Environmental features related to the decadal variation of South China Sea tropical cyclogenesis in the context of summer monsoon

This study investigates the changes in environmental conditions related to the decadal variation of genesis frequency of tropical cyclones (TCGF) over the South China Sea (SCS) during summer. Corresponding to the decadal increase in the TCGF, the seasonal mean anomalies of the genesis potential and environmental fields over the SCS are not favorable for tropical cyclogenesis, indicating their limited role in the decadal variation of SCS TCGF. It is found instead that the decadal change in tropical cyclogenesis over the SCS is more attributed to the local environmental fields associated with atmospheric intraseasonal oscillation (ISO). The decadal change in SCS TCGF is closely linked to the northward extension of ISO-related convection from the central SCS, which is contributed by the horizontal advection by background monsoon flow and the vertical advection by ISO-related vertical motions. Further analyses indicate that the anomalous upper-level cyclonic circulation over East Asia and the lower-level anticyclonic circulation over the western Pacific produce the unfavorable seasonal mean environmental fields in the SCS, whereas the resultant strong summer mean SCS monsoon flow facilitates the northward extension of ISO-related convection. This study highlights the importance of ISO activity for projections of the long-term change in SCS TC activity.