臺灣西南海域春、秋季之流場低頻變化

Abstract

本研究根據錨碇式都卜勒流剖儀(Acoustic Doppler Current Profiler, ADCP)及RCM海流儀(Recording Current Meters, RCMs)現場海流觀測，並搭配船測海流資料、沿岸水位資料、風速資料、漂流浮標資料以及數值模式模擬結果，藉以探討臺灣西南海域低頻流場型態之變化特性。所用之RCM錨碇觀測資料包含1997、1998年間於此海域東、西兩側各一組位於水下50公尺處的海流記錄(總計共有三段資料，時間上約落在夏季、冬季與春季，每段資料時間長度約為一個月)，RCM觀測資料顯示靠近臺灣端的東側測站海流主要沿西北──東南走向變化，而位於臺灣淺灘南方的西側流場變化則主要在西南──東北走向上，二者大致與當地海底地形走向一致，變動之週期略小於一個月，暗示呈現類似於中尺度之順鐘向(clockwise)或逆鐘向(anti-clockwise)渦流等兩種流場型態，後二者亦見諸於船測海流資料中。此外，2004年至2013年間在高雄外海尚有一組300kHz ADCP單點觀測記錄(該測站係於每年春、秋兩季分別觀測一次，每次測量時間至少為一個月，測量水層厚度約為100公尺)，本文採用主軸分析(principal analysis)探討此組資料所呈現的海流低頻變化，以統計方法估算此組流場資料的低頻變化方向、動能以及流速變化量；分析結果顯示，低頻海流之主要變化方向為西北──東南走向，海流動能與流速變化量呈現隨深度而遞減的情形。另方面將兩站沿岸水位資料以及海峽風速資料預作截止週期為48小時之低通過濾(low-passed)後，再搭配深度60公尺處之低頻流速資料，利用經驗正交函數(Empirical Orthogonal Function, EOF)方法共取得4個模組，其中Mode 1和Mode 2共可解釋最少七成以上之總相關係數(correlation coefficient)，而Mode 1之振幅時序變化則與沿岸水位時序變化趨勢十分相近。 為了解臺灣西南海域流場在水平空間上之分布態勢，本文進一步分析漂流浮標資料以及呂宋海峽海域現報預報系統模式(Luzon Strait Ocean Nowcast/Forecast System, LZSNFS)之計算結果，發現本海域較常出現順鐘向型態的流況，後者之形成，可能源於黑潮經呂宋海峽入侵南海北部所形成之向西北方傳遞的渦旋(Eddy)、或是受黑潮入侵南海北部後之行徑所影響。當由南海向北傳遞的渦旋進入臺灣西南海域後，由於受到當地海底地形之限制且渦旋能量在傳遞過程中會逐漸消散，此種渦旋型態流況之存在時間約為一個月左右，而後海域又回復為平均狀態，此種類型之事件目前統計共有五次。

Flow patterns of the southern opening of Taiwan Strait were investigated by incorporated moored observations, wind speed measurement (from the weather station in Taiwan Strait) and sea-level measurements (from two nearby tide station), among them the moored current data include two sets of Recording Current Meters(RCMs) observations (from 1997 to 1998) and one set of ADCP data (from 2004 to 2013). Aside from these, trajectories of surface drifters and model results from the Luzon Strait Ocean Nowcast/Forecast System (LZSNFS), a data-assimilating, dynamical ocean model routinely operated by Naval Research Laboratory, USA, were used to display the simultaneous spatial structure of the flow field, which facilitates the interpretation of the significance of the variations of currents, from moored observations, associated to the larger scale flow pattern. All the time series data were pre-processed by low-passed filtration (the cut-off period of the filter used is 48 hours ), afterwards the orientation of the low-passed current data were projected to the principal and minor axes, in which the direction of the former is roughly in parallel to local isobaths. From the current data, five events could be identified, if an event is defined by the current reversal, i.e. as the direction of the low-passed current offshore Kaohsiung reverses from northwestward to southeastward within a few days. Model results from LZSNFS suggest this phenomenon is closely related to the process of the intrusion of Kuroshio from the Luzon Strait to the northeastern South China Sea (SCS), this process may induce an anti-cyclonic (clockwise) flow pattern offshore the southwest Taiwan. Statistics from the Empirical Orthogonal Function analysis (among low-pass filrated ADCP currents, sea-level and winds) and simultaneous results from LZSNFS suggest that, once an anti-cyclonic flow pattern moved into the observation region offshore southwest Taiwan, the costal sea-level near Kaohsiung descended and the low-frequency currents offshore Kaohsiung turned toward the southeast. This kind of current reversal events occurred frequently in both spring and autumn.