Estimating the Occurrence of Wind-Driven Coastal Upwelling Associated with “Aoshio” on the Northeast Shore of Tokyo Bay, Japan: An Analytical Model

Abstract

“Aoshio” in Tokyo Bay is a hydroenvironmental phenomenon in which seawater appears milky blue due to reflection of sunshine off surface water which contains lots of sulfur particles. Its appearance is due to coastal upwelling of bottom oxygen-depleted water, which causes many deaths of shellfish and other aquatic animals around the bay. In this study, we derived some analytical solutions in the context of a two-layered fluid and used them to make a simple analytical model to estimate the occurrence of “Aoshio” phenomenon on the northeast shore of Tokyo Bay. Comparison with observation data suggested that this model was valid to a certain degree.

Applying a Simple Analytical Solution to Modelling Wind-Driven Coastal Upwelling of Two-Layered Fluid at the Head of Tokyo Bay, Japan

“…At long timescales of the northeasterly blowing wind, the two-layered fluid will be deflected to the right of the wind direction due to the Coriolis force. This simple hydrodynamic explanation to the blue tide occurrence was found to be consistent with some previous studies [5][6][7]9,12,15]. Although it seems difficult to determine the specific time at which the Coriolis force starts to play a role, this time scale could be simply estimated as two days in this study based on the numerical experiments of Matsuyama et al [15] and Ueno et al [28] concerning coastal upwelling in Tokyo Bay.…”

“…Under anaerobic water leads to the occurrence of blue tide. On the other hand, although the proposed analytical solution showed its validity, its comparison with real cases without blue tide was not performed in the study of Zhu and Yu [9], which seemingly implies that the wind conditions presented by the analytical solution are not the critical conditions for the occurrence of blue tide at the head of Tokyo Bay. In this study, we relaxed the small-amplitude assumption and derived a simple analytical solution to determine the wind conditions for the occurrence of blue tide on the northeast shore at the head of the bay in the context of a two-layered fluid.…”

“…During this process, a large concentration of dissolved oxygen at the bottom is consumed, and the bottom water consequently becomes oxygen-depleted and even anoxic. On the other hand, along the seasonal temperature stratifications, fresh water inflows around the bay and intermittent precipitation result in a high-temperature, low-salinity upper water layer and a low-temperature, high-salinity lower water layer [6][7][8][9]. As a result, a stable density stratification forms in which the lighter upper water overlies the heavier lower water, hindering the vertical circulation that brings dissolved oxygen from the surface to the bottom and thus enhances the oxygen-depleted state of the lower water [5,8].…”

Minimum Wind Stress for the Occurrence of Blue Tide on the Southeast Shore of Tokyo Bay

“…As presented in Zhu and Yu (2014), surface wind stress in Equation (9) is generally expressed to be a quadratic function of surface wind speed (Matsuyama et al, 1990;Wilson, 1960): , so we simply chose the empirical value 10 −4 in this preliminary study. Observation data sets of blue tide at the head of Tokyo Bay from 2003 to 2010 are available (Zhu and Isobe, 2012), and, among these data sets, those data sets of blue tide observed on the southeast shore of the bay are only chosen.…”

“…Observation data sets of blue tide at the head of Tokyo Bay from 2003 to 2010 are available (Zhu and Isobe, 2012), and, among these data sets, those data sets of blue tide observed on the southeast shore of the bay are only chosen. Wind data set measured at Edogawarinkai shown in Figure 1(a) is simply adopted to represent wind condition across the whole bay (Zhu and Yu, 2014). By using this raw data set, the average speed of the northeasterly-oriented wind w and the total duration are calculated.…”