Climate change impacts on linkages between atmospheric blocking and North American winter cold spells in CanESM2 and CanESM5

This study investigates changes in linkages between atmospheric blocking and winter (December–February) cold spells over the Pacific-North America region in two large-ensembles of Canadian Earth System Models (CanESM2 and CanESM5 under high-emission scenarios). The two ensembles show decreases in winter blocking frequency over the North Pacific from 1981–2010 baseline to 2071–2100, with larger decreases in CanESM5 (− 3.08%/decade) than CanESM2 (− 1.73%/decade). Using a time-invariant (stationary) threshold estimated from the baseline to define cold days, the two ensembles project a decline in cold spell events as future air temperature increases; the occasional occurrence of cold spell events is still projected to occur at the end of the century. Using a time-dependent (nonstationary) climatological threshold, CanESM2 and CanESM5 ensembles project modest decreases in cold spell days over North America (− 2.0 and − 2.3%/decade). With the nonstationary threshold, the two ensembles project decreases in winter cold spell frequency during blocking, with larger decreases in CanESM5 (13%) than CanESM2 (3%) for 2071–2100 period compared to the baseline. The two ensembles display similar blocking-cold spell linkages between the baseline and future periods; however, the linkage is weaker and exhibits larger uncertainty in the future. Moreover, temperature advection and net heat flux anomalies during blocking are generally weaker for the future period, resulting in weaker impacts on North American cold spells with larger uncertainty associated with increases in internal-variability.