The bimodality of the Luzon Strait deep water

Combined conductivity-temperature-depth (CTD) casts and Argo profiles, 3 086 historical hydrocasts were used to quantify the water column characteristics in the northern South China Sea (SCS) and its adjacent waters. Based on a two-dimensional "gravest empirical mode" (GEM), a gravitational potential (φ, a vertically integrated variable) was used as proxy for the vertical temperature profiles T_G(p, φ).φ integrated from 8 MPa to the surface shows a close relationship with the temperature, except in the deep layer greater than 15 MPa, which was caused by the bimodal deep water in the region. The GEM temperature profiles successfully revealed the bimodality of the Luzon Strait deep water, that disparate hydrophic vertical profiles can produce distinct specific volume anomaly (δ) in the SCS and the western Philippine Sea (WPS), but failed in the Luzon Strait, where different temperature profiles may produce a same δ. A significant temperature divergence between the SCS water and the WPS water confirmed that the bimodal structure is strong. The deepwater bifurcation starts at about 15 MPa, and gets stronger with increasing depth . As the only deep channel connecting the bimodal-structure waters, water column characteristics in the Luzon Strait is in between, but much closer to the SCS water because of its better connectivity with the SCS. A bimodal temperature structure below 15 MPa reveals that there was a persistent baroclinic pressure gradient driving flow through the Luzon Strait. A volume flux predicted through the Bashi Channel with the hydraulic theory yields a value of 5.62×106 m3/s using all available profiles upstream and downstream of the overflow region, and 4.03×106 and 2.70×106 m3/s by exclusively using the profiles collected during spring and summer, respectively. No volume flux was calculated during autumn and winter because profiles are only available for the upstream of the Bashi Channel during the corresponding period.