Sea surface salinity (SSS) can be considered as an indicator of the water cycle and the flux of freshwater across the air sea interface (Bingham et al., 2011). The low SSS areas are usually occupied by precipitation, freshwater and melting glaciers (Dickson et al., 1988; Durack and Wijffels, 2010). Currently, the main methods for obtaining SSS are in situ observation and satellite remote sensing (Bai et al., 2013, 2014).

In situ observations are widely used for scientific researches and ocean operational monitoring. The temperature and salinity from Argo profiles are often utilized to address the seasonal and interannual variability of SSS of many regions, such as the eastern Equatorial Indian Ocean and Southeastern Arabian Sea (Subrahmanyam et al., 2011). Liu and Feng (2012) studied the salinity data, which were collected on two zonal sections near the Jeju Island, to analyze the impact of wind on the transport of the Changjiang River freshwater plume in August. However, the in situ observation cannot describe the variations of SSS at finer spatial and temporal scales. Increasing availability of the satellite observations of SSS data will undoubtedly provide us with more accurate method to observe SSS. At present, several satellites such as AQUARIUS, SMOS and SMAP have been launched for observing SSS. SMOS is the first satellite mission to address the challenge of measuring SSS from space by the European Space Agency in 2009 (Berger et al., 2002; Font et al., 2010). Hasson et al. (2018) and Guimbard et al. (2017) analyzed the 2015–2016 El Niño event with SSS from SMOS, found that the SSS is strong anomalies in the tropical Pacific. AQUARIUS is lunched for observing the SSS from space with a combined passive/active L-band microwave instrument in 2011. The SSS data supports us to investigate the coupling relationships between ocean circulation, global water cycle, and climate (Le Vine et al., 2007; Lagerloef et al., 2008; Yueh et al., 2014). The SMAP mission is one of the first Earth observation satellites of National Aeronautics and Space Administration (NASA) to detect soil moisture from January of 2015. The SMAP can distinguish frozen from thawed land surfaces, it improving the estimates of water, energy, and carbon transfers between the land and the atmosphere. The SMAP mission provides high resolution and high accuracy soil moisture and frozen/thaw state data using L-band radar and radiometer instruments (Entekhabi et al., 2010, 2014; Das et al., 2011; Yueh et al., 2016). The SMAP can directly exhibit the flood assessment and drought monitoring of many large rivers through the distributions of SSS observations, such as the Niger River, the Amazon River, the Nujiang River, the Mississippi River and the Gangs River (Yueh et al., 2016). The SSS from SMAP was used to focus on the May 2015 severe flooding in Texas (Fournier et al., 2016).

SSS satellite missions provide new opportunities to study SSS variation and their potential applications in terms ocean monitoring and forecasting need to be explored, especially in coastal regions where the riverine freshwater discharge plays a significant role in oceanic processes and in situ salinity observation is often sparse in space. In present research, the SSS observation from SMAP mission is used to study SSS variation around Changjiang River Estuary. Section 2 introduces the materials we used in the study. Section 3 validates SMAP and SMOS observations using in situ observation and analyzes SSS variability of SMAP on seasonal and sub-seasonal time scales. In section 4, we summarize results.

In present study, the SMAP data we used is from September 1, 2015 to August 31, 2018 while the SMOS data is from September 1, 2015 to August 31, 2016. The SMAP is a gridded product with 1-day temporal resolution and 0.25° horizontal resolution while the temporal resolution of SMOS is 4-day and the spatial resolution is 0.26°.

The in situ measurement of sea-bird SBE CTD used for all measurements is from the R/V Rongjiang No. 1. The two observation time periods are from December 20 to 30, 2015 and August 3 to 13, 2016. For each station, the CTD instrument is put in the water for 3–5 min, and descends with a speed around 1 m/s. The procedure was repeated 1–2 times to measure temperature and salinity. Our data quality control procedure includes two steps. First, we removed all the data with the pressures smaller than 0. Second, the temperature and salinity with variability larger than 3 standard deviations are removed.

The daily freshwater discharge data of the Datong hydrological station over the Changjiang River from September 2015 to August 2018 is used in this study (http://xxfb.hydroinfo.gov.cn).

In this study, we compare SSS from SMAP and SMOS missions, with in situ observation, and find that the RMSDs between satellite SSS and in situ observations are about 3. However, the SSS derived from SMAP mission may be more suitable for describing the spreading of the freshwater plume from the Changjiang River than SMOS, especially in summer, since SMOS SSS has a larger bias and RMSD than that of SMAP. In addition, we also analyze the spatial distribution and temporal evolutions of SSS around the Changjiang River Estuary using SMAP data. The results show that the SMAP SSS has obvious seasonal cycle. The annual range of SSS, the difference between the maximum value in winter and minimum value in summer, exceeds 16. The averaged SSS over the region (32°–33°N, 122°–124°E) is negatively related with freshwater discharge from the Changjiang River. The Changjiang River freshwater spreads northeastward during summer time and southwestward during winter time. Our analysis demonstrates the potential of satellite SSS for studying and monitoring the Changjiang River freshwater plume in the East China Sea.

We thank Xianqiang He and the crew member of R/V Rongjiang No. 1 for making in situ CTD observations in the winter of 2015, and summer of 2016. The SMAP data are downloaded from the National Snow and Ice Data Center (NSIDC, http://nsidc.org/the-drift/data-set/smap/).