Characteristics and Formation of the Luzon Undercurrent in the Western North Pacific: Observational Study

The subsurface circulation system in the western North Pacific is dominated by the convergence of the Luzon Undercurrent (LUC) and the Mindanao Undercurrent, which forms the basin-scale eastward North Equatorial Undercurrent jets. The southward LUC, situated to the east of Luzon Island, plays a significant role in transporting subsurface waters equatorward in the western boundary of the subtropical North Pacific. Despite its crucial contributions to the water, heat, and energy distributions in the subsurface layer, its characteristics and formation mechanism remain unclear. In this study, we utilize monthly climatological temperature and salinity data and the derived geostrophic velocity to investigate the spatiotemporal variability of the LUC and its formation dynamics. The LUC transport exhibits weak seasonality and strong intra-seasonal variation, peaking in May/June and October. The origin of the LUC is closely connected with the southward veering of the Deep North Equatorial Current (DNEC) off the Ryukyu Islands and the Luzon Strait middle-layer outflow (LSMO). The DNEC dominates transport in the LUC, and the LSMO of the South China Sea (SCS) Intermediate Water contributes to the water mixing in the LUC. Locally, the appearance of the LUC is dynamically linked with the barotropic effect associated with the sea surface height gradient and the baroclinic effect raised from the downward tilting of the zonal isopycnal off the Philippine coast. Spatially, the core of the LUC shifts from in-shore to off-shore waters from May/June to October. This study reveals the spatiotemporal characteristics of the LUC and identifies its separate origins from the interior North Pacific and the SCS middle layer.

Automated summary

The study investigates the subsurface circulation system in the western North Pacific and reveals the characteristics and formation mechanism of the Luzon Undercurrent (LUC). The LUC transport shows weak seasonality and strong intra-seasonal variation, and its origin is closely connected with the Deep North Equatorial Current (DNEC) and the South China Sea Intermediate Water. The appearance of the LUC is dynamically linked with the sea surface height gradient and the downward tilting of the zonal isopycnal off the Philippine coast. The core of the LUC shifts from in-shore to off-shore waters spatially.