Emerging Interannual Variability of Compound Heat Waves over the Yangtze River Valley since 2000
Authors: Xie Wenxin , Zhou Botao , Li Hua 
Published: December 2024

Abstract
The variability of daytime–nighttime compound heat waves (CHWs) is a highly concerning issue due to severe impacts on human and natural systems. Although several studies surveyed physical processes for the CHW occurrence, its interannual variability and associated mechanisms have not been well understood. Focusing on CHWs in the Yangtze River valley (YRV, a hotspot across China), this paper indicates an emergence of enlarged interannual variability after entering into the twenty-first century, before which the interannual variability was quite small. The possible mechanism underlying the high interannual variability is further explored in terms of atmospheric and oceanic backgrounds. The results show that the atmospheric background associated with higher-than-normal CHWs over the YRV features anticyclonic circulation anomalies tilting southeastward from the north of the YRV in the upper troposphere to the western Pacific in the lower troposphere. Accordingly, the upper-tropospheric easterly and lower-tropospheric southwesterly anomalies dominate the YRV, causing anomalous subsidence and increased humidity in situ, respectively, which benefit the increase in CHWs. The tripole (positive–negative–positive) sea surface temperature (SST) anomalies in the North Atlantic (NA) and the positive SST anomalies in the Maritime Continent (MC) also play roles in increasing the YRV CHWs by influencing the above atmospheric circulations. The NA tripole SST anomalies tend to affect the upper- and midtropospheric anticyclonic anomalies through the eastward-propagating wave train across Eurasia. The warming of the MC SSTs can impact the lower-tropospheric anticyclonic anomaly over the western Pacific via local meridional circulation. The opposite situations are applicable for decreased CHWs over the YRV.

Significance Statement
Daytime–nighttime compound heat waves (CHWs) refer to persistent processes with abnormally high temperatures occurring both in daytime and nighttime. Compared with heat waves (HWs) occurring only at daytime or nighttime, the CHWs exert more severe damage to natural ecosystems and human society. Thus, understanding the physical mechanisms of CHWs is urgently needed. This study examines the interannual variability of Yangtze River valley (YRV) CHWs and finds that it exhibits a pronounced enlargement after 2000. During this period, an anomalous anticyclone tilting southeastward from the north of the YRV in the upper level to the western Pacific in the lower level is a favorable atmospheric circulation background for the increase in CHWs over the YRV, and vice versa. Sea surface temperatures (SSTs) in the North Atlantic (NA) and the Maritime Continent (MC) also play important roles. The positive–negative–positive SST anomalies in the NA and the warming SSTs in the MC tend to increase the YRV CHWs, through their influences on CHW-related atmospheric circulations in the upper troposphere and lower troposphere, respectively. These findings are expected to deepen our understanding of CHW variability, which are of great importance for disaster prevention and mitigation.