Upper-tropospheric forcing on the western North Pacific summer monsoon

• Main Authors: Chi-Hua Wu, 吳奇樺
• Other Authors: 柯文雄
• Format: Others
• Language: en_US
• Published: 2010
• Online Access: http://ndltd.ncl.edu.tw/handle/01244011812513581627

博士 === 臺灣大學 === 大氣科學研究所 === 98 === This study investigates the maritime monsoon onset by identifying the rapid inten-sification of convections at the subtropical western North Pacific (SWNP, 130-150°E; 15-25°N). Relating to the onset, the westerly wind in the tropical western North Pacific (TWNP, 125-140°E; 5-15°N) increases and the monsoon trough (MT) deepens and ex-tends northeastward to the SWNP. Corresponding to the strengthening of the MT, the ridge of the subtropical high-pressure shifts northward by ~10° to the south of Japan, and the rain band at the northern flank of the ridge weakens. The summer monsoon in the western North Pacific-East Asia (WNP-EA) sector shifts to a new monsoon phase corresponding to the SWNP onset. Normally the monsoon onset occurs in mid to late July (normal onset). At the nor-mal onset, clouds move into the SWNP from the east, which coincides with a significant change in the upper tropospheric circulation. It is likely that the moving convective clouds are induced by the westward and southwestward movement of up-per-tropospheric high potential vorticity from the mid-oceanic trough. Correspondingly, there is a rapid northeastward expansion of strong convections from the TWNP. Prior to the onset, the sea surface temperature (SST) over the SWNP increases due to strong surface heating associated with weak winds and clear skies, while winds remain diver-gent. 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 west-ward-moving upper-level disturbances might trigger onset of monsoon in mid to late July when low-level atmospheric conditions favor development of deep convections. In some years, the onset of maritime monsoon occurs earlier in late June to early July (early onset). Prior to the early onset, there is no significant development of con-vections east of the SWNP as in the normal onset, and the convection is very active over the equatorial West Pacific (east and northeast of New Guinea). These convections ra-pidly move northward and northeastward to the SWNP, and trigger maritime monsoon onset. Despite the different triggering between the normal onset and the early onset, the surface heating and lower-tropospheric conditions surrounding the two onsets are very similar. It is concluded that the westward moving clouds associated with the up-per-tropospheric disturbance in mid-July play a critical role in triggering monsoon onset in normal years. In some years when tropical convection over the equatorial West Pacific is active above normal in early summer, the onset at the maritime monsoon might occur much earlier. The rapid intensification of convection in the SWNP during the maritime monsoon onset is remotely forced instead of locally forced by high SST as suggested by some studies.