Diverging Shifts in the Frequency and Intensity of Compound Daytime and Nighttime Hot Extremes under Anthropogenic Forcings

Daytime and nighttime temperatures are often influenced by different mechanisms: daytime heat is often associated with reduced soil moisture and cloud cover, while nighttime heat is connected to high humidity and increased cloud cover. Due to these differing mechanisms, compound daytime and nighttime hot events (e.g. hot daytime temperatures and dry precipitation conditions; hot nighttime temperatures and high humidity conditions) may respond differently to the major anthropogenic forcings (e.g. greenhouse gas emissions, anthropogenic aerosol emissions, land use and land cover change). We examine how anthropogenic forcings have driven changes in compound hot events specifically compound hot maximum temperature and dry precipitation, hot minimum temperature and dry precipitation, hot maximum temperature and high humidity, and hot minimum temperature and high humidity events - using monthly output from GISS ModelE2.1-G CMIP6 DAMIP historical (1955-2014) single forcing runs. With our results, we found that the frequency and intensity of compound daytime and nighttime hot events respond differently to individual anthropogenic drivers. Relative to historical natural-only conditions, under greenhouse gas-only conditions, the frequency and intensity of nighttime hot and humid conditions have increased more rapidly than daytime hot and humid conditions, especially in the Arctic and tropical latitude bands. On the other hand, anthropogenic aerosols and human-driven land use and land cover changes have reduced the frequency and intensity of daytime hot and dry events more than nighttime hot and dry events. These divergent responses in daytime and nighttime compound events demonstrate how multivariate drivers and regional interactions between anthropogenic forcings govern heat extremes. Due to the differing impacts associated with compound daytime and nighttime hot events, characterizing how these events have diverged in response to anthropogenic drivers is valuable for a more comprehensive understanding of these important climate change impacts.