黑潮水文流速時空結構之研究

Abstract

本研究使用於2012年至2023間在臺灣東部黑潮海域進行的26個研究船航次觀測所得的溫鹽深與流速資料，探討黑潮流速的統計狀況與結構模態的變化，並分析黑潮在受到渦旋撞擊、海表面梯度變化、風應力、風應力旋度等作用下之變化。 根據實測流速資料計算結果顯示此區域黑潮流量值在11.99–33.09 Sv (1 Sv=106 m3 s−1)之間，平均流量為 21.95 Sv，標準差為5.8 Sv。透過渦旋偵測方法，將航次觀測期間的黑潮分類成受渦旋(CE)與反渦旋(AE)的案例，結果顯示在CE的影響下黑潮上層300 m內有−0.05 m s−1流速減小，在水深200 m內121.9–122.2°E間有−0.1 m s−1的流速變異。在AE的影響時下層300 m內流速增加0.05 m s−1，在100 m內的水層於121.9−122°E間流速增加0.1 m s−1。CE (AE)的影響下黑潮東側等密面有+10 (−10) m的垂直位移，西側等密面則有−9 (+5) m的垂直位移，由熱力風關係裡的東西向密度梯度變化，造成北向流速與相應的流量減小(增大)，計算平均的實測流量為16.85(25.78) Sv。先前的研究結果示此斷面的海表面高度東西向梯度與北向流量的關係並不顯著，此研究中使用新出版的海表面高度計資料，重新計算並發現實測流量與海表面高度東西向梯度的相關係數R提升為0.74 (p=0.0001)。透過動力模態分析拆解流速，我們發現此斷面的北向流(V)正壓流與東西向海表面梯度相關係數為R=0.71 (p= 0.0003)，在斜壓流上此相關僅有R=−0.02 (p=0.9312)。此研究亦透過理想渦旋來計算渦旋於測線上之地轉流強度，並將其與動力模態分析結果作比較，結果發現渦旋的影響以正壓流場為主。透過動力模態分析，此研究分析風應力旋度與風應力和此區域黑潮的相關性，結果發現6個月前的北太平洋環流區域低緯度平均風應力旋度與黑潮的流速、流量強度有相關，此外也顯示本區域風應力會造成黑潮於此區域的擺動。

This study utilizes conductivity temperature depth (CTD) data and current measurements obtained during 26 cruises conducted off eastern Taiwan between 2012 and 2023 to investigate variations in hydrography and velocity structures of the Kuroshio and the underlying dynamics. The study explores how the Kuroshio responds dynamically to the impingement of westward-propagating mesoscale eddies, sea surface height (SSH) zonal gradients, wind stress, and wind stress curl variability. Statistics of measured current velocities indicate that the Kuroshio transport ranges from 11.99−33.09 Sv (1 Sv=10⁶ m³ s−1), with an average of 21.95 Sv and a standard deviation of 5.8 Sv. An eddy detection algorithm was applied to assist the analysis of eddy influences on the Kuroshio. Under the influence of cyclonic eddy (CE), northward velocity anomaly is approximately 0.05 m s−1 within the upper 300 m, with more pronounced variations up to 0.1 m s−1 occurring between 121.9−122.2°E at depths shallower than 200 m. Conversely, influences of anticyclonic eddy (AE) increase northward by approximately 0.05 m s−1 within the upper 300 m, and increases up to 0.1 m s−1 are noted within the upper 100 m between 121.9− 122°E. Under the influence of CE(AE), the isopycnal offshore side of the Kuroshio varied vertically by approximately +10(−10) m, while the onshore side isopycnal varied by −9(+5) m. Through the thermal wind relation, CE (AE) induced zonal isopycnal slope decrease (increase) leads to that the average measured Kuroshio transport was 16.85 (25.78) Sv. While previous studies have suggested a weak relationship between the zonal SSH gradient and Kuroshio transport along this section, our reanalysis using updated satellite altimetry data reveals a significant positive correlation between observed transport and zonal SSH gradient with correlation coefficient R of 0.74 and p=0.0001. Further analysis by applying dynamical mode decomposition (DMD) to the velocity structure shows a significant correlation between the barotropic (depth-independent) northward flow component and the zonal SSH gradient (R=0.71, p=0.0003), whereas the baroclinic (depth-dependent) component exhibits no significant correlation (R=−0.02, p=0.9312). Additionally, the study employed an idealized eddy model to calculate the geostrophic flow induced by eddies along the observational transect and compared the results with the DMD outcomes. The comparison suggest that eddy influences are primarily exhibited in the barotropic flow field. Finally, DMD was also used to assess the relationship between wind forcing and Kuroshio dynamics. Results show that the mean wind stress curl over the low-latitude North Pacific, lagged by −6 months, is correlated with the strength of the Kuroshio. Moreover, regional wind stress was found to influence the zonal migration of the Kuroshio maximum velocity axis.