Uncertainty of the mixing coefficient investigated with microstructure surveys in coastal oceans

Eiji Masunaga, Ibaraki University, Mito, Japan, Koyo Wada, Tokyo University of Marine Science and Technology, Japan, Subhas Karan Venayagamoorthy, Colorado State University, Department of Civil and Environmental Engineering, Fort Collins, CO, United States and Hidekatsu Yamazaki, Tokyo University of Marine Science and Technology, Tokyo, Japan
This study presents estimates of the mixing coefficient G obtained from direct microstructure observations in coastal oceans. G is one of the key factors required for estimating diapycnal mixing in the oceans. A constant value of Γ = 0.2 is commonly used for estimating vertical diffusivity in the ocean. However, recent observational and numerical studies suggest that G can vary widely in the ocean and that it can be parameterized using a ratio of relevant turbulent length scales namely: the Ellison scale LE, (or equivalently the Thorpe scale) and the Ozmidov scale LO. Analyses of four microstructure data sets from coastal regions were performed. These include the Tokyo Bay (weakly stratified and strongly stratified cases), Ostuchi Bay (strongly stratified case), the vicinity of Izu-Oshima island (weakly stratified case). For weakly stratified conditions, Γ ~ (LE/LO)4/3 when LE/LO is less than unity. On the other hand, under strongly stratified conditions (LE/LO greater than one), Γ does not change with LE/LO. These findings are consistent with results from Direct Numerical Simulations of stratified turbulence by Garanaik and Venayagamoorthy (2019, JFM). In addition, observational results show that Γ is enhanced when nonlinear internal bores break on a shallow slope. But we suspect that the increase is only a transitional phenomenon since mixing is not completed.