What is North Atlantic Oscillation (NAO)?

For many of the past 15 years, a recurring pressure pattern has resulted in milder than normal winter temperatures in in western Europe. After El Nino, this pattern is one of the most dominant modes of global climate variability - referred to as the North Atlantic Oscillation (NAO pronouned "en-ay-oh"). In a diary which he kept in Greenland during the years 1770-78, the missionary Hans Egede Saabye made the following observation: "In Greenland, all winters are severe, yet they are not alike. The Danes have noticed that when the winter in Denmark was severe, as we perceive it, the winter in Greenland in its manner was mild, and conversely." This temperature see-saw is now known to be a manifestation of the NAO. The high index winter/springs of 1989, 1990, and 1995, were caused by a net displacement of air from over the Arctic and Icelandic regions towards the subtropic belt near the Azores and the Iberian peninsula, and had strengthened westerlies over the North Atlantic ocean. Stronger westerlies bring more warm moist air over the European continent and gives rise to milder maritime winters. The low index winter/springs of 1917, 1936, 1963, and 1969 had weaker mean westerlies over the North Atlantic ocean with corresponding colder than normal European winters. The strengthened or weakened westerlies over the North Atlantic are also known to have major impacts on oceanic ecosystems and ultimately North Atlantic fish stocks.

After more than 100 years of scientific research, the fundamental mechanisms behind the variability of the North Atlantic Oscillation still remain intriguing mysteries. However, some things are however becoming clearer. For example, it appears that the link between the notorious bad boy of the tropical Pacific, El Nino, and his nordic cousin, the NAO, is relatively weak. It is also becoming clearer that some of the current day climate models are showing some encouraging ability to make probabilistic forecasts of the NAO a season ahead. What is not clear is why the NAO has become more positive over the last 30 years and there is some speculation that this may be a sign of human induced global warming. On the other hand, it could merely be natural climate variability. This question is currently being adressed by by analysing long simulations of the NAO using state-of-the-art climate models running on the world's fastest supercomputers. Because of its climatic importance, the NAO is currently generating intense scientific interest and this will undoubtably lead to further advances in our understanding of this intriguing phenomenon and hopefully our ability to forecast it.

Further reading

• "What is NAO ?" keynote opening speech by D.B. Stephenson at AGU NAO conference, Ourense, 28 Nov - 1 Dec 2000.
• "NAO weather affects Europe" Article in Metropole Paris Magazine (30 March 1998)
• White paper on Atlantic Climate Variability by J. Marshall et al. (April 1997)
• Other links (updated regularly)