1. Introduction
The Southern Ocean is a key region for the ventilation and formation of intermediate and deep water masses.
Tilted density surfaces associated with the Antarctic Circumpolar Current (ACC) allow for the adiabatic
upwelling of Circumpolar Deep Water (CDW) that has been sequestered from the surface for
O
(100–1000 years).
At the surface, CDW exchanges heat and gases with the atmosphere, outgassing natural carbon stocks and acting
as a sink for anthropogenic CO
2
(Gruber et al.,
2019
; Landschützer et al.,
2015
). Numerical models suggest that
ventilation is spatially heterogeneous within the ACC (Tamsitt et al.,
2017
; Viglione & Thompson,
2016
). Inter
-
actions of the ACC with underwater topography can result in the diversion and compaction of frontal currents,
creating standing meanders (Sokolov & Rintoul,
2007
) associated with enhanced mesoscale eddy kinetic energy
(EKE; Figures
1a
and
1b
) (Gille & Kelly,
1996
). The ACC's major standing meanders are present at the Kergeulen
Plateau, Campbell Plateau, Eastern Pacific Rise, Crozet Plateau, and Drake Passage; these regions are thought to
shape uptake and sequestration of heat and carbon (Brady et al.,
2021
; Klocker,
2018
; Roach et al.,
2016
; Sallée
et al.,
2012
).
Ventilation in the ACC depends on the local density structure as well as advection and stirring along isopyc-
nals, and thus responds to a variety of processes and scales. Standing meanders lead to sloped isopycnals that
store available potential energy (Bischoff & Thompson,
2014
; Chapman et al.,
2015
; Klocker,
2018
), which is
released by baroclinic instability, producing a rich mesoscale
O
(100 km) eddy field approximately 100 km down-
stream of the standing meander (Rintoul,
2018
; Thompson & Naveira Garabato,
2014
). These eddies then stir
Abstract
Flow-topography interactions along the path of the Antarctic Circumpolar Current generate
standing meanders, create regions of enhanced eddy kinetic energy (EKE), and modify frontal structure. We
consider the impact of standing meanders on ventilation based on oxygen measurements from Argo floats and
the patterns of apparent oxygen utilization (AOU). Regions of high-EKE have relatively reduced AOU values at
depths 200–700 m below the base of the mixed layer and larger AOU variance, suggesting enhanced ventilation
due to both along-isopycnal stirring and enhanced exchange across the base of the mixed layer. Vertical
exchange is inferred from finite-size Lyapunov exponents, a proxy for the magnitude of surface lateral density
gradients, which suggest that submesoscale vertical velocities may contribute to ventilation. The shaping of
ventilation by standing meanders has implications for the temporal and spatial variability of air‒sea exchange.
Plain Language Summary
The circulation of the Southern Ocean is dominated by the
eastward-flowing Antarctic Circumpolar Current (ACC). The characteristics of the ACC are not uniform around
the Southern Ocean. Rather, when the ACC encounters underwater mountain ranges the flow is diverted,
which causes these regions to be more energetic through the generation of ocean eddies in a process similar to
atmospheric storm tracks. Numerical models have suggested that the exchange of properties, such as heat and
carbon dioxide, between the atmosphere and the interior ocean is enhanced in these energetic regions. In this
study, data from freely floating robotic floats in the Southern Ocean is used to observe the vertical structure of
dissolved oxygen. Transfer of properties between the ocean's surface and the interior ocean preferentially occurs
in high energy regions of the ACC. Most previous work has relied on numerical models of the ocean that, due
to computational limits, do not represent all aspects of the ACC's energetic regions. This study has implications
for how the Southern Ocean's ability to take up excess carbon dioxide from the atmosphere will evolve in the
future.
DOVE ET AL.
© 2022. American Geophysical Union.
All Rights Reserved.
Enhanced Ventilation in Energetic Regions of the Antarctic
Circumpolar Current
Lilian A. Dove
1
, Dhruv Balwada
2
, Andrew F. Thompson
1
, and Alison R. Gray
3
1
Division of Geological and Planetary Sciences, Environmental Science and Engineering, California Institute of Technology,
Pasadena, CA, USA,
2
Lamont-Doherty Earth Observatory, Columbia University, Palisades, NY, USA,
3
School of
Oceanography, University of Washington, Seattle, WA, USA
Key Points:
•
The relationship between apparent
oxygen utilization (AOU) and eddy
kinetic energy (EKE) is assessed in
the Antarctic Circumpolar Current
•
AOU has relatively reduced values
below the mixed layer in high-EKE
standing meanders as compared to
low-EKE regions
•
Modification of the density
structure and enhanced meso- and
submesoscale motions enhance
ventilation in standing meanders
Supporting Information:
Supporting Information may be found in
the online version of this article.
