Mixing observations at a constricted channel of a semi-closed estuary: Tokyo Bay

In this study, turbulence and mixing are investigated in a constricted channel at the mouth of Tokyo Bay, using two instruments: a microstructure profiler, TurboMAP-L, and an ADCP. We observed three different stratified conditions, i.e., SSC (Strongly Stratified Condition), MSC (Moderately Stratified Condition) and WSC (Weakly Stratified Condition) over a complete tidal period. The water exchange at the mouth of Tokyo Bay was in the transitional regime between the hydraulic and the diffusive limit with SSC and MSC close to the hydraulic limit and WSC close to the diffusive limit. For SSC and MSC, the enhancement of the stratification and vertical mixing showed asymmetry over a semidiurnal tide cycle. Mixing was stronger during ebb than during flood for SSC. On the other hand, mixing was elevated during flood for MSC. For WSC, weak stratification and strong mixing occurred during both flood and ebb. The SSC asymmetry resembled the strongly stratified condition of Columbia River (Kay and Jay, 2003a, 2003b), and the MSC asymmetry resembled the Strain-Induced Periodic Stratification condition (Simpson et al., 1990). A principle component analysis applied to the observed velocity profiles showed that both barotropic and baroclinic flow played a role during SSC and MSC, and in turn, barotropic flow dominated during WSC. The probability density function of the turbulent kinetic energy dissipation rate is practically expressed as a single lognormal population for all three conditions.