Interannual variability of summer monsoon onset over the western North Pacific

The western North Pacific summer monsoon (WNPSM) is an important element in the climate variability of western North Pacific (WNP) and East Asia. The summer monsoon onset is a critical stage in the seasonal evolution of monsoon.; The summer monsoon onset displays stepwise northeastward progress over South China Sea and WNP due to combined effects of the seasonal cycle and intraseasonal oscillation. The onset commences over South China Sea-Philippines around mid-May. It affects southwestern Philippine Sea in early- to mid-June. It reaches northeastern Philippine Sea after mid-July.; The large interannual variation of WNPSM onset is primarily due to large year-to-year change of the seasonal cycle. The role of the intraseasonal oscillation is secondary. A late (an early) onset usually corresponds to a late (an early) transition of the seasonal cycle from the dry to the wet phase in the WNP.; The large-scale sea surface temperature (SST) anomaly pattern of the tropical Pacific in boreal spring is an important factor in the interannual variation of WNPSM onset. When El Niño events occur, the anomalous heating over the equatorial central Pacific warm SST anomalies causes compensating anomalous downward motion over the WNP. The cold SST anomalies in the WNP induce anomalous low-level anticyclonic circulation and decreases mixed-layer temperature and moisture, through low-level divergence and surface heat fluxes. The remotely-forced and locally-forced changes can amplify through positive feedbacks. These changes lead to a delayed seasonal migration of convection and large-scale circulation and cause a late transition of the seasonal cycle from the dry to the wet phase in the WNP. The delayed setup of large-scale background leads to a late onset of WNPSM. Similar processes may work in La Niña cases except with opposite anomalies.; During El Niño, the remote SST forcing in the equatorial central-eastern Pacific has a dominant role in generation of large-scale upper-level circulation anomalies. Both the remote and local forcings are important for changes of low-level circulation and convective instability. During La Niña, the remote SST forcing is less effective that during El Niño, whereas the local SST forcing induces obvious changes of low-level circulation and convective instability.