Surface Atmospheric Circulation Over Europe Following Major Tropical Volcanic Eruptions, 1780–1995

We identify monthly circulation patterns of variability over the European continent within the first year after eight large tropical volcanic eruptions since the early 19th century. For this purpose, we apply an empirical orthogonal function (EOF) technique to a gridded sea level pressure data set covering the period 1780-1995. The post-eruption months with above-normal variance represented by the leading two EOFs are retained. Afterwards, we mapped their rotated equivalents to simplify physical interpretation of the results. Following the eight tropical eruptions, four months are identified during the first year fulfilling the criterion of exceptional values of variance in EOF1: January, June, July and September. January shows an anomalously dominant zonal pattern responsible for warmer temperatures over continental Europe. The summer months tend to show a more persistent south-westerly or westerly circulation over the continent, indicating a likely reduction in the strength of the Azores high. The results showing modification of the atmospheric circulation following large volcanic eruptions are consistent with previous studies based on instrumental records for more recent periods. This underscores the robustness of the circulation changes as a response to volcanic forcing. The analysis also demonstrates that empirical orthogonal functions provide a useful tool to detect these dynamical features.