Mapping the Changjiang Diluted Water in the East China Sea during summer over a 10-year period using GOCI satellite sensor data

Abstract

The Changjiang River loads freshwater into the East China Sea (ECS) and the Yellow Sea (YS). A low-salinity plume known as Changjiang Diluted Water (CDW) is formed by mixing with saline ambient waters; it disperses toward Jeju Island and the East/Japan Sea due to the dominant southerly wind and tide during summer. To map the offshore surface CDW in the ECS, a proxy was developed using surface water beam attenuation (c) and salinity from 18 annual summer cruises matched with synchronous Geostationary Ocean Color Imager (GOCI) satellite data. We followed a two-step empirical approach. First, a relationship between in situ salinity and c was obtained from hydrographic cruises (1998, 2003–2018, and 2021). Second, in situ c was matched with GOCI remote-sensing reflectance (Rrs) band ratios of all available blue-to-green wavelengths (2011–2018). Finally, satellite-derived surface salinity was determined directly by combining the two empirical relationships, providing a robust estimate over a range of salinities (22–34 psu). Our algorithm was then compared and validated with five previous satellite-derived salinity algorithms based on colored dissolved organic matter and particle concentrations. In the first step, surface salinity was matched and well correlated with c using an 18-year full conductivity–temperature–depth (CTD) dataset (N = 1,345, R2 = 0.93). In the second step, c was synchronously matched and well correlated with GOCI Rrs band ratios using an 8-year summer CTD dataset (N = 391, R2 = 0.93). The satellite-derived surface salinity based on the GOCI was compared with five other approaches and validated using an 8-year summer CTD dataset (2011–2018, N = 707, RMSE = 0.43, bias = 0.053, mean ratio = 1.002). We mapped satellite-derived surface salinity using monthly GOCI images during the summer from 2011 to 2020. The spatial patterns of the CDW were connected to the Changjiang River mouth and extended to the east–northeast during summer. Saline water was confined to the warm current and the upper slope in the eastern part of the study area. Anomalous dispersion of the CDW occurred in August 2016, 2017, and 2020, and limited dispersion occurred in August 2014 and 2019.

The record-breaking 2022 long-lasting marine heatwaves in the East China Sea

“…Figure 1(a) shows the spatial distribution of anomalous July SST and accumulated land precipitation around the Yangtze-Huaihe Rivers from June 23-27 2022. Because heavy rainfall continues throughout the summer over southern China in general, the river discharge remarkably increased during monsoon season [6]. The Yangtze (Changjiang) River discharge flow in June 2022 was the second largest since…”

“…Because the intrusion of the CDW significantly affects the physical and biological processes in the East China Sea (ECS) [4], it is crucial to understand how the CDW is affected by the inland hydrological cycle [5]. Thus, the effect of extreme rainfall over land is not limited to the land area but is extended to the oceanographic and ecological environment [6,7], such as through marine heatwaves (MHWs) and loss of biodiversity. These oceanic extremes have remarkably increased and intensified, especially over most of the East Asian marginal sea [8][9][10].…”

“…In 2022, La Niña was recorded as a rare 'triple-dip' , which had persisted since September 2020. Although multiple extreme events for 2022 summer occurred concurrently, the relationships between the MHWs, extreme rainfall-CDW in inland China [6], and La Niña have not been addressed.…”

Water mass structure determine the prokaryotic community and metabolic pattern in the Korea Strait during fall 2018 and 2019

“…According to previous studies, 70% of CDW is transferred annually via KS to the East Sea (Chang and Isobe, 2003;Senjyu et al, 2006;Moon et al, 2019). As a result, CDW influence plays a significant role in the large biological effects on the ECS and Korean coastal environment (Son and Choi, 2022) and alters phytoplankton composition (Yoon et al, 2022). Within the past ten years, the CDW has had the largest FW discharge in 2019-2020 followed by 2016 and 2017, but there was no significant FW discharge in 2018 (Son and Choi, 2022).…”

Factors controlling the distribution of dissolved organic carbon and nitrogen in the coastal waters off Jeju Island

“…The CDW is not strictly defined as an independent water mass but it is generally treated as one due to its significant influence. The Changjiang River discharge is the fifth largest in the world and has a strong influence on the hydrography and currents of the East China Sea, which has a width of approximately 800 km (Son and Choi, 2022). The CDW has long been known to extend toward Jeju Island during summer every year, though serial hydrographic surveys around Jeju Island show that the characteristics of the CDW are considerably different every year (Kim and Rho, 1994;Moon et al, 2009).…”