本研究使用1958-2002年的ECMWF再分析資料(ERA40)分析夏季西北太平洋副高的緯向變化，目的是要瞭解造成副高異常緯向變化的物理機制。由850hPa高度場可知西北太平洋副高西側(110°-150°E,15°-30°N)有著相當明顯的緯向年際變化，因此選定此區域之夏季850hPa高度場平均做為一副高指標，當指標偏高(低)時，代表夏季西北太平洋副高異常西伸(東退)的現象。由於西北太平洋副高在1979年後有更為西伸之趨勢，因此又分為ID1 (1958-78) 和ID2 (1979-02) 兩時期來各自討論其異常緯向變化。 藉由合成分析法探討ID1與ID2夏季西北太平洋副高西伸(東退)時的SST與環流場，發現影響副高異常緯向變化主要有兩個機制：一是東太平洋有暖(冷)SST距平，改變東西向的沃克環流，並於副熱帶西北太平洋產生反氣旋(氣旋)式環流場，此機制出現在ID1副高西伸以及ID2副高東退合成年。另一是當印度洋、印尼一帶SST增暖(變冷)，伴隨對流活動增強(減弱)，在副熱帶西太平洋出現低層輻散沉降(輻合上升)場，有利副高西伸(東退)，此機制出現在ID1副高東退、ID2副高西伸與東退合成年。 由環流場的季節發展可發現，在ID1副高東退以及ID2副高西伸合成年，位於西太平洋有異常環流場從前一個冬季持續至夏季。在ID1副高東退的前一個冬季，熱帶中、東太平洋有異常冷SST，改變東西向的沃克環流於菲律賓一帶產生氣旋式環流，到了春季之後，印尼一帶冷SST距平使氣旋式環流得以延續至夏季。在ID2副高異常西伸年前一個冬季至春季，熱帶中、東太平洋有異常暖SST，並於西太平洋形成反氣旋環流，到了夏季，印度洋暖SST距平增強，伴隨著東印度洋至印尼一帶強對流運動發展，使西太平洋反氣旋環流可延續至夏季。 The extreme zonal variations of the Western North Pacific subtropical high in summer are studied using ECMWF reanalysis data (ERA40) for 1958-2002. It has been shown that the North Pacific subtropical high displays substantial interannual variability at its western edge, so we defined a WNPSH index as the June-July-August (JJA) mean geopotential height anomalies at 850hPa averaged over the west edge (110°-150°E, 15°-30°N) of WNPSH. According to the WNPSH shows westward extention after 1979, the datasets are partitioned into two periods, 1958-78 (ID1) and 1978-96 (ID2). We use composite analysis to examine the SST and circulation when the WNPSH moves westward (eastward) in these two period. The results show that the enhanced (suppressed) convection caused by warmer (colder) SST in the equatorial eastern Pacific or tropical Indian ocean, leading to an anomalous anticyclonic (cyclonic) circulation over the subtropical western Pacific. In a ID1 WNPSH eastward year and a ID2 WNPSH westward year, an anomalous circulation located in the western North Pacific would persist from preceding winter to summer. In a ID1 eastward year, cold SST appears in the tropical eastern Pacific in the previous winter, whereas cold SST associated with weak convective activity over the Indonesia is contributed to the anomalous cyclonic circulation from the spring to the summer. An anomalous anticyclonic flow in a ID2 westward year is preceded by a warm tropical eastern Pacific from the previous winter to the spring and followed by a warm eastern Indian ocean in the summer.
