Wind-driven stratification patterns and dissolved oxygen depletion off the Changjiang (Yangtze) Estuary
<p>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.</p> <p>Two oceanographic cruises, conducted in July 2015 and August–S...
| Main Authors: | , , , |
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| Format: | Article |
| Language: | English |
| Published: |
Copernicus Publications
2020-05-01
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| Series: | Biogeosciences |
| Online Access: | https://www.biogeosciences.net/17/2875/2020/bg-17-2875-2020.pdf |
| Summary: | <p>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.</p>
<p>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 <span class="inline-formula">1.3×10<sup>4</sup></span> km<span class="inline-formula"><sup>2</sup></span> 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
(<span class="inline-formula"><10</span> 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<span class="inline-formula"><sup>−1</sup></span>) 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.</p>
<p>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.</p>
<p>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.</p> |
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| ISSN: | 1726-4170 1726-4189 |