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Subtropical‐Tropical Transfer in the South Atlantic Ocean

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Here we explore the water transfer between the subtropical and tropical gyres of the South Atlantic Ocean to better understand its unique equatorward heat delivery. A Lagrangian technique is applied to the reanalysis product GLORYS2V4 in order to trace back the western boundary flow in the tropical (North Brazil Undercurrent, NBUC) and subtropical (Brazil Current) gyres. Most of the northward NBUC core transport (14.9 Sv at 8°S) arrives from the eastern boundary subtropical current (Benguela Current) via the zonal South Equatorial Current. This subtropical‐tropical transfer represents the core of the returning limb of the Atlantic meridional overturning circulation and accounts for most of the observed increase in heat and salt‐volume transports (0.18 PW and 0.19 Sv from 30°S to 8°S, respectively) across the South Atlantic. The NBUC also includes Antarctic Intermediate Water below 400 m (7.4 Sv at 8°S) coming from the interior subtropical gyre, as well as water from the current's surface and peripheral components coming from the tropical gyre (13.3 Sv at 8°S). The Brazil Current (9.9 Sv at 29°S) is mostly composed of subtropical water originating in the upper 800 m west of the eastern boundary current at 30°S (8.5 Sv), with a minor contribution of surface tropical water that transfers to the subtropics (1.4 Sv).
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SubtropicalTropical Transfer in the South Atlantic Ocean
A. Cabré
1
, J. L. Pelegrí
1
, and I. VallèsCasanova
1
1
Institut de Ciències del Mar, CSIC, Barcelona, Spain
Abstract Here we explore the water transfer between the subtropical and tropical gyres of the South
Atlantic Ocean to better understand its unique equatorward heat delivery. A Lagrangian technique is
applied to the reanalysis product GLORYS2V4 in order to trace back the western boundary ow in the
tropical (North Brazil Undercurrent, NBUC) and subtropical (Brazil Current) gyres. Most of the northward
NBUC core transport (14.9 Sv at 8°S) arrives from the eastern boundary subtropical current (Benguela
Current) via the zonal South Equatorial Current. This subtropicaltropical transfer represents the core of the
returning limb of the Atlantic meridional overturning circulation and accounts for most of the observed
increase in heat and saltvolume transports (0.18 PW and 0.19 Sv from 30°S to 8°S, respectively) across the
South Atlantic. The NBUC also includes Antarctic Intermediate Water below 400 m (7.4 Sv at 8°S) coming
from the interior subtropical gyre, as well as water from the current's surface and peripheral components
coming from the tropical gyre (13.3 Sv at 8°S). The Brazil Current (9.9 Sv at 29°S) is mostly composed of
subtropical water originating in the upper 800 m west of the eastern boundary current at 30°S (8.5 Sv), with a
minor contribution of surface tropical water that transfers to the subtropics (1.4 Sv).
1. Introduction
The South Atlantic plays a crucial role in the returning limb of the Atlantic meridional overturning circula-
tion (AMOC), being the only basin that transfers heat equatorward from the subtropics to the tropics to com-
pensate for the southward transfer of North Atlantic Deep Water (NADW; Ganachaud & Wunsch, 2000;
Garzoli et al., 2013; Talley, 2003). However, the exchange pathways between the subtropical and tropical
gyres in the South Atlantic are poorly understood, due to both an historical lack of observations and the com-
plex interaction between the upperthermocline gyres and the AMOC.
The South Atlantic subtropical gyre is characterized by an anticyclonic (anticlockwise) circulation, with the
isopycnals (sea surface height) deepening (rising) toward the gyre center located in the western half of the
basin (e.g., Garzoli et al., 2013; Schmid, 2014). The western boundary current of this subtropical gyre is
the Brazil Current (BC), and its southern limit is the South Atlantic Current, starting at the Brazil
Malvinas Conuence near 3940°S and owing east between about 43° and 47°S, on the northern side
of the subantarctic fronts (e.g., Boebel et al., 1999; Garzoli, 1993; Gordon, 1989; Jullion et al., 2010;
OrúeEchevarría et al., 2019). The subtropical water masses continue northward in the eastern side as the
Benguela Current (Garzoli & Gordon, 1996) and nally ow back to the western boundary as the South
Equatorial Current (SEC).
The tropical gyre meets with the subtropical gyre along the SEC, before turning north along the western
boundary as the North Brazil Undercurrent (NBUC). A fraction of this nearsurface current recirculates into
the equatorial ocean via the Equatorial Undercurrent (and its southern and northern branches) and upwells
in the central and eastern equatorial ocean. This upwelled water is then carried southward mostly by Ekman
surface ows before subducting back toward the northwest, hence joining the SEC and completing what is
called the subtropical cell (STC, not to be confused with subtropical gyre; Hazeleger & Drifjhout, 2006;
Schott et al., 2004; Schmid, 2014).
The bifurcation latitude between the northward and southward western boundary currents, dening the
western end of the tropicalsubtropical border, shifts poleward with depth (Stramma & England, 1999).
Surface water masses (0100 m) bifurcate around 14°S (Rodrigues et al., 2007) where the southward BC ori-
ginates. The South Atlantic Central Water bifurcates around 21°S and the Antarctic Intermediate Water
(AAIW) near 28°S (Stramma & England, 1999). This results both in the downstream deepening of the BC
towards the BrazilMalvinas Conuence (Rocha et al., 2014) and the shoaling of the NBUC toward the tro-
pics (see schematics in Soutelino et al., 2013).
©2019. American Geophysical Union.
All Rights Reserved.
RESEARCH ARTICLE
10.1029/2019JC015160
Key Points:
We study the paths and intensity of
recirculation and northward
transfer in the subtropical and
tropical waters of the South Atlantic
The change in heat and freshwater
transport is calculated along these
pathways
Most heat is incorporated by
intermediate waters that upwell
several hundred meters as they ow
north along the eastern boundary
Correspondence to:
A. Cabré,
annanusca@gmail.com
Citation:
Cabré, A., Pelegrí, J. L., &
VallèsCasanova, I. (2019).
Subtropicaltropical transfer in the
South Atlantic Ocean. Journal of
Geophysical Research: Oceans,124,
48204837. https://doi.org/10.1029/
2019JC015160
Received 19 MAR 2019
Accepted 17 MAY 2019
Accepted article online 25 MAY 2019
Published online 12 JUL 2019
CABRÉ ET AL. 4820
The presence of the AMOC modies the picture of two semiisolated winddriven gyres, accounting for the
transfer of water masses from the subtropical to the tropical gyre mostly along the SEC (Garzoli & Matano,
2011; Lazar et al., 2001). Another consequence of the AMOC is that a portion of subtropical water masses
originates or recirculates into other basins as part of a Southern Hemisphere supergyre (Fratantoni et al.,
2000; Laurian & Drijfhout, 2011; Speich et al., 2007). According to Gordon et al. (1992) and most recent
literature (e.g., Speich et al., 2007), a major proportion of the surface and central water masses that end
up in the Benguela Current and SEC are rst carried to the Indian Ocean through the South Atlantic
Current and then recirculate back via the Agulhas Current past South Africa. Some of the deeper AAIW
that eventually becomes part of the AMOC recirculates even further east into the Pacic basin (Speich
et al., 2007).
Our goal is to expand our knowledge on the interconnections between the subtropical and tropical gyres in
the South Atlantic down to a depth of 1,500 m, which includes all surface, central, and intermediate water
masses. For this purpose, we use the hydrographic and velocity elds from GLORYS2V4, an eddypermitting
reanalysis model. From the release of particles along selected sections, we infer the predominant pathways
and ages for water parcels transferring between the subtropical and tropical gyres as well as for water parcels
recirculating within each gyre. In particular, we focus on the pathways followed by the subtropical water
masses that reach the NBUCwhat may be considered as the returning branch of the AMOC in the
South Atlanticand investigate the mechanisms responsible for the associated changes in heat and
freshwater transports.