Correspondence to:
L. A. Dove,
dove@caltech.edu
Citation:
Dove, L. A., Balwada, D., Thompson,
A. F., & Gray, A. R. (2022). Enhanced
ventilation in energetic regions of
the Antarctic Circumpolar Current.
Geophysical Research Letters
,
49
, e2021GL097574.
https://doi.
org/10.1029/2021GL097574
Received 22 DEC 2021
Accepted 16 JUN 2022
10.1029/2021GL097574
Special Section:
Southern Ocean and Climate:
Biogeochemical and Physical
Fluxes and Processes
RESEARCH LETTER
1 of 12
Geophysical Research Letters
DOVE ET AL.
10.1029/2021GL097574
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and strain the surface density field, leading to frontogenesis and influencing submesoscale motions (Bachman
& Klocker,
2020
; Balwada et al.,
2018
; Klein & Lapeyre,
2009
; Rosso et al.,
2015
). Through both lateral
(Abernathey & Marshall,
2013
; Roach et al.,
2018
) and vertical (Adams et al.,
2017
; Klein & Lapeyre,
2009
)
motions, mesoscale and submesoscale eddies contribute significantly to ventilation in the ACC. Throughout this
work, we refer to “ventilation” as any process or combination of processes that work to transfer surface waters
and tracers into the pycnocline, which as described above, can occur on a variety of temporal and spatial scales
(Morrison et al.,
2022
). Additionally, stirring refers to the advection of tracers by an eddying velocity field, while
mixing is an irreversible process that removes tracer variance; only the former contributes directly to ventilation
although mixing influences the interpretation of ventilation from tracer distributions (Villermaux,
2019
).
Numerical models demonstrate that regions with higher EKE have enhanced capacity for submesoscale transport
of tracers across the base of the mixed layer (Balwada et al.,
2018
; Lévy et al.,
2018
; Uchida et al.,
2020
) and
can have an outsized impact on ventilation (Naveira Garabato et al.,
2011
; Rintoul,
2018
; Tamsitt et al.,
2016
;
Viglione & Thompson,
2016
). Standing meanders have also been identified as regions where older waters
Figure 1.
(a) Bathymetry and major fronts of the Southern Ocean. Gray contours show 1,000, 2,000, and 3,000 m isobaths. Fronts are the Subantarctic Front (blue),
Polar Front (orange), Southern Antarctic Circumpolar Current Front (green), and the Southern Boundary (red). (b) Base-10 logarithm of eddy kinetic energy (EKE)
[log
10
m
2
s
−2
]. Black solid lines show the Antarctic Circumpolar Current (ACC) boundaries used in this study. Black dotted lines denote regions of high EKE. Standing
meanders are labeled by the corresponding bathymetric feature: Crozet Plateau (CrP), Kerguelen Plateau (KP), Campbell Plateau (CP), East Pacific Rise (EPR), and
Drake Passage (DP). (c) Spatial distribution of float profiles containing oxygen data across the Southern Ocean within the ACC; Δlatitude = 1.25°, Δlongitude = 2.5°.
Black dotted lines show the ACC boundaries used in this study. (d) Histogram of the number of float profiles as a function of longitude within the ACC boundaries in
panel (b). Profiles categorized as low-EKE are in orange, with high-EKE profiles in blue. Standing meanders are labeled the same as in panel (b). (e) Histogram of the
number of float profiles at a given latitude within the ACC boundaries in panel (b). Colors are the same as in panel (d).
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enriched in dissolved inorganic carbon are preferentially transported to the surface (Brady et al.,
2021
; Tamsitt
et al.,
2017
), which can potentially create local regions of enhanced air‒sea gas exchange. Observational studies
are needed to validate these largely numerical results.
Due to coarse, ship-based sampling, examination of spatial variations in ventilation has focused on the basin (or
ACC sector) scale (Morrison et al.,
2022
; Sallée et al.,
2012
). More recently, observations from floats have shown
that air–sea fluxes of carbon (Gray et al.,
2018
) and oxygen (Bushinsky et al.,
2017
) vary across the Southern
Ocean. Evidence for finer-scale variability in biogeochemical distributions comes from the analysis of Bioge-
ochemical Argo (BGC-Argo) profiles, in which subsurface tracer anomalies are found to be more prevalent in
high-EKE regions, suggesting stronger ventilation and export (Llort et al.,
2018
). High-resolution glider obser
-
vations near the Southwest Indian Ridge also showed reduced vertical tracer gradients in the standing meander
as compared to the low-EKE region downstream (Dove et al.,
2021
). Although these observational studies have
provided initial evidence for the importance of standing meanders in ventilation, the physical processes in the
ACC that set the dominant spatial and temporal scales of variability in surface-interior exchange have not yet
been fully explored.
