An Intermediate Current in Tropical North Pacific Observed by Moored Current Meters

Six years of current meter observation from a subsurface mooring at 130 degrees E, 8.5 degrees N captured an eastward flow below the North Equatorial Undercurrent (NEUC) between 1500-3000m. The mean velocity of this flow at 2000m and 2500m are 0.73 +/- 0.08cm/s and 1.05 +/- 0.08 cm/s, respectively. Its spatial distribution is studied using Argo absolute geostrophic velocity data and ocean reanalysis. In the northwestern Pacific, this Lower-NEUC shifts poleward and is wider in Argo absolute geostrophic velocity data (around 300km) and narrower in reanalysis (around 150km). Vertically, the core of Lower-NEUC is shallower compared to observation, lies mostly between 27.5-27.6 sigma theta in reanalysis. 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. Plain Language Summary Ocean circulation in tropical Pacific is known to have complex structure with undercurrents and alternating zonal flows. Information of circulation below 2,000m is scarce due to limitation of observation methods. This study uses long-term mooring current meter observation to identify an eastward Lower-North Equatorial Undercurrent (L-NEUC) at 130 degrees E, 8.5 degrees N. This flow lies between 1,500 and 3,000 m with a maximum velocity of 1 cm/s at 2,500 m. The existence of L-NEUC is also confirmed in Argo derived velocity product and reanalysis. The nonlineared dyprocesses are responsible for the generation of this flow. The existence of L-NEUC suggests that the ocean is not sluggish at intermediate layer but has significant mean flow and mesoscale processes