The structure of the article is as follows. In section 2 we introduce the data and methods, and in section 3 we
present the pathways and the heat and salt transports associated with the different water masses. We
conclude in section 4 with a discussion of the new perspectives brought about by the Lagrangian approach
and of the mechanisms necessary to produce the observed changes in heat and salt transport.
2. Materials and Methods
2.1. Reanalysis Model GLORYS2V4
The Mercator Ocean team computed and provided monthlyaveraged values from the reanalysis model
GLORYS2V4 (eddypermitting with 0.25° resolution; CMEMS, 2017). The monthly climatology includes
values of salinity, temperature, and the threedimensional velocity eld, as calculated using the 23year data
set (1993 to 2015). Our region of study is the South Atlantic Ocean between 30°S and 5°S and down to 1,500
m. The position of the northern and southern boundaries ensures that the entire subtropicaltropical bound-
ary is contained in the domain and further warrants that the particles circulate only once through either the
subtropical or the tropical gyre before leaving the domain.
Mignac et al. (2018) have recently performed an exhaustive comparison between current data assimilation
products (including GLORYS2V4), freerunning models, and observations. This comparison shows that
the reanalysis products are more accurate than the freerunning models at representing the AMOC's
strength and meridional heat transport at 35°S and the NBUC volume transport at 11°S. Although the heat
and volume transports are systematically underestimated across models, GLORYS2V4 is the one that shows
the highest agreement with observations (Mignac et al., 2018). Drijfhout et al. (2003) have also found that
eddypermitting models are essential to correctly represent the Lagrangian pathways in the thermocline.
These results endorse the idea that GLORYS2V4 will be a very valuable tool for examining the exchange
of subtropical and tropical waters in the South Atlantic Ocean; further information on the model's perfor-
mance can be found in von Schuckmann et al. (2016).
2.2. Lagrangian Experiments
We use the Connectivity Modeling System code v2.0 (Paris et al., 2013) to track ~10,000 particles pathways
ofine. The inputs for this code are the timevarying horizontal (u,v) and vertical wvelocity component
elds and xed values for the horizontal (k
h
) and vertical (k
v
) diffusion coefcients. We choose two latitudi-
nal sections near the western boundary (black lines in Figure 1a) where we release the particles and trace
their trajectories back in time to study the transport between the subtropical and tropical gyres.
1. Section 8°S experiment traces back the northward ow at 8°S in the upper 1,400m layer, between the
South American coast and 15°W (Figure 1d). This experiment is designed to include both the NBUC,
10.1029/2019JC015160
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CABRÉ ET AL. 4821
which is located between the continental slope and about 32.5°W, and the northward thermocline
transport (50200 m) found in the interior ocean as far as 15°W.
2. Section 29°S experiment traces back the southward ow at 29°S in the upper 1,000m layer, between
the coastline of South America and 46°W (Figure 1f). This experiment is designed to track the BC,
isolated from the deeper northward countercurrent and from the deeper southward deep western
boundary current.
Figure 1. (a) Meridional transport (Sv) in the top 1,500 m, accumulated from the eastern boundary, as obtained from climatological averaged meridional velocities.
The experiment release sections are shown in black, the northern and southern boundary sections in gray, and the schematic representations of the four
possible trajectories in blue. Two sections approximately perpendicular to the main ow are shown in yellow (western) and green (eastern). (b) Westernboundary
meridional velocity, averaged within a 2.5° band off the coast (m/s) as a function of depth (m) and latitude. Meridional velocity (m/s) depthlongitude prole at
(c) the northern boundary (5°S), (d) the 8°S release section, (e) the southern boundary (30°S), and (f) the 29°S release section.
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CABRÉ ET AL. 4822
The number of particles Nreleased at each pixel (lon,depth) is set to be proportional to the meridional
transport at that location, vA(where vis the meridional velocity and Ais the vertical area of the pixel,
Δlon Δdepth), that is, more particles are released wherever and whenever the meridional transport is higher.
This way, each particle carries the same water transport (vA/N), which optimizes the sampling and facili-
tates the calculation of Lagrangian streamlines.
Our focus is on the climatological and yearaveraged velocities, hence leaving aside the impact of interannual
variations. The difference between both cases will reveal the role of seasonality on the net transfer of water
between the tropical and subtropical gyres. We use the monthlyaveraged velocity and density data interpo-
lated every 3 days, as several tests show that the trajectories are fairly insensitive to a higher temporal resolu-
tion while a lower resolution causes some signicant differences in the fastcurrent regions (not shown).
The model is eddypermitting but not entirely resolved, so the level of diffusion is probably underestimated
(see a more detailed analysis of diffusion parameters in PeñaIzquierdo et al., 2015). We have studied the
sensitivity of our results when the following diffusion parameters are added to the Lagrangian experiments:
k
h
= 0, 100, 500, 1;000 m
2
/s, and k
v
=0,10
5
,10
6
,107m
2
/s. We nd that a value of 100 m
2
/s for horizontal
diffusivity minimizes the number of particles that are lost near coastal regions or near the surface due to
methodological limitations. This is also the preferred value in PeñaIzquierdo et al. (2015), who analyzed
in detail the differences between horizontal diffusivity coefcients in the ECCO2 model, with the same
horizontal resolution as GLORYS2V4. Changes in vertical diffusion do not affect our results signicantly,
so we set it to 0 for the nal experiments. Importantly, our general conclusions remain the same when using
a different set of diffusion parameters (see the sensitivity of the Lagrangian transports to a selected set of
diffusion parameters in Table 1).
2.3. Lagrangian Stream Function
The Lagrangian stream function differs from the traditional Eulerian stream function because it only shows
the transports associated to pathways that end up at the release section (Blanke et al., 1999, and references
therein). The calculation is straightforward if we assume conservation of volume transport, which holds true
if particles leave the studied domain. This is the case for our experiments, where nearly all released particles
(>97%) leave the domain in less than 50 years.
To calculate the Lagrangian stream function in the South Atlantic, we rst dene an output grid (0.5° reso-
lution) and evaluate, at each time step (backward in time) and for each particle, whether the particle has
moved meridionally to a different output grid cell. If so, the initial meridional transport (vA/N), positive
for the 8°S experiment and negative for the 29°S experiment, is added to the intersection between cells
(multiplied by 1 if northward, +1 if southward to account for backtracking of the particle trajectory). If
more than one cell is crossed in a single time step, transport is added to each crossed intersection.
Eventually, the particles leave the domain either through the northern or southern borders (Figures 1c
and 1e). The meridional transport is cumulatively integrated with longitude (from the eastern side of the
Atlantic) and results in our nal Lagrangian stream function.
2.4. Mass, Heat, and Salt Transport Calculation
The total volume Vand mass Mtransports (m
3
/s and kg/s) are calculated as
V¼N
i¼1Vi;M¼N
i¼1ρiVi;
where V
i
is the iparticle volume transport (m
3
/s) and ρ
i
the water density at the ilocation. Volume transport
is conserved by construction in our experiments, and mass transport is almost conserved because particles
approximately follow constant density lines.
The total heat transport H(in watts) across a section at any xed latitude is calculated as
H¼cp
N
i¼1
ρiViTiTr
ðÞ
;
where T
i
is the particle potential temperature, T
r
is the reference temperature (set as 0 °C), and c
p
is the sea
water heat capacity (c
p
= 3,985 Jkg
1
K
1
).
