Abstract:

Hydrological and turbulent microstructural observations were conducted in summer 2016 to analyze the summertime stratification and turbulent mixing in the Yangtze Estuary. The temporal and spatial distributions of salinity, temperature, stratification, and turbulent mixing showed highly-stratified structures in the estuary. The vertical distributions of density vertical gradient in different layers demonstrated bottom mixed layer and upper pycnocline, where a division of the stratification was approximately along the contour line of lgN ² = - 4.0. The bottom mixed layer had a stronger dissipation rate and unstably-stratified structure, while the upper pycnocline had a weaker dissipation rate and stably-stratified structure, where the formation and spread of internal waves would occur. Under the condition of stratified structure, the dissipation rate, turbulent shear production and buoyancy flux were found to satisfy the local turbulent kinetic energy balance. Turbulent Reynolds number Re_t and turbulent Froude number Fr_t in the bottom mixed layer and the upper pycnocline were distributed in different regions of the Re_t-Fr_t space, indicating that the stratification and mixing in the estuary was in line with the classical theory of Ivey and Imberger (1991).

Key words: stratified flow, buoyancy frequency, bottom mixed layer, upper pycnocline, Yangtze Estuary