南亞高壓/西北太平洋高壓秋季東南位移的年際與年代際變化

Abstract

南亞高壓起始於南亞地區青藏高原的非絕熱加熱，產生上升氣流促使上對流層出現反氣旋結構。此高層反氣旋會在南亞地區與西北太平洋上對流層之間來回移動。春季至夏季，位在低緯度西北太平洋上空的反氣旋，往西北方移至青藏高原西側，秋季時由南亞地區往東南方移至低緯度西北太平洋地區。本論文主要探討北半球秋季南亞高壓往東南位移的年際與年代際變化。 定義北半球秋季為九、十、十一月共三個月份，利用200hPa水平風場決定南亞高壓中心位置。將研究時間區分成前、後20年（1958-1977、1978-1997），進行南亞高壓往東南位移的年代際變化討論。再選取1965、1972、1982、1986、1991、1997等六個聖嬰年，1964、1970、1973、1975、1988等五個反聖嬰年，討論南亞高壓往東南位移於聖嬰／反聖嬰年的變化。 在年代際的部份，前、後期南亞高壓的東南位移，均於夏末由青藏高原西側出發，冬初時移至西北太平洋副熱帶地區。十月上旬開始，前期則仍為平滑的東南位移過程，後期則呈現明顯向東跳躍的現象。由中南半島西北端跳躍過中南半島，出現在菲律賓上對流層。十月在中南半島出現低層輻散場，在菲律賓則出現非絕熱加熱，促使該區域產生低層西風距平、高層東風距平的反沃克環流結構，不利後期南亞高壓的東南位移，而產生向東跳躍。 在聖嬰／反聖嬰年的部份。南亞高壓於十月上旬之前，均平滑地往東南位移至中南半島北端。十月中旬開始，聖嬰年呈現明顯向東跳躍的現象，突然由中南半島東北端越過南中國海，出現在菲律賓上對流層。十月菲律賓出現非絕熱冷卻，同時伴隨低層反氣旋結構，加上聖嬰年於太平洋熱帶地區的反沃克環流結構，不利聖嬰年南亞高壓的東南位移，而產生向東跳躍。

Since late 1960s, the tropical east-west circulation including the South Asian anticyclone originated from the Tibetan Plateau during boreal summer is obtained. Following this finding, several studies find northwestward movement of the upper anticyclone from Southeastern Asia to the western edge of the Tibetan Plateau during boreal summer. In this study, on the other hand, the South Asian anticyclone moving southeastward to the western North Pacific during boreal autumn is discussed. The ERA40 data are used to decide the centers of the South Asian/Western North Pacific anticyclone (hereafter SAA) for the recent 40 years which are separated into two periods（1958~1977 and 1978~1997）based on several previous studies suggesting a clear climate jump occurred in late 1970s. Our results show that there is an obvious eastward jumping near the northern edge of the Indo-China Peninsula during southeastward moving of the SAA in October in later period, while it moves southeastward smoothly in the earlier period. Following this, the interannual variation of such movement and the role of ENSO on the southeastward movement of the SAA are studied. There is an abrupt eastward jump in El Nino years. However, the SAA moves smoothly to the area near the Philippines in early October and stays until November in La Nina years. Since the later period（1978~1997）contains more El Nino cases and the earlier period （1958~1977）has more La Nina cases, it explains the differences in the two period separated by 1977-78.