Iowa climate reconstructed from tree rings, 1640–1982

Abstract

Tree ring indices from an expanded network of 17 white oak (Quercus alba} sites in eastern and central Iowa were used to reconstruct state average July Palmer hydrological drought index (PHDI), annual precipitation (previous August to current July), and other climate variables for 1640–1982. We removed nonclimatic variance trends caused by changing sample size and senescent growth. July PHDI correlated better with tree growth than annual precipitation. Occurrence of prolonged droughts throughout the reconstruction suggests that decades like the 1930s occur about twice per century in Iowa. Iowa climate is correlated with the Southern Oscillation Index (SOI) from June in the year of El Niño onset (Yr0) through the next February (Yr+1), with negative SOI (El Niño) associated with wetter conditions. When the June (Yr0) to February (Yr+1) average SOI reaches extremes ≥ + 1.0 or ≤−1.0, it correlates significantly with observed and reconstructed July PHDI (r = −0.37 and −0.56, respectively). Climate during solar cycles centered on sunspot minima alternates between wet and dry regimes that differ by an average of 1.21 units of observed July PHDI and 46.7 mm of annual precipitation for 1877–1982. The solar relationship has been stable since 1640. Combining solar and SOI influences in forecasts may improve prediction of Iowa climate.