Coastal hypoxia response to the coupling of catastrophic flood, extreme marine heatwave and typhoon: a case study off the Changjiang River Estuary in summer 2020
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Abstract: Massive bodies of low-oxygen bottom waters are found in coastal areas worldwide, which are detrimental to coastal ecosystems. In summer 2020, the response of coastal hypoxia to extreme weather events, including a catastrophic flooding, an extreme marine heatwave, and Typhoon Bavi, is investigated based on multiple satellite, four cruises, and mooring observations. The extensive fan-shaped hypoxia zone presents significant northward extension during July−September 2020, and is estimated as large as 13 000 km2 with rather low oxygen minimum (0.42 mg/L) during its peak in 28−30 August. This severe hypoxia is attributed to the persistent strong stratification, which is indicated by flood-induced larger amount of riverine freshwater input and subsequent marine heatwave off the Changjiang River Estuary. Moreover, the Typhoon Bavi has limited effect on the marine heatwave and coastal hypoxia in summer 2020.
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Key words:
- coastal hypoxia /
- Changjiang River Estuary /
- extreme weather events /
- seasonal evolution
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Figure 1. a. Schematic of the summer time circulation in the East China Sea [modified from Yang et al. (2012, 2013) and Tian et al. (2022)]. CDW: Changjiang River Diluted Water; YSCC: Yellow Sea Coastal Current; TWC: Taiwan Warm Current. Four Kuroshio intrusion branches in summer are also represented (Yang et al., 2012): Kuroshio Branch Current (KBC), Offshore Kuroshio Branch Current (OKBC), Nearshore Kuroshio Branch Current (NKBC) and the westward Kuroshio branch (upwelling, UW). b. Historical bottom hypoxia frequency (1998−2020; shading, hypoxia records are listed in Table 1), cruise and mooring sampling stations (marked), and seabed topography (thin contour) off the Changjiang River Estuary. The black cycles, blue stars, red rectangles, and black rectangles indicate sampling stations observed during 14–22 July, 17–22 August, 28–30 August, and 19–25 September, respectively. The red star denotes SIO-HOTS mooring. The dashed curve indicates surface suspended sediment front off the Changjiang River Estuary (redrawn by Li et al., 2021). The dashed gray box indicates the domain average area for the climatic and hydrographic factors.
Figure 2. The time series of Changjiang River discharge (a), CCMP wind vector (b), SMAP salinity (c), and OISST temperature (d) during July−September 2020. Gray lines in a–d denote the climatology mean. The average period is 1998−2020 for river discharge, wind, and sea surface temperature (SST), and 2015−2020 for sea surface salinity (SSS). The average domain for wind, SST, and SSS is shown in Fig. 1b. Gray shading areas indicate four cruise periods during July−September 2020.
Figure 3. Hypoxia events observed off the Changjiang River Estuary during July–September 2020. The observed periods is 14−22 July (a), 17−22 August (b), 28−30 August (c) and 19−25 September (d), respectively. Red and blue curves denote the hypoxia and low DO thresholds (2 mg/L and 3 mg/L, respectively).
Figure 6. SST and SSS off the Changjiang River Estuary during July to September 2020. The observed periods are 14−22 July (a, e), 17−22 August (b, f), 28−30 August (c, g), and 19−25 September (d, h), respectively. Solid contour in e–h denotes the isoline of CDW (S = 30). Red and blue curves denote bottom hypoxia and low-DO areas (blue: 3 mg/L; red: 2 mg/L).
Figure 7. Pycnocline intensity (Δδ/Δz) and pycnocline layer thickness (Δz) during 14−22 July (a, e), 17−22 August, (b, f), 28−30 August (c, g) and 19−25 September (d, f), respectively. The solid contours indicate bottom hypoxia and low-DO areas (blue: 3 mg/L; red: 2 mg/L). The contour interval is 0.05 kg/m4 in a–d and 5 m in e–h.
