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

Abstract

This work uses reanalysis and NOAA Geophysical Fluid Dynamics Laboratory's Coupled Model Intercomparison Project 6 model, CM4, to investigate the colocation of heat extremes and atmospheric blocking in the current climate and an end of 21st century, extreme-emissions projection. In the present day, the colocation of heat events and blocking is greatest for the strongest heat events. Block-heat extreme colocation is found to be less prevalent over ocean than land, exhibiting regional variation throughout the Northern Hemisphere. Over North America, colocation is greatest near the northwestern and northeastern coasts, minimizing near the center; over Eurasia, colocation is most prevalent in northern regions. In an RCP 8.5 projection, the historical 90th percentile temperature decreases to 0–70th percentile, depending on the region. This is primarily driven by mean state warming. Blocking is found to decrease along with the colocation of blocking and heat extremes, suggesting that in some regions, the mechanisms driving heat extremes will change in future climates.