臺灣東部外海氣旋渦引起黑潮截斷之研究

Abstract

本研究探討氣旋渦西移至臺灣東部外海，撞擊黑潮後，對黑潮造成的影響。研究結果顯示，在研究期間 (1993年1月至2016年4月)，衛星高度計資料指出，氣旋渦西移至臺灣東部外海 (123°E) 共發生18次，並且其中8次發生後有伴隨黑潮入侵呂宋海峽事件。氣旋渦西移路徑大致可以分3種：(1) 從呂宋島東側往西北移動(18°N~20°N) (2)從呂宋海峽東側向西移(20°N~22°N) (3)從臺灣東部外海西移(22°N~23°N)。本研究針對第三種氣旋渦移動路徑：從臺灣東部外海西移的氣旋渦事件進一步分析，發現當氣旋渦西移至123°E時，臺灣東部外海的黑潮受到氣旋渦影響，導致黑潮向北的流速減弱，隨後在黑潮上游處發生黑潮入侵呂宋海峽事件，並在臺灣西南產生順時針渦流。另一部份黑潮則向東沿著氣旋渦以逆時針方向向北流動，與Kuo& Chern (2011)有相同的結果。氣旋渦由黑潮東邊向西傳遞時，向西移動到經度123°E氣旋渦強度就會開始減弱，可能是受到黑潮北向流的影響而減弱並往北移，與Liang et al. (2003)所定義的黑潮東邊邊界123°E相符合。進一步用ROMS模式（Regional Ocean Modeling System）模擬，也同樣發現氣旋渦西移到臺灣東岸撞擊黑潮後，臺灣東岸的黑潮流速減弱，且發現黑潮上游有入侵呂宋海峽的現象，在臺灣西南也發現一順時針渦流產生。由ROMS模式結果輸出的地轉流場，計算呂宋海峽區域內的渦度情形，發現當氣旋渦碰撞黑潮時，渦度值從低值有顯著增加的現象發生，為黑潮入侵呂宋海峽所造成。進一步藉由渦度變化來解釋黑潮入侵呂宋海峽現象，當氣旋渦西移撞擊黑潮，導致臺灣東岸的黑潮向北流速減弱，進而造成呂宋海峽東側的黑潮流速減弱，使得呂宋海峽區域的渦度產生改變(正渦變小)，為了維持區域內渦度守恆，因此黑潮向西轉而入侵呂宋海峽(正渦增加)。

This research focus on the influence of cyclonic eddy on Kuroshio on outer region of eastern Taiwan sea. According to data of satellite altimeter, during the study period (January 1993 to April 2016), cyclonic eddy moved westward to the outer region of eastern Taiwan sea (123°E) for 18 times, among eight of them. When there is Kuroshio intrusion at Luzon Strait. The eddy westward path can be roughly divided into these three types: (1) moves from east of Luzon Island to northwest (18°N~20°N) (2) moves from east of Luzon Strait to west (20°N~22°N) (3) moves westward from Taiwan eastern sea (22°N~23°N). This study focus on the third cyclonic eddy moving path: a further analysis on west moving cyclonic eddy event at outer eastern region of Taiwan sea, and found that when the cyclonic eddy moves westward to 123°E, the Kuroshio met a cyclonic eddy, it results a decrease in velocity for the flow of Kuroshio. Followed by an intrusion into the Luzon Strait at upstream of Kuroshio, and occurs a clockwise eddy in the southwest of Taiwan. The other part of the Kuroshio flows eastward, and then northward when it flows along the cyclonic eddy counterclockwise, attaining a same result as Kuo& Chern (2011). When the cyclonic eddy is transmitted from the east to the west of the Kuroshio, and move westward to the longitude of 123°E, the vorticity of the cyclonic eddy begins to weaken, which may be influenced by a northward flowing Kuroshio, results it to move northward, according to definition by Liang et al. (2003), where the boundary for east of Kuroshio is 123°E. After further simulation using ROMS model (Regional Ocean Modeling System), a same result appeared that the eddy at eastcoast of Taiwan weakened, when met Kuroshio and appeared the phenomenon of intrusion into Luzon Strait at upstream of Kuroshio. A clockwise eddy was also found in the southwest of Taiwan. From the geostrophic field output by ROMS model, the vorticity calculated for Luzon Strait shows when the cyclonic eddy collides Kuroshio, the vorticity value increases significantly, so the changes could be concluded that the vorticity is influenced by the intrusion of Kuroshio into the Luzon Strait. Furthermore, the vorticity change could be used to explain the phenomenon of Kuroshio intrusion into Luzon Strait. When the west shifted cyclonic eddy met Kuroshio, resulted Kuroshio has a weakened northward flow on east coast of Taiwan, which in turn weakens the velocity of Kuroshio flow on the eastern side of the Luzon Strait. The vorticity in Luzon Strait area changes (positive vorticity becomes smaller), in order to maintain the conservation vorticity of in the region, and so when westward Kuroshio moves in and invades Luzon Strait too (positive vorticity increases).