Effect of ENSO and volcanic events on the Sun–cloud link Original Research Article

Results of correlation studies between solar proxies and clouds suggest that there is a solar effect on the occurrence of clouds. However, there is a possibility that terrestrial quasi-periodic and sporadic phenomena, such as ENSO and/or major volcanic eruptions, which have an effect on the cloud formation, may influence the results of statistical studies of the Sun–cloud relation. We show that removing ENSO and volcanic years from the full-set analysis does not alter the results. Moreover, the correlation between clouds of different type and two solar proxies, UV irradiance and cosmic ray induced ionisation, is partly improved. This supports the idea that the solar signal affects clouds directly. An interesting result relates to an area in the eastern Pacific where the full-set analysis showed that the relationship between clouds and cosmic ray induced ionization is opposite to the global one. When ENSO and volcanic years are removed this odd correlation disappears, suggesting that in this particular area, the ENSO effect prevails over solar effects.