A pollen-based reconstruction of summer temperature in central North America and implications for circulation patterns during medieval times

摘要

We present a reconstruction of mean summer temperature for the northern Midwest of the USA based on lacustrine pollen records from three different lakes in Wisconsin. The results suggest a relatively warm period during the earlier part of the record (～1200-1500 CE) followed by a cooler Little Ice Age (～1500-1900) and a subsequent warming to modern conditions. The reconstructed modern summer mean temperature is in good agreement with observations, and the decades of the 1930s to 1950s appear to be the warmest such period in the proxy record (through 1974). Analyses of circulation features associated with the warmest summers in the recent climate record suggest a prevalence of continental ridging accompanied by generally dry conditions during these warm summers in the Midwest Drought reconstruction using the Palmer Drought Severity Index (PDS1) and tree-ring records as predictors also yield relatively dry conditions in medieval times for the central US. As reported in a number of recent studies, possible forcing mechanisms include La Nifla-like conditions in the equatorial Pacific and warmer than average waters in the tropical Indo-western Pacific Ocean possibly coupled to a positive mode of the AMO/ NAO North Atlantic circulation pattern.