Compound Meteorological and Hydrological Hot‐Dry and Hot‐Wet Extremes Over Global Land Areas: Spatiotemporal Changes, Attribution, and Drivers

ABSTRACT Global warming increases the occurrences of compound hot‐dry and hot‐wet extremes that pose amplified socioeconomic and environmental impacts compared to univariate extremes. Compound meteorological hot‐dry extremes have made substantial progress at the global scale, while compound meteorological hot‐wet extremes are still poorly understood. Meanwhile, there remains a research gap on the compound hydrological hot‐dry (hot‐wet) extremes based on high temperature and low (high) runoff, which is more directly related to damages on social and natural systems. This study investigated the spatiotemporal variations and attribution (e.g., associated univariate extremes and their dependence) of compound meteorological hot‐dry (CMHDE) and hot‐wet (CMHWE) extremes and compound hydrological hot‐dry (CHHDE) and hot‐wet (CHHWE) extremes and effects of large‐scale oceanic modes and SST variability on compound extremes across globe during the warm season (i.e., May–September in the Northern and November–March in the Southern Hemispheres) over 1901–2019. Results indicate that the likelihood of compound meteorological and hydrological hot‐dry and hot‐wet extremes showed an increasing trend in more than 90% of global land during 1901–2019. The frequency of compound extremes from high to low was in the order of CHHWE, CMHWE, CMHDE, and CHHDE. The spatial extent of compound meteorological and hydrological hot‐dry and hot‐wet extremes showed an increasing trend in almost all climate regions over the last 119 years, with higher increasing rates in compound hot‐wet extremes. The positive (negative) dependence between meteorological and hydrological hot‐dry (hot‐wet) extremes was detected in 74.1% and 87.1% of global land, with stronger dependence observed for hydrological than meteorological compound extremes. The high temperature contributed to the reduction of return period of compound hot‐dry and hot‐wet extremes in more than 92% of global land, except for western South America, southern Asia, and central‐eastern North America. The contribution of dependence and precipitation/runoff had high spatial heterogeneity, resulting in increases or decreases of return period in regions across global land. The decreases (increases) in runoff under combined high temperature and low (high) precipitation showed in about 95% (70%) of the globe. The effect of high (low) precipitation on increases (decreases) in runoff was higher than that of high temperature on runoff decreases, indicating that precipitation dominates runoff change more strongly than temperature. The conditional probability of low runoff given CMHDE was higher than that of high runoff given CMHWE. La Niña (El Niño) dampened (enhanced) the likelihood of compound hot‐dry extremes in the northern and central South America, southern and central Africa, Sahara, southern Asia, and Australia. The low (high) Dipole Mode Index (DMI) and Atlantic Multidecadal Oscillation (AMO) dampened (enhanced) the likelihood of compound hot‐dry and hot‐wet extremes in more than 70% of global land, indicating that low (high) DMI and AMO were negatively (positively) related to high temperature. The leading modes between global SST and compound hot‐dry and hot‐wet extremes showed positive correlation and explained 75.9%–84.0% of compound extreme variability during 1901–2019. This study contributes to the understanding of characteristics and driving mechanisms of compound extremes at the global scale under climate warming.