南海深層水流量之探討

Abstract

根據海底地形量測資料顯示，在太平洋與南海 (South China Sea, SCS)之間，在大約東經121.5 度的位置，存在兩個深水通道。此兩個深水通道推測為深層 (大於2000 公尺)海水交換的路徑；其中，較北方的通道位於台灣的東南邊，以『D1』表示，而較南方的通道則位於呂宋海峽 (Luzon Strait)的東邊，以『D2』表示。透過實際觀測當地海流變化發現，深層海水主要是經由『D2』通道流入南海內部，平均流量約為1.08 (±0.32)Sv (1 Sv=106 m3s-1)，而經由『D1』通道傳輸的流量由於過小 (僅約-0.09±0.03 Sv)故予以忽略。根據容積守恆 (volume conservation)原理，可藉由流入的流量估算出底部冷水的滯留時間 (residence time)，以及在水深2000 公尺等深線處的平均湧昇速度 (upwelling)。根據研究結果指出，流入的深層冷水需要花費約31∼58 年的時間才能將舊有的南海海盆深層水替換完成，這與前人估算年限頗為接近；而在估算湧昇速度方面，水深2000 公尺處的湧昇速度約為1.10 (±0.32)×10-6 ms-1，與透過QSCAT 風場資料 (Quick Scatterometer)計算出的艾克曼抽送 (Ekman Pumping)速度 (1.30×10-6 ms-1)，以及由氣候平均值GDEM (Generalized Digital Environmental Model)計算出的地轉流 (Geostrophic flow)湧昇速度 (0.30×10-6 ms-1)的流速總合相比，約略近似。經由水團比對分析後發現，流入南海內部的深層海水可能是由太平洋副北極中層水 (Pacific Subarctic Intermediate Water, PSIW)與繞極深層水 (Circumpolar Deep Water, CDW)兩種水團所混合而成。 對於南海內部熱平衡之探討，深層冷水由『D2』通道注入南海內部的熱通量約為-10.5 Wm-2，其值約能抵消40 %的海表面淨熱通量 (net heat flux)，顯示其冷水的湧升現象，對於維持南海內部的強分層 (stratification)扮演著十分重要的角色。

Geographically, two deep channels which are both located around 121.5°E could lead deep water exchange (>2000 m) between Pacific Ocean and South China Sea (SCS). The north one, named as D1, is located east of southern Taiwan, while the south one, named as D2, is located east of Luzon Strait. The presented current observations indicate the deep water continuously flow into the SCS through D2. The volume transport through D2 was 1.08±0.32 Sv (1 Sv = 106 m3s-1), while the transport through D1 was negligible. By assuming volume conservation, the inflow transport was used to estimate the residence time and vertical velocity on the 2000m isobaths. The estimated residence time in the deep SCS (>2000m) is from 31 to 58 years. The estimated vertical velocity at 2000m is (1.10±0.32)×10-6 ms-1. It is close to the sum of the mean Ekman pumping (1.30×10-6 ms-1) and the mean geostrophic vertical velocities (0.30×10-6 ms-1) which were calculated by using Quick Scatterometer (QSCAT) wind and Generalized Digital Environmental Model (GDEM), respectively. The result indicates the deep current at D2 could be a primary channel providing the cold water into SCS. Examined the historical hydrography around SCS, the origination of SCS deep water could be mainly mixed by the water mass of Circumpolar Deep Water (CDW) and the water mass of Pacific Subarctic Intermediate Water (PSIW). Heat flux through D2 channel was also calculated. It could be balanced for 40% of net heat flux on sea surface. The result was implied that the upwelling at deep SCS as well as intermediated depths could be important for maintaining strong stratification and energetic internal motion in the SCS.