Estimation of vertical mixing with Thorpe scale on the continental slope of the southwestern East Sea

Abstract

Internal tide of about 30 m amplitude was observed from two sets of 25-hour-long hourly CTD data obtained on the continental slope of the southwestern East Sea. To quantify the mixing that may have induced by the internal tide we conducted Thorpe scale (L_T) analysis using the raw CTD data. The data were processed carefully to reduce instrument noises and measurement errors. In each profile, segments with low fall speed (<0.25 m/s) or pressure inversion were removed without introducing artificial inversions. In each density inversion, the sum of the positive displacement is compared to that of the negative one. Only when the two values are comparable the inversion is considered as a physical one. The value of K_z below the main thermocline estimated with the Osborn parameterization (K_(z-os)=αε/N^2&#8776 0.128N&#12310 L^2&#12311 _T) was 5.3 ×&#12310 10&#12311 ^(-4) (1.1 ×&#12310 10&#12311 ^(-4)) m^2/s and that with the Shih parameterization (K_(z-sh)=2ν&#12310 (ε/νN^2)&#12311 ^(1/2)&#8776 1.6ν^(1/2) L_T N^(1/2)) was 5.9 ×&#12310 10&#12311 ^(-5) (2.4 ×&#12310 10&#12311 ^(-5)) m^2/s during the spring (neap) tidal period. Here ν is molecular viscosity, ε dissipation rate, and N^2 the buoyancy frequency. The Richardson numbers were low (0&#8722 0.25) in layers where the value of K_z is large. These results suggest that internal tides can enhance vertical mixing in the observation region.