Oxygen Variability During ENSO in the Tropical South Eastern Pacific

Francois Colas,Jorge Tam,Jesús Ledesma,Vincent Echevin,Michelle Graco,Dimitri Gutierrez,Carlos Quispe-Ccalluari,Dante Espinoza-Morriberón

doi:10.3389/fmars.2018.00526

Abstract

The Oxygen Minimum Zone (OMZ) of the Tropical South Eastern Pacific (TSEP) is one of the most intensely deoxygenated water masses of the global ocean. It is strongly affected at interannual time scales by El Nino (EN) and La Nina (LN) due to its proximity to the equatorial Pacific. In this work, the physical and biogeochemical processes associated with the subsurface oxygen variability during EN and LN in the period 1958 – 2008 were studied using a regional coupled physical-biogeochemical model and in situ observations. The passage of intense remotely forced coastal trapped waves caused a strong deepening (shoaling) of the OMZ upper limit during EN (LN). A close correlation between the OMZ upper limit and thermocline depths was found close to the coast, highlighting the role of physical processes. The subsurface waters over the shelf and slope off central Peru had different origins depending on ENSO conditions. Offshore of the upwelling region (near 88°W), negative and positive oxygen subsurface anomalies were caused by Equatorial zonal circulation changes during LN and EN, respectively. The altered properties were then transported to the shelf and slope (above 200 m) by the Peru-Chile undercurrent. The source of nearshore oxygenated waters was located at 3°S – 4°S during neutral periods, further north (1°S – 1°N) during EN and further south (4°S – 5°S) during LN. The offshore deeper (< 200 – 300 m) OMZ was ventilated by waters originating from ~ 8°S during EN and LN. Enhanced mesoscale variability during EN also impacted OMZ ventilation through horizontal and vertical eddy fluxes. The vertical eddy flux decreased due to the reduced vertical gradient of oxygen in the surface layer, whereas horizontal eddy fluxes injected more oxygen into the OMZ through its meridional boundaries. In subsurface layers, remineralization of organic matter, the main biogeochemical sink of oxygen, was higher during EN than during LN due to oxygenation of the surface layer and in spite of a reduced primary production. Sensitivity experiments highlighted the larger impact of equatorial remote forcing with respect to local wind forcing during EN and LN.

Highlights

Oxygen Minimum Zones (OMZ) are large water bodies of the open ocean with low concentrations of dissolved oxygen (DO < 22 μmol kg−1) at subsurface depths (∼50–900 m; Levin, 2003; Karstensen et al, 2008)
The nearshore ZO2 was deeper in the model (∼50–70 m between 6◦S and 14◦S), than in the observations (∼30 m in IMARPE and ∼ 50 m in CARS)
Eddy Oxygen Fluxes As eddy activity is enhanced during El Niño Southern Oscillation (ENSO) (Chaigneau et al, 2008; Espinoza-Morriberón et al, 2017), we investigate the impact of El Niño (EN) and La Niña (LN) events on the oxygen vertical and horizontal fluxes

Summary

Introduction

Oxygen Minimum Zones (OMZ) are large water bodies of the open ocean with low concentrations of dissolved oxygen (DO < 22 μmol kg−1) at subsurface depths (∼50–900 m; Levin, 2003; Karstensen et al, 2008). The oxycline, which separates the well-mixed surface waters with the lowoxygenated subsurface waters, impacts marine life by limiting the habitat of several species in the water column (Diaz and Rosenberg, 2008; Gutiérrez et al, 2008; Bertrand et al, 2011) These features, along with the recently observed increase of their vertical extent (Stramma et al, 2008, 2010; Ito and Deutsch, 2013; Schmidtko et al, 2017; Breitburg et al, 2018) have made OMZ studies an oceanographic hot topic in the recent decades. The oxycline is found at 20–50 m water depth (Graco et al, 2007; Gutiérrez et al, 2008; Fuenzalida et al, 2009)

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