Significant slope change but insignificant correlation change between heatwaves and blocking under future global warming

Extra-tropical continental summer heatwaves often occur under persistent anticyclones or blocking.  Here, we study the heatwave–blocking relation under future global warming projected by CMIP6 SSP585 experiment.  We define both recent-past and future heatwaves using detrended temperature anomalies and recent-past thresholds.  We employ an optimized blocking index that best correlates with heatwave frequency (Pearson correlation of 0.7).  The index is from Dole and Gordon 1983, which identifies persistent positive geopotential anomalies. With detrended temperature and recent-past thresholds, we find a significantly steepened (flattened) heatwave–blocking slope over Europe (Greenland) under future global warming.  The steepened heatwave–blocking relation over Europe could be a result of depleted soil moisture and enhanced land–atmosphere coupling, which enhance the capacity of blocking in driving more heatwaves.  The flattened heatwave–blocking relation over Greenland is likely because the melting of ice or snow and the absorption of latent heat, which limit blocking’s capacity in driving heatwaves.  Considering the contribution to heatwave frequency change, the heatwave–blocking slope change likely dominates over decrease in blocking frequency.  Despite the statistically significant change in heatwave–blocking slope, we find statistically insignificant change in heatwave–blocking correlation.  In other words, the ratio of heatwave variability explained by blocking will not change significantly.  We also find the same type of blocking found in the recent past, i.e., identified with the same amplitude threshold and duration threshold, continues to be most relevant to future heatwaves.  We revisit previous evidences that suggest future heatwaves being caused by different circulation patterns, in order to understand more about the reasons behind the apparent discrepancy.  Studies on future heatwave frequency should focus on the heatwave–blocking relation change, and the thermodynamical factors behind.