On the subsurface countercurrents in the Philippine Sea

The subsurface countercurrents in the Philippine Sea and their roles in water mass transportation have been reported in previous studies. Their existence is still controversial, and the underlying dynamics remains unclear. This study investigates the climatological structures and relationships of three subsurface countercurrents, namely the Mindanao Undercurrent (MUC), the Luzon Undercurrent (LUC), and the North Equatorial Undercurrent (NEUC), using recently available hydrographic and satellite altimeter data. The three subsurface currents below and opposite to the surface currents are confirmed by multisections analysis. The MUC, as traced at zonal sections between 6.5°N and 10.5°N, shows two northward velocity cores, both with maximum speed larger than 10 cm s−1. The LUC exhibits an obscure core with southward velocity larger than 2 cm s−1 under the Kuroshio at 18°N and 16.25°N sections. The eastward flowing NEUC also has two separated cores at 128.2°E and 130°E sections with velocity larger than 1 cm s−1. Analyses of θ‐S relationship suggest that the southern part of NEUC is fed by the MUC with the South Pacific water and South/North Pacific water mixture, while the northern NEUC is likely a destiny of the North Pacific water carried by the LUC. Tightly associated with the opposite horizontal gradients between sea surface height (SSH) and the depth of thermocline (DTC), the subsurface countercurrents exist in connected zones where the baroclinic adjustment below the thermocline overcomes the barotropic forcing at the sea surface, which indicates the dynamical linkages among the three subsurface countercurrents. Key Points: Three subsurface countercurrents (UCs) in PS exist at multisections with new data South and north NEUC fed by MUC and LUC with SP and NP water, respectively UCs link to each other where baroclinic adjustment overcomes barotropic forcing