黑潮中水團層疊交錯和紊流特性之觀測與研究

Abstract

本研究使用水下滑翔觀測儀(Seaglider)在臺灣東南部海域進行高解析度的水文觀測資料，探討北太平洋中層水(North Pacific Intermediate Water)和南海中層水(South China Sea Intermediate Water)兩水團之間層疊交錯現象(interleaving)的動力過程。Seaglider為自主式水下載具，利用改變自身浮力的方式，往返於海面至水深1000 m之間取得水文剖面資料。本研究使用在2011、2012及2016年在臺灣東南部海域以沿著三角形測線進行連續性的觀測，其中每航次中皆有數百回從海面至水深1000 m的水文剖面資料。 從Seaglider觀測的水文資料顯示，在東西向N1測線上的西測經常觀測到南海中層水，而在測線的東側經常觀測到北太平洋中層水，並且可在121.8°E至122.1°E之間觀測到分隔兩個不同水團的鋒面。利用觀測的水文資料計算跨等密度面鮮度曲線(diapycnal spiciness curvature)判別水團層疊交錯的現象，從結果顯示N1測線中在中層水(intermediate water) 400 m到800 m之間有南海中層水入侵北太平洋中層水的水團層疊現象，其水平尺度為O(10-100) km，垂直尺度為O(10-100) m。此外，由Turner angle判別海水在穩定分層下是否有鹽指擴展(salt fingering)或擴散對流(diffusive convection)的現象，結果顯示當北太平洋中層水在南海中層水上面時，在介面上多發生擴散對流的現象。相對的，當南海中層水在北太平洋中層水之上時，在介面上多發生鹽指擴展的現象。說明雙重擴散驅動南海中層水和北太平洋中層水在黑潮橫向方向互相入侵，使得在靠近臺灣東岸可以發現到北太平洋中層水，而在124°E還能發現類似南海中層水的原因。 然而，此海域的垂直渦流擴散率κ_ρ (vertical eddy diffusivity of density)會影響上下兩層不同水團混合的速度，但由於缺乏紊流儀在臺灣東部的黑潮裡直接量測紊流，本研究使用2012至2017年研究船在臺灣東部海域三條東西向測線KTV1、KTV2和KTV3上觀測的高垂直解析度水文資料，藉由Thorpe scale方法推算發生密度翻轉(overturning)區域的紊流動能消散率ε (turbulent kinetic energy dissipation rate)和垂直渦流擴散係數。Thorpe scale方法推算結果顯示臺灣東部海域在中層水區域的ε為O(10-9-10-7) W kg-1，κ_ρ為O(10-4-10-3) m2 s-1，而紊流儀量測的結果顯示ε為O(10-10-10-8) W kg-1，κ_ρ為O(10-6-10-3) m2 s-1，相對於一般大洋中κ_ρ在O(10-5) m2 s-1 大的許多，可能造成在水團層疊交錯的區域較快的垂直混合了南海中層水和北太平洋中層水。

To examine the water masses exchanges in the Kuroshio, thousands of high-resolution CTD profiles (salinity, temperature and depth) from the sea surface down to 1000 m depth where acquired by Seagliders across the Kuroshio off the southeast coast of Taiwan. Results from the observations quantify vigorous layered intrusions of South China Sea Water to North Pacific Water transported by the Kuroshio, resulting in the interleaving of the two water masses, particularly in the intermediate layer. The diapycnal spiciness curvature was used to characterize the magnitude of interleaving, suggests that significant interleaving mostly occurred in the intermediate layer between 400 m and 800 m, and the vertical and horizontal length scales were O(10-100) m and O(10-100) km, respectively. The Turner angle, which is a useful parameter for classifying salt fingering and diffusive convection processes, suggests that double diffusion is a dominant process in the interleaving regions. In addition, in order to quantify the turbulence property in the Kuroshio, shipboard hydrographic surveys at the KTV1、KTV2 and KTV3 transect were analyzed. The turbulent kinetic energy dissipation rates (ε) and vertical eddy diffusivity of density (k_ρ) estimated using the Thorpe scale, average value of ε and k_ρ is O(10-7-10-9) W kg-1 and O(10-4-10-3) m2 s-1 at depths between 600 m and 1000 m. This is a plausible cause for the fast mixing of South China Sea Intermediate Water and North Pacific Intermediate Water in the Kuroshio.