利用雷達測流推估台灣海峽在冬季之海水傳輸量

Abstract

台灣海峽為連接南海與東海的重要通道，海峽流量及物質通量，像是營養鹽的輸送或熱量的交換等，決定了兩區域物質交換的方向與速率。冬季時因海況不佳，研究船之觀測資料難以逐日推估台灣海峽之海水傳輸量Q(Sv)，而傳統利用底置式錨錠陣列之量測方式，從以往至現在僅有2.5月的完整資料。藉由往返台灣及馬祖的台馬輪底部裝設船載ADCP，在橫跨台灣海峽都會收集當時當地的全深度流速資料也可以估算Q。本文嘗試使用高頻雷達測流儀所量測到的表層流場資料與風場資料來推估台灣海峽的日平均流量。該組高頻雷達測流儀是由台灣海洋科技研究中心(TORI)架設於台灣西北海岸。 經過驗證以及測試雷達測流儀的資料後，評估該資料的適用海域，並試著將風驅效應從雷達測表層流中去除掉，計算出正壓海流之大小以及地轉流的海水單位寬度傳輸量(Sv/m)。在台灣海峽東側接近雷達站的表層流場資料品質較佳，故只擷取120.854°E至121.335°E範圍的海流流量與sb-ADCP所量測到的流量做比較。 分別利用(A)相關係數分析以及(B)Matsuoka et al. (2003)的風速轉換風驅流的經驗係數(風驅流大小為風速的3.4%，方向則為風向順時針轉37度)，扣除2012年9月至12月的風驅流效應後所推估120.854°E至121.335°E範圍之流量q為(A)0.67 Sv、(B)0.78 Sv。同時段由台馬輪量測的q=0.64 Sv， 如果雷達資料可以涵蓋全台灣海峽，則q 等同於 Q。

Taiwan Strait (TS) connects the South China Sea and East China Sea (ECS). The transport of water, nutrient and heat through TS is important to the marine ecosystem in TS and ECS. However, measuring the flow velocity and the transport in Taiwan Strait is a very difficult task. There are much fewer winter surveys in TS, and there is only one set of 2.5-month data from bottom-mount ADCP array. The ship-board ADCP on TaiMa ferry collects nearly daily velocity field U between Keelung and Matsu, except during days of severe sea states. The total volume transport Q through TS is derived from integrating U across TS. High frequency radars, like the CODAR stations that were built and operated by Taiwan Ocean Research Center may provide hourly data of surface current velocity Vc. After testing the accuracy of Vc, finding the area where Vc gives good M2 tidal current data, and removing the wind induced current velocity from Vc, one may derive (1) the barotropic velocity Vbt, or the vertically averaged velocity and (2) the transport per unit width. The result is compared with the sb-ADCP data from Taima ferry. The volume transport q crossing the cruise line between 120.854°E and 121.335°E is (A) q=0.67 Sv, if Vbt is derived from the correlation between Vbc(z) =Vc-Va(z) and the wind, or (B) q=0.78 Sv if Vbt = Vc – Vw, where Vw is the wind-driven surface velocity that was proposed by Matsuoka (2003), i.e. Vw equals 3.4% of wind speed, in the direction 37 degrees clockwise from the wind direction. Taima ferry measured q being 0.64 Sv. If CODAR coverage may extend across TS, then q becomes Q.