Effect of the solar cycle on cloud cover variations over the United States

In recent years there has been renewed interest in studying the effects of solar cycle on climate. Svensmark and Friis-Christensen (1997) (SFC) found a positive correlation of satellite cloud measurements with measurements of the cosmic ray flux at Climax,Colorado. However, SFC's short data record raised concerns regarding the validity of their conclusions. Our results support the conclusion by Kuang et al. (1998) that the cloud variations presented by SFC were likely to be influenced by an ENSO event.

We derive a consistent longterm cloud data set over the United States based on observations from 90 stations from the sunrise-sunset data set (Steurer and Karl, 1991). We perform a spectral analysis of the data between 1900 and 1982. Results of the spectral analysis reveal a statistical significant peak at a period of 11.3 years; the coherence spectrum between cloud cover and sun spot numbers (which is an index for solar variability) has a value of 0.8 at this period. Furthermore, the cloud cover is positively correlated with sun spot numbers and thus anticorrelated with cosmic rays. Therefore, it is unlikely that the cosmic rays influence cloud cover variations. We will discuss possible alternative mechanisms for a connection between solar variability and cloud cover.

Kuang, Z. and Y. Jiang and Y.L. Yung, Cloud Optical Thickness variations during 1983-1991: Solar cycle or ENSO?, Geophys. Res. Lett., 25, 1415-1417,1998.

Steurer, P., and T. Karl, Historical sunshine and cloud data in the United States, Report for the Carbon Dioxide Research Program, ORNL/CDIAC-43, Oak Ridge National Laboratory, Environmental Sciences Division, Pub. No. 3689, Oak Ridge, TN, 1991.

Svensmark, H., and E. Friis-Christensen, Variation of cosmic ray flux and global cloud coverage--A missing link in solar-climate relationships, J. Atmos. Sol. Terr. Phys., 59, 1225-1232, 1997.