Compound Drought and Heatwaves at a Global Scale: The Role of Natural Climate Variability-Associated Synoptic Patterns and Land-Surface Energy Budget Anomalies

Compound drought and heatwave (CDHW) events have garnered much attention in recent studies. However, thus far, the identification of such events is oversimplified, and their association with natural climate variability is not fully explored. Here, we derive anomalies in the weekly self-calibrated Palmer Drought Severity Index (sc_PDSI) and daily maximum temperatures to identify CDHW events from 1982 to 2016 over 26 climate regions across the globe. Using a Poisson Generalized Linear Model (GLM), we analyze yearly occurrences of seasonal CDHW events and their association with the warm and cold phases of El Nino Southern Oscillation (ENSO), Pacific Decadal Oscillation (PDO), and North Atlantic Oscillation (NAO). ENSO exhibits robust association with CDHW events over the Southern Hemisphere during the austral summer and fall, while PDO influences their occurrences over the Western North America in the Northern Hemisphere during the boreal summer, which is supported by the composites of anomalies in the atmospheric circulations and surface energy budget. However, NAO association with CDHW events is relatively weak. The CDHW occurrence over other regions is driven by a combination of these large-scale natural forcing. Our analyses also highlight that the cooccurrence of weekly to submonthly scale anomalies in the observed temperature and precipitation may not be always aligned between the observations and the reanalysis. Therefore, caution must be exercised while explaining such observed anomalies on the basis of reanalysis-based circulations and surface energy budget. Overall, our analyses provide a new insight towards concurrent extremes and should help foster research efforts in this area. Plain Language Summary The cooccurrence of drought and heatwave events has increased considerably during the last few decades over many regions of the globe. Therefore, it is imperative to develop an understanding of the factors that influence their regional occurrences. In this study, using a Poisson Generalized Linear Model framework, we identify major regional drivers that are associated with the occurrences of compound drought and heatwave events. Our results show that seasonal occurrences of these compound extremes over many regions in the Southern Hemisphere is associated with the positive phase of ENSO while negative phase of PDO influences their occurrences in the Western North America. Such occurrences in other regions exhibit influence of more than one mode of natural variability. The associations of ENSO and PDO with compound drought and heatwave events can be explained in terms of atmospheric circulations and surface energy budget anomalies.