This study uses the broad spatial coverage of subsurface dissolved oxygen measurements collected by the
BGC-Argo array, as well as remote sensing products, to consider controls on apparent oxygen utilization (AOU)
patterns in the Southern Ocean and its relationship to ventilation of surface waters. Both vertical and isopycnal
distributions of AOU exhibit substantial variations along the path of the ACC that can be linked primarily to
enhanced ventilation in the ACC's major standing meanders. We identify several physical mechanisms that are
consistent with these distributions. This work is a critical step for validating ocean models and observationally
describing key regions of ventilation of climatologically important tracers in the Southern Ocean.
2.
Data and Methods
2.1.
Biogeochemical-Argo Floats
The Argo program has deployed over 10,000 profiling floats across the global ocean since 1999 (Riser et al.,
2016
)
with the Southern Ocean Carbon and Climate Observations and Modeling (SOCCOM) program playing a vital
role in increasing the BGC-Argo population of the Southern Ocean (Claustre et al.,
2020
; Johnson et al.,
2017
).
Argo floats sample the upper 2,000 m of the ocean every 10 days. In between profiles, the floats drift at 1,000 m
and follow a quasi-Lagrangian trajectory (Roemmich et al.,
2009
).
This study uses 21,941 profiles of dissolved oxygen, along with the associated temperature and salinity profiles,
that were collected within the boundaries of the ACC (defined in Section
2.3
) during the period 15 January
2003–16 May 2021 (Figures
1c–1e
). Only data that have undergone delayed-mode quality control procedures
and have been flagged as “good” are used in this analysis. All profile data were obtained from the “Sprof” files
provided by the Argo Global Data Assembly Center, which merge biogeochemical samples that are measured at
slightly different vertical positions onto a single common pressure axis.
2.2.
Derived Variables
AOU is the difference between oxygen saturation concentration and observed dissolved oxygen concentra-
tions (AOU
퐴퐴
=
푂푂
sat.
2
−
푂푂
obs.
2
), where the oxygen saturation is a function of the observed conservative temperature
and absolute salinity. AOU in the surface ocean is typically close to 0 due to equilibration with the atmos-
phere. Bushinsky et al. (
2017
) showed that AOU ≈ 0 is generally true for the ACC, but small variations of
±5–10 μmol kg
−1
exist due to biological activity, surface heat fluxes, or rapid entrainment of thermocline waters
(Ito et al.,
2004
). Lower AOU values are used as a proxy for younger age, signaling recent ventilation, since
respiration in the ocean interior is a persistent oxygen sink. AOU is a non-conservative tracer with its value deter
-
mined by several processes, for example, remineralization, along-isopycnal stirring, cross-isopycnal mixing, and
the non-conservative nature of solubility. AOU has been used to trace pathways between the surface and interior
(Llort et al.,
2018
), and both vertical and along-isopycnal variations provide insight into ventilation dynamics.
We study the distribution of AOU in both density and depth coordinates. Additionally, to account for temporal
and spatial variations in mixed layer depths (MLDs), vertical variations in AOU are also considered as deviations
from the observed values at the base of the mixed layer in each profile. Depth below the base of the mixed layer
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is given by Δh, and ΔAOU refers to the difference in AOU between the value at Δh and at the MLD. The MLD
was defined by a density difference criterion of 0.03 kg m
−3
from the surface (de Boyer Montégut et al.,
2004
).
Other derived variables, such as potential density, were calculated from temperature and salinity using the Ther
-
modynamic Equation of Seawater 2010 (McDougall & Barker,
2011
).
2.3.
Satellite Data
EKE was calculated as EKE
퐴퐴
=
‒
−
√
푢푢
′2
+
푣푣
′2
, where
u
′ and
v
′ are the zonal and meridional eddy geostrophic veloc-
ities estimated from the time-varying sea surface height anomaly field, and
퐴퐴
(
.
