Local Tree-Rings and Teleconnections: Ways to Improve the Reconstruction of Lake Superior Water Supplies

Abstract

Hydrological records can be extended back in time using tree-rings. A basic assumption is that tree growth and the reconstructed variable respond in a similar fashion to a given combination of climate variables. It was documented in an earlier study that this assumption poses a problem in reconstructing water supplies to Lake Superior. Low summer temperatures at times of drought reduce the stress on trees induced by a lack of water. The water supplies to Lake Superior, on the other hand, are largely unaffected by air temperature; evaporation is low in summer because of low water temperatures. Tree-rings therefore tend to overpredict water supplies at times of cool summer drought. It was shown in the earlier study that using air temperature as an additional predictor will improve the modeling of Lake Superior water supplies. The observed air temperature record is however too short to be useful. Results of the present study show that the well-known teleconnection between atmospheric circulation patterns over western North America and air temperature over the Great Lakes region can be used to improve the modeling. Including western tree-ring chronologies–as a surrogate for local temperature–in regression models results in an increase in explained variance of 0.14 above that obtained by using only local chronologies, and most of this improvement in explanation occurs in the lower (drier) half of the water supply distribution. There is therefore information in the western tree-ring chronologies that could be useful in reconstructing Great Lakes water supplies. Many of the chronologies were however collected in the 1960s and 1970s. Updated chronologies would lengthen the record available for model calibration and verification by more than 20 years which is about a third of the length of the record presently available.