台灣海峽冬季流量觀測

Abstract

台灣海峽為連接南海與東海兩海域間重要的通道，海峽通量觀測對於兩區域海水之間物質交換非常重要，如營養鹽的輸送、熱通量的交換，然而台灣海峽冬季流量觀測非常稀少，其中船載式ADCP (sb-ADCP) 實測資料受限於冬季海況更是少有，本論文使用裝設於台馬輪之sb-ADCP觀測台灣海峽北部一到四月之流場，並以OSU 正壓潮流模式濾潮求得殘餘流及其流量，模式對於正壓潮之濾除效果良好，殘餘流顯示在流出海峽後分為兩支，分界點位於25.6°N，121.3°E左右，主流東北偏北、支流以東北偏東流出，且支流流量與當地風場無太大關係。實測總流量(TVT) 在一到四月的月平均為1.35、1.20、.1.54、1.54 Sv。在冬季，東北季風主宰海峽流場及流量變化，台馬輪大多在海況較好或風速較小的日子出航，在強烈東北季風的海況下停航，因此在一、二月頻頻觀測到2 Sv以上高流量，表示平均流量有正的系統偏差。分析中央氣象局四個氣象站的風速資料與流量之關係，得到彭佳嶼測站風速與流量相關性最高，沿海峽方向風速與每層深度上平均流速之相關性以水深50~60 m最高之後隨深度增加迅速減少。沿海峽方向風應力 (tau)與總流量變化的相關係數為0.74，其線性迴歸關係式為 TVT = 1.27 *tau (N/m2) + 1.63 Sv。為彌補台馬輪無法強烈風速下觀測資料，以風應力代入關係式估計流量一到四月平均分別是0.75、1.25、1.15、1.44 Sv，如以風速估算流量，兩種結果相差甚小，唯有風應力呈現較大波動，有負流量及過高值。 以實測資料及估算流量發現此次之流量觀測與過去觀測不同，流量較以往以空間解析度不足之bm-ADCP估計高，在冬季東北季風稍歇時，流量最高可達2.7 Sv，流量月平均值皆大於0.75 Sv，意味著台灣暖流在冬季也從台灣海峽流向東海。

Taiwan Strait (TWS) connects the South China Sea and East China Sea (ECS). The transport of water, nutrient and heat through TWS is important to the marine ecosystem in TWS and in ECS. Due to heaving fishing activity and severe sea states in TWS, there are very few direct measurement of current velocity in TWS, and none in wintertime. The ship-board ADCP on TaiMa ferry collected nearly daily velocity field between Keelung and Matsu since January 16, 2009. The OSU barotropic tidal model TPXO was applied to remove tidal contribution and to get the residual current and total volume transport (TVT in Sv) through TWS. The STD ellipse of current velocity shows that the procedure of detide is effective and accurate. The residual northward flow has two branches, the primary flows in NNE direction and the secondary is near Taiwan and flows in NEE direction. They split near (25.6°N, 120.3°E). The primary flow correlates well with the along strait wind stress tau (N/m2) near Pengiayu meteorological station, but the secondary does not. Their regression relation is TVT = 1.27*tau + 1.63 Sv with correlation coefficient 0.74. The estimated TVT from daily wind stress has monthly mean of 0.75, 1.25, 1.15 and 1.44 Sv. from January to April. TVT of 2 Sv or higher was observed frequently in January and February when the sea was relatively calm. The correlation coefficient between the mean velocity of each layer and tau is high in upper 60 m, and drops rapidly below 60m depth. The correlation between TVT and along-strait wind velocity at 10 meter height is equally good, and their regression relation may also be used to estimate TVT. The result is similar except with smaller daily fluctuation. The wintertime TVT of this study differs significantly from the published ones, because our estimated TVT is based on daily, direct measurements and there is no preference to the sea states in estimating TVT. Positive values of TVT from January to April, means that Taiwan Strait current feeds constantly the Taiwan Warm Current through all seasons.