Abstract: There are two major sea surface temperature anomaly (SSTA) patterns over the tropical Indian Ocean (TIO): basin-wide SSTA variation which peaks in the boreal spring, and the dipole mode that is phase-locked in the boreal autumn. This paper explores possible impacts of these two SSTA patterns on the South China Sea (SCS) summer monsoon onset. Using composite analyses, it is revealed that basin-wide warming (cooling) in the TIO apparently induces an anomalous reversed (intensified) Walker circulation over the tropical Indo-Pacific region, leading to anomalous descending (ascending) motion, and hence suppressed (increased) convection over the western Pacific. The intensified (weakened) western Pacific anticyclone in April and May prevents (favors) the extension of the Indian Ocean westerly flow into the SCS region, thereby causing a late (an early) SCS summer monsoon onset. The results suggest that the basin-wide SSTA mode in the Indian Ocean should play an important role in prolonging the El Niño-Southern Oscillation (ENSO) effects on the subsequent Asian summer monsoon, mainly through modifying the strength of the western Pacific anticyclone. Further, impacts of the Indian Ocean dipole (IOD) case on the SCS summer monsoon onset in 1994 are carried out. Coupled to the exceptionally strong positive IOD over the tropical Indian Ocean in early May, the Asian summer monsoon circulation around the Indian Ocean is significantly weakened, especially the Somali jet along the east coast of Africa and the low-level westerlies across the equatorial Indian Ocean. Anomalous anticyclones dominate the 850-hPa wind field in the tropical Indian Ocean, accompanied by easterly anomalies along the equatorial Indian Ocean. These features, induced apparently by the positive IOD event, are unfavorable for the establishment and advancement of the Asian monsoon westerlies, and lead to a delay of the SCS summer monsoon onset in 1994.