)
represents a time average calcu-
lated over 1993–2016. Regions with EKE greater than 250 cm
2
s
−2
were considered “high-EKE” (Figures
1b
and
S1 in Supporting Information
S1
), and individual float profiles were tagged as “high” or “low” EKE based on
their surfacing locations. Previous studies have identified distinct dynamical regimes within individual standing
meanders (Barthel et al.,
2017
; Youngs et al.,
2017
), but we do not distinguish these here.
The ACC boundaries were defined using absolute dynamic topography (ADT) with the northern and southern
boundaries given by the −0.1 m and the −1.05 m ADT contours, respectively. These boundaries were selected
in part to avoid inclusion of the Agulhas Retroflection, which is a region of enhanced EKE but is not considered
in this study. Several definitions of the northern and southern boundaries of the ACC were tested, including
hydrographic definitions of frontal boundaries (shown in Figure
1a
) as opposed to sea level anomaly (Kim &
Orsi,
2014
), but these led to minimal differences in the results.
Finite-size Lyapunov Exponents (FSLEs) describe the orientation and timescale of strain fields by quantifying
stretching and compression (d’Ovidio et al.,
2004
,
2010
). They are a Lagrangian diagnostic, and for a given flow
field are defined as the separation growth rate for particle pairs,
퐴퐴퐴퐴
푑푑
0
,푑푑
푓푓
)
=
1
휏휏
log
(
푑푑
푓푓
푑푑
0
)
, where
d
0
and
d
f
are the
initial and final separation distances and
τ
is the first time where the separation distance
d
f
is reached. Here we
use FSLE estimates provided by AVISO+ that were computed from satellite-derived geostrophic velocities. We
use the FSLEs from 1 January 2018 to 31 December 2020, but the exact choice of the period does not impact
the
results.
3. Results
3.1.
Subsurface Signatures of Ventilation
Variations in AOU with depth and density may arise from various mechanisms, some reflecting differences in
advection and stirring at scales smaller than the standing meander (discussed in Section
3.2
), and others related
to the larger-scale density structure of the ACC. A simple partitioning of individual float profiles between high-
and low-EKE regions reveals striking differences in the vertical structure of AOU between these two regimes
(Figure
2
). Just below the mixed layer, for example, Δh = 100 m, high- and low-EKE regions both have low ΔAOU
values with similar distributions (Figure
2a
), although low-EKE regions have a longer tail. At Δh = 300 m, high-
and low-EKE regions have distinct peaks with the high-EKE region having a lower median; the difference in the
distributions' medians is 27 μmol kg
−1
and the difference in the modes is 68 μmol kg
−1
(Figures
2b
and
2d
). For
values of Δh ≥ 700 m, the two regions have approximately the same distribution, with a difference in medians of
only 2 μmol kg
−1
(Figures
2c
and
2d
). The largest disparity in ΔAOU between the high- and low-EKE regions
is present for 200 < Δh < 700 m (Figure
2d
). This ΔAOU structure is set, in part, by meanders of the ACC that
horizontally transport lighter waters southward into energetic regions downstream of topography. At the level of
individual standing meanders, the high-EKE regions associated with the Kerguelen Plateau, Campbell Plateau,
and Eastern Pacific Rise have distributions of ΔAOU that most closely align with the median distributions for
the entire ACC (Figures
2fiii–2fv
). The distinction between high- and low-EKE regions is weakest at the Crozet
Plateau (Figure
2fii
), although data availability is reduced here.
Changes in hydrographic properties along the path of the ACC provide insight into the origin of subsurface
low-AOU waters found in high-EKE regions. CDW is distinguished by high salinity (>34.6 g kg
−1
) and low
temperature (∼2°C). Comparatively, Antarctic Intermediate Water (AAIW), a more recently ventilated water
mass, is characterized by lower salinity as a result of sea ice melt. Differences in hydrographic properties are
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Figure 2.
Probability density functions across the full Antarctic Circumpolar Current of ΔAOU [μmol kg
−1
] where (a)
Δh = 100 m, (b) Δh = 400 m, (c) Δh = 700 m. (d) Median in ΔAOU [μmol kg
−1
] at values of Δh. (e) Variance of AOU
[μmol
2
kg
−2
] on potential density surfaces. (f) Locations of profiles used to create probability density functions of ΔAOU
[μmol kg
−1
] at Δh = 300 m at (i) Drake Passage, (ii) Crozet Plateau, (iii) Kerguelen Plateau, (iv) Campbell Plateau, and
(v) Eastern Pacific Rise. In all panels, blue colors denote high-eddy kinetic energy (EKE) regions and orange colors denote
low-EKE regions.
