Numerical simulation of Blue tide and high turbidity in the middle layer of the coastal low-oxygen area.

To reproduce Aoshio (Blue tide) and high turbidity in the middle sea layer at the head of Tokyo Bay, a physical-biogeochemical model including a sulfur cycle is proposed in this study. The sulfur-cycle model comprises anaerobic sulfate (SO₄^2-) reduction to sulfide (HS^-) that occurs in the sediments and the bottom sea layers, bacterial oxidation of HS^- to elemental sulfur (S⁰), and bacterial oxidation of S⁰ to SO₄^2-. Appearance and disappearance of the high turbidity in the middle layer, which are caused by an upwelling of anoxic water, were reproduced well through distributions of simulated S⁰. It is observed that the breaking of internal Kelvin waves (as indicated by the propagation of upwelling and counterclockwise circulation in the surface layer) can lead to the mixing of the oxic-anoxic interface. Majority of the high turbidity is caused by the mixing occurring above the anoxic layer because the affected area by the outflow to the middle layer along the Chiba ship channel is limited. Furthermore, it is estimated that at least 50% area of the head of Tokyo Bay is hypoxic during summer.