Influence of NAO and clouds on long‐term seasonal variations of surface solar radiation in Europe

Abstract

This study is an analysis of the seasonal all‐sky surface solar radiation variability in Europe during 1970–2000 using surface observations from the Global Energy Balance Archive (GEBA). On the basis of the annual means period 1970–1985, there is a statistically significant decline of −3.0% decade−1 (−3.8 Wm−2 decade−1) followed by a nonsignificant rise of 0.3% decade−1 (0.4 Wm−2 decade−1) during 1985–2000. The behavior of the solar radiation for spring is similar to the annual series and has the strongest increase of 1.6% decade−1 (2.5 Wm−2 decade−1) during 1985–2000. In summer a similar evolution to the annual and spring time series is shown but has a stronger decline of −3.2% decade−1 (−6.8 Wm−2 decade−1) during 1970–1985. A small positive nonsignificant trend is reported for the winter means time series while a statistically significant negative trend of −2.5% decade−1 (−2.1 Wm−2 decade−1) was found in autumn during 1970–2000. By comparing variations in all‐sky solar radiation with changes in cloud cover and NAO, we attribute the winter and autumn trends mainly to the NAO through the modification of mid‐to‐low cloud cover in southern Europe and the spring and summer trends to mid‐to‐low cloud cover in northern Europe. However, because the cloud cover and solar radiation relationship weakens in the low‐frequency variability, it suggests that other effects such as aerosols may also play a role. In addition, aerosols could be interfering with the relationship between solar radiation and NAO, contributing to a strengthening of their correlation in the low‐frequency variability during winter and autumn.