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particularly distinct around Δh = 300 m, consistent with large differences in ΔAOU medians between the high-
and low-EKE regions (Figure
3
). In both the high- and low-EKE regions at Δh = 300 m, the distributions of mean
AOU as a function of temperature and salinity are similar (Figures
3a–3c
), suggesting that ΔAOU is predom-
inantly tied to the relative contributions of water masses below the mixed layer, with variations due to biology
secondary. Stronger differences between the two regions are found, however, when considering the frequency
distribution in conservative temperature-absolute salinity space (Figures
3d–3f
). In the low-EKE regions, CDW
properties dominate, with a temperature of 2°C and high salinity (34.4–34.8 g kg
−1
; Figure
3d
). In the high-EKE
regions a greater fraction of the observations have lower values of salinity (34.0–34.2 g kg
−1
) and also warmer
temperatures (3–5°C; Figures
3e
and
3f
), consistent with intermediate waters that have been subducted from the
surface. The increased presence of waters consistent with AAIW at these depths in the high-EKE regions suggests
that more intermediate water is subducted in high-EKE regions of the ACC as compared to low-EKE regions.
This hydrographic analysis indicates that mixing of old CDW and recently ventilated AAIW at the basin-scale
contributes to the patterns in ΔAOU described in Figure
2
. Yet, coupled processes on the submesoscale‒mesos-
cale spectrum may still play a role in setting these subsurface ΔAOU distributions, as described in previous obser
-
vational work in standing meanders (Dove et al.,
2021
). Using an oxygen utilization rate (OUR) for the upper
mesopelagic zone of 40 μmol kg
−1
year
−1
(Hennon et al.,
2016
), low ΔAOU waters with a median of O(70 μmol
Figure 3.
Absolute salinity (S
A
)-conservative temperature (CT) diagrams. Average apparent oxygen utilization (AOU) for each S
A
-CT position at Δh = 300 m in (a)
the low-eddy kinetic energy (EKE) regions and (b) the high-EKE regions. (c) Difference in AOU between the low- and high-EKE regions. Joint histogram of profile
locations at at Δh = 300 m in (d) the low-EKE regions, and (e) the high-EKE regions. (f) Difference in joint histograms between the low- and high-EKE regions. Gray
contours are potential density [kg m
−3
], with the black contour at 27.2 kg m
−3
. In all panels, only where there were more than five points at a given CT-S
A
value that
could be averaged are shown. ΔCT = 0.2°C, ΔS
A
= 0.025 g kg
−1
.
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kg
−1
) in high-EKE regions would have an age of ∼2 years, suggesting there may be recent injection from the
mixed layer. However, estimates of OUR in the Southern Ocean are sparse and there is a good deal of uncertainty
in the estimate of this time scale. Specifically, an OUR of 40 μmol kg
−1
year
−1
represents a regional, near-surface
value that may not be representative of values at greater depths or over the broader Southern Ocean. Therefore
this OUR value should be considered an upper bound, and the low ΔAOU waters observed in high-EKE regions
likely include waters that have been subducted below the surface boundary layer for periods longer than 2 years.
It is important to also consider AOU variations on density surfaces because the mean density structure between
the high- and low-EKE regions is different: the lower EKE regions host denser isopycnals linked to deeper depths
and higher AOU values in the mid- and low-latitude basins to the north (Figures S2a and S2b in Supporting
Information
S1
). These variations along the path of the ACC are related to changes in outcropping density classes
as well as the steepening of lateral density gradients within standing meanders (Chapman et al.,
2015
; Thompson
& Naveira Garabato,
2014
), which may enable recently ventilated surface waters to be displaced downward in the
water column. Despite the different density ranges between the regions, the vertical stratification, measured by
the vertical buoyancy gradient
N
2
, is similar (Figure S2c in Supporting Information
S1
). Considering the Argo
observations in density space show that the heavier isopycnals have relatively homogeneous mean AOU distri-
butions along the path of the ACC, which is likely a result of the rapid along-ACC circulation (Figures S3 and
S4 in Supporting Information
S1
). However, lighter isopycnals and regions where isopycnals are shallower show
inhomogeneities in mean AOU along the path of the ACC, due in part to the outcropping of denser isopycnals
in low-EKE regions. Some of the signal of lower mean AOU concentrations, particularly at deeper depths, may
be attributed to adiabatic heaving rather than ventilation by advection and mixing. In the next section, we offer
evidence that high-EKE regions are subject to more energetic stirring, leading to enhanced along-isopycnal vari-
ance of AOU, suggesting that AOU variations do not result from isopycnal heaving alone.
3.2.
