In this paper, we first elucidate the significant seasonality in long-term trends in the Asian monsoon on a monthly mean basis. Advanced monsoon onsets over the Bay of Bengal and the western Pacific were evident in recent decades. Increasing rainfall in May along 10°N reflected the advanced monsoon onset. Decreasing rainfall trends in June along 10°N were also detected. Because the rainfall trends in July and August showed less significance, the monsoon transition phase should be discussed in the context of climate change rather than boreal summer mean field. The advanced monsoon onset and weakening of the monsoon during early summer are most likely to be attributed to the heat contrast between the Asian landmass and the tropical Indian Ocean. The heating trend over the Asian landmass primarily contributes to the heat contrast variability with the persistent SST increase in the Indian Ocean throughout the season.