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

• Published in: Journal of geophysical research. Oceans 2024-11, Vol.129 (11), p.n/a
• Main Authors: Wang, Zhenxiao, Zhang, Linlin, Mu, Lin, Hui, Yuchao, Song, Weiqi, Li, Wenjuan, Hu, Dunxin
• Format: Article
• Language: English
• Subjects: Acoustic Doppler Current Profiler; Baroclinic instability; Barotropic instability; Barotropic mode; Boundary currents; Climate change; Coastal currents; Doppler sonar; Eddies; Equatorial currents; Equatorial regions; In situ measurement; Instability; intraseasonal variability; Life span; Mesoscale eddies; mesoscale eddy; Mooring; Ocean currents; Oceans; Regional climates; Seasonal variations; Shear; Subtropical countercurrent; Surface stability; Undercurrents; Velocity; Vertical shear; Vortices; western boundary current; Western boundary currents
• ISSN: 2169-9275; 2169-9291
• DOI: 10.1029/2024JC021206

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. Plain Language Summary The Kuroshio and Luzon Undercurrent (LUC) have a far‐reaching impact on regional climate change as a crucial part of the western boundary current (WBC) system. In situ measurements east of Luzon Island have revealed that the currents exhibit significant intraseasonal variabilities (ISVs). However, the vertical difference of the ISVs is unclear, and we still do not know if subsurface ISVs exist in this region. In this work, we found the currents east of Luzon Island are characterized by two types of ISVs with a period of approximately 60 days: surface‐intensified and subsurface‐intensified. The two types of ISVs are linked to the surface and subsurface mesoscale eddies, respectively. Previous studies have revealed that baroclinic instability associated with the vertical shear of velocity in the North Equatorial Current/subtropical countercurrent (NEC/STCC) system is the primary generation mechanism of surface eddies. This study suggests that baroclinic instability of the NEC/STCC system also dominates the generation of subsurface eddies in the interior ocean, while barotropic instability associated with the horizontal shear of currents near the Luzon coast is non‐negligible