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The persistence of heat waves and droughts is a key factor in determining their societal impact. Here, the local effect of atmospheric blocks on the persistence of summer hot and dry spells is quantified by comparing their climatological daily survival probability, that is, the probability to survive the next day, to their daily survival probability when they co-occur with a block. The survival odds of hot spells are increased by more than 50% over most of the Northern Hemisphere extratropical land masses when co-occurring with blocks. Dry spell persistence is also strongly increased by colocated blocks over western North America, Europe, and southern Russia, while it is significantly decreased over the western North Atlantic and the western North Pacific. These spatial differences in the effect of blocks on both spell types are explained by considering the spatially varying surface temperature and precipitation anomalies induced by the blocks.

Plain Language Summary The persistence of heat waves and droughts strongly affects how challenging these weather events are to societies. However, the meteorological phenomena that foster the persistence of heat waves and droughts are not yet fully understood. Here we focus on one such meteorological phenomenon, namely, stationary high-pressure systems known as atmospheric blocks, and quantify their effect on the persistence of hot and dry spells that co-occur with the blocks. For hot spells, this effect is positive almost everywhere blocks occur in the Northern Hemisphere, most strongly so over land. Dry spell persistence is also strongly affected by blocks, but the effect can be either positive or negative. Over land, the strongest positive effects of blocks on dry spell persistence are found over western North America, Europe, and southern Russia. There, blocks also increase the persistence of so-called compound hot and dry spells, that is, periods with both hot and dry weather occurring concomitantly. Our results show that changes in the number and geographical distribution of blocks with climate change would significantly affect hot and dry spell persistence in a future climate.