Heatwave–blocking relation change likely dominates over decrease in blocking frequency under global warming

Abstract

Extra-tropical continental summer heatwaves often occur under persistent anticyclones or blocking. Here we partition heatwave changes into contributions from blocking changes, heatwave–blocking relation change and mean temperature increase, under global warming in climate models. We employ an optimized blocking index that best correlates with heatwaves (Pearson correlation of 0.7) and find heatwave-driving blocking decreases but the change in heatwave–blocking relation likely dominates. Over Europe, with a historical heatwave frequency of 2.5%, less blocking will cause 0.6% fewer heatwaves, steepened heatwave–blocking relation will cause 1.4% more heatwaves, and the mean temperature increase will cause 60% more heatwaves. Over Greenland, flattened heatwave–blocking relation will dominate over the insignificant decrease in blocking. The future increase in heatwave frequency is not caused by changes in blocking frequency, but by factors such as thermodynamics, that enhance the capacity of blocking to drive heatwaves.

Northern Hemisphere Heat Extremes in a Warmer Climate: More Probable but Less Colocated With Blocking

“…On the other hand, blocking is projected to decrease, as is the colocation of blocking and heat extremes. This is consistent with recent findings by Chan et al., 2022, who also found less blocking and block‐heat extreme association, but increased heatwave occurrence driven primarily by mean state warming. Decreases in block‐heat extreme colocation could point to changes in the dominating mechanical and thermodynamic drivers of persistent heat extremes in a warmer world.…”

“…et al. (2022), who found heat extremes over land in RCP 8.5 to be less correlated with blocking across CMIP6 models. Together, our findings along with Chan et al.…”

“…Atmospheric blocks are large (∼10 6 km 2 ), persistent (5+ days), quasi‐stationary anticyclones (Lupo, 2021; Rex, 1950) linked to hazardous weather phenomena. In particular, case studies and global statistical analyses have found that blocks can be drivers of persistent heat extremes (Chan et al., 2019, 2022; Pfahl & Wernli, 2012; Suarez‐Gutierrez et al., 2020; Trenberth & Fasullo, 2012). When colocated with heat extremes, blocked flow can generate warmer temperatures through (a) adiabatic compression of subsiding air, and (b) diabatic heating effects induced by clear skies (i.e., drying of soil moisture, greater sensible heat flux; e.g., Horton et al., 2016); When offset with heat extremes, blocking can also drive temperatures via warm horizontal advection (Pfahl, 2014).…”

Typicality of the 2021 Western North America summer heatwave

“…This was compounded by the presence of an intense atmospheric Ω-block, like the one associated with the 2010 Russian heatwave [27][28][29]. Atmospheric blocks [30] are often the culprits for individual heatwaves as a result of possibly concurring processes, such as anomalies in clear-sky incoming radiation, warm air advection, and subsidence [31,32]. The Western North America heatwave (WNAHW) blocking developed on 25th June in association with a large scale pattern over the mid-latitudes dominated by low zonal wavenumbers [19,33].…”

“…While this is obviously a serious simplification, because one neglects all the possible impacts of shifting climate conditions on climate variability (e.g. changes in the dynamics of the atmosphere, water cycle, soil properties [32]), shifting the climatic mean has been shown to be a good first order approximation of the impact of climate change on the statistics of heatwaves [19].…”