he southern South China Sea is located at the edge of the Western Pacific Warm Pool and plays a crucial role in the interaction of the Asian winter and summer monsoons. Understanding the evolution of the upper water structure during glacial-interglacial cycles is essential for investigating the East Asian monsoon and tropical ocean-atmosphere coupling processes. This study is based on sediment samples from the MD01-2392 core(43.3 m, 09° 51.13' N, 110° 12.64' E; water depth: 1966 m) and uses long-chain unsaturated ketone and glycerol dialkyl glycerol tetraether(GDGT) proxies to reconstruct surface and subsurface seawater temperatures over the past 500 ka. The study also estimates thermocline depth changes based on the temperature difference between surface and subsurface waters.

The results reveal a two-stage evolution of thermocline depth over the past 500 ka: between 480.8 ka and 215.8 ka, the thermocline depth was significantly higher than during the period from 214.8 ka to the present. Specifically, during the 480.8~215.8 ka period, the surface seawater temperature fluctuated by 4.4 ℃, while the subsurface temperature fluctuated by 5.7 ℃. The thermocline depth varied between 103.8 m and 159.8 m, with an average depth of 133.6 m, showing a total variation of 56.0 m. In contrast, during the 214.8 ka to present period, the sea surface temperature fluctuation increased to 5.7 ℃, and the subsurface seawater temperature fluctuation rose to 6.0 ℃. The thermocline depth exhibited a greater variation, ranging between 89.1 m and 167.0 m, with an average depth of 128.7 m, showing a total change of 77.9 m.

This difference in the depth of thermocline between the two stages may be attributed to changes in the local ocean circulation system, which triggered a shift in the mechanisms controlling the depth of thermocline. During the 480.8~215.8 ka period, the study site was likely located at the edge of the upwelling zone, where thermocline depth variation was influenced by local circulation. When the site was in a non-upwelling region, strong winter monsoons promoted vertical mixing, leading to a deeper thermocline. In contrast, when the site was located within the upwelling region, stronger winter monsoons led to a shallower thermocline.

During the 214.8 ka to present period, the study site was positioned within the upwelling zone of mesoscale eddies. In this stage, thermocline depth changes were jointly influenced by the winter monsoon and the La Niña phenomenon. Stronger winter monsoons intensified upwelling, leading to a shallower thermocline, while La Niña may have further influenced the depth of thermocline by enhancing the intensity of the winter monsoon. In conclusion, this study demonstrates that long-term changes in the upper water structure of the southern South China Sea are influenced by both the winter monsoon and local ocean circulation systems. The results provide key geological evidence for exploring the tropical ocean-atmosphere coupling mechanisms and their role in regional climate change. Future research could further extend the understanding of thermocline dynamics, particularly on longer timescales(e.g., millions of years), to reveal the deep connections between ocean heat transport and global climate change.