Three types of East Asian summer rainfall associated with monsoon circulation and tropical cyclone activities: unique features and major influential factors

Most previous studies have investigated East Asian summer rainfall as one element. This study investigates the East Asian summer monsoon (EASM), Western North Pacific (WNP) tropical cyclones (TCs), and the concurrent rainfall occurring between May and September from 1983 to 2021. Three distinct types of rainfall are identified: monsoon-only rainfall, TC-only rainfall, and monsoon-TC joint rainfall (MS-TC rainfall), each exhibiting its own unique characteristics. Monsoon-only rainfall is characterized by positive anomalies along the East Asian subtropical front, while a decrease in TC activity leads to negative rainfall anomalies in the tropical WNP. TC-only rainfall, on the other hand, contributes to positive rainfall anomalies in the tropical WNP, as more TCs exhibit westward movement. In the case of MS-TC rainfall, positive precipitation anomalies are observed in the Philippines, the Korean Peninsula, and southern Japan. These anomalies can be attributed to an enhanced northward movement of TCs during such rainfall events. During strong monsoon-only (TC-only) rainfall years, the tropical western Pacific experiences anticyclonic (cyclonic) anomalies, along with a westward (eastward) shift of monsoon trough and WNP subtropical high (WNPSH). In strong MS-TC years, a localized cyclonic anomaly dominates the Philippine Basin, resulting in an eastward shift of the monsoon trough and WNPSH. Additionally, a significant increase (decrease) in vertical wind shear (VWS) is observed in the tropical WNP during strong monsoon-only (TC-only) years, while a moderate decrease is observed in strong MS-TC years. Distinct influential factors are associated with each rainfall type. Preceding positive sea surface temperature (SST) anomalies in the offshore China seas and WNP, in conjunction with the El Nino-Southern Oscillation (ENSO), play a primary role in driving interannual variations of monsoon-only rainfall. ENSO serves as the principal modulator of interannual variations in TC-only rainfall. Additionally, anomalous thermal conditions in the Maritime Continent (MC) act as major drivers for MS-TC rainfall. This study enhances our understanding of the underlying mechanisms and influential factors contributing to the diverse patterns of East Asian summer rainfall.