The observed North Equatorial Counter Current in the far western Pacific Ocean during the 2014-2016 El Niño

Hui Zhou,Yao Li,William K Dewar,Shuwen Tan,Xueqi Liu,Hengchang Liu,Wenlong Yang,Qiang Ren

doi:10.1175/jpo-d-20-0293.1

Abstract

AbstractThe structure and variations of the North Equatorial Counter Current (NECC) in the far western Pacific Ocean during 2014-2016 are investigated using repeated in-situ hydrographic data, altimeter data, Argo data, and reanalysis data. The NECC shifted ~1 degree southward and intensified significantly with its transport exceeding 40 Sv (1 Sv = 106 m3 s-1), nearly double its climatology value, during the developing phase of the 2015/16 El Niño event. Observations show that the 2015/16 El Niño exerted a comparable impact on the NECC with that of the extreme 1997/98 El Niño in the far western Pacific Ocean. Baroclinic instability provided the primary energy source for the eddy kinetic energy (EKE) in the 2015/16 El Niño, which differs from the traditional understanding of the energy source of EKE as barotropic instability in low latitude ocean. The enhanced vertical shear and the reduced density jump between the NECC layer and the subsurface North Equatorial Subsurface Current (NESC) layer renders the NECC–NESC system baroclinically unstable in the western Pacific Ocean during El Niño developing phase. The eddy-mean flow interactions here are diverse associated with various states of the El Niño Southern Oscillation (ENSO).

Highlights

The western equatorial Pacific Ocean plays an important role in the initiation of El Niño–Southern Oscillation (ENSO) due to its active air–sea interactions (Wyrtki 1975, 1985; Weisberg and Wang 1997; Wang et al 1999; Kessler et al 2003)
Since the North Equatorial Countercurrent (NECC) has very strong seasonal variations, to compare the amplitudes of the seasonal and interannual anomalies, we present the seasonal geostrophic velocity anomaly (GVA) of the NECC derived from 14-year-long Argo data around its main axis (Fig. 6b)
The hydrographic data collected during three research cruises carried out on board of R/V Kexue during 2014–16, combined with the Argo data, altimetry data, and Ocean Reanalysis system 4 (ORAS4) product in the western equatorial Pacific Ocean are used to investigate the structure and variability of the NECC at its birthplace

Summary

Introduction

The western equatorial Pacific Ocean plays an important role in the initiation of El Niño–Southern Oscillation (ENSO) due to its active air–sea interactions (Wyrtki 1975, 1985; Weisberg and Wang 1997; Wang et al 1999; Kessler et al 2003). Three cruises were carried out across the NECC along the 1308E transect during the summer/fall of 2014, 2015, and 2016 (Table 1) During these three years, the widely anticipated major 2014/15 El Niño event failed to materialize but helped push the 2015/16 El Niño event to extreme magnitude (Levine and McPhaden 2016). Since extreme El Niños stand out for their powerful impacts and for their

28 Sep–8 Oct 2016

Data and methods

Summary and conclusions