Mesoscale and Submesoscale Contributions to Ventilation
Variations in AOU due to the ACC's density structure occur at standing meander and larger scales (≥1,000 km);
below these scales, mesoscale and submesoscale motions can impact ventilation through a number of differ
-
ent mechanisms. These include (a) increased along-isopycnal stirring as a result of enhanced EKE; (b) frontal
subduction as a result of frontogenesis in the standing meander; and (c) enhanced vertical transport by submesos-
cale motions. Here, we investigate how these processes shape along-ACC differences in AOU distributions.
Differences in isopycnal AOU variance between high- and low-EKE regions offers insight into how along-isopycnal
stirring contributes to ventilation within the ACC. To remove the effects of vertical isopycnal displacement (i.e.,
heave; Figure S3 in Supporting Information
S1
), we consider deviations from a longitude-dependent (10-degree
longitude bins), along-isopycnal mean AOU value. The variance in AOU on density surfaces <27.4 is up to 18%
larger in high-EKE regions than in low-EKE regions, with a peak in variance at 27.3 kg m
−3
in both regions.
The observed enhanced AOU variance in high-EKE regions is consistent with along-isopycnal stirring bringing
low-ΔAOU waters to depth, as opposed to this signal solely occurring due to variations in the ACC's large-scale
density structure. Enhanced variance in the high-EKE regions may arise from both stirring processes and injec-
tion of tracer anomalies from the surface layer onto density surfaces below the mixed layer. With regard to
exchange out of the mixed layer, seasonal or along-stream changes in mixed layer properties may be expected to
modify ventilation. However, the float data indiciate that MLD and stratification at the base of the mixed layer
are similar in high- and low-EKE regions and therefore do not contribute to the disparity in subsurface ΔAOU
distributions (Figure S5 in Supporting Information
S1
).
In addition to being regions of energetic mesoscale eddies, ACC standing meanders are regions of strong
surface frontogenesis that give rise to large mixed layer lateral density gradients. These gradients are reser
-
voirs of potential energy that can give rise to instabilities that lead to intense submesoscale vertical motions
and increase the efficiency of tracer transport between the surface and interior ocean, contributing to ventila-
tion (Klein & Lapeyre,
2009
; Lévy et al.,
2018
; Mahadevan,
2016
). While these instabilities typically occur
on spatial and temporal scales consistent with the submesoscale, they are shaped by the mesoscale flow field
(Balwada et al.,
2018
,
2020
; Rosso et al.,
2015
). We investigate the potential of enhanced ventilation occurring
via frontogenesis and submesoscale subduction by considering the relative magnitude of lateral density gradients
between low- and high-EKE regions. Measuring lateral density gradients can be achieved with high temporal and
spatial resolution measurements, but such observations are sparse in the Southern Ocean. Siegelman et al. (
2020
)
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empirically showed that maximum stretching FSLEs (hereafter FSLEs) calculated from satellite-derived flow
fields can be used to approximate the magnitude of lateral density gradients and derived a relationship between
the two quantities. Specifically, density anomalies are physically aligned with FSLEs, so larger magnitude FSLEs
are correlated with stronger lateral density gradients. While FSLEs have previously been linked to mixed-layer
density gradients, Siegelman et al. (
2020
) demonstrated that this relationship may extend below the mixed layer
in the Southern Ocean, particularly in energetic regions.
Consistent with the heterogeneous distribution of EKE in the ACC, lateral density gradients (as inferred from
FSLEs) undergo abrupt transitions in standing meander regions (Figure
4a
). The probability distribution of FSLE
has a log-normal distribution within both low- and high-EKE regions (Figure
4b
). However, in the high-EKE
region, the median value is shifted to larger magnitudes and the distribution has a longer tail, which we link to
stronger and more frequent small-scale surface density gradients. The FSLE probability density function also
differs for each individual standing meander (Figure
4c
). The standing meander at the Kerguelen Plateau has the
most negative (strongest) FSLE values, implying an increased frequency of strong lateral density gradients and
potentially enhanced vertical transport. The standing meanders associated with the Crozet Plateau and Eastern
Pacific Rise have the least negative (weakest) mode of FSLE probability, with the Campbell Plateau and Drake
Passage falling between the extremes.
To consider the relationship between FSLEs and ΔAOU within individual standing meanders, we define local-
ized low-EKE regions that surround each high-EKE standing meander, defined between the north-south ACC
boundaries and extending 5 degrees of longitude to either side of the meander. For each of the five major stand-
ing meander regions, the median difference in ΔAOU between the high- and
localized
low-EKE regions at
Figure 4.
