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The East Asian summer monsoon in Northeast Asia (NEA) has experienced an increase in summer rainfall and a delayed end to the rainy season after 2000, suggesting a trend of enhancement. Based on the data analyses spanning 1979–2022, our results show that the increased rainfall amounts are associated with a more pronounced Mongolian cyclone (MC) in July–August, a manifestation of a portion of the Eurasian barotropic Rossby wave train. Sea surface temperature (SST) anomalies in the North Atlantic (NA) regulate this wave train, with SST increases leading to its amplification. Somewhat independently, a delayed end to the rainy season in September is related to an enhanced anticyclone over the Kuril Islands (ACKI) in the Russian Far East. This anticyclone originates in the Arctic region, possibly induced by the loss of sea ice in the East Siberian Sea, a condition that can be detected two months in advance. The stronger MC and ACKI jointly contribute to the observed enhancement in the East Asian summer monsoon in NEA since 2000 by facilitating ascending motion and moisture transport. Therefore, the SST anomaly in the NA, which is responsible for the intensified rainfall in the rainy season in NEA, coupled with the sea ice conditions in the East Siberian Sea, provides a potential prediction source for the retreat of the rainy season. [ABSTRACT FROM AUTHOR]