Any old wind that blows …
Originally posted by Wikipedia:
The North Atlantic oscillation (NAO) is a climatic phenomenon in the North Atlantic Ocean of fluctuations in the difference of atmospheric pressure at sea level between the Icelandic low and the Azores high. Through east-west oscillation motions of the Icelandic low and the Azores high, it controls the strength and direction of westerly winds and storm tracks across the North Atlantic. It is highly correlated with the Arctic oscillation, as it is a part of it.
The NAO was discovered in the 1920s by Sir Gilbert Walker. Unlike the El Niño-Southern Oscillation phenomenon in the Pacific Ocean, the NAO is a largely atmospheric mode. It is one of the most important manifestations of climate fluctuations in the North Atlantic and surrounding humid climates.
The North Atlantic Oscillation is closely related to the Arctic oscillation (AO) but should not be confused with the Atlantic Multidecadal Oscillation (AMO).
The NAO has multiple possible definitions. The easiest to understand are those based on station differences, such as:
Lisbon, Portugal and Stykkisholmur/Reykjavik, Iceland
Ponta Delgada, Azores and Stykkisholmur/Reykjavik, Iceland
Azores (1865-2002), Gibraltar (1821-2007), and Reykjavik, Iceland
These definitions all have in common the same northern point (because this is the only station in the region with a long record), in Iceland; and various southern points. All are attempting to capture the same pattern of variation, by choosing stations in the "eye" of the patterns shown in the graphic.
A more complex definition, only possible with more complete modern records generated by numerical weather prediction, is based on the principal empirical orthogonal function (EOF) of surface pressure. This definition has a high degree of correlation with the station-based definition. This then leads onto a debate as to whether the NAO is distinct from the North American Mesoscale Model (NAM) or AO, and if not, which of the two is to be considered the most physically-based expression of atmospheric structure (as opposed to the one that most clearly falls out of mathematical expression).
Which is all very well if you can understand it.
The trouble with old weather data is that once it has been distributed to the Weather forecast manufacturers for tuning into presentations, the stuff becomes the property of climatology.
And they love averages.
The senior meteorologist having done his best to rid the computer model of averages in the runs he chooses for his prophecies, this seems a dichotomy.
That's because it is:
Originally posted by Wikipedia:
Westerly winds blowing across the Atlantic bring moist air into Europe. In years when westerlies are strong, summers are cool, winters are mild and rain is frequent. If westerlies are suppressed, the temperature is more extreme in summer and winter leading to heatwaves, deep freezes and reduced rainfall.
A permanent low-pressure system over Iceland (the Icelandic Low) and a permanent high-pressure system over the Azores (the Azores High) control the direction and strength of westerly winds into Europe. The relative strengths and positions of these systems vary from year to year and this variation is known as the NAO. A large difference in the pressure at the two stations (a high index year, denoted NAO+) leads to increased westerlies and, consequently, cool summers and mild and wet winters in Central Europe and its Atlantic facade. In contrast, if the index is low (NAO-), westerlies are suppressed, these areas suffer cold winters and storms track southerly toward the Mediterranean Sea. This brings increased storm activity and rainfall to southern Europe and North Africa.
So what do we do with it?
By controlling the position of the Azores high, the NAO also influences the direction of general storm paths for major North Atlantic tropical cyclones: a position of the Azores high farther to the south tends to force storms into the Gulf of Mexico, whereas a northern position allows them to track up the North American Atlantic Coast.
It does more than that:
When the so called anomaly is in the negative phase…. (What a set of unwieldy terms this syndrome encourages.) ….when the so called anomaly is in the negative phase…. as far as I know there is an High pressure area over Greenland.
Since this affects temperatures there -which even with a Low is cold, the area being snowbound more or less all the year through; sea temperatures seem to be greater than in more "normal conditions".
But what IS normal?