136 F. Schütte et al.: Hidden vortices: near-equatorial low-oxygen extremes driven by high-baroclinic-mode vortices 
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Figure 9. Model snapshots of PV on isopycnal surface 26.6 kg m”? (a, e, i, m), relative vorticity over f, DO and salinity on isopycnal 
surface 26.5 kg m7} (b, c, d, f, g, h, j, k, 1, n, o, p) for different phases (different columns) of an anticyclonic HBV (respective time indicated 
above each column with T = 0/211/346/580 d: formation/strongest peculiarity/weakening/ decay. Black-white dashed box in each sub panel 
denotes HBV position. 
This underlines that in understanding the Earth system, a 
,etter understanding of small-scale ocean dynamics (smaller 
chan the first baroclinic Rossby radius of deformation) is es- 
sential, as they play a crucial role in the distribution of energy 
and tracers as well as the regulation of biogeochemical pro- 
cesses. In particular, below the surface layer — where satellite 
observations are ineffective — our understanding of the fre- 
quency, magnitude, and impact of these small-scale ocean 
dynamics remains limited. 
In the vicinity of the equator (< 5° N/S), mesoscale dy- 
namics dominantly appear as horizontally anisotropic waves 
(e.g. tropical instability waves) rather than closed circular 
structures. These wave-like structures, however, are not 1so- 
lated enough to effectively transport or develop low-oxygen 
environments. The eddies with DO anomalies that we ob- 
served are relatively small and long-lived high-baroclinic 
vorticies (HBVs). Ship sections along 23° W exclusively re- 
vealed anticyclonic HBVs, whereas both anticyclonic and 
cyclonic HBVs were found from moored observations at 
[1°N/21° W and in the model. 
Ocean Sci., 22, 119-143. 2026 
5.1 
Vertical and horizontal structure of the low-oxygen 
events and the associated high-baroclinic mode 
vorticies 
The observed anticyclonic HBVs had a pronounced low-DO 
core that vertically extended from the base of the mixed layer 
down to several hundred meter depth (with minimum DO 
at depths between 45 and 90 m). The anomalous horizontal 
velocity of the observed anticyclonic HBVs was at maxi- 
mum (maximum EKE) at the depth of the DO minimum 
and extended from 50 to roughly 250m. Stratification in 
'*he observed anticyclonic HBVs’ core was weak over this 
depth range with upward and downward displaced isopyc- 
nals above and below the depth of EKE maximum, respec- 
tively. We found an average radius of about 34 km (between 
20 and 45 km) for the observed HB Vs. A decomposition into 
vertical baroclinic modes showed, that modes 4 to 10 fit best 
to low-DO events that are related to these HBVs. The asso- 
ciated 4th to 10th baroclinic Rossby radii of deformation are 
between 34 and 13km (at 9° N) and in good agreement with 
the observed eddy radii. The observed radii appear well be- 
low the first baroclinic Rossby radius of deformation (more 
than 100km in the region) and corresponding eddies can be 
considered as higher baroclinic mode vorticies. Rossby num- 
bers were below 1, with values of approximately 0.3-0.7 es- 
timated from shipboard observations (one eddy crossing is 
https://doi.org/10.5194/os-22-119-2026