THE NORTH ATLANTIC Oscillation (NAO) is quasi-periodic shifts in the relative strength of the quasi-permanent centers of high atmospheric pressure over the Azores and low atmospheric pressure between Greenland and Iceland (the Azores High and Iceland Low). It is a major large-scale mode of the atmospheric circulation over the extratropical ocean in the Northern Hemisphere. The NAO intensity is at a maximum in winter. It governs the large-scale variability of hydrometeorological fields over the North Atlantic, North America, and northern Europe, as became clear after a pioneer paper published by Sir Walker and co-author in 1932. Recently, the NAO has been set in a broader spatial context of hemispheric climatic variations, both at the surface and aloft as follows, for instance, from generalized publications of ...