分析1958-2005 年NCEP/NCAR 月平均再分析資料結果顯示，夏 季北半球副熱帶高壓（簡稱副高）在西北太平洋區域有明顯的西伸或 東退的年際變化，其主要變化週期為2-3 年及3-5 年。 副高的2-3 年震盪在1990 年代之後較顯著，其海溫變化有向東傳 的特徵，北印度洋、海洋大陸到澳洲北部海域與中、東太平洋海域反 位相，前者從前一個夏季的冷海溫轉變成副高增強時的暖海溫，而後 者則從前一個夏季的較弱暖海溫，發展到副高增強夏季轉為冷海溫， 類似La Nina 發展。副高的3-5 年震盪主要發生在1980 年代，伴隨類 似長週期El Nino 的海溫發展，副高在El Nino 衰退的夏季增強，其前 一個夏季，中、東太平洋有較暖海溫而其周圍有較冷海溫，並維持到 副高增強的夏季，印度洋及南中國海則從前一個冬季出現暖海溫，之 後持續增強到夏季且往中國東南沿海延伸。 伴隨副高2-3 年震盪的對流及環流場變化有下列特徵，副高增強的 前一個冬季，菲律賓海附近出現反氣旋距平，到春季此系統明顯減弱， 副高增強的夏季，海洋大陸一帶出現強對流，其所伴隨的局部哈德里 環流於此上升，在西北太平洋下沉，維持西北太平洋地區顯著的高度 場正距平，此區內中、高層的下沉增溫與長波輻射冷卻維持高度場相 當正壓結構。前述副高增強的夏季對流及環流場特徵跟前一個夏季反 相位，類似熱帶準兩年發展。 副高的3-5 年震盪伴隨一個西北太平洋反氣旋環流，以及熱帶東 西向雙圈反轉環流，一圈由東太平洋上升，至西太平洋下沉，另一圈 由印度洋上升，至西太平洋下沉。這兩個環流可以從前一個冬季維持 到副高增強的夏季。持續到夏季的菲律賓反氣旋距平，中心北移至西 北太平洋，其高度場接近斜壓結構。其維持機制可以從局部冷海溫抑 制對流加熱所產生反氣旋距平（Rossby wave）開始，此反氣旋距平環 流透過風-蒸發-海溫正回饋機制可以從前一個冬季維持到夏季。 QTCM 大氣模式及簡單陸面模式證實，3-5 年震盪中之持續存在 的西北太平洋冷海溫距平，是夏季副高西伸的主要因素，2-3 年震盪 中海洋大陸一帶的暖海溫距平貢獻，有待其他模式的進一步分析。 This study analyzes the NECP/NCAR reanalysis data for the period of 1958-2005 to examine the interannual variability of the western North Pacific Subtropical High (WNPSH) in summer. The analysis reveals the WNPSH exhibits significant 2-3 years and 3-5 years oscillations with interdecadal variability. The 2-3-year oscillation is most evident after 1990. The associated SST distribution shows contrasting sighs in the region of northern Indian Ocean, maritime continent, north of Australia, and the Central-East Pacific region. SST in the former (latter) region evolves from cold (weak warm) anomalies in the preceding summer to warm (cold) anomalies in the concurrent summer of strong WNPSH. The above change exhibits an eastward propagation, and is similar to the La Nina development. In this oscillation period, it is also accompanied by anomalous meridional overturning circulation characterized by ascending motion in the maritime continent and anomalous descending motion near the Philippine Sea. The stronger downward motion causes drier air and more outgoing longwave radiation. Such a heat balance maintains the anomalous high with equivalent barotropic vertical structure The 3-5-year oscillation is most pronounced during the 1980s. The associated SST evolution is similar to that of El Nino with longer life cycle when WNPSH intensifies in the El Nino decaying summer. In the preceding summer, warm SST appears in the central-eastern Pacific, which is surrounded by cold SST to its north and south. The SST distribution persists till the concurrent summer of strong WNPSH. In Indian Ocean and South China Sea, warm SSTAs appear in the preceding winter and persist till following summer when warm SST extend to SE China coast. In this period, it is accompanied by anomalous descending motion over the maritime continent and ascending motion in the central-eastern equatorial Pacific and Indian Ocean that persists from the preceding winter to the summer. The complementary cooling and descending motion in the western Pacific are related to anomalous east-west circulation associated with ENSO. The anomalous WNPSH as part of the wave response exhibits baroclinic vertical structure The analysis suggests that the 2-3-year oscillation is due to the monsoon-warn ocean interaction in the maritime continent, and the 3-5-year oscillation is related to the suppressed convection over the cold SST in the western North Pacific, causing Rossby wave response (anticyclonic flow) that is maintained by wind-evaporation-SST feedback mechanism. An intermediate atmospheric model which is forced by SST anomalies obtained from the analysis is used to further examine the above mechanisms. The model results support our mechanism for 3-5-year oscillation, but no clear evidence for 2-3-year oscillation. More model studies are required to investigate the relevant mechanism.