(a) Snapshot of Finite-size Lyapunov Exponents (FSLEs) from 1 March 2020 centered on the Crozet Plateau region of the Antarctic Circumpolar Current
(ACC). Blue regions are high-eddy kinetic energy (EKE) while orange are low-EKE, using the EKE definition defined in the methods. (b) Probability density function
of maximum stretching FSLE in the high- (blue) and low- (orange) EKE regions across the full ACC. Gray lines are expanded in panel (c). (c) Probability density
function of maximum stretching FSLE in the Kergulen Plateau (red), the Crozet Plateau (black), the Campbell Plateau (yellow), Drake Passage (green), and the Eastern
Pacific Rise (EPR; blue). Vertical lines represent 75th percentiles with the same colors as above. (d) Plot of the mode of the FSLE in the high-EKE region versus the
differences in the high and low medians of the ΔAOU probability density functions (
δ
AOU) at Δh = 250 m. Error bars are standard deviations of
δ
AOU and colors are
the same as in panel (c).
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Geophysical Research Letters
DOVE ET AL.
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Δh = 250 m is calculated; we refer to this as
δ
AOU. A large magnitude of
δ
AOU represents large differences
in ΔAOU distributions between the high-EKE standing meander and the surrounding low-EKE region, while a
negative
δ
AOU indicates a greater volume of low-AOU water in the high-EKE region. In other words, a large,
negative value of
δ
AOU suggests that the high-EKE region experiences enhanced ventilation as a result of the
stirring and submesoscale subduction processes described above, as compared to the surrounding low-EKE
region. Differences in FSLE distributions between meanders are correlated with differences in
δ
AOU (Figure
4d
)
for depths of Δh up to 500 m. The standing meander that has the largest FSLE mode magnitude (implying strong-
est stirring), Kerguelen Plateau, is associated with the largest
δ
AOU. Standing meander regions with smaller
magnitude FSLE modes, the Eastern Pacific Rise and Crozet Plateau, have
δ
AOU values closer to zero. While
five meanders dominate the high-EKE regions in the ACC, this analysis suggests that contributions of low ΔAOU
waters to depth may be localized to only one or two intense standing meanders, Kerguelen and Campbell plateaus,
indicating these standing meanders may play the dominant role in ventilation of the ACC.
4. Discussion
Ventilation of surface properties and tracers can arise from a combination of large-scale circulation features,
for example, shaping of density surfaces through flow-topography interactions, as well as smaller-scale stirring
by mesoscale and submesoscale motions. There is increasing evidence from both observational and numerical
studies that motions occurring in the mesoscale and submesoscale range are tightly coupled. Mesoscale strain,
through the process of frontogenesis, is responsible for the generation of near-surface lateral density gradients
that are precursors for strong vertical submesoscale velocities (Archer et al.,
2020
; Su et al.,
2020
). A possible
scenario is that these enhanced submesoscale velocities in high-EKE regions rapidly inject surface properties
to depths of 300 m or even deeper. Llort et al. (
2018
) did indeed find evidence of deep, unmodified waters
(anomalously low AOU values), which only occurred in high-EKE standing meander regions. Yet, these deep
anomalies were found in <1% of all float profiles. An alternate scenario, in line with Balwada et al. (
2018
),
Balwada et al. (
2021
), and Freilich and Mahadevan (
2021
), and one that is more consistent with the observed
ΔAOU values, is that submesoscale motions play the essential role of efficiently carrying surface properties
across the base of the mixed layer. After this, stirring along isopycnals, by the same eddies that create the surface
density gradients, enhances the transfer of these surface properties to depth. Thus, while attributing ventilation to
different physical processes is important for ensuring that they are represented accurately in climate models, the
coupling of motions across scales likely makes this task challenging. Accordingly, numerical models that do not
fully resolve mesoscale and submesoscale processes may misrepresent the formation of intermediate waters, as
well as the concentration of oxygen in the thermocline.
While this study has focused on ventilation pathways of oxygen in the ACC, these results likely have important
implications for the spatial variability of air‒sea CO
2
fluxes. Oxygen has an equilibrium timescale that is at least
an order of magnitude shorter than that of CO
2
, which has an equilibration timescale of
O
(6 months). Combining
this study with evidence that ACC standing meanders are also sites of enhanced upwelling (Brady et al.,
2021
;
Tamsitt et al.,
2017
) suggests that recently ventilated deep waters in these regions may have short surface resi-
dence times, and therefore full equilibration with atmospheric CO
2
may not be reached (Jones et al.,
2014
). This
provides further motivation for exploring how localized high-EKE regions impact exchange of waters between
the surface and interior and the larger Southern Ocean carbon cycle.
