Wind-driven stratification patterns and dissolved oxygen depletion off the Changjiang (Yangtze) Estuary

Abstract

Abstract. Multiple factors have been accused of triggering coastal hypoxia off the Changjiang Estuary, and their interactions lead to high yearly variation in hypoxia development time window and distribution extent. Two oceanographic cruises, conducted in July 2015 and August–September 2017, were complemented by river discharge, circulation simulation, remotely sensed wind, salinity and sea level anomaly data to study the dissolved oxygen (DO) depletion off the Changjiang Estuary from synoptic to interannual timescales. Intensification of the Chinese Coastal Current and Changjiang Diluted Water (CDW) spreading to the south together with coastal downwelling caused by the northerly wind was observed in the summer of 2015. This physical forcing led to a well-ventilated area in the north and a hypoxic area of 1.3×104 km2 in the south, while in 2017 the summer monsoon (southerly winds) induced offshore transport in the surface layer that caused a subsurface intrusion of Kuroshio-derived water to the shallower areas (<10 m depth) in the north and upwelling in the south. Wind-driven Ekman surface flow and reversal of the geostrophic current related to the upwelling compelled alteration of the Chinese Coastal Current. Consequently, intense hypoxia (DO down to 0.6 mg L−1) starting from 4 to 8 m depth connected to CDW and deep water intrusion in the north and coastal hypoxia linked to the upwelling in the south were observed in 2017. Distinct situations of stratification and DO distributions can be explained by wind forcing and concurrent features in surface and deep layer circulation, upwelling and downwelling events. Enhanced primary production in the upper layer of the CDW or the upwelled water determines the location and extent of DO depletion. Likewise, the pycnocline created by Kuroshio subsurface water intrusion is an essential precondition for hypoxia formation. Wind forcing largely controls the interannual change of hypoxic area location and extent. If the summer monsoon prevails, extensive hypoxia more likely occurs in the north. Hypoxia in the south occurs if the summer monsoon is considerably weaker than the long-term mean.

Advances on Coastal and Estuarine Circulations Around the Changjiang Estuary in the Recent Decades (2000–2020)

“…There are also observations showing that the hypoxia can occasionally last longer to the midautumn (Wang et al, 2012;Chen C. C. et al, 2020). In winter, the buoyant waters are well-ventilated and hypoxia is thereby prevented (Zheng et al, 2016;Liblik et al, 2020). It is considered that the physical environment favors the development of hypoxia in summer, when the Changjiang river discharge maximizes (section Changjiang estuarine circulation), an intensified northeastward TWWC appears (Wang et al, 2012), upwelling of the shelf waters is established and the coastal waters are more stratified (Zheng et al, 2016;Chen et al, 2017).…”

“…In the synoptic timescale, the increment of Changjiang river discharge is believed to stimulate the expansion of hypoxia (Zheng et al, 2016;Zhou et al, 2017), although there are also studies suggested that the changes in hypoxia are not response to the changes in the river discharge instantly (Chen et al, 2015). The offshore detachment of buoyant waters in form of "cloudy patches" synthesized in the section 3.1, played critical roles in regulating the hypoxia condition (Liblik et al, 2020;FIGURE 6 | Maps for (A) the observed distributions of bottom hypoxic waters (Zhang et al, 2019c) and (B) events of harmful algae bloom (HAB) between 2000 and 2014 . "The color and size of the solid circle represent the date and affected area of each event.…”

Hypoxic Effects on the Radiocarbon in DIC of the ECS Subsurface Water

“…In addition, high-speed surface wind brought by Typhoon Nasset during our survey did enhance the bottom water upwelling (Liblik et al, 2020). However, the similarity of the two surface samples (YE-12 and YEN1-7) with middle and bottom samples was also likely due to upwelling.…”

“…Therefore, the northward movement of KBC subsurface water has strong influence on the location and the magnitude of hypoxia occurred north of 31°N. Southerly wind enhanced by Typhoon, such as Typhoon Nesat during our survey, provides the driving force for KC subsurface water upwells to the submarine delta (Liblik et al, 2020;Wang et al, 2017). In addition, high nutrients brought by the CR stimulate primary productivity on surface and then accelerate the oxygen consumption near bottom (Zhang et al, 2010 and references therein).…”

“…Salinity, temperature and DO are available through Liblik et al (2020), and the other analyzed result are shown in Table 1 in this paper.…”