| Summary: | 碩士 === 國立臺灣大學 === 大氣科學研究所 === 88 === Abstract
The western North Pacific monsoon locates within an area (110E~170E, 5N~25N) over the Philippine Sea, an oceanic monsoon by nature. The rainy season starts from late July to September, a month after the peak of Indian monsoon outbreak. The bulk of cloud clusters take path from dateline toward northwestern direction. The rain-bearing systems mostly comprise typhoon and tropical storm type, all embedded in a giant monsoon gyre.
To study interannual variability of the WNPM, it is crucial to measure monsoon strength. We found that the annual summer rainfalls over WNPM are highly correlated to low-level JJA zonal wind anomalies in South China Sea (SCS); hence a SCS index is defined. The fact that WNPM rainfall is predominately controlled by flow pattern over the SCS, instead of in situ SST (as generally conceived among the typhoon community), suggests that the interannual variability of the WNPM is resulted from large-scale dynamical process. It also relates closely to annual cycle, since the SCS is noted for an endurable cross-season memory. A scattering diagram, the diagram of the SCS dynamic index (summer) vs. SCS SST in previous winter, shows a linear relationship exists, specially for cases from NIINO(0)/NINO(1) winter and NINO(1) summer. A group of 6 cases of NINO (66, 73, 83, 88, 95, 98) are chosen to compose this particular half-year cycle.
During NINO(0)/NINO(1) winter, the high SST anomaly was found in tropical eastern Pacific, so did and the rainfall anomaly. An anomalous anticyclone, stressed by Wang et al (2000) as to be maintained by remote NINO forcing and local SST gradient, appears most prominent. This anomalous anticyclone sends southwestern wind anomaly against the East Asian winter monsoon, such that cold surges along coast are severely suppressed. Since the Aleutian low is enhanced by a PNA pattern and the Siberia High shifts toward Japan, the course of winter storms also deviates to central Pacific, leaves the South China Sea a void witnessing fewer cold fronts penetrating south of 30N. A warm SST section extends from the Indo-China Peninsula, along Kuroshio, to Japan.
In NINO(1) spring the eastern Pacific SST anomaly becomes rapidly wilting whereas the anomalous anticyclone starts to adjust to local boundary condition. The WNP anticyclone anomaly migrates to cold SST in lower latitude and develops a vertical structure that of a baroclinic gravest mode. Its intensity keeps increasing even when the remote forcing dwindling. Evidently this spring warm water in the SCS and subtropical Pacific High ridge have been phase-locked in a coherent way. Since the anomalous high is now superimposed by the seasonal SE flow, it leads to more stirring and positive sensible heat flux from air to sea. The heat stored in the SCS subsurface since winter is expected to be brought to surface. In NINO(1) summer the anomalous convection caused by warm SST anomaly induces easterly wind which, in turn, shifts the confluence zone toward west to the SCS and Indo-China peninsula, the WNPM is occupied by strong Subtropical Pacific high, hence a dry zone. The dry anomaly in the WNPM emits a teleconnection pattern, the Pacific-Japan pattern, a multi-belt structure alternating a dry zone in Yang-Tze region and a wet zone near Yellow River.
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