ENSO-Driven Variability of Deep Ocean Circulation in the Southeast Pacific

Abstract. The Southeast Pacific is crucial for transporting Pacific Deep Water toward the Southern Ocean, primarily through a persistent deep southward flow off the Chilean coast. However, the variability and underlying dynamics of this flow remain poorly understood. Here, we investigate its interannual variability and explore its relationship with El Niño–Southern Oscillation (ENSO). Using the GLORYS12 global ocean reanalysis (1993–2020) and repeated GO-SHIP/WOCE P06 hydrographic sections, we focus on the 2200–4000 m depth range, covering the steep continental slope and the Atacama Trench region. Our analysis confirms a robust southward transport of approximately 2.5 Sv within ~300 km from the coast, predominantly concentrated within 100 km of the continental margin. ENSO exerts a significant influence on this current, with an intensification of the deep southward flow during El Niño events and a weakening during La Niña. This modulation is linked to large-scale oceanic adjustments, including the westward propagation of extratropical Rossby waves, which transfer energy from the surface and thermocline to the deep ocean along steep ray paths consistent with linear theory. Complex Empirical Orthogonal Function (CEOF) analysis and dynamical mode decomposition further reveals that high order baroclinic modes play a key role in shaping deep current variability. Mesoscale processes also seem to contribute to the observed variability. Enhanced deep eddy kinetic energy (EKE) occurs near the continental margin and in the coastal transition zone, which is more prominent during ENSO events. The Analysis of Reynolds stresses and vertical buoyancy fluxes (VBF) indicates that energy is transferred between mean flow and turbulence, with VBF being a dominant contributor to deep variability of EKE tendency. These findings emphasize the complex interplay of large-scale climatic forcing, topography, and mesoscale turbulence in shaping deep ocean circulation. Future work should integrate sustained deep-ocean observations and high-resolution modelling to refine our understanding of the DESPC and its role in the Pacific Meridional Overturning Circulation.