The response of the Western Pacific to high-and low-sea levels: based on ROMS experiments

Superimposed on tectonic movements, sea level changes during geologic history caused coastline advancement and retreat, exposing shallow shelf or submerging coastal regions, and thus influencing regional ocean currents and global ocean circulation. The Regional Ocean Model (ROMS) was used to simulate the response of the West Pacific thermohaline pattern and major ocean currents in the low sea level scenarios, e.g., the SLdrop120 (sea level drop by 120 m) case during the last glacial maximum (LGM), and the high sea level scenarios, e.g., the SLrise65 (sea level rise by 65 m) case during the global ice sheet melting. Results show that sea level rise and fall have more significant non-linear influences on ocean currents than on temperature and salinity schemes. Compared with the modern sea level, the extremely low sea level led to the exposure of the near-shore shelf sea and the closure of the Taiwan Strait, cutting off the invasion of the western boundary current into the South China Sea, causing the Kuroshio transport in the East China Sea to move toward the open sea and reducing the flow rate in the main axis section. Compared with previous research results considering the LGM glacial climate state, this decreasing trend indicates that the effects of sea level drop during the LGM period and glacial climate driving offset each other. The high sea level pushes the shoreline landward, the area of coastal waters such as the Bohai Sea increases, and the Taiwan Strait widens, causing the western boundary current to expand westward, which would diverse the mainstream of the Kuroshio. Regarding the Indonesian Throughflow (ITF), which controls the exchange between the Pacific and Indian Oceans, its western branch path responds more significantly to sea level changes due to its shallower depth. Under the extreme low sea level scenario, the Karimata Strait is closed and the western branch of the ITF is cut off, so the freshwater blocking effect from the western branch disappears, which leads to an increase of 2.31 Sv (1 Sv=1×10⁶ m³/s) in the flow through the Makassar Strait. On the contrary, under the high sea level scenario, both the Karimata Strait and the Makassar Strait on the west side are widened, and the Torres Strait is opened, which makes the flow into the Indian Ocean larger than the modern sea level scenario. This study demonstrated that the responses of the Kuroshio and the Indonesian Throughflow in the western Pacific to the change of the coastline is nonlinear, and emphasized the important role of sea level changes in regional ocean currents during geological evolution.