NAO index. Positive and negative phases

How to Live Aligned with we already introduced previously, the strength of the NAO is described by the NAO index. The NAO index is difference between pressure at sea level between two stations located in the vicinity of the centers of the Lowlands of Iceland and the Highlands of the Azores. Stykkisholmur (Iceland) is used as the north station, while Ponta Delgada (Azores), Lisbon (Portugal) and Gibraltar are used as the south station.

La positive phase of the NAO index  presents some pressures higher than usual in central subtropical highs and deeper than low pressures in the Atlantic. The increase in the temperature difference leads to more frequent and intense winter storms that cross the Atlantic Ocean, as well as a movement further north. As a result of this, there are hot and humid winters in Europe and dry and cold winters in Canada and Greenland. In this case, the eastern US experiences mild and humid winter conditions. For example, the high winter / spring rates 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 and westerly winds intensified over the North Atlantic Ocean. Other stronger winds from the west cause a more humid and warmer air over the European continent and lead to milder maritime winters.

NAO POSITIVE

La negative phase of the NAO index show a weak subtropical high pressure center as well as weak Icelandic low pressure center. The reduced pressure gradient results in a smaller and weakened flow traversing along a more west-east path. They carry humid air to the Mediterranean and in the absence of an intense flow from west to east, cold Siberian air can descend to very low latitudes, therefore more frequent snowfalls may occur in southern Europe.  However, Greenland has warmer winter temperatures. The low winter / spring rates of 1917, 1936, 1963 and 1969 had softer westerly winds over the North Atlantic Ocean corresponding to colder than usual European winters.

NAO NEGATIVE

Source: WAIT


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  1.   Sergio said

    thanks, very interesting!