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Journal of Geophysical Research: Oceans
CABRÉ ET AL. 4823
The salt transport S(kg/s) is calculated as
S¼
N
i¼1
ρiSiVi;
where S
i
is the particle salinity (in units of kilogram salt per kilogram water). In this paper, the salt transport
is also expressed in mass transport units (Sv), following Talley (2008) and using 35 as the average salinity,
S¼
N
i¼1
Si
35 Vi:
2.5. Trajectory Clustering
We also average the spatial trajectories (longitude, latitude, and depth) and properties (temperature, salinity,
and density) for all the particles that belong to one same group. For example, in section 3.4 we consider all
particles that belong to the same density bin (group) and combine the individual trajectories based on the
fraction of distance traveled from the origin to the end section (after normalizing the traveled distance for
each particle from 0 to 1). These results are not an exact representation of the real averaged trajectories
but help visualize the differences between groups.
3. Transfer of Water Masses Between the Tropical and Subtropical Gyres
We can use the velocity elds at any time to calculate the instantaneous depthintegrated (01,500 m)
Eulerian stream function. As for the Lagrangian streamlines, we do so by accumulating the meridional
transport from the eastern side of the basin. Since we are not integrating the full water column, and there
is a small fraction of water particles that upwells from deeper layers, the Eulerian transport is not strictly
conserved. However, the nonconservative fraction is small so that the Eulerian stream function becomes a
useful complementary tool.
The mean velocity elds (Figures 1b1f), and the Eulerian (Figure 2e) and Lagrangian streamlines
(Figures 2a2d) reect the existence of the western and eastern boundary currents. These boundary
currents are identied as a connected region, adjacent to the continental slope, where the ow has one pre-
dominant direction. At the reference sections, they are located as follows: The NBUC is found west of
33.0°W at 5°S (Figure 1c) and west of 32.5°W at 8°S (Figure 1d), the BC is placed west of 46.2°W at
29°S (Figure 1f) and west of 47.0°W at 30°S (Figure 1e), and the Benguela Current is located east of
8.0°E at 30°S (Figure 1e).
For our analysis we follow Ganachaud (2003) and Talley (2003) and split the particles among surface and
nearsurface, central, and intermediate layers, considering their initial or nal vertical location; along their
trajectory, particles often change greatly their location in the water column (Table 2). The surface particles
occupy roughly the upper 60 m of the water column (potential density σ
0
less than about 25.0/25.5 kg/m
3
in
the western/eastern basins), and the nearsurface particles are found immediately below, down to some
Table 1
Sensitivity of Experiments to a Selection of Horizontal and Vertical DiffusionParameters (m
2
/s) and to the Use of Different Velocity Fields (Climatological CLIM Versus
YearAveraged YRAV)
Section Pathway
CLIM CLIM CLIM CLIM YRAV YRAV
k
h
=0 k
h
= 100 k
h
= 100 k
h
= 500 k
h
= 100 k
h
= 500
k
v
=0 k
v
=0 k
v
=10
5
k
v
=0 k
v
=0 k
v
=0
8°S 5°S 8°S 12.9 13.3 13.1 14.0 13.1 13.6
(38.5 Sv) 30°S 8°S 21.9 22.3 22.4 23.1 22.4 23.4
z> 1,500 m 8°S 3.6 2.9 3.0 1.4 3.0 1.5
29°S 5°S 29°S 1.5 1.4 1.5 1.6 1.6 1.6
(9.9 Sv) 30°S 29°S 8.4 8.5 8.4 8.3 8.3 8.3
Note. The table shows the water volume transport (Sv) associated to each Lagrangian experiment. The rst column shows the collecting section (the meridional
water transport is shown in parenthesis, with positive/negative values indicating northward/southward transports), the second column indicates the pathway,
and the remaining columns quantify the associated northward/southward (positive/negative) transports.
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Journal of Geophysical Research: Oceans
CABRÉ ET AL. 4824
100200 m (σ
0
up to 26.0 kg/m
3
). The central layers correspond to the Subtropical Mode Waters,
corresponding to thermocline waters reaching down to less than 600 m (σ
0
from 26.0 to 26.8 kg/m
3
). The
intermediate layers travel at depths of roughly 5001,500 m, including both Subantarctic Mode Water
(SAMW, with σ
0
from 26.8 to 27.2 kg/m
3
) and AAIW (σ
0
from 26.8 to 27.55 kg/m
3
).
The main objective of the forthcoming discussion is to understand and quantify the four possible pathways
for water departing in the tropical/subtropical gyres and ending at the NBUC and BC: the subtropical and
tropical recirculations and the exchange between the tropical and subtropical gyres (blue trajectories in
Figure 1a). Throughout this paper, we will refer to those water masses transferred from the subtropical to
the tropical gyre as AMOC water masses.
Figure 2. Lagrangian streamlines (Sv) for particles that (a) transfer from 30°S to 8°S, (b) transfer from 5°S to 8°S, and (c) end at the subtropical 29°S section. The
release sections are shown in black, the two sections perpendicular to the main ow are shown in yellow and green as in Figure 1a, and the schematic of the
trajectories are shown in blue. The sum of the four Lagrangian streamlines is shown in (d). The Eulerian stream function down to a depth of 1,500 m is shownin
(e) for comparison.
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CABRÉ ET AL. 4825
3.1. Velocity Fields and Water Masses
The boundary current system off the eastern coast of South America (Figure 1b) is similar to the one simu-
lated by Rodrigues et al. (2007), who studied in detail the bifurcation between northward and southward cur-
rents at the western boundary. The western boundary currents at 5°S and 11°S are consistent with Schott
et al. (2005) and Hummels et al. (2015) observations, both in terms of depth structure and volume transports.
Further, the southward deepening and strengthening of the BC in GLORYS2V4 approximately coincides
with real estimates obtained from moorings in Rocha et al. (2014). They nd 5.7 Sv at 25°S (mooring C3,
July and August) and 10.0 Sv at 28°S (mooring W333, December and January), while we respectively nd
5.1 and 10.4 Sv when considering the same sampling periods.
Figure 3 shows the spatial distribution of the different water masses when crossing the South Atlantic
westward through oblique sections centered at 25°W and 5°E (hereafter western and eastern sections;
yellow and green lines in Figure 1a, respectively). Consider rst the eastern section, which is clearly domi-
nated by the subtropical pathways to the tropics (30°S to 8°S, AMOC water; Figure 2a and red dots in
Figure 3b); these water masses account for most of the transport to the northwest (18 Sv), while subtropical
recirculating water masses represent only 1.5 Sv. The subtropical recirculation takes place further south and
shallower than the AMOC water, its surface layers reaching only as far north as about 20°S (orange dots in
Figure 3b). We nd that the direction of the velocity at 30°S determines whether the particle will continue to
the north or will recirculate within the subtropical gyre: The more northward the velocity at 30°S, the more
likely it will leave the subtropical gyre and reach 8°S (not shown).
Table 2
The Average Depths at Which the Given Water Masses (Left Column) Reach Their Maximal Potential Density Value σ
0
for
the Different Latitudinal Sections; This Also Includes the Dependence With Longitude
Depth (m)
Water mass σ
0
max 8°S 29°S 5°S 5°S 30°S 30°S
(kg/m
3
) section section west of 0°E east of 0°E west of 8°E east of 8°E
Surface 25 104 13 82 35 25
Near surface 26 148 203 111 79 162 103
Central 26.8 306 578 339 364 590 456
Intermediate 27.2 848 1,080 837 831 1,096 906
27.55 1,390 1,765 1,371 1,372 1,712 1,510
Figure 3. Depthlatitude distribution of particles owing west in the (a) western and (b) eastern cross sections of Figure 1a, colored by the water route as labeled.
