| Summary: | 碩士 === 國立臺灣大學 === 大氣科學研究所 === 96 === Interannual variation of summer monsoons is strongly influenced by tropical oceans. Therefore, the Asian monsoon and the Australian monsoon, which separate by 6 months and by ten thousand kilograms, are well correlated, due to the impact of the SST anomaly in the tropical eastern Pacific and the tropical Indian Ocean.
The monsoon index defined by precipitation shows that the Asian-Australian monsoon (the AA monsoon) linkage abruptly changed from positive to negative correlation after 1990. To identify the variation of the AA monsoon linkage and take the asymmetry of the Asian-Australian monsoon and the Australian-Asian monsoon into account, we used the large scale OLR field, by putting two monsoon prevailing period together, JJA and DJF as the Asian-Australian monsoon linkage, and D(-1)JF and JJA as the Australian- Asian monsoon linkage, to do EOF analysis. The result indicates that after the abrupt change in 1990/91, the Australian monsoon had similar reaction to ENSO, while the Asian monsoon had distinct response in two periods.
Before 1990s, the AA monsoon was connected in ENSO developing and decay year. For example, in the preceding winter and the spring of the El Nino developing year, the whole tropical Indian Ocean became cool, which resulted in less water vapor, and leaded to dry Asian monsoon. In winter, El Nino matured, and suppressed the Australian monsoon rainfall by reversed Walker cell. As a result, the Asian monsoon and the Australian monsoon were both dry and in phase in the El Nino developing year. However, an anomalous anti-cyclonic circulation over Western North Pacific developed when El Nino matured, and persisted till the following summer to block the formation of the monsoon trough, which caused dry Asian monsoon, especially the Western North Pacific monsoon (WNPM).The Asian-Australian monsoon and the Australian-Asian monsoon were asymmetric owing to the impacts of ENSO to the Asian monsoon were different. Low frequency ENSO was dominate in this period, and made the AA monsoon in phase by combining two effects of ENSO.
However, in 1990s, Indian Ocean dipole (IOD) turned active. IOD and the Asian monsoon, especially the WNPM, became the main part of the air-sea interaction in summer. In the El Nino developing year, the positive IOD, which accompanied El Nino in summer, made the low level atmosphere over the western ocean of Sumatra dry and cool, and convert this place into a subsidence region to support the Asian monsoon in the north. The Asian monsoon and the western ocean of Sumatra formed a reversed local Hadley cell and intensified by each other. Consequently, the Asian monsoon was wet rather than dry in the El Nino developing year (the El Nina year was opposite to the El Nino year) and thus the AA monsoon linkage changed from positive to negative correlation, and simply responded to ENSO in developing year. The Asian-Australian monsoon and the Australian-Asian monsoon were anti-symmetric and had TBO frequency because of the interaction between the Asian monsoon and IOD.
The monsoon index also implies that Indian Ocean influenced the tropical eastern Pacific actively in 1990s. When IOD and ENSO are well-coupled, the low level wind over tropical western Pacific would be changed, and accelerates the phase turning of ENSO through the oceanic Kelvin waves. In 1990s, the Indian Ocean turned active, and had a feedback mechanism of the monsoon- tropical ocean system, the interaction with the Asian monsoon, which not only change the period of ENSO into TBO frequency, but also the AA monsoon linkage and the asymmetry of the monsoons.
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