As previously introduced , the strength of the NAO is described by the NAO index. The NAO index is the difference in sea level pressure between two stations located near the centers of the Icelandic Low and the Azores High . Stykkisholmur (Iceland) is used as the northern station, while Ponta Delgada (Azores), Lisbon (Portugal), and Gibraltar are used as the southern stations.
The positive phase of the NAO index indicates higher-than-usual pressures in the center of subtropical highs and deeper pressures than those in the Atlantic lows . The increased temperature difference leads to more frequent and intense winter storms across the Atlantic Ocean, as well as a more northward shift. As a result, Europe experiences warm, wet winters, while Canada and Greenland experience dry, cold winters . In this case, the eastern United States experiences mild, wet winter conditions. For example, the high winter/spring indices of 1989, 1990, and 1995 were caused by a net displacement of air from the Arctic and Iceland to the subtropical belt near the Azores and the Iberian Peninsula, intensifying westerly winds over the North Atlantic. These stronger westerly winds bring warmer, wetter air over the European continent, resulting in milder maritime winters.
The negative phase of the NAO index shows a weak subtropical high-pressure center as well as a weak Icelandic low-pressure center . The reduced pressure gradient results in a smaller and weaker flow moving along a more west-east route. This draws moist air into the Mediterranean, and without a strong west-east flow, cold Siberian air can descend to very low latitudes, potentially leading to more frequent snowfall in southern Europe. However, Greenland experiences milder winter temperatures. The low NAO indices during the winters/springs of 1917, 1936, 1963, and 1969 were associated with weaker westerly winds over the North Atlantic Ocean, corresponding to colder-than-usual European winters.
Source: WAIT