Quantifying the Risk of Compound Heat Wave Events

Heat waves with high impact on human welfare are often compound events, that is threshold-exceeding hot days interspersed with short breaks of cooler days. Little prior work has focused on the compound nature of heat waves, and how allowing such compounding of shorter heat extremes in heat wave definitions might impact heat wave probabilities. Here we quantify the probability of temporally compound heat wave events, by developing definitions of such events derived from the Warm Spell Duration Index. We analyze frequency of heat wave days using the original and compound definitions in observations and 50 km land/atmosphere GFDL GCM CM2.5-FLOR runs (Control, 2xCO2, 30-member ensemble along RCP 4.5). Both the spatial pattern of changes, and greater percent increase of compound heat wave days with global warming compared to the original definitions will be discussed. Despite the complexity of these definitions, a simple shift of the Control run's mean climate can recreate the changes with warming. We will explore the reasons for the success of this simplification, and its relevance for projection of these heat extremes.