The black lines show the potential density contours (σ
0
), and the gray line shows the zero zonal velocity contour.
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CABRÉ ET AL. 4826
Only a small fraction of tropical recirculating water masses (1.1 Sv) reach the eastern section (blue dots in
Figure 3b). These water masses ow either (a) within the surface (upper 50 m), on top of AMOC water
masses (0.3 Sv) and interacting with surface recirculating subtropical water masses from 24°S to 21°S, (b)
at similar depths as AMOC water (0.2 Sv), or (c) through deep zonal jets at 1517°S that do not interact with
AMOC water (0.6 Sv).
Consider now the western section where all four pathways are clearly separated (Figure 3a). The Vitória
Trindade Ridge, which along this section is located at 21°S, denes the nearsurface bifurcation latitude
between the subtropical gyre (5.5 Sv) and the AMOC water mass (21.8 Sv), in agreement with Soutelino
et al. (2013). The bifurcation latitude shifts south with depth (see also black line in Figure 1b). Note that
the subtropical gyre penetrates to the intermediate layers (Figure 3a), in agreement with results by
Schmid et al. (2000). Moreover, this western section clearly illustrates that the entire AMOC transfer, from
the subtropics (30°S) to the tropics (8°S), occurs mainly within central and intermediate layers.
The transport of recirculating tropical water masses that reach the western section equals 3.9 Sv, with about
half arriving directly from 5°S and the other half after having transited southeast into the Angola Dome
(Figure 2b), returning west partly as deep zonal jets along 1012°S (1.0 Sv; blue dots in Figure 3a). The tro-
pical water masses that are transferred to the subtropics (1.4 Sv) ow on top of the recirculating tropical
water masses (cyan dots in Figure 3a) and eventually ll the surface part of the southward ow in the wes-
tern boundary (Figure 1b).
3.2. Transport at the Tropical 8°S Section
A total of 38.5 Sv follow north through the 8°S section, with most of it (35.0 Sv) occurring at all depths within
the NBUC and a small fraction (3.5 Sv) owing through the interior thermocline near the surface (50150
m). The origins of this northward current are shown in Figure 4a. Most of it, 22.3 Sv (58% of the total volume
transport), enter the domain in the subtropics (30°S, black dots and contours in Figure 4a). These subtropical
water masses feed the core of the NBUC (21.4 Sv at depths 100 to 1,200 m) and the nearsurface interior
layers (0.9 Sv at depths 50 to 150 m). The 8°S current also transports 13.3 Sv of tropical origin (34.5% of
the total volume transport), feeding the surface (3.8 Sv) and eastern edge (6.2 Sv) layers of the NBUC, the
central and intermediate layers near 32°W (1 Sv at depths 200 to 1,000 m), and the nearsurface (50150
m) interior ow as far as 15°W (1.8 Sv; red dots and contours in Figure 4a). The remaining fraction of the
NBUC (2.8 Sv, 7%) originates below 1,500 m in the subtropics (green dots in Figure 4a).
As noted in the methods section, the results are qualitatively similar when using different diffusivities
(Table 1). However, as the horizontal diffusivity increases, the border between the tropical and subtropical
Lagrangian streamlines diffuses and the number of particles originating deeper than 1,500 m decreases
(not shown).
3.2.1. Subtropical to Tropical Transfer (30°S to 8°S)
Most of the water of subtropical origin (originating at 30°S) that ends in the NBUC at 8°S comes from the
deep ow (down to 1,500 m) in the eastern basin (east of 8°E) at 30°S (14.9 Sv; black dots in Figure 4c).
This ow constitutes the core of the returning limb of the AMOC, arriving mainly from the Indian Ocean
via Agulhas rings and laments (e.g., Donners & Drijfhout, 2004; Gordon et al., 1992; Speich et al., 2007).
Finding the exact origin of this northward ow at 30°S would require running the Lagrangian experiments
over a much larger domain, which is beyond the scope of this paper.
Interior water masses (departing west of 8°E at 30°S) also reach 8°S along the intermediate layers, carrying a
total of 7.4 Sv. In particular, we nd that 6 Sv of these intermediate water masses are transported at σ
0
>27
kg/m
3
, comparable to the 4 Sv reported by Boebel et al. (1999) or the 5 Sv by Schmid et al. (2000) at the same
densities. These water masses bifurcate around 28°S in the western boundary, at a location known as the
Santos bifurcation (Boebel et al., 1999; Stramma & England, 1999).
Figure 2a shows the Lagrangian streamlines for water masses that transfer from 30°S to 8°S. In general, these
water masses ow northwestward along a wide band from the eastern side at 30°S to the western boundary
where they feed into the northward boundary current. Those water masses that reach the western boundary
at the northern latitudes cross the South Atlantic more to the north (Figure 2a) and come from the subsurface
eastern boundary at 30°S (25.5 < σ
0
<26.5kg/m
3
; blue and green dots in Figure 5a), while water masses ori-
ginating more to the west and deeper at 30°S ow moredirectly westward and arrive to the western boundary
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Figure 4. Spatial distribution of particles at (a) the tropical section (8°S), (b) the subtropical section (29°S), (c) the southern boundary (30°S), and (d) the northern
boundary (5°S), colored depending on the origin and ending as labeled. In (a) and (b), for the sake of clarity, the colored dots are replaced by contour lines wherever
the number of particles is large (>10, 25, 60, and 80 particles in pixels of size 0.2° × 20 m). The potential density lines (σ
0
) are shown in gray.
Figure 5. (a) Depthlongitude distribution in the eastern margin of 30°S for AMOC particles transferred from 30°S to 8°S, colored by the latitude crossing the wes-
tern section (yellow line in Figure 1a), with density and temperature contours overplotted in gray and black, respectively. (b) Depthlatitude distribution along the
western section for AMOC particles, colored by their longitude at the southern boundary (30°S). In both panels, black colored dots show data outside the longitudes
or latitudes in the color bars.
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further south (Figure 5a). Another way of understanding the large spatial range of the South Atlantic north-
westward subtropicaltotropical transfer is through a latitudedepth distribution of particles through the
westernbasin section (which is roughly perpendicular to the main ow, yellow line in Figure 1a), illustrating
the departing longitude at 30°S (Figure 5b). The deeper the transfer, the further south it occurs, in agreement
with Stramma and England (1999). At any depth, the westward transfer occurs over a latitudinal band about
10° wide, the latitude of transfer varying both with depth and with the original longitude at 30°S.
Crossing the South Atlantic from 30°S to 8°S takes 8.9 years on average, with a long tail in the distribution
and the 20% slowest particles taking over 21.2 years to cross (dark red line in Figure 6a). On average it takes
4.7 years for the surface and nearsurface particles, 6.2 years for the central water masses, and 12.8 years for
the intermediate water masses (Figure 6b). Lazar et al. (2001) found timescales of 56 years for anomalies
propagating through the thermocline from 30°S to the equator, in good agreement with our results.
3.2.2. Tropical Recirculation (5°S to 8°S)
The tropical water masses (originating at 5°S, Figure 1c) that end up northward at 8°S (red dots in Figure 4a)
come either from the surface (2060 m) throughout the entire 5°S section (4.6 Sv), from the nearsurface and
upperthermocline layers (4.0 Sv east of 32°W) or from the intermediate western boundary layers (4.6 Sv
west of 32°W; black dots in Figure 4d).
