Xiaoluan Yan, Linlin Zhang, Yuchao Hui, Fan Wang

Published in Journal of Geophysical Research: Oceans, March 2025

Six years of current meter observation from a subsurface mooring at 130°E, 8.5°N captured an eastward flow below the North Equatorial Undercurrent (NEUC) between 1,500 and 3,000 m. This flow, named Lower-NEUC, has mean velocity of 0.73 ± 0.08 cm/s and 1.05 ± 0.08 cm/s at 2,000 and 2,500 m, respectively. Its spatial distribution is studied using Argo absolute geostrophic velocity data and ocean reanalysis. In the northwestern Pacific, Lower-NEUC shifts poleward with increasing longitude and is wider in Argo absolute geostrophic velocity data (around 300 km) and narrower in reanalysis (around 150 km). Vertically, the core of Lower-NEUC lies mostly between 27.5 and 27.6σθ in reanalysis, shallower compared to mooring observation. Reanalysis data shows L-NEUC's generation is connected to deep eddies. After proving that L-NEUC is not a result of averaging west-translating eddies, we showed that L-NEUC is generated by eddy potential vorticity flux convergence.

Fig. Surface (black arrows), subsurface (red solid arrows), and intermediate (red dashed arrow) currents in the northwestern tropical Pacific. The stars represent locations of IOCAS full-depth moorings at 130°E with the filled yellow star being the mooring at 8.5°N. Abbreviations are NECC for North Equatorial Countercurrent, MC for Mindanao Current, MUC for Mindanao Undercurrent, KC for Kuroshio Current, LUC for Luzon Undercurrent, NEC for North Equatorial Current, NEUC for North Equatorial Undercurrent, L-NEUC for Lower-North Equatorial Undercurrent.

Yan, X., Zhang, L., Hui, Y., & Wang, F. (2025). An intermediate current in tropical North Pacific observed by moored current meters. Journal of Geophysical Research: Oceans, 130, e2024JC021960. https://doi.org/10.1029/2024JC021960