09:00 〜 09:15
[AOS15-07] Exploring Kuroshio Paradox – a study of mixing processes over topographic features
キーワード：Mixing processes, TKE dissipation, Lee-wave breaking, Shear instability, KH billows
The mixing processes over topographic features are important agents for nutrient delivery to the primary producers and supporting migratory fish species along the route of Kuroshio. Lee-wave breaking provides a mechanism for extracting energy from the geostrophic flow and transferring energy to small-scale turbulence. Proper parameterization of mixing processes is crucial to the spatial and temporal variation of biological productivity and biodiversity in the ecosystem. A seven-day two-ship field experiment was conducted to investigate turbulent mixing behind the topographic ridge in the upstream of Kuroshio in the east coast of Taiwan. Under high Kuroshio velocity (up to 2m/s), lee wave breaking induced shear instability results in persistent vertical nitrate flux behind the seamount. The observed NO3 profile can be explained by the estimated nitrate flux associated with mixing at subsurface layer (100-150m) where shear instability/KH billows are observed. The data analysis indicates that shear unstable occurs behind the seamount when the reduced shear squared is greater than zero (S2-4N2>0, where S represents the flow shear and N represents the buoyancy frequency). Additionally, the estimated eddy diffusivity based on TKE dissipation rate and density observed by VMP-500 depends on the gradient Froude number (SN-1), and the theory of Kunze et al., (1990) captures the observed TKE dissipation rate of approximately 10-5 to 10-4 m2s-1 in unstable shear layers.
Fig. (a) a schematic diagram shows the process of lee wave breaking and irregular turbulent perturbations in the lee of the topographic ridge. The color shows normalized density (ρ-ρ0)/Δρ, where Δρ is the density difference between the top and bottom boundary and ρ0 is the background density (see the color legend in (c)). (b) The shipboard observation of echo sounder intensity shows that the occurrence of lee waves and irregular oscillations in the lee of the topographic ridge. (c) Model results of KH billows-like structures. The coherent structures associated with shear instabilities are visualized by Q-method (Jeong and Hussain, 1995). The vertical coordinate is scale up five times for the visualization purpose.
Fig. (a) a schematic diagram shows the process of lee wave breaking and irregular turbulent perturbations in the lee of the topographic ridge. The color shows normalized density (ρ-ρ0)/Δρ, where Δρ is the density difference between the top and bottom boundary and ρ0 is the background density (see the color legend in (c)). (b) The shipboard observation of echo sounder intensity shows that the occurrence of lee waves and irregular oscillations in the lee of the topographic ridge. (c) Model results of KH billows-like structures. The coherent structures associated with shear instabilities are visualized by Q-method (Jeong and Hussain, 1995). The vertical coordinate is scale up five times for the visualization purpose.