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Some notes on the North Atlantic Oscillation Index

At the end of 1997 El Niņo was in full swing. However, its influence on the weather was mostly felt in the tropics. (See the note on El Niņo.) For Europe the weather is mainly "made" on the Atlantic. What happens over there to a large extent determines the weather and the climate of Europe.

And here another oscillition, the so called North Atlantic Oscillation, comes into play. Two extremes in winter can be distinguished:

  1. A strong Iceland low combined with a well developed Azores High.
    The so called North Atlantic Oscillation Index, the difference in air pressure between Portugal and Iceland, is high.
  2. A much weaker Azores High and higher pressures on the North Atlantic.
    Now the North Atlantic Oscillation Index is low.
In case 1. Strong South-Westerly winds bring warm air deep into Europe. It results in mild winters. Sometimes very deep and active mid latitude storms can develop South of Iceland. The winters of the late 80-ties and Early 90-ties were very good examples of this weather pattern. On 10 Januari 1993 an all time low barometric pressure of 912 mbar was recorded South East of Iceland. In the winter of 1990 England was hit by a couple of severe winter storms. The one on 25 Januari 1990 being a notable example.

In case 2. High pressure area's are present over the North Atlantic or Scandinavia. In case of high's over Scandinavia cold waves can engulve Europe. This doesn't mean that no severe weather can arrive from the sea. Northern Scotland in these cases can be hit by 'polar lows', very small hurricane like storm systems which form when very cold air moves over open sea. In Februari 1969 in such a polar low a gust of 118 Kts (218 km/h) was recorded on the airport of Kirkwall, Orkney Islands, during a violent snow shower. Sailors call this type of weather "The Barber". If you venture on deck you get "a nice shave" ...

North Atlantic Oscillation Index
North Atlantic Oscillation Index anomalies.

In The figure above the North Atlantic Oscillation Index anomalies for the winters of 1865 - 2007 are shown. Very striking are the low values in the 60-ties and the very high values in the late 80-ties and 90-ties up to 1995. In the years 1961 - 1965 England experienced 5 years with a "skating Christmas". The 1963 winter was an historic winter. In the late 80-ties and 90-ties the winters, in general, were mild. The winters of 1989 and 1990 were, together with the recent 2007 and 2014 one, "subtropical".

This makes it difficult to prove the point that these winters are a harbinger of the present warming of the climate on Earth. The very warm winters were caused by a "high North Atlantic Oscillation Index" weather pattern. The cold one's (e.g. 1963, and more recently 1979, 1985, 1986, 1987 and 1996) by a low "NAOI" weather pattern.

The present warming of the climate on Earth can not easily be proven by means of local observations and conclusions. Weather tends to slap about from one extreme to the other opposite extreme over the World. While we had balmy weather in Europe during the 1998 New Year, people were dying of hypothermia in Bangla Desh.
In the winter of 1996 NW Europe had a cold winter. The West coast of Greenland had one of the mildest winters of the century.

The most solid proof of the present warming is the overall average picture. The overall World temperature of 1998 was 0.58 °C above the 1961 - 1990 average, which is an all time high on record.

Recently a report from the KNMI, the Netherlands, ("De toestand van het klimaat in Nederland 1999"), doesn't rule out the possibility of a shift to "high NAOI" weather patterns in case of global warming. This will become a subject for further reasearch in the coming years


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