Emerging interannual variability of compound heat waves over the Yangtze River Valley since 2000

Abstract Variability of daytime-nighttime compound HWs (CHWs) is a highly concerned 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 the CHWs in the Yangtze River valley (YRV, a hotspot across China), this paper indicates an emergence of enlarged interannual variability after entering into the 21st century, before which the interannual variability is quite small. The possible mechanism underlying the high interannual variability is further explored in terms of atmospheric and oceanic background. The results show that the atmospheric background associated with higher-than-normal CHWs over the YRV features anticyclonic circulation anomalies tilting southeastward from 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 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 middle-tropospheric 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.