||This study is focused on the temporal and spatial variations of turbulent mixing in the seas off southeastern Taiwan. Historical hydrographic data of 1986 ~ 2000 and in-situ data from six cruises were used to estimate indirectly vertical eddy diffusivity (Kρ) and turbulent kinetic energy dissipation rate (ε) by means of three methods, that is, Thorpe scale method and parameterization method with shear and strain values. Based on the 730 CTD profiles which were selected to meet the criteria in the study region as well as the QuikSCAT satellite-derived winds, a number of factors such as mixed-layer depth, wind stress, surface-layer temperature, current speed and bottom topographies were investigated about their influence on the turbulent mixing of upper oceans and bottom boundary layer. The role of Kuroshio on the turbulent mixing of this region is further explored. The CTD and LADCP data from six cruises were divided into three transects, i.e., along 21.9°N, 22.05°N and 22.2°N, to investigate variations of eddy diffusivities induced by the Kuroshio with and without the interference of the Lanyu Island. |
Among the three methods examined in this study, the parameterization method showes the most satisfactory result whose mean value is stable for the large scale evaluation of turbulent mixing distribution. Analysis of historical CTD profiles shows that seasonal variations of turbulent mixing in the upper water column (30 ~ 200 m depth) can be classified into two different parts. When the Kuroshio intrudes into the area off eastern Taiwan (121.3 ~ 122 °E), turbulent mixing is enhanced and its spatial variation is consistent with the seasonal migration of Kuroshio. In the area farther from the Kuroshio (east of 122 °E), the dominant factor affecting turbulent mixing is the wind which exerts great influence in northern Luzon Strait, the north of Southern Longitudinal Trough, and southern and northern parts of Lanyu Island. Turbulent mixing in the bottom boundary layer (< 100 mab) has close correlation with the bottom topography. At steep slopes currents will be forced to rise which will cause strong mixing. Under the influence of Kuroshio Current (0 ~ 200 m depth), Kρ can reach 10-4~10-2 m2s-1, which is much larger than those in the open ocean of the Western Pacific and South China Sea, evidencing the role played by the Kuroshio in enhancing the turbulence. Turbulent mixing could also be greatly enhanced when the Kuroshio flows through the channel between Lanyu Island and Taiwan, causing the current speed to increase. The depth that Kuroshio Current exerts marked influence on the turbulent mixing is 300 m or deeper, and the vertical shear is increased at the front of Kuroshio, affecting the turbulence in 0 ~ 200 m depth.