PHYSICAL STRUCTURE AND MIXING PROCESSES ASSOCIATED WITH HYPOXIA IN THE INNER PART OF TOKYO BAY

Abstract

 This study presents physical structures and mixing processes and thier effect on the oxygen distribution in the inner part of Tokyo Bay. A one-year analysis was conducted using long-term observation data from Chiba light. The ratio of the vertical density gradient and vertical shear, namely the Richardson number(R_i) was negatively correlated with bottom disssolved oxygen (DO), indicating that vertical mixing conditions strongly influence the bottom DO. The bottom DO was also negatively correlated with the horizontal Richardson number(R_ix), which represents the mixing state considering the horizontal gradient, wind, tidal current, and buoyancy. R_ix exceeded the index of stable stratification in 76% hypoxia events. The wind stress largely influenced vertical mixing, rather than tidal currents. R_i and R_ix was significantly correlated, therefore, the horizontal buoyancy gradient and wind stress can be used to explain the vertical mixing and bottom DO conditions in the inner part of Tokyo Bay.