| Summary: | An enhanced greenhouse effect could possibly result in frequency changes of extreme events. Observations of variations and changes in the frequency of extreme air pressure is an example of such events. Extreme air pressures are here defined as pressure lower than or equal to 990 hPa or higher than or equal to 1036 hPa. A gridded dataset from UKMO (United Kingdom Meteorological Office) with a spatial resolution of 5° latitude, 10° longitude, have been used. Analyse of the variation in relative frequency of extreme pressures during the period 1881-1995 has been performed in two study areas. One area covers the North Atlantic, Europe and the western part of Russia (60°W-60°E och 45°N-75°N) and another covers Scandinavia (10°E-20°E and 55°N-70°N). A third area (l0°E-20°E and 45°N-75°N) was analysed for each 5° latitude with respect to the latitudinal distribution. The most obvious change is detected in winter during the subperiod 1961-199S of extreme low pressures. A marked rise in the relative frequency is observed over the entire investigation area. A corresponding change is only detected during winter over Scandinavia. A marked rise is observed in the frequency of pressure lower than or equal to 970 hPa over Scandinavia, particularly after 1965. The extreme high pressures does not show any marked changes in either of the areas. The latitudinal distribution of extreme low pressures shows a marked maximum around 65°N latitude during all seasons. The maximum of the latitudinal distribution of extreme high pressures moves during the year and reaches the most northern position, 65°N. during April, May and June. During autumn the maximum moves south and reaches its most southern position. 50°N, during December and January. There is an obvious qualitative agreement between the observed rise in hemispheric air temperature during the last twenty years and the marked rise in relative frequency of extreme low pressures. This fact possible indicate a connection between the enhanced greenhouse effect and a change in frequency ...
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