Estimation of sea surface salinity in the East China Sea using GOCI-II

Abstract

Sea surface salinity (SSS) is one of the important variables to monitor ocean environment and is intimately related to climate change. In particular, low salinity water causes a lot of damage as it travels along the ocean current without mixing with other seawater because of different density. Low salinity water in the East China Sea is caused by a significant volume of fresh water from the Yangtze River. This flows into Jeju Island and the South Sea of the Korean Peninsula, causing significant harm to the fish farms. L-band microwave sensors, Soil Moisture Ocean Salinity (SMOS) and Soil Moisture Active Passive (SMAP), observe the worldwide SSS, but daily monitoring is not practicable due to long revisit time (3-8 days) and detailed distribution analysis is difficult due to low spatial resolution (25-100 km). Choi et al. (2021) complemented these issuses and estimated the low salinity water of East China Sea using Geostationary Ocean Color Imager (GOCI). GOCI collects 8 images per day, every hour at a resolution of 500 m, and hourly and daily monitoring is conceivable with this high spatio-temporal resolution. Choi et al. (2021) developed empirical equation between remote sensing reflectance (Rrs) of four bands (490, 555, 660, 680 nm) of GOCI and in-situ data. However, as GOCI’s mission was ended in March 2021, low salinity water monitoring with the new GOCI-Ⅱ has become necessary. The purpose of this study is to estimate SSS in the East China Sea using GOCI-Ⅱ with higher resolution than GOCI. The GOCI-Ⅱ, which was launched in February 2020, observed twice per day more than the GOCI and improved spatial resolution from 500 m to 250 m. After establishing the empirical correlations between GOCI Rrs and GOCI-Ⅱ Rrs, the emprical equation of Choi et al. (2021) was applied to GOCI-Ⅱ Rrs. There was a high correlation between GOCI-derived SSS and the GOCI-Ⅱ- derived SSS (R2 was 0.94 and RMSE was 0.87 psu). This demonstrated that GOCI-Ⅱ may use the same empirical equation as GOCI to continually monitor low salinity water in the East China Sea.