Interannual variability of the East Asian trough in summer

The middle-tropospheric East Asian trough (EAT) is one important component of the East Asian monsoon system. Previous studies have focused on the variability of the EAT in winter and have highlighted its crucial role in the climate over East Asia and beyond; however, little attention has been given to the EAT in summer. This study investigates interannual variability of the summer EAT using the European Centre for Medium-Range Weather Forecast (ECMWF) Reanalysis version 5 (ERA5) dataset for 1979-2019 and reveals its impact on rainfall and the underlying dynamic mechanisms. The results show that the interannual variability of the EAT is characterized by intensity variation. Related to an enhanced EAT, rainfall increases significantly within the East Asian subtropical rainy belt extending from the Yangtze-Huaihe River valley to western and central Japan and decreases in North China. The rainfall anomalies are triggered by the anomalous ascending motion due to warm advection in the east of the EAT and descending motion due to cold advection in the west. The interannual variation in the EAT intensity is significantly modulated by the extratropical Arctic Oscillation. In addition, it is also affected by tropical rainfall anomalies in South Asia and the western North Pacific, through a zonally-oriented circumglobal teleconnection and a meridional East Asia-Pacific teleconnection, respectively. Moreover, the EAT intensity is significantly connected with summer sea surface temperature (SST) anomalies in the tropical eastern Pacific, bridged by the SST-induced rainfall anomalies in South Asia. The summer SST anomalies can be traced back to the preceding spring, implying potential predictability of the summer EAT intensity. Finally, the difference between the EAT and northern East Asian low in the lower troposphere is discussed.