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

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.