Xiao Ma, Qicheng Meng, Dewang Li, Yuanli Zhu, Xiaobo Ni, Dingyong Zeng, Di Tian, Ting Huang, Zhihao Jiang, Haiyan Jin, Feng Zhou. Coastal hypoxia response to the coupling of catastrophic flood, extreme marine heatwave and typhoon: a case study off the Changjiang Estuary in summer 2020[J]. Acta Oceanologica Sinica. doi: 10.1007/s13131-024-2311-1
Citation:
Xiao Ma, Qicheng Meng, Dewang Li, Yuanli Zhu, Xiaobo Ni, Dingyong Zeng, Di Tian, Ting Huang, Zhihao Jiang, Haiyan Jin, Feng Zhou. Coastal hypoxia response to the coupling of catastrophic flood, extreme marine heatwave and typhoon: a case study off the Changjiang Estuary in summer 2020[J]. Acta Oceanologica Sinica. doi: 10.1007/s13131-024-2311-1
Xiao Ma, Qicheng Meng, Dewang Li, Yuanli Zhu, Xiaobo Ni, Dingyong Zeng, Di Tian, Ting Huang, Zhihao Jiang, Haiyan Jin, Feng Zhou. Coastal hypoxia response to the coupling of catastrophic flood, extreme marine heatwave and typhoon: a case study off the Changjiang Estuary in summer 2020[J]. Acta Oceanologica Sinica. doi: 10.1007/s13131-024-2311-1
Citation:
Xiao Ma, Qicheng Meng, Dewang Li, Yuanli Zhu, Xiaobo Ni, Dingyong Zeng, Di Tian, Ting Huang, Zhihao Jiang, Haiyan Jin, Feng Zhou. Coastal hypoxia response to the coupling of catastrophic flood, extreme marine heatwave and typhoon: a case study off the Changjiang Estuary in summer 2020[J]. Acta Oceanologica Sinica. doi: 10.1007/s13131-024-2311-1
Coastal hypoxia response to the coupling of catastrophic flood, extreme marine heatwave and typhoon: a case study off the Changjiang Estuary in summer 2020
State Key Laboratory of Satellite Ocean Environment Dynamics, Second Institute of Oceanography, Ministry of Natural Resources, Hangzhou 310012, China
2.
Observation and Research Station of Yangtze River Delta Marine Ecosystems, Ministry of Natural Resources, Zhoushan 316022, China
3.
Key Laboratory of Marine Ecosystem Dynamics, Second Institute of Oceanography, Ministry of Natural Resources, Hangzhou 310012, China
4.
School of Oceanography, Shanghai Jiao Tong University, Shanghai 200240, China
5.
College of Oceanography, Zhejiang University, Zhoushan 316021, China
Funds:
The National Natural Science Foundation of China under contract Nos U23A2033 and 42230404; the National Program on Global Change and Air–Sea Interaction (Phase II) under contract No. GASI-01-CJK; the Key Research & Development Program of Zhejiang Province under contract No. 2022C03044; the Joint Funds of the Zhejiang Provincial Natural Science Foundation of China under contract No. LZJMZ23D050001; Long Term Observation and Research Plan in the Changjiang Estuary and the Adjacent East China Sea Project under contract No. SZZ2007; the Project of State Key Laboratory of Satellite Ocean Environment Dynamics under contract No. SOEDZZ2105; the Zhejiang Provincial Natural Science Foundation under contract No. LR16D060001, and the Zhejiang Provincial Ten Thousand Talents Plan under contract No. 2020R52038.
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 Estuary. Moreover, the typhoon Bavi has limited effect on the marine heatwave and coastal hypoxia in summer 2020.
Figure 1. (a) Schematic of the summertime circulation in the East China Sea (modified from Yang et al., 2012, 2013 and Tian et al., 2022). CDW: Changjiang 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 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 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 (a) Changjiang river discharge, (b) CCMP wind vector, (c) SMAP salinity, and (d) OISST temperature 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. Gary shading areas indicate four cruise periods during July−September 2020.
Figure 3. Hypoxia events observed off the Changjiang Estuary during July-September 2020. The observed periods is (a) 14−22 July (b) 17−22 August (c) 28−30 August and (d) 19−25 September, respectively. Red and blue curves denote the hypoxia and low DO thresholds (2 mg/L and 3 mg/L, respectively).
Figure 4. DO value (with unit of mg/L) along the repeated section A (~31.5°N) in (a) 20 July (b) 19 August (c) 29 August and (d) 21 September, respectively. Red curves denote the hypoxia threshold (2 mg/L).
Figure 5. Bottom hydrographic parameters observed by SIO-HOTS during 1 August-30 September 2020. The bold curves denote daily mean time series. (a) Bottom temperature, with unit ℃; (b) Bottom Salinity; (c) Bottom DO value, with unit mg/L. Shading area denotes the passage period (24−27 August) of typhoon Bavi.
Figure 6. SST (with unit of ℃) and SSS off the Changjiang Estuary during July to September 2020. The observed periods are (a, e) 14−22 July, (b, f) 17−22 August, (c, g) 28−30 August, and (d, h) 19−25 September, 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, with unit of kg/m4) and pycnocline layer thickness (Δz, with unit of m) during (a, e) 14−22 July (b, f) 17−22 August (c, g) 28−30 August and (d, f) 19−25 September, 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 8. Cross-shelf distributions of water properties measured along section A (~31.5°N) in (a) 20 July (b) 19 August (c) 29 August and (d) 21 September, respectively: (a-d) temperature (with unit of ℃); (e-h) salinity; and (i-l) potential density (with unit of kg/m3). White contour indicates bottom hypoxia water (DO<2 mg/L).
Figure 9. (a) Changjiang river discharge /(m3∙s-1) in July during 1998 to 2020; (b) domain averaged SST (℃) and SST maxima in August off the Changjiang Estuary during 1998 to 2020; (c) domain averaged SSS in August off the Changjiang Estuary during 2015−2020; (d) historical records about the hypoxia area (histogram, with unit of km2) and DO minimum (dotted, with unit of mg/L) during 1998；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); 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.
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