| Summary: | 博士 === 中國文化大學 === 地學研究所 === 90 === Abstract
The tropical low-frequency oscillation involves complex interactions of ocean-atmosphere-land of multiple temporal and spatial scales. Its study has became a major focus of atmospheric sciences in recent decades. So far, two significant low-frequency signals are found in the tropics: one is the 30~60 days "Intra-Seasonal Oscillation", while the other is the so-called "Southern Oscillation". In spite of the fact that the above phenomena are resulted from different mechanisms, yet both phenomena involve significant convective latent heating and air-sea energy exchange. Thus examining the role played by large-scale circulation, tropical cumulus heating, and sea surface temperature variations becomes a key to understand the above tropical low-frequency oscillations. In this study, we utilize NCEP reanalysis data of atmosphere and SST, CMAP precipitation to examine the evolution and structure of atmospheric low-frequency oscillations, along with the relationship with SST variations. We also utilize UCLA AGCM to simulate the atmospheric response to SST variations in equatorial eastern Pacific, north western Pacific, and Indian ocean, respectively.
Observations indicate that the 30~60 days oscillation consists of wavenumber 1 or 2 precipitation signal developing in Tropical Indian ocean. The precipitation signal propagates eastward with a phase speed of 10~15 m/s along the equator. Its strength peaks in winter and spring. During the El Nino years, from September to the following May, the signal is weaker than normal in Indian ocean and western Pacific but stronger in eastern Pacific. Estimating the sea surface evaporation-wind feedback flux exhibits a 15 days lead time against the precipitation field which favors eastward propagation. The magnitude of transient evaporation flux shows that its not the major energy source for the maintenance of 30~60 days oscillation, but to play a secondary role in modulating the amplitudes and seasonal variations of tropical 30∼60 days oscillations. Analysis using singular value decomposition further indicates that the evaporation-wind feedback flux interacts positively with surface westerly in the tropical western Pacific and Indian Ocean, while interacts negatively in the tropical eastern Pacific and Atlantic Ocean.
During the El Nino year , SST increases in the eastern Pacific between 10°S to 10°N , the maximum amplitude is about 4ºC. SST cooling occurs in the subtropical western Pacific at the same time. After mature phase, the negative SST anomalies switch to positive anomalies in the south western Pacific, while the negative SST anomalies in the north western Pacific can maintain several months later, showing asymmetric SST pattern in the western Pacific. An anti-cyclonic circulation is found over the north western Pacific which may increase moisture flux and precipitation in the regions of Taiwan and southeast China in February , March and April.
Numerical simulations indicate that the eastern Pacific SST variations is the major factor responsible for interannual variability while the Indian Ocean and Western Pacific SST play the secondary role in modulating the regional circulation and climate. The north western Pacific experiment shows that compensate cooling will increase local anti-cyclonic circulation and thermally indirect circulation over the western Pacific in the northern hemisphere. Indian ocean experiment shows that the convective activities are locked in equatorial which may delay the Indian monsoon onset. The atmospheric non-linear response occurs mostly in mid-latitude of winter hemisphere.
The relationship between Taiwan''s spring precipitation and Nino3.4 winter SST anomaly exhibits high correlation in the period of 1979-2001, especially in strong El Nino years, and so does the relationship between Taiwan summer precipitation and the western North Pacific summer monsoon index (WNPSMI). In normal year, Strong (weak) WNPSM results in the increase (decrease) of Taiwan summer precipitation. During ENSO years, major rain bands move eastward to 130°E∼160°E、10°N∼30°N which decreases the summer precipitation in Taiwan. On the other hand, the WNPSM tends to be out of phase with the East Asian summer monsoon.
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