First Baroclinic Mode Variability of Kuroshio in the Luzon Strait

Originating from the North Equatorial Current, the Kuroshio plays a crucial role in transporting heat and transmitting climate signals from the tropics to the mid-latitudes. As the Kuroshio flows northward through the Luzon Strait, its pathway, velocity, and vertical structure are strongly modulated by the complex bathymetry and the strait's gap. To examine how the vertical velocity structure of the Kuroshio changes during its passage through the Luzon Strait, this study utilizes the Global Ocean Physics Reanalysis dataset (GLORYS12V1) provided by the Copernicus Marine Environment Monitoring Service. This dataset has a horizontal resolution of 1/12° and comprises 50 vertical layers, ranging in depth from 0.5 m to 5728 m. The data used in this study spans from 1993 to 2024. Empirical orthogonal function analysis was employed to identify the dominant modes of variability in the vertical velocity structure. The result reveals that the zero-crossing depth of the first baroclinic mode of the Kuroshio’s vertical velocity in the Luzon Strait lies approximately between 180 and 320 m, with stronger velocities associated with shallower zero-crossing depths. Furthermore, wavelet analysis of the corresponding principal component indicates that the influence of interannual variability originating from the tropical Pacific weakens near 20°–21°N in the Luzon Strait and is progressively replaced by decadal variability signals as the Kuroshio continues northward. These findings offer new insights into the vertical structure and climate variability of the Kuroshio, contributing to a deeper understanding of how climate signals are conveyed from the tropics into the ocean interior and toward higher latitudes. This is closely related to the United Nations 2030 Sustainable Development Goal (SDG) 13.