North Equatorial Current Influence on Ocean Mixing Dynamics in the Western Tropical Pacific Along the 149 E Transect

Abstract

This study examines the relationship between the strength of the North Equatorial Current (NEC) and local ocean mixing along the 149 E transect from 4 N to 16 N. Data was collected in the Western Tropical Pacific aboard the R/V Thomas G. Thompson from 29 December 2024 to 10 January 2025 using an Acoustic Doppler Current Profiler (ADCP) and Conductivity-Temperature-Depth (CTD) sensor. Zonal velocity data from the upper 700 m revealed distinct eastward and westward velocity patterns. The NEC was present from 8 N to 16 N, with peak velocities between 9 N and 12 N in the upper 200 m. The North Equatorial Counter Current (NECC) was observed between 4 N and 6 N in the upper 200 m. Zonal jets were noted at 10 N to 11 N and 13 N to 14 N, with an eddy observed at 15 N. The mixed layer depth (MLD) averaged 82 m across the transect. Although no clear relationship between MLD and NEC strength was found, velocity patterns suggested significant variations in mixing and stratification. Depth profiles of buoyancy frequency (N²), shear magnitude, and Richardson number (Ri) highlighted regions of shear-driven turbulence, particularly where Ri values fell below 0.25, indicating potential vertical mixing. This effect was predominantly observed in the mixed layer, with only one station showing deeper ocean mixing. The shear in the region was not strong enough to overcome buoyancy forces, suggesting a strongly stratified water column that inhibits vertical mixing. These findings underscore the need for further research on the NEC’s impact on shear and buoyancy interactions in ocean mixing dynamics.