120 F. Schütte et al.: Hidden vortices: near-equatorial low-oxygen extremes driven by high-baroclinic-mode vortices

welling systems (e.g. Stramma et al., 2013; Karstensen et
al., 2015; Schütte et al., 2016b; Frenger et al., 2018). A
strong isolation and the longevity of such eddies over several
 months favor the existence of a DO depleted eddy core.
The DO depleted core may result from (i) trapped water,
which is transported westward within the eddy core from
a region of initially low DO, typically from the eastern
vDoundary (dynamic effect) and/or (ii) enhanced DO consumption
 (production effect) due to a biologically high productive
 regime above the eddy core (McGillicuddy, 2016).
The latter is associated with high phytoplankton productivity,
 which leads to enhanced respiration and reduction of
DO beneath the mixed layer directly in the isolated core
reaching down to about 200 m (Karstensen et al., 2017). Res-5iration
 rates in the eddy’s interior (at around 80m depth)
were found to be substantially increased, with up to 3 to
5 times the values of ambient conditions for the tropical
North Atlantic (approximately 0.04-0.06 umolkg”! d—1),
e.g. subsurface intensified anticyclonic eddies (subsurface
ACES): 0.19 +0.08 umolkg ! a7! and surface intensified
cyclonic eddies (CEs): 0.10 + 0.12 umolkg”! a7! (Schütte
et al., 2016b). The increased respiration within these isolated
 mesoscale eddies may result in anoxic conditions (<
5 umolkg”!) in the otherwise hypoxic (> 60 umolkg”!)
ETNA. Such eddies can locally modulate biogeochemical
processes and influence marine organisms (Fiedler et al.,
2016; Hauss et al., 2016; Löscher et al., 2015). Moreover,
it is suggested that the increased DO consumption within the
isolated mesoscale eddy cores promote the formation and existence
 of a broad-scale shallow DO minimum zone (SOMZ)
at about 80 m (Schütte et al., 2016b), that is most pronounced
off the nutrient-rich Mauritanian upwelling system in the
ETNA, located between 15 and 23°N (Karstensen et al.,
2008; Brandt et al., 2015) (Fig. 1). For such low DO extremes
to develop highly isolated eddies must form and propagate
over a relatively long period through regions of relatively low
dynamical activity, e.g. as stated in the mentioned literature
north of 12° N in the eastern Atlantic and Pacific oceans.
The occurrence of DO depleted long-lived coherent eddies
 in near-equatorial waters (< 12°) is not intuitive and
contrasts theoretical considerations of equatorial dynamics,
which suggest a dominance of anisotropic wave like strucures
 (Eden, 2007). However, several extreme low DO events
have been observed in the ETNA at latitudes between 6 and
12°N (Brandt et al. 2015), where Christiansen et al. (2018)
associated one of these events (at 8°N, 23° W) with a subsurface
 ACE. These eddies are expected to be less isolated
and shorter-lived compared to eddies poleward of these low
iatitudes. The first baroclinic Rossby radius of deformaon
 (Ra,1), which is a characteristic threshold size of a dynamical
 regime to be in a geostrophic balance on meso- and
larger scales, strongly increases towards the equator (Chelton
et al., 1998). Global eddy studies, mainly based on altimeter
 sea surface height data, show a strong equatorward decrease
 of long-lived (>= 35 d) eddies (Chaigneau et al.. 2009:

Ocean Sei... 22. 119-143. 2026

Chelton et al., 2011). Less isolated eddies more readily entrain
 DO from surrounding waters, while short-lived eddies
do not persist long enough to substantially deplete DO in
their cores. Both factors inhibit the development of a low
DO extreme within eddy cores. Additionally, the equatorial
region — compared to the eastern parts of the oceans north of
12°N — is highly dynamic. It features the energetic equatorial
 zonal current system with associated instabilities as well
as various wave phenomena (e.g. Urbano et al., 2006; Brandt
et al., 2015; Pefa-Izquierdo et al., 2015; Calil, 2023; Köhn
et al., 2024). Nevertheless, our observations occasionally reveal
 DO values significantly below the climatological value
(Fig. 1).
While mesoscale eddies of the first baroclinic mode can
hardly exist at near-equatorial latitudes, smaller-scale eddies
 might. In the following, we refer to these smaller eddies,
 which still exhibit similar dynamics to mesoscale eddies
 - ie., they are dominantly in geostrophic balance — as
high-baroclinic mode vortices (HBVs). They have radii below
 the first baroclinic Rossby radius, Ra,1, and baroclinic
modes larger than one (D’Asaro, 1988; McCoy et al., 2020;
McWilliams, 1985). These HBVs, often referred to in the literature
 as subsurface or submesoscale coherent vortices, are
observed to have isolated cores and can therefore advect tracers
 (Gula et al., 2019). Due to their small spatial scales and
since they often appear at subsurface depth, these eddies are
not necessarily detectable from satellite observations (Mc-Coy
 et al., 2020). HBVSs are not typically known to persist
for extended durations in near-equatorial waters. However,
here we provide evidence that HBVs may serve as a potential
 mechanism driving the observed low-oxygen extremes at
these low latitudes. When linked to biogeochemical anomalies,
 HBVs may play a role in shaping local biogeochemical
conditions and ecosystem variability. However, ocean models
 are often submesoscale “permitting” only, in the sense
that the model has sufficient resolution to begin representing
submesoscale processes but does not fully resolve them, particularly
 with increasing distance from the equator. Understanding
 the frequency and behavior of HBVSs is essential for
understanding tracer distributions, developing effective parameterizations
 and improving model accuracy.
In this study, we identify the characteristics, origin and
temporal evolution of low-oxygen extremes in the upper
200 m of the tropical Atlantic Ocean and discuss the role of
HBVSs in driving these DO deficient zones. We use a comprehensive
 dataset of in situ moored and shipboard observations
combined with an actively eddying ocean-biogeochemistry
model (respective data and methods introduced in Sects. 2
and 3 in order to investigate the frequency distribution and
magnitude of these events (Sect. 4.1). We show that the lowoxygen
 events are mostly associated with subsurface intensified
 HBVs (Sect. 4.2), both anticyclonic and cyclonic. We
derive the demography of these structures from an eddy detection
 algorithm applied to in-situ observations (Sect. 4.3)
and a vertical baroclinic mode analysis (Sect. 4.4). The core

https://doi.org/10.5194/os-22-119-2026