Variability of the surface atmospheric circulation over Europe, 1774-1995

Long series of monthly surface pressure observations are analysed from the 1770s to 1995, using empirical orthogonal function (EOF) analysis and circulation indices. Analyses of the associated principal components show that the variability of surface atmospheric circulation over Europe (35°-70°N and 30°W-40°E) is well captured, using only 20 stations. The first three EOF patterns of the study are the central tendency of European pressure (EOF 1), a zonal flow pattern over Europe (EOF 2), and a blocking/cyclonic pattern in the eastern North Atlantic (EOF 3). EOF analyses on sub-periods of the station records suggest episodes of more intense meridional circulation (EOF 3) from 1822 to 1870, and stronger zonal westerlies from 1947 to 1995. Simple zonal circulation indices were also constructed for a North Atlantic Oscillation (NAO) index using Gibraltar and Reykjavik (1821-1995), a western European zonal index using Madrid, Barcelona, Trondheim and Lund (1786-1995), and a Paris-London index (1774-1995). Correlation analysis suggests that the NAO may be a better indicator of eastern North Atlantic blocking or cyclonicity (EOF 3) than of European zonal flow, especially outside the winter months. Both the western European and Paris-London zonal indices were highly correlated with the time series principal components (PCs) of EOF 2, extending the construction of a reliable monthly index of European surface westerlies to 1774. The zonal flow indicated by these circulation indices appears to have been considerably more variable, with more extreme values, in the late 18th and early 19th centuries than in the 20th century. From the 200-year perspective presented here, the recent positive trend in the NAO does not appear unusual