The role of internal variability and external forcing on the emergence of hot and dry compound extremes in the CESM2 large ensemble

Extreme hot and dry compound events pose significant hazards to human health, agriculture, and ecosystems, making it critical to better understand what drives their occurrence and spatiotemporal variability. Although the role of internal climate variability in driving compound events has been previously studied, we leverage a large ensemble to enable a more robust understanding of the response of hot and dry events to both large-scale internal climate variability and external forcing. We explore the influence of well-known large-scale climate modes including the El Niño Southern Oscillation (ENSO), Pacific Decadal Oscillation (PDO), Indian Ocean Dipole (IOD), and the North Atlantic Oscillation (NAO) on the occurrence of hot and dry compound events in the Community Earth System Model 2 Large Ensemble (CESM2-LE). We also investigate when anthropogenic changes in hot and dry compound events emerge from internal variability. Overall, we find that internal climate variability is, and will continue to be, a strong influence on compound event occurrence, although external forcing will likely cause changes in frequency patterns over the 21 st century. We also find that under SSP 3-7.0, frequencies of compound events in most of the world will emerge from internal variability by 2100. Knowledge of drivers from an internal variability perspective combined with an understanding of greenhouse gas forced changes can aid in quantifying the predictability of extreme compound events on regional scales.