Multicentury reconstruction of the Canadian Drought Code from eastern Canada and its relationship with paleoclimatic indices of atmospheric circulation

Abstract

Inter-annual and -decadal scale variability in drought over the Abitibi Plains ecoregion (eastern Canada) was investigated using a 380-year dendrocli- matic reconstruction of the Canadian Drought Code (CDC; July monthly average) i.e., a daily numerical rating of the average moisture content of deep organic layers. Spectral analyses conducted on the reconstructed CDC indicated a shift in spectral power after 1850 leading toward a reduction in interdecadal variability and an increase in interannual variability. Investigation on the causes for this shift suggested a decrease in North Pacific forcing after the mid-nineteenth century. Cross- continuous wavelet transformation analyses indicated coherency in the 8-16 and 17-32-year per cycle oscilla- tion bands between the CDC reconstruction and the Pacific Decadal Oscillation (PDO) prior to 1850. Fol- lowing 1850, the coherency shifted toward the North Atlantic Oscillation (NAO). Principal component anal- ysis conducted over varying time windows reaffirmed that the Pacific forcing was restricted to the period about 1750-1850. Prior to and after this period, the CDC was correlated with the NAO. The shift around 1850 could reflect a northward displacement of the polar jet stream induced by a warming of the sea surface temperature along the North Pacific coast. A northward displace- ment of the jet stream, which inhibits the outflow of cold and dry Arctic air, could have allowed the incursion of air masses from the Atlantic subtropical regions.