The Observed North Equatorial Countercurrent in the Far Western Pacific Ocean during the 2014-16 El Niño

Hui Zhou , Hengchang Liu , Shuwen Tan , Wenlong Yang , Yao Li , Xueqi Liu , Qiang Ren , William K. Dewar (1)

1 IGE - Institut des Géosciences de l’Environnement

The structure and variations of the North Equatorial Countercurrent (NECC) in the far western Pacific Ocean during 2014-16 are investigated using repeated in situ hydrographic data, altimeter data, Argo data, and reanalysis data. The NECC shifted ~1° 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 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 El Niño-Southern Oscillation (ENSO).

Domains: Sciences of the Universe [physics]

Hui Zhou, Hengchang Liu, Shuwen Tan, Wenlong Yang, Yao Li, et al.. The Observed North Equatorial Countercurrent in the Far Western Pacific Ocean during the 2014-16 El Niño. JOURNAL OF PHYSICAL OCEANOGRAPHY, 2021, 51, pp.2003-2020. ⟨10.1175/JPO-D-20-0293.1⟩. ⟨insu-03706455⟩