Figure 9. a. Changjiang River discharge in July during 1998 to 2020; b. domain averaged SST and SST maxima in August off the Changjiang River Estuary during 1998 to 2020; c. domain averaged SSS in August off the Changjiang River Estuary during 2015−2020; d. historical records about the hypoxia area (histogram) and DO minimum (dotted; mg/L) during 1998 to 2020. The statistical rectangle area for b and c is shown in Fig. 1b. Solid cycles denote the severe hypoxia events happened in summers of 1999, 2006, 2013, 2016, 2017 and 2020. The climatology mean values for river discharge and SST are also given in a and b.
Figure 10. The SST anomaly (℃) and summer typhoon activities in summers of six severe hypoxia years: August 1999 (a), August 2006 (b), August 2013 (c), August 2016 (d), August 2017 (e), and August 2020 (f). Red curves indicate observed hypoxia area in literatures. The colors of the typhoon dots (within the interval of 6-hours) indicate the “Saffir-Simpson” wind scale. The typhoon data are provided by Joint Typhoon Warning Center.
Table 1. Hypoxia area and dissolved oxygen minimum off the Changjiang River Estuary reported in literatures
Investigation
periodHypoxia extent
(2 mg/L
threshold)/km2Dissolved oxygen
minimum/
(mg·L–1)Reference August 1998 600 1.44 Wang and Wang (2007) August 1999 13 700 1.00 Li et al. (2002) August 2002 579a 1.73 Wang (2009) August 2003 100a 1.8 Chen et al. (2007) August 2004 no data 2.30 Li et al. (2011) August 2005 45a 1.56 Li et al. (2011) August 2006 19 600 0.98 Zhou et al. (2010) August 2007 7 600a 0.90 Li (2015) August 2008 3 000a 1.40 Liu et al. (2012) August 2009 2 800a 1.79 Liu et al. (2012) August 2010 1 968 1.20 Liu et al. (2021) August 2011 no data 2.10 Zhu et al. (2017) August 2012 4 162 1.54 Luo et al. (2018) August 2013 11 150 0.73 Zhu et al. (2017) August 2014 1 000a 0.85 Zhou et al. (2020) August 2015 no data 1.92b Chi et al. (2017) August 2016 22 800 0.08 Chen et al. (2020) August 2017 10 071 0.33 Chen et al. (2020) August 2020 13 000 0.42 this study Note: a. The hypoxia areas are digitized from the listed references; b. the dissolved oxygen minimum appears in the subsurface layer. Table 2. Hypoxia and low DO status observed off the Changjiang River Estuary during July–September 2020
Cruise period 14−22
July17–22
August28–30
August19–25
SeptemberLow DO area/km2
(DO < 3 mg/L)21 000 25 000 18 000 9 700 Hypoxia area/km2
(DO < 2 mg/L)700 8 800 13 000 1 800 DO concentration
minimum/(mg∙L−1)1.60 1.05 0.42 1.26 Table 3. Indices for coastal hypoxia and the associated hydrographic factors for 2020, climatology mean, severe hypoxia years, and normal hypoxia years
Factor 2020 Climatology (1998−2020) Severe hypoxia year Normal hypoxia year Hypoxia Area/(103 km2) 13 000 5 900 15 200 1 680 Minimum oxygen/(mg∙L-1) 0.42 1.27 0.59 1.58 River discharge Transport in July/(m3∙s−1) 71 500 50 000 57 400 47 300 SSTa August mean SST/℃ 27.0 26.7 27.3 26.5 August maximum SST/℃ 28.9 28.1 28.7 27.8 Note: a. The statistical rectangle area is shown in Fig. 1b; b. severe hypoxia years: 1999, 2006, 2013, 2016, 2017, and 2020; c.normal hypoxia years: 1998, 2002, 2003, 2004, 2005, 2007, 2008, 2009, 2010, 2011, 2012, 2014, and 2015. -
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