The North Atlantic Oscillation (NAO) has seized the attention of the news media in Europe and North America during the last two years. The general thrust of most of the stories is that this "new phenomenon" is responsible for much of the unusual weather which has affected countries on both sides of the ocean recently, including the floods which hit Britain and adjacent parts of Europe during the autumn and winter of 2000-2001. Some even try to suggest that the present warming trend in the climate of western Europe can be explained by changes in the NAO. So that's all right then.
Such stories are based on a fundamental misunderstanding of what the North Atlantic Oscillation is. First of all, it is not new; nor is it a phenomenon in the sense that it has any specific cause or effect. It is not even an "oscillation" because it has no pattern, regularity or predictability - "fluctuation" would be a more accurate description.
Let us first of all determine what the NAO really is. Climatologists use the term to measure the strength of the atmospheric circulation in the northern hemisphere in the Atlantic/Europe sector. A variety of indices are used, the most common of which is the difference in barometric pressure between Iceland and the Azores. The higher this index is, the stronger the prevailing westerly winds are, while a strongly negative index means that the normal westerlies are replaced by winds from other quarters; these indices can be calculated on a daily, monthly, seasonal, or annual basis. It may be called the North Atlantic Oscillation now, but until the early-1990s it was called - rather more accurately - the mid-latitude zonal index in longitudes 40W-20E, and has been a tool of climate researchers for almost a century.
In other words the NAO is nothing more than a simple description of the character of the atmospheric circulation in one region over a specific time period. It is no different from other indices or summaries that we come across is our day-to-day lives, like the FTSE share index, or even the Radio Times. The NAO is not to blame for unusual weather any more than the FTSE is responsible for the UK's economy, or the Radio Times can be held to account for the content of our television programmes. Indeed the Radio Times is rather more useful: it can predict quite accurately what we are likely to see on the TV whereas the NAO predicts nothing.
So why has there been so much fuss about the North Atlantic Oscillation in recent years? There are two main reasons. The holy grail of climatological research is successful long-range forecasting and the ability to pin down and explain long-term fluctuations in the atmospheric circulation is regarded as the key. Sadly, these days, publicity helps get research funding. Secondly, those who deny the anthropogenic contribution to global climate change will grab any other possible explanation, however inappropriate or implausible. Their logic presumably does not require them to discover where the fluctuations in the NAO come from.
The NAO in the web:
University of Washington
University of Reading