At 5°S, the surface waters (4.6 Sv) ow southwest before turning north along the western boundary
(Figure 2b), on average taking 2.6 years to get to 8°S (yellow line in Figure 6c). Of these, many particles
(40%, 1.9 Sv) recirculate along the surface but 60% (2.7 Sv) escape from the surface winter mixed layer depth,
constituting the STC. About 1.1 Sv of these STC subsurface water masses ow along the upperthermocline,
and the remaining 0.7 Sv reach much deeper, in the intermediate levels. We observe that the STC water
remains largely separated from the AMOC water masses, in agreement with Hazeleger & Drifjhout
(2006), who found that STC water masses feed the surface part of the northward current at 6° and 10°S while
AMOC water masses feed the subsurface part of the current (>100 m).
The water masses that originate below the thermocline at 5°S (4.0 Sv) move preferentially toward the
southeast before winding west and northwest along the western boundary (Figure 2b), in agreement
with Hazeleger & Drifjhout (2006). These water masses end up at surface and near surface at 8°S (red dots in
Figure 4a), mostly above AMOC water masses (black dots in Figure 4a), taking on average 14.1 years to ow
from 5°S to 8°S, the traveling time increasing with depth (purple line in Figure 6c).
Finally, there is a tight recirculation of deep central and intermediate water near the western boundary
(western black dots in Figure 4d), with 4.6 Sv of water owing south off the boundary current and recircu-
lating northward at 8°S (red dots and contours east of the core NBUC in Figure 4a). These water masses take
on average 5.1 years to ow from 5°S to 8°S, the longer, the deeper in the water column they are (orange line
in Figure 6c).
3.2.3. Comparison With Literature
Hazeleger & Drifjhout (2006) found 16 Sv of AMOC transport using a simulation with the same horizontal
resolution as ours (0.25°). They found that around one third of the AMOC water upwells in the tropics and
part of it (2 Sv) recirculates into the STCs before continuing northward. We nd a similar amount of STC
water masses (2.7 Sv) in our experiments.
Blanke et al. (1999) studied the transport of water from 10°S to 10°N, down to a depth of 1,200 m, with a low
resolution simulation (2° zonal and 0.51.5° meridional). They found 37.3 Sv of volume transport owing
northward at 10°S, mostly through the western boundary, similar to our 38.5 Sv at 8°S (down to 1,500 m),
with 17.4 Sv reaching all the way to 10°N. This interhemispheric handover, which is mainly associated to
the AMOC, is comparable but less than the transfer we nd from 30°S to 8°S: 14.9 Sv through the eastern
boundary (east of 8°E at 30°S, z=01,500 m) and 7.4 Sv from intermediate layers (west of 8°E at 30°S, z
> 700 m).
The AMOC water masses continue into the North Atlantic subtropical gyre (out of our domain), either
directly along the western boundary for the deep water masses (<7 °C; Blanke et al., 1999) or after meander-
ing around the tropics multiple times for warmer particles (Blanke et al., 1999; Hazeleger & Drifjhout, 2006).
In particular, Blanke et al. (1999) found that 19.9 Sv (out of the 37.3 Sv northward transport at 10°S) recir-
culate in the tropics and return back to 10°S without contributing to the interhemispheric transfer of heat.
When comparing their results with ours, we deduce that their recirculating surface and upper
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thermocline water coincides roughly with our southward surface tropical
water at 5°S. Blanke et al. (1999) also show that intermediate tropical
water masses recirculate close to the western boundary, similarly to
Schott et al. (2005) who observed this deep tight counterow at 511°S
with an associated transport (5 Sv) similar to our result (4.6 Sv).
3.3. Transport at the Subtropical 29°S Section
The southward western boundary BC carries a total of 9.9 Sv at 29°S, split
between 1.4 Sv at the surface and nearsurface layers coming from the
tropics (red dots and contours in Figure 4b), and 8.5 Sv at the upper
thermocline layers coming from the subtropics (black dots and contours
in Figure 4b). The subtropical water masses reaching 29°S originate
mostly west of 8°E at 30°S (west of the AMOC eastern boundary ow)
in the surface and central layers (red dots in Figure 4c), taking an average
of 4.4 years (orange line in Figure 6a). The tropical water masses transfer-
ring to 29°S come mainly from the surface (1.0 Sv) and nearsurface (0.4
Sv) layers at 5°S (red dots in Figure 4d); these water parcels on average
take 5.8 years, the further west they start, the quicker the transfer (cyan in
Figure 6a).
These results indicate that the timescale needed for thermocline particles
to cross the South Atlantic is typically 510 years. This applies to the water
parcels recirculating the tropical gyre (dark blue in Figure 6a), to those
beginning at 30°S as part of the AMOC and arriving to 8°S (red in
Figure 6a), and to the particles recirculating within the subtropics (orange
in Figure 6a).
3.4. Heat and Freshwater Transfer
3.4.1. AMOC Returning Water
We rst focus our analysis on the AMOC water (e.g., transfer from 30°S to
8°S in our experiment). The total heat transport at any latitude and long-
itude (cumulative with respect to the western boundary) is the sum of the
conserved heat transportcalculated considering that the water particles
maintain their 30°S original temperature along their trajectory (contour
lines in Figure 7)and the heat gain at any latitude as compared with
30°S (color shading in Figure 7). Water masses owing from 30°S to 8°S
transport 0.90 PW of heat at 30°S and gain an additional 0.20 PW along
the South Atlantic, most of it (0.15 PW) occurring gradually from 30°S
to 18°S. Water masses departing 30°S west of 8°E originally transport
0.64 PW and only warm by 0.02 PW (Table 3); in contrast, water masses
departing east of this latitude initially only carry 0.26 PW and warm by
0.18 PW (Table 3). After their incorporation into the western boundary,
water masses actually cool slightly (0.01 PW). See Table 3 for a compari-
son among water masses.
Temperature and salinity increase toward the tropics up to 8°S along iso-
pycnals and also toward the west for light water masses (σ
0
< 26 kg/m
3
;
not shown). Hence, as particles cross the South Atlantic approximately
along isopycnals, we expect them to gain not only heat but also salt. In
particular, subtropical (30°S) water reaching 8°S gains 0.22 Sv of salt
volume transport (Table 3).
Within the central water stratum, temperature and salinity bear a linear
relation, with both properties increasing toward the sea surface (light blue
and green dots in Figure 8a; Poole and Tomczak, 1999). A scatter plot of temperature and salinity changes
between 30°S and 8°S displays an approximate linear relation (Figure 8b). Since the slope of the central
Figure 6. (a) Histograms showing the transfer time for the four different
routes (arbitrarily normalized to a unit area), as labeled (bin width is 0.5
years). (b) Contributions of different density bins in the 30°S to 8°S course, as
labeled. (c) Contributions of the different depth levels in the 5°S to 8°S
course, as labeled (bin width is 0.2 years). The legend also shows the time
that divides the 80% quickest particles from the 20% slowest particles.
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water TS relation is greater than the slope of the isopycnals (Figure 8a), we expect the particles to move
toward lighter isopycnals to match the observed temperature and salinity increase (Figure 8b). Indeed, as
particles ow north, the temperature increase overcompensates the salinity changes so that water masses
gain some buoyancy (Figure 8c). However, we nd that changes in density are relatively small when
compared to changes in spiciness (Flament, 2002), showing that most of the diffusive and advective
motions occur along quasiisopycnals. We also nd that, for the same change in temperature, surface
Figure 7. The colorlled contours show the northward heat transport (PW) that is gradually accumulated from the
western and southern boundaries as particles increase their temperature while moving from 30°S to 8°S. In contrast,
the black solid contours show the conservative northward heat transport accumulated from the western boundary, cal-
culated considering that the water particles maintain their 30°S original temperature along their trajectory.
