Climatologically, summer monsoon starts in mid-May over the South China Sea (SCS). The period from April to June signifies the transition from spring to summer over the SCS. Advanced or delayed spring to summer transition may have important implication for summer climate. Two types of abnormal spring to summer transitions over the SCS are presented in this talk. The first type is associated with persistent anomalies during the transition season. This type of abnormal transition is related to large-scale sea surface temperature (SST) anomalies in the tropical Indo-Pacific region. The other type of abnormal transition is related to the anomaly change from April to June. This type of abnormal transition is related to regional air-sea interactions around the SCS. A clear signal of local air-sea interaction is identified over the SCS.