Various estimates of air‒sea exchange of CO
2
in the Southern Ocean have identified interannual to decadal-scale
variations in the region's ability to provide an atmospheric carbon sink (Gruber et al.,
2019
; Landschützer
et al.,
2015
). Notably, these estimates are obtained after some form of interpolation or mapping, for example,
neural network (Landschützer et al.,
2016
), of CO
2
measurements from repeat shiptracks that typically do not
sample the strongest and most variable EKE regions. Decadal-scale variations in the Southern Ocean carbon
sink have been largely attributed to large-scale processes, such as a change in the Southern Annular Mode
(Le Quéré et al.,
2007
; Lovenduski et al.,
2008
), a southward shift and strengthening of the westerly winds
(DeVries et al.,
2017
), and enhanced stratification due to increased northward advection of sea ice and southward
advection of warmer waters (Landschützer et al.,
2015
). Localized processes may shape the temporal evolution
of air‒sea fluxes across the Southern Ocean. For instance, Zhang et al. (
2021
) illustrate that the standing mean-
der
associated with the Campbell Plateau governs trends in EKE over the full Pacific basin. This may also extend
to air‒sea flux properties; for example, Langlais et al. (
2017
) show that standing meanders dominate the transfer
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Geophysical Research Letters
DOVE ET AL.
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of anthropogenic carbon to AAIW. This sequestration is underresolved in models and underobserved in situ.
Processes at the standing meander level, including enhanced localized winds and jet-submesoscale interactions,
may play a vital role in shaping air‒sea exchange of climatologically important properties such as CO
2
(Bachman
& Klocker,
2020
). This aligns with our findings that standing meanders are likely hotspots for ventilation and
suggests that dynamics occurring at scales currently unresolved by most climate models are critical for the trans-
fer of atmospheric anomalies to the ocean interior.
5. Conclusions
We provide observational evidence of heterogeneous subsurface vertical distribution of AOU along the full path
of the ACC. In both depth and density space, we find substantial differences in AOU linked to enhanced ventila-
tion in high-EKE regions associated with the ACC's major standing meanders. While shifts in the ACC's density
surfaces due to topographic steering at standing meanders explain some of the observed distribution, we also
identify mechanisms on the submesoscale‒mesoscale spectrum that can contribute to the ventilation of AOU.
Data from BGC-Argo floats, especially those deployed by the SOCCOM project, have enhanced our ability to
understand the processes impacting ventilation across the entire Southern Ocean, and here we use those data
to suggest that localized regions of high EKE play an outsized role in such ventilation. Accordingly, it is vital
to
consider sub-basin-scale variability and, in particular, how temporal variations in high-EKE standing mean-
ders can impact global Southern Ocean properties that influence and reflect biogeochemical cycling.
Data Availability Statement
The float data were collected and made freely available by the International Argo Program and the national
programs that contribute to it (
https://argo.ucsd.edu
,
https://www.ocean-ops.org
). The Argo Program is part
of the Global Ocean Observing System. The float profiles used in this manuscript are available in the Argo
Global Data Assembly Center 10 June 2021 screenshot (
https://doi.org/10.17882/42182%2385023
). Sea level
anomaly products were produced and distributed by the Copernicus Marine 360 and Environment Monitoring
Service and are available at
https://resources.marine.copernicus.eu/product-detail/SEALEVEL_GLO_PHY_L4_
MY_008_047/INFORMATION
. Finite-size Lyapunov Exponents were produced and distributed by AVISO+
and are available at
https://www.aviso.altimetry.fr/en/data/products/value-added-products/fsle-finite-size-lyapu-
nov-exponents.html
.
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Acknowledgments
L. A. Dove and A. F. Thompson acknowl-
edge funding from National Science
Foundation (NSF) award OCE-1756956,
the David and Lucille Packard Foun-
dation, and the Resnick Sustainability
Institute. L. A. Dove was additionally
supported by an NSF Graduate Research
Fellowship. D. Balwada and A. R.
Gray were supported by NSF Award
OCE-1756882. A. R. Gray acknowledges
additional funding through National
Oceanic and Atmospheric Administra-
tion Award NA20OAR4320271 and
through NSF's Southern Ocean Carbon
and Climate Observations and Modeling
Project under Awards PLR-1425989 and
OPP-1936222. The authors are grateful
for the comments of three anonymous
reviewers, which led to the improvement
of this manuscript.
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