- 學位論文
台灣附近海域對西行颱風通過反應之數值模擬研究
Numerical Study of the Westward Moving Typhoon Induced Upper Ocean Responses in the Vicinity of Taiwan
摘要
颱風對海洋輸入大量能量並引起劇烈海洋反應，其中包括偏向路徑右側有冷尾跡以及顯著的慣性內波運動，而在背景流場與地形的影響下海洋反應過程會更為複雜。本文根據台灣附近海域觀測資料，進一步以數值模式探討更整體的動力過程，共分為三個子題: 1. 颱風向西通過呂宋海峽所引起海洋反應。 2. 颱風在台灣西南海域所引起海洋反應。3.颱風引起之渦旋與西方邊界流的交互作用。本研究所使用的數值模式為一個三維原始方程的海洋模式並採用二階的紊流閉合方法計算垂直紊流黏滯係數以適當模擬強風下海水中動量向下傳遞的過程。研究結果如下: 衛星觀測資料顯示颱風向西通過呂宋海峽後，在呂宋海峽東側的海表溫降逐漸往南方發展，有別於典型開放海域中海洋冷卻會偏向颱風路徑右側的現象，數值模式結果顯示北上的黑潮暖水與颱風在呂宋海峽以東所引起逆時針環流發生輻合可強化黑潮東測冷水的南流同時熱力風效應也有助於冷水往南傳送，另一方面，黑潮東側流切帶負渦度的區域容易聚集慣性內波的波能，由慣性內波引起的垂直混合也是冷水往南發展的因素之一。 在許多觀測資料中顯示台灣西南海域所產生的上層海洋溫度變化可依颱風路徑分為兩類: 1. 通過台灣及其以北的路徑將引起西南海域顯著的溫降(路徑A)，2. 若是颱風西行通過呂宋海峽並經由台灣以南海域靠近中國大陸沿岸(路徑B)，可能在西南海域引起增溫的效應。研究結果顯示若為路徑A的颱風，於西南海域感受到的風向主要為偏西風，則由風力引起的艾克曼流與非線性效應增強作用下使台灣灘處海流往外(東)側偏移伴隨內側下層海水湧升。若為路徑B的颱風，西南海域感受到的風向為偏東風，此時颱風引起的輻散海流則可能在此近岸淺水區域堆積並產生下沉流與混合層增厚的現象。 本文第三部分使用一個reduced-gravity model來解釋颱風所引起中尺度低壓渦旋進入西側邊界流域後逐漸衰退的過程。西方邊界流流軸上的水平流切以及渦度梯度是使渦旋發生不穩定並開始衰退的一項重要因素。低壓與高壓與西方邊界流交互作用中會有不一樣的能量交換過程，低壓將能量轉換至背景流場，而高壓可以自背景場取得能量。若為呂宋海峽內黑潮的情況，當中尺度低壓接近呂宋島東北側時，部分黑潮海流將向東繞過低壓渦旋，而呂宋海峽內黑潮則會偏向西使流幅增加流速大幅減弱，在β效應下套流(loop current)會向西沿伸。然而若低壓的強度較弱則呂宋海峽內黑潮的變化便不顯著。
並列摘要
As a typhoon moves over an open ocean, it generated cold wakes toward the right of its track. Recent researches also demonstrated that the background current and topography have strong influence on the ocean responses. This study uses a numerical model to investigate the dynamic process of some interesting responses observed in the vicinity of Taiwan. Three topics are addressed in this study: 1. the ocean response to a typhoon moving zonally across the Luzon Strait (LS), 2. the typhoon induced ocean response off southwest of Taiwan, and 3. the interactions between a typhoon induced mesoscale eddy and western boundary current. The numerical model used in the study is a 3-dimensional hydrostatic primitive equation model with a level-2 turbulence closure scheme. The conclusions for the three topics are as follow: The satellite measured SST shows that in the relaxation period of a typhoon zonally passing the LS, some cold anomaly water also present toward left (south) of the storm track to the east of the LS, different to the classic ‘right bias cooling’ behavior. Model results show that to the east of the LS, the convergence between the warm Kuroshio water and the cold wakes in the post-storm period will enhance the southward spreading of cold anomaly water. The enhanced vertical mixing, induced by the southward propagation of nearly inertial waves associated with the cold wakes, can also produce some cold anomaly to the south of a storm track in the post-storm period. Both mechanisms can contribute to the occurrence of some cold anomaly water to the south of the storm track east of the Kuroshio Current. Because of the strong influence of complex topography in the southern Taiwan Strait (TS), there are two kinds of upper ocean temperature responses, cooling/warming, relating to different typhoon’s tracks. Both the satellite and mooring measurements show that when a typhoon moved westward or northwestward and passed through Taiwan (track ‘A’), the cooling occurred in the southern TS; but when a typhoon moved westward or northwestward across the LS and passed through the area to the south of TS (track ‘B’), warming was induced. The model result shows that in the track ’A’ case, the southern TS is mainly under strong westerly. The Ekman flow and nonlinear effect will trigger upwelling around east of the Taiwan Bank. In addition, when the typhoon is passing through the TS, the strong southerly induces northward upslope bottom flow in the northern Penghu channel which also contributes to the cooling. In the track B case, the southern TS topography will confine the typhoon induced divergent flow, and thus the warm water transported to this area accumulates, resulting in downwelling. In other words, the heat content in the shelf area was redistributed. In the third part, a reduced-gravity model is used to study the interactions between a typhoon induced eddy and western boundary current (WBC). It shows that the gradient of the relative vorticity to the east of the WBC is an important factor which causing the eddy to become unstable. The cyclonic eddy loses its energy to the mean field, whereas an anticyclonic eddy can obtain energy from the mean flow during the WBC–eddy interaction. When a cyclonic mesoscale eddy approaches to the northeast of Luzon Island, a small branch of Kuroshio deflect toward the east, turn counterclockwise with the eddy flow, and then rejoin the Kuroshio main path at the northwest of the eddy. The Kuroshio in the LS deflect toward west and is weakened, resulting in westward extension of the loop by the β effect.