Two Types of Intraseasonal Variability With a Vertical Difference in the Currents East of Luzon Island and Their Sources

Intraseasonal variabilities (ISVs) of the western boundary currents (WBCs) east of Luzon Island were explored using acoustic Doppler current profiler (ADCP) measurements from three moorings at 18°N during 2018–2020. Besides the traditionally known surface-intensified ISV, subsurface-intensified ISV with a typical period of approximately 60 days was also detected in the currents, and the strongest signal appeared between 400 and 800 m. Further analysis indicates that they are highly associated with subsurface eddies. Based on their lifespan, subsurface eddies are classified into two categories: short-lived and medium-to long-lived eddies. The short-lived eddies are primarily generated locally near the eastern coast of Luzon Island, whereas the medium-to long-lived eddies are mainly generated away from the western boundary, in the region west of 135°E. Additional energy diagnosis suggests that baroclinic instability induced by the velocity shear of the North Equatorial Current (NEC)/subtropical countercurrent (STCC) system dominates the generation of medium-to long-lived subsurface eddies in the interior ocean, while barotropic instability and baroclinic instability play a comparable role in the generation of short-lived eddies near the eastern coast of Luzon Island.