Observations of Volume Transport in Taiwan Strait in Winter

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.