Structure and dynamics of the Pacific North Equatorial Subsurface Current

Xiang Li,Dongliang Yuan,Rui Li,Linlin Zhang,Ya Yang

doi:10.1038/s41598-020-68605-y

Abstract

The North Equatorial Subsurface Current (NESC) has recently been found to flow westward below the North Equatorial Countercurrent in the subsurface layer across the Pacific Ocean. The structure, water mass properties, and the dynamics of the NESC are studied using Argo profiles and geostrophic currents, combined with moored current meter observations. The mean westward geostrophic currents of the NESC has been validated with moored current meter measurements at 4.7° N, 142° E in the far western tropical Pacific Ocean. Sizable seasonal-to-interannual variability of the NESC is indicated by the observations, with strong transports in boreal summer and during La Niña events, whereas weak transports in boreal winter and during El Niño events. The water masses of the NESC appear to be the mixture of the North and South Pacific intermediate waters, with the waters immediately below the thermocline closer to the North than to the South Pacific waters. A simulation using a linear continuously stratified model of ocean circulation suggests that the mean NESC is forced by wind curl through low baroclinic mode responses of the ocean.

Highlights

The equatorial currents in the Pacific Ocean play an important role in the distribution of water mass and heat and freshwaters of the warm pool, which is key to the evolution of anomalous climate events, like El Niño/ Southern Oscillation (ENSO)
The Argo absolute geostrophic currents based on the P-vector calculation[1] show the existence of the North Equatorial Subsurface Current (NESC) across the entire basin of the tropical North Pacific Ocean (Fig. 1a)
The subsurface westward NESC is surrounded by strong eastward currents of the North Equatorial Countercurrent (NECC) on the top and the NSCC in the south and can be distinguished from the westward North Equatorial Current (NEC) to the north at the surface (Fig. 1b–d)

Summary

Introduction

The equatorial currents in the Pacific Ocean play an important role in the distribution of water mass and heat and freshwaters of the warm pool, which is key to the evolution of anomalous climate events, like El Niño/ Southern Oscillation (ENSO). A westward sub-thermocline current in the latitudinal range of 4° N–7° N (Fig. 1a) was found based on Argo geostrophic currents and ship-board acoustic Doppler current profiler (ADCP) measurements, which was named the North Equatorial Subsurface Current (NESC) by Yuan et al.[1]. The water mass properties, the variability, and the dynamics of the NESC, which are important for Pacific Ocean circulation and warm pool variability, have not been disclosed so far. We use recent mooring measurements in the far western equatorial Pacific Ocean (Fig. 1a) and Argo profile measurements across the tropical Pacific basin to study the structure of this subsurface current. A linear continuously stratified model (LCSM) is used to investigate the dynamics of the NESC

Methods

Results

Conclusion