What is the North Atlantic Oscillation?
(This note prepared with the help of: Dr. Rob Wilby, Department of Geography, University of Derby.) A. In very crude terms, it is possible to visualise the mean sea level pressure patterns affecting the north-east Atlantic as varying (or ‘oscillating’) between two extremes: At one extreme is a minimally perturbed westerly type, with disturbances rattling swiftly across the Atlantic, hurried along by very strong winds. At the other extreme lies a weak, sometimes ill-defined pressure pattern, but with a strong tendency for stagnation of weather types over and downwind of the north-east Atlantic. In climatological studies, some attempt must be made to quantify such variations, frequency, periodicities etc., to judge, for example, whether patterns are changing, and to find correlations to particular rainfall or temperature regimes, not only over maritime NW Europe, but also further afield. One method is to use a measure of the 500 mbar strength between defined latitudes: By taking the diff
