Abstract
AbstractObservations have revealed strong variability of shortwave (SW) irradiance at the Earth?s surface on decadal time scales, referred to as global dimming and brightening. Previous studies have attributed the dimming and brightening to changes in clouds and atmospheric aerosols. In this study, we assess the influence of atmospheric circulation on clouds and SW irradiance to separate the influence of ?natural? SW variability from direct and, to some extent, indirect aerosol effects. We focus on SW irradiance in northern Europe in summer and spring because there is little high-latitude SW irradiance during winter. As a measure of large-scale circulation, we use the Grosswetterlagen (GWL) data set, a daily classification of synoptic weather patterns. Empirical models of normalised SW irradiance are constructed based on the GWL, relating the synoptic weather patterns to the local radiative climate. In summer, a temporary SW peak in the 1970s and subsequent dimming is linked to variations in the synoptic patterns over Scandinavia, possibly related to a northward shift in the North Atlantic storm track. In spring, a decrease of anti-cyclonic and increase of cyclonic weather patterns over northern Europe contributes to the dimming from the 1960s to 1990. At many sites, there is also a residual SW irradiance trend not explained by the GWL model: a weak non-significant residual dimming from the 1950s/60s to around 1990, followed by a statistically significant residual brightening. We conclude that factors other than the large scale circulation, e. g., decreasing aerosol emissions, also play an important role in northern Europe.
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