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The position of the high (H) and low (L) pressure
and their effects on the westerly flow...

NAO INDEX

This oscillation can be quantified by an index. The index is calculated with the easiest method which is the difference in the atmospheric pressure of a meteorological station near the Azores (Lisbon and Ponta Delgada or or Gibraltar, etc.) and a station near the depression of Iceland (Reykjavik and Akureyri or Stykkisholmur, etc.) according to the formula Rogers (1984). The index is calculated for each winter and it is between -5 and +5. To the right a diagram shows the geostrophic circulation associated with the NAO. Fp indicates the pressure force, Fc the Coriolis force and Vg geostrophic wind parallel to isohypses lines (increased pressure in red solid line, drop in pressure in blue dotted line). The traditional places for the calculation of the NAO index are marked by color points (red for the Azores and blue for the Iceland) .

Here are details of the calculation :

Am,y 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) :

Then after each monthly data is normalized by subtracting the mean of the corresponding month then it is divided by its standard deviation :

We do the same exercise for Dm,y and the NAO index for a month m and a year y given is defined by :

NAOm,y = A'm,y - D'm,y

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.

_1_ (NAOdec,y - 1 + NAOjan,y + NAOfev,y) =NAOhivy .. 3

This graph is created following the reconstruction of the index by UEA from
years 1500 to 1658 and is updated according to the data recorded by NOAA

SEASONAL FORECAST

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.

Projection of the anomalies in the end of summer for each year in the period 1949-2003 on the two modes of Figures a and b. The blue rounds are the winters with NAO+ (two months on three for the light blue, three out of three for dark blue), yellow and orange rounds to winters dominated by the NAO- (two out of three months for the yellow, three in three for orange). White circles represent the winters where the NAO aren't predominant. Source : Cassou and al. (2004).