Tree‐ring reconstructed sunshine duration over central USA

Tree‐ring chronologies of baldcypress (Taxodium distichum) and post oak (Quercus stellata) from the central United States of America are directly correlated with precipitation, and inversely with percentage possible sunshine during the growing season (March‐June). Partial correlation analysis indicates that precipitation is the most important climate signal in the growth of these trees, but sunshine duration accounts for an additional 22 per cent of the radial growth variance independently of the growth influences of precipitation or temperature. Because the internal moisture balance of trees and ultimately the width of annual tree‐rings assimilate the direct and interactive effects of sunshine duration, temperature, and especially precipitation, the tree‐ring chronology can explain 60 per cent of the variance in regional sunshine duration. Consequently, the derived reconstruction retains more sunshine duration information than is reflected by the simple covariance between rainfall, cloud cover, and sunshine. A tree‐ring reconstruction of sunshine duration from AD 1700 to AD 1980 suggests that short‐period changes in the mean, variance, and persistence of cloud cover have occurred over the past 281 years. Analysis of the reconstruction indicates that strong El Niño events are often associated with cloudy growing season conditions over the central USA. The full 281‐year reconstruction and the 85‐year instrumental sunshine data also contain prominent 5‐ to 10‐year sunshine oscillations, which frequently help to induce trends lasting up to 20 years. The sunshine reconstruction does not, however, reveal any statistically significant 30‐year‐long linear trends in sunshine duration since 1700.