Table 3
The First Row Shows the 30°Sto8°S Volume Transport, Total Heat Gain and Total Salinity Gain
Volume transport Heat gain Salt gain
Total 22.3 Sv 0.20 PW 7.8 × 10
6
kg/s (0.22 Sv)
Eastern boundary water 66.9% 88.8% 86.7%
(East of 8°E at 30°S)
Intermediate water 45.2% (25.6) 36.6% (30.1) 19.5% (16)
(σ
0
> 26.8)
Central water 39.8% (31.0) 41.6% (41.0) 41.3% (40.0)
(26 < σ
0
< 26.8)
Surface and near surface 15.0% (10.2) 21.8% (17.7) 39.2% (30.7)
(σ
0
< 26)
Water masses that downwell 54.7% (36.2) 43.9% (39.0) 63.9% (53.2)
(from 30°S to western section)
Water that warms the most 13.5% (13.2) 50.4% (49.5) 47.2% (46.5)
(ΔTemp > 5 °C)
Interior arriving water 4.6% 16.8% 15.3%
(East of 33°W at 8°S)
Note. The following rows show the percentage of the total for the different water masses (dened in the rst column). In
parentheses are percentage values of the total contributed by the eastern boundary current (east of 8°E at 30°S).
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particles increase their salinity more than deep particles (Figure 8b and Table 3). This means that, for a
similar increase in spiciness, the surface particles decrease their density less than the deep particles,
essentially indicating the more stable character of the highly stratied nearsurface waters (Figure 8c).
Where and how are particles warming? The majority of the AMOC particles crossing the ocean gain heat,
apart for a small fraction that cool (Figures 8b and 9a) once inside the western boundary. As we have already
said, most of the heat gain (88.8%) occurs for 30°S particles departing east of 8°E (Table 3 and green and yel-
low dots in Figure 9a). This heat gain occurs through the full depth of the water column, in central (41.6%),
intermediate (36.6%), and surface/nearsurface layers (21.8%; Table 3). About 50% of the heat gain occurs
through particles that warm over 5 °C (Table 3) mostly in the central and intermediate layers (green and blue
in Figure 8b). In order to identify how these particles gain so much heat, we group together the particle
trajectories based on the total temperature change from 30°S to 8°S, as explained in section 2.5. The
average trajectories show again that the eastern particles warm most and travel further north (red lines in
Figure 10d). They warm by upwelling several hundred meters when crossing the South Atlantic (especially
from 30°S to 22°S, red lines in Figures 10e and 10f; see also upwelling in Figure 9b) and mixing with surface
warm water masses. Such large upwelling is associated with the rising of the isopycnals at the boundary
between the subtropical and tropical gyres, with most warming occurring in the easternmost central and
Figure 8. (a) Temperaturesalinity scatter plots at 30°S for AMOC water masses transferring from 30°S to 8°S. (b) Scatter plot of the change in temperature and
salinity for particles starting at 30°S near the eastern boundary (longitude > 8°E; colored by initial temperature). (c) Scatter plot of the change in density and spi-
ciness for particles, as in (b).
Figure 9. Depthlongitude distribution of the Atlantic meridional overturning circulation particles (30°S to 8°S) in the eastern side of 30°S, colored by (a) tempera-
ture increase from 30°S to the western cross section and (b) depth change (m), positive if particles upwell and negative if particles downwell. Note that the color
tables are linear, but the stretch depends on the sign of the data points. Potential density lines (σ
0
) are overplotted in gray.
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intermediate layers (Figure 9a). On the other hand, surface and nearsurface layers downwell along isopyc-
nals and experience small or moderate warming (Figure 9).
3.4.2. Other Pathways
We next repeat the procedure of last section and explore the heat and salt gain in the other three pathways:
recirculations within the subtropical and tropical gyres and transfer from the tropics to the subtropics. In
particular, following section 2.5, we group the particle trajectories based on their total temperature and salt
changes and summarize here the main ndings.
Subtropical recirculating water masses (30°S to 29°S) transfer northward 0.51 PW of heat at 30°S and gain
0.04 PW on their way to 29°S (salmon pink trajectory in Figure 11). While crossing the Atlantic, these par-
ticles in average downwell without much warming except for the easternmost water masses, which initially
upwell and warm (similar to the eastern AMOC water masses) before submerging back to the west.
However, water masses that cross 30°S west of 10°W warm only once inside the western boundary.
Tropical recirculating water masses warm and get saltier when circulating around the tropical gyre from
5°S to 8°S (three darkest blue trajectories in Figure 11). The warming (0.08 PW) occurs only for water
masses below the surface mixed layer, which rst ow southeast and then warm as they move toward
the western boundary (not shown). In the process, these waters also increase their salinity by 0.08 Sv.
Surface water masses that subduct in the STC (into the saltier nearsurface layers) also increase their
salinity by 0.10 Sv.
Tropical water masses that feed into the subtropical southward ow do not experience any signicant
change in either heat or salinity (light blue trajectory in Figure 11).
4. Discussion and Concluding Remarks
We have combined the Connectivity Modeling System code v2.0 ofine tracking software (Paris et al., 2013)
and the GLORYS2V4 velocity elds (CMEMS, 2017) to backtrack particles released in the western boundary
region of the tropical (8°S) and subtropical (29°S) South Atlantic gyres. For our analyses, we have explored
Figure 10. Histograms of (a) volume transport and (b) heat transport at 30°S and (c) heat gain for these particles after reaching 8°S, binned as a function of the total
temperature change. We present, for each temperature bin, (d) the mean trajectories (longitude and latitude) as well as the (e) average depth, (f) temperature,
and (g) density (as a function of latitude) along these pathways; the curves are colorcoded as in panels (ac). The zero Lagrangian streamline in Figure 2d is shown
as a dashed line in panel (d).
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four paths of propagation down to 1,500 mwithin the subtropical and tropical gyres and the exchange
between both gyres. The water masses are split among surface, nearsurface, upperthermocline (central),
and intermediate strata (Table 2).
Figure 11 and Table 3 summarize the main pathways of transfer, and the volume and heat transport
associated to each of these, when using meanseasonalcycle velocity elds and xed diffusivities
(k
h
= 100 m
2
/s, and k
v
=0m
2
/s). We nd that the results are qualitatively similar when adopting dif-
ferent diffusivities (Table 1). The conclusions are also similar when using yearly averaged velocity elds
instead of climatological ones (Table 1), which suggests that seasonality does not play a signicant role
in the mean transfer between gyres. However, there are substantial differences across seasons, which
need to be considered in order to avoid biases when looking at any particular set of eld observations
not averaged throughout the whole year.
The tropical gyre represents the pathway from the transoceanic 5°S section to the 8°S western boundary sec-
tion, while the tropicalsubtropical transfer corresponds to the transfer from the transoceanic 5°S section to
the 29°S western boundary section. All the pathways starting at 5°S are shown in blue in Figure 11. The
initial watervolume (Sv) and heat (PW) transports are labeled in white at 5°S, and the heat gain (PW)
and salt transport gain (Sv) are labeled along the pathways when higher than 0.01. Most subsurface tropical
water masses starting at 5°S end up recirculating into the tropical gyres and become warmer (0.08 PW) and
saltier (0.18 Sv) on their way to 8°S. In contrast, most surface tropical water masses end up transferred to the
subtropical gyre, carrying 1.4 Sv and 0.14 PW southward.
Figure 11. Schematic showing the average transfer between tropical and subtropical gyres on top of the Eulerian streamlines in Figure 1a. In red, water masses
coming from the subtropical 30°S section (darker red if deeper). In blue, water masses coming from the tropical 5°S section (darker blue if deeper). The initial
watervolume (Sv) and heat (PW) transports are labeled in white; the heat gain (PW) and salt transport gain (Sv) are labeled along the pathway if higher than 0.01.
The collecting sections are shown as dashed lines.
