The rainy season over southern China (SC) begins with the onset of the South China Sea monsoon around mid-May. We demonstrate here that the SC precipitation exhibits a notable drying (wetting) trend prior to (after) the monsoon onset, leading to a more abrupt thus amplified hydrological transition over SC in early summer. The contrasting precipitation trends are largely driven by the recent decline in mid-latitude synoptic eddy activity. Weakened mid-latitude synoptic disturbances suppress SC frontal activities before the monsoon onset, facilitating the drying trend, and enhance the spatial stationarity of the SC monsoonal front after the monsoon onset, leading to the wetting trend. CMIP6 models in general fail to capture the contrasting trends between the two periods. In models where both the trends in precipitation and mid-latitude synoptic eddy activity are well captured, the amplification of the hydrological transition over SC is projected to continue well into the late 21^st century. Our results highlight the footprints of hemispheric-scale climate change in regional hydroclimate and the obvious challenges faced by climate models in capturing such footprints.