Pacific low-latitude western boundary currents region east of the Philippine coast is characterized by abundant meso-scale eddies in the subsurface layer, which play crucial roles in the intraseasonal variation of the currents and transport of subsurface waters. Based on hydrographic transects and model outputs, the basic characteristics and dynamics of these subthermocline eddies were investigated. CTD transects along 8°N during the December 2012 and December 2017 cruises captured two subthermocline lens-like features centering at about 800 m, with isopycnals above the center doming upward and those below depressing downward (Song et al., 2022). Model suggests that most Subsurface Eddies (SSEs) originate at ~138 E furthest with a mean lifespan of 55 days and a westward translation speed of 6 cm/s. The zonal volume transport of SSEs is around 12 Sv, 30-40% of the transport of North Equatorial Current. SSEs-induced stirring heat/salt transport is 1-2 orders larger than the trapping component and is mainly concentrated near the Mindanao coast. At the 8 N section, the mean salt transport associated with SSEs is about 24% of that induced by the Mindanao Undercurrent (MUC, Zhang et al., 2024). Energy and instability analysis suggest that besides the baroclinic instability reported in previous studies, barotropic instability is also essential in the generation of SSEs, especially near the Mindanao coast (Zhang et al., 2021; Hui et al., 2022; Wang et al., 2022).

subsurface eddy, eddy-induced transport, western Pacific