Regulation of Water Masses to Hypoxia Zones in the Changjiang Estuary

The regulating ways of different water masses affecting the locations and intensities of hypoxia zones were studied based on the time-space continuum data from August 2011 to 2013–2017. The 6-year distribution of the hypoxic area in the Changjiang Estuary (CE) and its adjacent waters show that the hypoxic area can be divided into two segments. The southern segment is out of the south branch of the CE, whereas the northern segment is in the junction zone between the South Yellow Sea and the CE. The two segments were divided along the 31.5°–32 °N latitude line. The northern and southern segments were dominated by the East China Sea shelf water (ECSSW) and Kuroshio subsurface water (KSW), respectively. When the KSW (salinity > 34) intrusion reached the east of 123 °E and south of 31 °N, hypoxia zones mainly occurred in the southern segment covered by the Changjiang Diluted Water (CDW), meanwhile the Yellow Sea cold water mass may emerge in the northeastern area. When the KSW intensely invaded westward to the region between 122° and 122.5°E and northward to 31.5°N or further north, hypoxia zones appeared in the northern segment. The strength of the KSW with low dissolved oxygen concentration is the basic driving factor for the hypoxia occurrence in the CE. Moreover, the stratification is crucial for the southern segment, whereas the organic matter decomposition is dominated for the northern segment, even with severe hypoxia across the sea surface in the study area.