Correlations and trends of the surface solar radiation, the cloud coverage and the NAO. – A case study for the North Atlantic, Europe and North Africa

This study is an analysis of recently published monthly data records of solar radiation (SIS) from the SARAH-3 climate data record and monthly data records of total cloud fraction (CFC) from the CLARA–A3 climate data record, both comprising the 38-year period 1983–2020. The investigations in this study are concentrated on the region of the northern part of the METEOSAT disc covering the area of the North Atlantic and Europe between 65 °W – 65 °E/0 °N – 65 °N.In a first step long-term means and the standard deviations of SIS (SISSD) and CFC (CFCSD) are studied. In a second step the spatial and temporal distributions of the long-term trends of SIS (SIST) and CFC (CFCT) are investigated. For further understanding of the spatiotemporal variability, the research area is divided into 30 sub-areas. In autumn SIST reaches values of 2 to 3 W/m²/decade in the southern regions and up to 5 W/m²/decade in spring in the northern regions of the research area. In the areas north of 50 °N, a positive trend of SIS and a corresponding negative trend of CFC were consistently found for all months of the year.The large-scale general atmospheric circulation in the North Atlantic and in the Central European region can be described by the NAO index (NI). This index is used here to study its influence on SIS and CFC and vice versa.For this reason, the correlations between these quantities were examined in two separate steps. On the one hand, the correlations between NI and SIS (RNI,SIS) and NI and CFC (RNI,CFC) were investigated. In winter, a small but significant correlation was found in southern Europe between 0.3 and 0.5. In summer, we found a significant correlation between 0.4 and 0.6 in central and northern Europe and about 0.4 in the eastern Mediterranean. The correlations in the western part of the Mediterranean in winter might be related to the activity of the low-pressure areas steering the formation of the Mistral.The spatial patterns of the trends in SIS and CFC have been compared to the anomalies of these quantities with respect to months with positive and negative values of NI; no correspondence of the spatial patterns of the trend and the corresponding anomalies / trends in NI are found indicating a negligible role of the NI in explaining the trends in SIS and CFC from 1983 to 2020. These results underline that the change in the atmospheric circulation in the Northern Atlantic, as described by NI, has a small to moderate influence on the spatial and temporal variability and trends of SIS and CFC. In addition, there are other influencing factors, such as other weather pattern, spatially different sea surface temperature (SST), and possibly also aerosol particles, which influence the spatial and temporal variability of surface solar radiation.