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The subtropical gyre is assessed as the route from the transoceanic 30°S section to the 29°S western boundary
section. This pathway is formed by surface, nearsurface, and central water masses that depart 30°S at long-
itudes mostly west of 8°E (red dots in Figure 4c). We nd that the subtropical recirculating water masses
warm by 0.04 PW from 30°S to the western boundary at 29°S where they ow southward, hence being
responsible for a small net southward heat transport. Talley (2003) estimated a net southward warming of
0.1 PW at 32°S when excluding AMOC water masses. She dened the subtropical gyre as the region with
σ
0
< 26.2 kg/m
3
starting from the western side until the longitude where the interior northward volume
transport equals the southward western boundary transport, hence leaving out the easternmost water
masses with net northward transport as AMOC. To compare this value to our results, we must add the heat
transport from the tropical gyre to the subtropical gyre (0.14 PW) to the net subtropical warming (0.04 PW).
Hence, we effectively nd a southward warming of 0.18 PW at 29°S, higher but comparable to the 0.1 PW
estimated by Talley (2003) at 32°S. Talley's method is different from ours, which grants condence to
our results.
The return branch of the AMOCthe subtropicaltropical transferresponds to the pathway from the
transoceanic 30°S section to the 8°S western boundary section. It is attained through parcels in the
Benguela Current spanning the entire water column (14.9 Sv), from the surface down to intermediate strata,
and through intermediate water owing across the entire subtropical basin (7.4 Sv). All this water ends up in
the NBUC and is transferred to the tropical gyre (Figure 2a), after gaining 0.20 PW and becoming saltier by
0.22 Sv. Most of the heat and salt gain occurs for water masses of eastern origin (0.18 PW and 0.19 Sv; Table 3
), while the rest of heat and salt is added by interior intermediate water masses (0.02 PW and 0.03 Sv). This
heat gain is mostly explained by the upwelling of water masses in the easternmost central and intermediate
layers that mix with more buoyant water masses along the Benguela Current and, to a lesser degree, by
surface and nearsurface water masses that head generally north as they depart from 30°S and gradually
experience moderate warming and downwelling. According to Hazeleger & Drifjhout (2006), AMOC water
masses gain an additional 0.22 PW of heat when crossing from 10°S to 10°N but freshen by 0.16 Sv due to
intense precipitation in the tropics.
The classical Eulerian view on the meridional extent of the subtropical gyre (zero Eulerian streamline,
located near 12°S in the western boundary in Figure 2e) is awed as the central Atlantic includes both
particles that recirculate in the subtropical gyre (orange dots in Figure 3a) and AMOC particles that ow into
the tropical region (red dots in Figure 3a). Using the Lagrangian perspective allows identifying that the
northern ank of the Eulerian subtropical gyre is lled with water parcels that continue north, constituting
the returning limb of the AMOC, and that the bifurcation latitude between the subtropical ow and the
AMOC ow shifts southward with depth (black line in Figure 1b, also see Figure 3a). The westernboundary
bifurcation between the tropical and subtropical gyres is located near 21°S, further south than expected from
the Eulerian perspective (border between orange and dark blue dots in Figure 3a). Note that north of 21°S,
the surface southward transport in the western boundary (Figure 1b) is explained entirely by the water
masses of tropical origin that transfer into the subtropical gyre (cyan dots in Figure 3a).
Baringer and Garzoli (2007) and Majumder et al. (2016) have shown that the fullcolumn northward heat
transport at 30°S is 1.5 PW when excluding the southward western boundary. Further, Mignac et al.
(2018) used several reanalysis models to show that the fullcolumn integrated net heat transport at 30°S is
around 0.5 PW. Both results together imply that there is a southward transport of 1.0 PW through the
western boundary. Our results down to 1,500 m show northward transports of 0.90 PW for AMOC particles
and 0.51 PW for the interior branch of the subtropical gyre, which together account for most of the full
column 1.5 PW of northward interior transport (excluding the western boundary). In the western boundary,
our results show southward transports of 0.55 PW for subtropical particles and 0.14 PW for tropical particles
(a total of 0.69 PW) down to 1,500 m, which means that the remaining southward 0.31 PW needed to account
for the fullcolumn southward 1.0 PW are transferred at depths greater than 1,500 m.
Finally, a comparison of the western boundary transports with the total transports through the southern
(30°S) and the northern (5°S) boundaries of the study area shows that the Lagrangian experiments account
for most of the total transport in the South Atlantic (Figures 2d and 2e), which means that nearly all water
masses in the central South Atlantic eventually channel through the western boundary. At 5°S, we nd 20.6
Sv of water owing north, which are mostly explained by the 22.3 Sv coming from 30°S (and captured at 8°S)
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minus the 1.4 Sv of tropical origin that escape to the subtropics (for a total of 20.9 Sv); the northward heat
transport at 5°S (1.0 PW) is essentially due to the heat carried by AMOC water masses (1.10 PW at 8°S), plus
the heat gained along the tropical gyre (0.08 PW) and minus the heat that is transferred from the tropics to
the subtropics (0.14 PW), for a total of 1.04 PW. At 30°S, the total integrated northward transport is 19.4 Sv,
roughly due to AMOC water masses (14.9 Sv along the eastern boundary and 7.4 Sv in the interior ocean for a
total of 22.3 Sv) minus tropical water masses transferred southward (1.4 Sv), a total of 20.9 Sv; the associated
northward heat transport at 30°S is equal to 0.70 PW, mostly due to the heat carried by AMOC water (0.90
PW northward) minus the heat gained along the subtropical gyre (0.04 PW southward) and minus the heat
transferred from the tropics to the subtropics (0.14 PW southward), a total of 0.72 PW. This means that only
about 1.5 Sv and 0.02 PW of the 30°S ow circulate off the western boundary.
Our study has shown that numerical models are a useful tool to assess the actual changes in heat and salt
content of water particles as they experience both alongisopycnal and diapycnal transformations. In parti-
cular, our results provide a good foundation for developing a future observational strategy of crossgyre
exchange. Past observations aimed at determining AMOC properties have focused mainly on the boundary
transports by means of zonal arrays. However, we have shown that the zonal component of the AMOC plays
a major role in the subtropical and tropical South Atlantic. Therefore, in order to best quantify the AMOC
and to understand its interaction with the tropical and subtropical gyres, it would be convenient to set up
meridional arrays that can monitor the zonal ows across the South Atlantic.
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CABRÉ ET AL. 4836
Acknowledgments
A. Cabré is grateful to the Beatriu de
Pinos fellowship and the program
Marie Curie Actions COFUND of the
7th Framework Program for Research
and Technological Development of the
European Union. I. VallèsCasanova
acknowledges an FPI contract (BES
2015071314) from the Spanish
government. We also acknowledge
funding from the Spanish government
through projects VADERETRO
(reference CTM201456987P) and
SAGA (reference RTI2018100844B
C33). GLORYS2V4 monthlyaveraged
data were obtained upon request from
https://www.mercatorocean.fr/en/
solutionsexpertise/howtoaccessthe
mercatoroceanservices/letsdene
yourneeds/. The authors are indebted
to the Mercator Ocean Service for
providing these reanalysis data.
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Journal of Geophysical Research: Oceans
CABRÉ ET AL. 4837
... Less is known about mixing in the ocean interior, away from rough topography and strong coastal winds, in particular in the central South Atlantic thermocline due to a historical lack of observations. Via a subtropical gyre, the South Atlantic transports surface water equatorward to compensate the southward flow of the North Atlantic Deep Water (Garzoli and Matano, 2011;Cabré et al., 2019) (Figure 1 inset). Previous research in the South Atlantic has mostly focused on low-frequency variability of its large-scale circulation (Stramma and England, 1999;Dong et al., 2015), or mesoscale variability near boundaries like the Brazil-Falkland confluence (Garzoli, 1993;Valla et al., 2018). ...
