The Role of Internal Variability and External Forcing on the Emergence of Compound Events in the CESM2 Large Ensemble

April 15, 2025

Ashley Dwyer

Committee: Elizabeth Barnes (Advisor); James Hurrell; Frances Davenport (Civil and Environmental Engineering)

Abstract

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 the noise of internal variability. 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.