F. Schütte et al.: Hidden vortices: near-equatorial low-oxygen extremes driven by high-baroclinic-mode vortices 123 
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Figure 2. Observation-model comparison of the minimum DO between 0 and 200m of the time-average distribution from (a) the World 
Ocean Atlas 2023 and (b) from last 20 years of GFDL CM2.6 model. Latitude-depth section, 0-500 m along 23° W, of mean DO from 
(c) repeat ship sections and (d) last 20 years of GFDL CM2.6 model. (e) and (f) are similar to (c) and (d), but mean zonal velocity is shown. 
The box in (a) and (b) illustrates the area of interest in this study; the line denotes the 23° W section that is shown in subpanels (c) to (f). 
This section has been surveyed by 15 individual shipboard observations that are used in this study for the latitude range 6-12° N. Diamond 
marks the mooring position (11° N/21° W). where data used in this study were taken. 
lower values at the eastern boundary compared to the clima- 
cological distribution from observations, which is particularly 
the case in the Gulf of Guinea region. In the interior basin, 
meridionally alternating bands of oxygen-poor and oxygen- 
rich water, that are associated with shallow east- and west- 
ward current bands, are pronounced in GFDL CM2.6., albeit 
more intensified. 
In the ETNA, the average DO distribution along 23° W 
in GFDL CM2.6 (Fig. 2d) shows a notable mismatch with 
observations (Fig. 2c). While observations from repeat ship 
sections reveal two distinct OMZ layers — a shallow OMZ 
above 200 m and a deeper OMZ at 300-700 m — the model 
instead simulates only a single OMZ spanning 100-600 m. 
This bias is also present in other coupled ocean circulation 
viogeochemistry models (e.g. Duteil et al., 2014) and can 
ve attributed, among other factors, to the limited represen- 
cation of physical transport processes such as submesoscale 
https:/doi.org/10.5194/o0s-22-119-2016 
eddies, which locally enhance oxygen minima. Additionally, 
simplified or parameterized remineralization and biological 
processes fail to reproduce rapid upper-ocean oxygen con- 
sumption. These discrepancies highlight the importance of 
direct observational studies, such as ours, which provide de- 
tailed insights into the shallow oxygen minimum and its con- 
nection to low-oxygen events and high-baroclinic vortices, 
thereby motivating the focus of this study. 
Further differences appear south of the equator, where the 
observed OMZ is absent in the model along 23° W. Instead, 
GFDL CM2.6 simulates lower DO levels between 24° N at 
depths below 150m compared to observations. The corre- 
sponding section of zonal velocity (Fig. 2f) indicates that the 
model represents upper-ocean currents (above 200m) well 
when compared to observations (Fig. 2e). However, below 
200 m in the equatorial region (5° S-5° N), zonal currents 
Ocean Sci... 22. 119-143, 2026