Summer monsoon onset in the subtropical western North Pacific

Monsoon onset in July over the subtropical western North Pacific (SWNP, 15–25°N; 130–150°E) is investigated by focusing on the westward movement of cloudy region east of the SWNP, which is associated with high vorticity in the upper troposphere. At least in seven of the 22 years between 1985 and 2006, the arrival of clouds at the SWNP from the east coincides with a significant change in the upper tropospheric circulation and a rapid northeastward extension of strong convections from the tropical western North Pacific, which essentially is monsoon onset. Before the monsoon onset, the sea surface temperature (SST) increases, but winds remain divergent over the SWNP. Right after the monsoon onset, winds turn convergent and convections enhance, leading to a rapid decrease of surface heating and SST. It is suggested that westward‐moving upper‐level disturbances might trigger onset of monsoon in July when low‐level atmospheric conditions favor development of deep convections.