The NAO (North Atlantic Oscillation) is a variation of the natural climate which has an important impact on the climate of the Western Europe, in the surrounding of Northern Africa and Eastern North America. The NAO has much more important effects in winter than in summer. It was around 1920 that the two meteorologists, the Austrian Friedrich and English Gilbert Walker discovered the North Atlantic Oscillation.
This oscillation also has some influence, because it determines the position and trajectory of depressions in the northern hemisphere. The variation of this phenomenon depends on the atmospheric pressure.
and Dm,y monthly
means of pressure which m is the month (m
= 1 to 12) and y the year (y
= y1 to y2).
For each month are calculated the average and standard deviation
for A (the Anticyclone of the Azores) and
D (depression of Iceland) separately and on
a reference period (we chose logically periods of 30 years as 1961-1990
or 1971-2000) :
We do the same exercise for Dm,y and the NAO index for a month m and a year y given is defined by :
Then the winter NAO index NAOhiv which represents the average of the months of December, January and February, we obtain a time series with a value annually.
The NAO is the climate phenomenon that has the most effect on winters. About 15% of the monthly variance of the NAO in early winter would seem predictable to several months in advance. According SST ( Sea Surface Temperature) anomaly in North Atlantic in July, August and September it is possible to predict if the NAO index is going to be positive or negative and therefore to have an idea if the winter is going to be correct or cold or mild.
The resemblance of the anomalies at the end of the summer is determined by their spatial projections on both structures figures a and b below on the right, projections that is transferred in the reduced space represented by the figure below left. When the round falls into the white space, a positive phase of the NAO is expected and reverse if the round falls into the gray area. The more the round deviates from the line of separation between the white and grey, more the NAO of the following winter will be intense. 22 forecast of the NAO on 26 winters are planned with successfully. Of course, bad forecasts exist like those winters, 1975 and 1983, where a positive NAO took place instead of the negative forecast, and years 1985 and 1986, where a positive NAO is false.