Abstract
Blue tides at the head of Tokyo Bay are a hydro-environmental phenomenon where seawater appears to be milky blue because of the reflection of the sunlight off surface water containing large amounts of sulphur particles. Its appearance is due to the coastal upwelling of bottom oxygen-depleted water induced by northeasterly wind-driven circulation. Blue tides cause the death of many shellfish and other aquatic animals across the head of Tokyo Bay and consequently result in substantial economic losses to coastal fisheries. This paper examines the occurrence of wind-driven blue tides in Tokyo Bay, based on a simplified hydrodynamic model and observational analysis. The model assumed a two-layer structure with a wind-driven upper layer and an oxygen-depleted lower layer. In this study, we derived a simple analytical solution to determine a critical wind condition for which the lower layer outcrops at the surface if the wind forcing is sufficiently strong, resulting in the mixing of the two layers and giving rise to blue tide. The results of sensitivity analyses of the analytical solution to all incorporated factors were found to be in accordance with a qualitative understanding of the blue tide phenomenon. More importantly, comparisons of observational data with real cases of blue tide during 1978–2016 and without blue tide during 2003–2016 suggested that this analytical solution was mostly valid. This study would be helpful for gaining a better understanding of the hydro-dynamical mechanism of blue tide.
Highlights
Tokyo Bay is a typical semi-enclosed embayment subject to eutrophication [1,2,3]
Compared to a mesh-based numerical modelling approach, the analytical solution adopted in this study can roughly estimate the critical wind conditions with lesser computational cost, and it facilitates a qualitative understanding of the factors that influence the occurrence of blue tide and what these effects are
Once the densities of the upper and lower layers, the thickness of the upper, well-mixed layer and the observed wind conditions are estimated based on real-time stratification data provided by the Chiba Light Beacon (CLB) and meteorological data provided by Japanese Meteorological Agency (JMA), the analytical solution can provide a simple evaluation of whether the blue tide could occur at the head of Tokyo Bay
Summary
Tokyo Bay is a typical semi-enclosed embayment subject to eutrophication [1,2,3]. The input of nutrient loads, with available sunlight at the surface and suitable temperatures in spring or summer, promote the excessive growth and decay of phytoplankton [1,2,4]. If the wind forcing is sufficiently strong, the density interface will intersect the surface At this time, a large amount of hydrogen sulphide contained in the bottom water layer will meet oxygen at the surface, and as a result, the blue tide phenomenon can occur on the northeast shore of the bay. It should be noted that there are some simplifications and assumptions involved in the derivation of the analytical solution, but it gives a simple relation for characterizing the dependence of wind conditions on the stratification properties and geometrical properties of the two-layered fluid This proposed analytical solution shows its validity by comparison with real cases with and without blue tides as presented . Comparison with Observational Data be consistent with a qualitative understanding of the wind-driven coastal upwelling
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