When fully established in mid-August, the western North Pacific summer monsoon (signified as WNPM) becomes as intense as, or even more dominant than, the Southeast Asian summer monsoon (abbreviated to SEAM). The boundary between the WNPM and SEAM appears to exist somewhere over the South China Sea where relatively dry weather persists with a climatological OLR exceeding 230 Wm<-2>, in contrast to a much smaller value of less than 190 Wm-2 at updraft portions of the WNPM and SEAM. The major downdraft leg is located over the central North Pacific where the divergent Pacific High is capped by the convergent Tropical Upper Tropospheric Trough. This implies a dominant E-W vertical overturning which extends from about 110°E (South China Sea) to 140°W (central North Pacific), with active convection occurring over the key WNPM domain (10°-20°N, 130°-150°E) of the world's highest SST (in excess of 29°C). This E-W overturning along l0°-20°N is baroclinic in the vertical, i.e., upper-level westerlies overlying low-level easterlies eastward of 150°E, while there is a superposition of upper-level easterlies above low-level westerlies westward of that longitude. Presumably, the WNPM is driven by combined effects of zonal asymmetry in SST at 10°to 20°N, and E-W (continent-ocean) heat contrasts between about 20°and 30°N. Absence of a large land mass poleward of the WNPM domain relegates the contribution due to N-S differential heating to that of secondary importance. In contrast, the SEAM can be envisioned as a distinct meridional vertical overturning, primarily driven by N-S (continent-ocean) heat contrasts. The SEAM withdraws before early October due to commencement of continental cooling. The WNPM lives much longer because of the persistence of a high SST (> 29°C) until the beginning of November.
Meteorological Society of Japan