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Significant salinity increase in subsurface waters of the South China Sea during 2016–2017

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Abstract

The South China Sea (SCS) is the largest semi-enclosed marginal sea in the North Pacific. Salinity changes in the SCS play an important role in regional and global ocean circulation and the hydrological cycle. However, there are few studies on salinity changes over the SCS due to lack of high-quality and long-term observations. In the past decade, the deployment of floats from the Argo program in the SCS and their accumulated temperature and salinity profiles have made it possible for us to examine salinity changes over the entire basin. In this study, salinity changes were investigated with Argo and underwater glider temperature and salinity observations and gridded temperature-salinity objective analyses (UK Met Office Hadley Centre EN4.2.1 objective analysis and China Argo Real-time Data Center BOA_Argo). The results indicated that the subsurface water in the entire SCS became significantly saltier during 2016–2017. The most significant salinity increase was found during 2016 in the northeastern SCS. The subsurface water in the northeastern SCS exhibited a salinity maximum above 35, which was recorded by three Argo floats during 2015–2016. Such high salinity water was rarely observed and reported prior to the Argo era. Average salinity of 2016–2017 along the 25.5σθ−23.5σθ isopycnal surfaces in the whole SCS is 0.014−0.130 higher than the climatology. Increases in subsurface salinity started from the northeastern SCS and extended southwestward gradually. Moreover, the subsurface salinity changes, especially in the northern SCS, exhibited a semiannual lead behind the subsurface Luzon Strait transport. Further analysis indicated that the predominance of advection, driven by subsurface Luzon Strait transport, led to salinification along the western boundary of the SCS. In other parts of the SCS, negative wind stress curl trends tended to preserve the high salinity characteristics of the subsurface water.

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Acknowledgments

This work benefited from numerous data sets that were freely available. The Argo data were collected and made freely available by the International Argo Program and the national programs that contribute to it (https://doi.org/www.argo.ucsd.edu, https://doi.org/argo.jcommops.org). The Argo Program is part of the Global Ocean Observing System (https://doi.org/10.17882/42182). All the Argo T/S profiles have been post-quality controlled by the China Argo Real-time Data Center (https://doi.org/www.argo.org.cn); the Sea-wing underwater glider data are provided by the State Key Laboratory of Robotics, Shenyang Institute of Automation, Chinese Academy of Sciences. The BOA_Argo gridded data set was developed and provided by the China Argo Real-time Data Center. The EN4.2.1 monthly data set were produced by the UK Met Office Hadley Centre for Climate Change and made freely available at https://doi.org/www.metoffice.gov.uk/hadobs/en4/. The GODAS model outputs were downloaded from https://doi.org/wwww.esrl.noaa.gov/psd/data/gridded/data.godas.html, and the SODA 3.4.2 outputs were from https://doi.org/wwww.atmos.umd.edu/~ocean/index_files/soda3.4.2_mn_download.htm. The monthly sea surface wind reanalysis was produced by the U.S. NCEP/NCAR and made available at https://doi.org/wwww.esrl.noaa.gov/psd/data/gridded/data.ncep.reanalysis.derived.surface.html. We thank Tina Tin and Kara Bogus from Liwen Bianji, Edanz Group China (https://doi.org/wwww.liwenbianji.cn/ac), for editing the English text of a draft of this manuscript.

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Authors and Affiliations

  1. National Marine Environmental Forecasting Center, Beijing, 100081, China

    Xingrong Chen & Haiyan Wang

  2. State Key Laboratory of Satellite Ocean Environment Dynamics, Ministry of Natural Resources, Hangzhou, 310012, China

    Zenghong Liu & Dongfeng Xu

  3. Second Institute of Oceanography, Ministry of Natural Resources, Hangzhou, 310012, China

    Zenghong Liu, Dongfeng Xu & Lei Wang

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  1. Xingrong Chen
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Correspondence to Zenghong Liu.

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Foundation item: The National Basic Research Program (973 Program) of China under contract No. 2016YFC0304105; the National Natural Science Foundation of China under contract Nos 41621064, 41606003 and U1811464; the Sino-German cooperation project “The Response of Circulation and Ecosystem of Northwestern South China Shelf Sea to the Anthropogenic and Nature Influences”.

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Chen, X., Liu, Z., Wang, H. et al. Significant salinity increase in subsurface waters of the South China Sea during 2016–2017. Acta Oceanol. Sin. 38, 51–61 (2019). https://doi.org/10.1007/s13131-019-1498-z

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