image: Rainfall anomalies over the Yangtze River Valley (YRV) in June 2020
(a), (b) Rainfall anomalies over the YRV in June 2020 using the CMAP dataset and PREC/L dataset. (c) Standardized time series of the June YRV (red rectangle) rainfall index using the CMAP dataset. The black line denotes the linear trend of the time series.
The rainfall over the Yangtze River Valley (YRV) in June 2020 broke the record since 1979 (Figure 1). As of June 28, the People's Daily Online reported that there were more than 12 million people affected by flood disasters related to this torrential rain, with deaths or disappearances of 78 persons and a direct economic loss of more than 25 billion CNY. Recently, scientists from South China Sea Institute of Oceanology (SCSIO), Chinese Academy of Sciences revealed the cause of the record-breaking rainfall over the YRV.
According to their study published in Science China Earth Sciences on March 19, 2021, all three oceans of the Pacific, Indian and Atlantic Oceans contribute to the record-breaking rainfall over the YRV in June 2020, which is unique and different from previous studies.
"There are negative 200-hPa relative vorticity anomalies over North China (NC) and negative 850-hPa relative vorticity anomalies in the South China Sea (SCS) in June 2020 (Figure 2). The rainfall anomalies in the YRV are mainly controlled by the NC vorticity. However, the SCS vorticity provides favorable moisture conditions for the rainfall anomalies in the YRV", explained by Dr. Jiayu Zheng from SCSIO, leading author of the study. Atmospheric processes associated with these two vorticity anomalies are essential for the record-breaking rainfall over the YRV.
Dr. Chunzai Wang, corresponding author of this study, added: "Atmospheric circulation anomalies are strongly influenced by sea surface temperatures (SSTs). The purpose of our study is to detect which ocean's SSTs are responsible for the atmospheric circulation and rainfall anomalies in June 2020 over the YRV."
In their study, they revealed that the positive SST anomalies in May over the western North Atlantic induce positive geopotential height anomalies in June over the mid-latitude North Atlantic, which affect the rainfall anomalies in the YRV by changing the NC vorticity via Atlantic-induced atmospheric wave train activity across Europe. The Indian Ocean and tropical North Atlantic, as capacitors of Pacific El Niño events, affect the SCS vorticity associated the anomalous anticyclone over the SCS and also facilitate the YRV rainfall by providing favorable moisture conditions. Thus, all three oceans make contributions to the rainfall over the YRV in June 2020, but the Atlantic plays a dominant role.
Dr. Wang concluded: "Previous studies focused on relationships of the Pacific or Indian Oceans with the rainfall in China. In our study, we found that the three oceans have a combined influence on the rainfall in China." These results indicate that it is of importance to investigate extreme weather and climate events from the view of three oceans. Further research is needed to advance our understanding of three-ocean interactions and extreme weather and climate events.