| Summary: | 碩士 === 臺北市立大學 === 地球環境暨生物資源學系環境教育與資源碩士班 === 104 === ABSTRACT
The observation revealed that the Mei-Yu rainfall over the South China in early boreal summer (June) experienced an abrupt increase and northward shift in the early 1990s. This abrupt change modified the dipole-like summer rainfall pattern in the East Asia to monopole-like structure (i.e., the out-of–phase relationship between the rainfall in South China and North China became in-of–phase). Analysis reveals that the enhanced and westward-extension of the western North Pacific high (WNPSH) was response for the abrupt change of Mei-Yu rainfall in the early 1990s.
Observation and numerical experiments further indicate that the enhanced WNPSH was primarily triggered and maintained by the low-level anticyclone anomaly in the western North Pacific (WNP), a Rossby wave response to the diabatic cooling anomaly in the WNP. The sea surface temperature and the diabatic heating in the eastern North Pacific, concurrently experiencing an abrupt increase in the late 1980s, forced an anomalous west-east Walker circulation in the Pacific, which in term the associated large-scale subsidence in the WNP may strengthen the anomalous diabatic cooling in the WNP and therefore contributes to enhance the anticyclone anomaly, that was also maintained and enhanced by the local air-sea interaction. The enhanced anticyclone anomaly can strengthen the Walker Circulation, form a positive feedback loop in the Pacific, lead to the surplus rainfall from the South China northward extension to the Yangtze River Valley region.
Keywords: interdecadal variation ; Mei-Yu rainfall ; western North Pacific subtropical high ; eastern North SST
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