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 tropi-
cal North Atlantic (ETNA), collected over a period of up to
15 years at three different mooring sites, reveal recurring eX-
treme low-oxygen events lasting for several weeks. Similarly,
observations from 15 individual meridional ship sections be-
cween 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. De-
spite the energetic equatorial circulation and the expected
dominance of wave-like structures in the near-equatorial re-
gion, these HBVSs persist as relatively long-lived and coher-
ent 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 vortic-
ity (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 substan-
tial portion of the near-equatorial eddy field may remain un-
detected 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, biogeochemi-
cal 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 equa-
tor (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 biogeochem-
ical model to investigate these small-scale processes below
the mixed layer in the tropical Atlantic. This integrated ap-
proach allows us to characterize their structure, variability,
and strong influence on DO distribution, with potential im-
plications for marine ecosystems and biogeochemical cycles.
Extreme events of low DO in isolated cores of large co-
herent mesoscale eddies have become a well-studied phe-
nomenon of the Atlantic and Pacific eastern boundary up-
Published by Copernicus Puhlications on behalf of the European Geosciences Union.
