The Seasonal Variation of the Anomalously High Salinity at Subsurface Salinity Maximum in Northern South China Sea from Argo Data

The large variations in salinity at the salinity maximum in the northern South China Sea (NSCS), as an indicator for the changes in the Kuroshio intrusion (KI), play an important role in the hydrological cycle. The high salinity here is more than 34.65 at the salinity maximum and is intriguing. In the past, the salinity was difficult to trace in the entire NSCS over long periods due to a lack of high-quality observations. However, due to the availability of accumulated temperature and salinity (T-S) profiles from the Argo program, it is now possible to capture subsurface-maximum data on a large spatiotemporal scale. In this study, the salinity maximum distributed in the subsurface of 80 to 200 m at a density of 23.0-25.5 sigma(theta) was extracted from decades of Argo data (on the different pressure surfaces, 2006-2019). We then further studied the spatial distribution and seasonal variation of the salinity maximum and its anomalously high salinity. The results suggest that a high salinity (salinity > 34.65, most of which is located at the shallow depths < 100 m) at the subsurface salinity-maximum layer often occurs in the NSCS, especially near the Luzon Strait, which accounts for about 23% of the total salinity maximum. In winter, the anomalously high salinity at the shallow subsurface salinity maximum can extend to the south of 17 degrees N, while it rarely reaches 18 degrees N and tends to locate at deeper waters in summer. The T-S values of the anomalously high-salinity water are between the mean T-S values in the NSCS and north Pacific subsurface water, implying that the outer sea water gradually mixes with the South China Sea water after passing through the Luzon Strait. Finally, our results show that the factors play an important role in the appearance and distribution of the anomalously high salinity at the subsurface salinity maximum, including the strength of the Kuroshio intrusion, the local wind stress curl and the anticyclonic eddy shedding from the loop current.

Automated summary

The study found that high salinity often occurs at the subsurface salinity maximum in the northern South China Sea, especially near the Luzon Strait in winter, while it tends to be located in deeper waters in summer. This suggests that outer sea water gradually mixes with South China Sea water after passing through the Luzon Strait. Factors such as the strength of the Kuroshio intrusion, local wind stress curl, and anticyclonic eddies play important roles in the appearance and distribution of anomalously high salinity in the subsurface salinity maximum in this region.