Ocean Sci., 22, 119-143, 2026
https://doi.org/10.5194/0s-22-119-2026
© Author(s) 2026. This work is distributed under
the Creative Commons Attribution 4.0 License.

7
Ocean Science :( EGU

Hidden vortices: near-equatorial low-oxygen extremes
driven by high-baroclinic-mode vortices

Florian Schütte'?, Johannes Hahn?, Ivy Frenger!, Arne Bendinger*, Ahmad Fehmi Dilmahamod', Marco Schulz!,
and Peter Brandt‘?
'GEOMAR Helmholtz Centre for Ocean Research Kiel, Kiel, Germany
’Faculty of Mathematics and Natural Sciences, Kiel University, Kiel, Germany
“Federal Maritime and Hydrographic Agency, Hamburg, Germany
‘Laboratoire d’Oc&anographie Physique et Spatiale, University Brest, CNRS, Ifremer, IRD, IUEM, Plouzane, France
Correspondence: Florian Schütte (fschuette @ geomar.de)
Received: 8 May 2025 — Discussion started: 21 May 2025
Revised: 17 October 2025 — Accepted: 27 October 2025 — Published: 13 January 2026

Abstract. Long-term time series of dissolved oxygen (DO)
measurements from the upper 500m of the eastern tropical
 North Atlantic (ETNA), collected over a period of up to
15 years at three different mooring sites, reveal recurring eXtreme
 low-oxygen events lasting for several weeks. Similarly,
observations from 15 individual meridional ship sections becween
 6 and 12°N along 23° W show DO concentrations
far below 60 umolkg”! in the upper 200m — significantly
lower than the climatological values in this depth range
(> 80 umolkg-!). Two-third of these low-oxygen events
could be related with high-baroclinic-mode vorticies (HB Vs)
with their cores located well below the mixed layer. Despite
 the energetic equatorial circulation and the expected
dominance of wave-like structures in the near-equatorial region,
 these HBVSs persist as relatively long-lived and coherent
 features. Based on moored and shipboard observations
{rom the ETNA, and supported by an eddy-resolving ocean-»0geochemistry
 model, we characterize their dynamics and
DO distribution. Observed water mass properties and model
analyses suggest that most HBVs originate from the eastern
boundary and can persist for more than six months. As they
propagate westward into regions of higher potential vorticity
 (PV), anticyclonic HBVs with low-PV cores remain more
effectively isolated and have longer lifespans compared to
cyclonic HBVs with high-PV core. The vertical structure of
*he dominant anticyclonic HBVs corresponds to baroclinic
modes 4-10, with associated Rossby radii ranging from 34 to
13km, respectively. This is consistent with observed eddy
sizes and is well below the corresponding lst baroclinic

Rossby radius of deformation (> 100km). Since none of
the observed HBVs exhibit a surface signature, a substantial
 portion of the near-equatorial eddy field may remain undetected
 by satellites, yet still exert significant influence on
local ocean ecosystems and biogeochemical cvcles.

1

Introduction

Dissolved oxygen (DO) concentration is a key component
of marine ecosystems, shaping biodiversity, biogeochemical
 cycles, and the survival of pelagic species (e.g. Deutsch
et al., 2020). From long-term moored observations in the
open Eastern Tropical North Atlantic (ETNA) near the equator
 (latitudes < 12° N), we repeatedly observe short-lived
extreme low-oxygen events in the subsurface, well below
the mixed layer. This DO variability is likely driven by
small-scale vortices, which is unexpected, as theory suggests
that wave-like structures should dominate at these latitudes
(Eden, 2007). In this study, we combine moored time series,
repeated ship transects, and an eddy-resolving biogeochemical
 model to investigate these small-scale processes below
the mixed layer in the tropical Atlantic. This integrated approach
 allows us to characterize their structure, variability,
and strong influence on DO distribution, with potential implications
 for marine ecosystems and biogeochemical cycles.
Extreme events of low DO in isolated cores of large coherent
 mesoscale eddies have become a well-studied phenomenon
 of the Atlantic and Pacific eastern boundary up-Published

 by Copernicus Puhlications on behalf of the European Geosciences Union.