAO)
The North Atlantic Oscillation (NAO) is a characteristic and powerful atmospheric circulation phenomenon in the Northern Hemisphere.
The North Atlantic Oscillation consists of opposing variations in atmospheric pressure near Iceland and the Azores. Accordingly, it reflects variations in the intensity of the westerlies over the Atlantic toward Europe, and therefore variations in their embedded low-pressure systems and corresponding frontal systems. The NAO is a characteristic and powerful atmospheric circulation phenomenon in the Northern Hemisphere. Because the NAO is closely coupled with the winds over the North Atlantic, it has a major influence on interactions between the ocean and atmosphere. As such, it directly shapes the weather and climate in Europe.
The intensity of the NAO is classified using an index. The NAO index is based on seasonal average atmospheric pressure differences between e.g. stations at Ponta Delgada (Azores) and Stykkisholmur (Iceland). The NAO index is subject to substantial variation. A high (positive) NAO index is connected to powerful westerlies in the middle latitudes and intensified trade winds in the tropics. A low (negative) NAO index produces weakened westerlies and moderate trade winds.
Rising wind speeds at the ocean’s surface intensify exchanges of heat, water vapour and momentum between the ocean and atmosphere. These exchanges can e.g. cause the ocean to cool, or the atmosphere to absorb more moisture. Beyond these direct effects, which concern the water temperature, changes in the wind field can lead to changes in ocean currents. The NAO is most pronounced in winter; in the other seasons, it is very weak.