... continuous east-west data transect (Figure 1). The transect sits at the center of the South Atlantic subtropical gyre and provides an opportunity to investigate the change of mesoscale mixing processes along a significant distance in a region that contains the returning limb of the Atlantic meridional overturning circulation (Cabré et al., 2019). ...
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We investigate the spatial distribution of diapycnal mixing and its drivers in the central South Atlantic thermocline between the Rio-Grande Rise to the Mid-Atlantic Ridge. Diapycnal mixing in the ocean interior influences the slowly evolving meridional circulation, yet there are few observations of its variability with space and time or its drivers. To overcome this gap, seismic reflection data are spectrally analyzed to produce a 1,600 km long full-thermocline vertical section of diapycnal diffusivity, that has a vertical and horizontal resolution of O (10) m and spans a period of 4 weeks. We compare seismic-derived diffusivities with CTD-derived diffusivities and direct observations from 1996, 2003, and 2011. In the mean and on decadal scales, we find that thermocline diffusivities have changed little in this region, retaining a background value of 1 × 10 –5 m ² s –1 . Imprinted upon the background rates, mixing is heterogeneous at mesoscales. Enhanced mixing, exceeding 10 × 10 –5 m ² s –1 and spreading between 200 and 700 m depth, is found above the Mid-Atlantic Ridge suggesting the ridge enhances diffusivity by at least one order of magnitude across the entire water column. Rapid decay of diffusivities within 30 km of the ridge implies local dissipation of tidal energy. Above smooth topography, patches of enhanced mixing are possibly caused by a recent storm that injects near-inertial energy into the water column and elevates mixing from 3 × 10 –5 m ² s –1 to 50 × 10 –5 m ² s –1 down to depths of more than 600 m. The propagation speed of near-inertial energy varies substantially from 17 to 27 m/day. Faster speed, and therefore greater penetration depths of 800 m, are probably facilitated by an eddy. Together, these data extend the observational record of central South Atlantic thermocline mixing and provide insights into drivers of mesoscale variability.
... Moreover, subtropical water masses within the Southern Atlantic Ocean Circulation (AMOC) at latitude ~30 • feed the NBUC core (~21.4 Sv) (Cabré et al., 2019). Our study region comprises the latitude band between 8 • S and 7 • S were the NBUC have intensifies, with its mean velocity reaching 1 m. ...
... The observed low vertical temperature and salinity gradient in the first 30 m or more within the SVs can be attributed to the mixing of water masses between TW and of the SACW (Aguiar et al., 2018;Araujo Limongi et al., 2011). This is verified by the TS diagram analysis, which shows water masses signature limits of CW, TW and SACW (Cabré et al., 2019;de Domingues et al., 2017;Schettini et al., 2017) (Fig. 9). While the late spring profiles show the largest vertical gradients of temperature and salinity, the shallow and weaker mixed layer allows the isopycnals to reach shallower depths during the late fall. ...
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... Recent observations depict the EUC as a crucial component of the AMOC return limb because it partly contributes to the zonally averaged upwelling within the upper layers (Tuchen et al., 2022). Previous studies have indeed demonstrated that most water that ventilates the equatorial thermocline through the NBC retroflection comes from the South Atlantic Ocean subduction region, where tropical and subtropical waters converge (Cabré et al., 2019). The different contributions from these southern sources, as well as waters originating in the North Atlantic, determine the composition of the surface and thermocline equatorial waters. ...
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The equatorial retroflection of the North Brazil Current (NBC) into the Equatorial Undercurrent (EUC) and its posterior tropical recirculation is a major regulator for the returning limb of the Atlantic Meridional Overturning Circulation. Indeed, most surface and thermocline NBC waters retroflect at the equator all the way into the central and eastern Atlantic Ocean, before they recirculate back through the tropics to the western boundary. Here, we use cruise data in the western equatorial Atlantic during April 2010 and reanalysis time series for the equatorial and tropical waters in both hemispheres in order to explore the recirculation pathways and transport variability. During the 1998–2016 period, the annual‐mean EUC transports 15.1 ± 1.3 Sv at 32°W, with 2.8 ± 0.4 Sv from the North Atlantic and 11.4 ± 1.3 Sv from the South Atlantic. At 32°W most of the total EUC transport comes from the western boundary retroflection south of 3°N (7.2 ± 0.9 Sv), a substantial fraction retroflects north of 3°N (5.6 ± 0.4 Sv), and the remaining flow (2.3 Sv) joins through the interior basin. The South Atlantic subtropical waters feed the EUC at all thermocline depths while the North Atlantic and South Atlantic tropical waters do so at the surface and upper‐thermocline levels. The EUC transport at 32°W has a pronounced seasonality, with spring and fall maxima and a range of 8.8 Sv. The 18 yr of reanalysis data shows a weak yet significant correlation with an Atlantic Niño index, and also suggests an enhanced contribution from the South Atlantic tropical waters during 2008–2016 as compared with 1997–2007.
... The STUW on the northern side of 21°S is generally located within the permanent thermocline, and STUW on the southern side of 21°S is generally located within the seasonal thermocline, persisting from October to the next June. The 21°S zonal line in the South Atlantic demarcates where the western subtropical and tropical gyre bifurcate (Cabré et al. 2019). ...
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The South Atlantic subtropical underwater (STUW) is a high-salinity water mass formed by subduction within the subtropical gyre. It is a major component of the subtropical cell and affects stratification in the downstream direction due to its high salinity characteristics. Understanding the interannual variability in STUW subduction is essential for quantifying the impact of subtropical variability on the tropical Atlantic. Using the output from the ocean state estimate of the Consortium for Estimating the Circulation and Climate of the Ocean (ECCO), this study investigates the interannual variability in STUW subduction from 1992 to 2016. We find that heat fluxes, wind stress, and wind stress curl cause interannual variability in the subduction rate. Heat fluxes over the subduction area modulate the sea surface buoyancy and regulate the mixed layer depth (MLD) during its deepening and shoaling phases. Additionally, the wind stress curl and zonal wind stress can modulate the size of the subduction area by regulating the probability of particles entrained into the mixed layer within 1 year of tracing. This analysis evaluates the influence of subtropical wind patterns on the South Atlantic subsurface high-salinity water mass, highlighting the impact of heat and wind on the interannual changes in the oceanic component of the hydrological cycle.
... The southern salinity maximum has a lower salinity/temperature, and its existence depends on the seasons; the northern salinity maximum is saltier, warmer, and more persistent throughout the year than the southern salinity maximum. Cabré et al. (2019) reported that the bifurcation latitude between the western subtropical and western tropical South Atlantic gyres is 21°S. Thus, the different characteristics between the southern and northern salinity maxima probably result from the different current systems in the western tropical and subtropical South Atlantic. ...
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The South Atlantic Subtropical Underwater (STUW) was identified from Argo floats between 2001 and 2019. Expressed as a vertical salinity maximum at the subsurface, the STUW spreads over the western subtropical and tropical South Atlantic. The mean salinity, conservative temperature, and potential density of the STUW are 36.66 ± 0.43 g/kg, 22.62 ± 2.27°C, and 25.14 ± 0.49 kg/m3, respectively. Based on the criteria derived from the STUW properties described above and the results from the Estimating the Circulation and Climate of the Ocean (ECCO) model, the production and fate of the STUW are calculated. The annual mean subduction rate is 3.26 Sv, and effective subduction occurs from September to November. Approximately 73% (2.38 Sv) of the subducted STUW is transported toward the tropical Atlantic, and 23% (0.75 Sv) of the subducted STUW is dissipated near the STUW formation region; 99% of the subducted STUW particles are dissipated within 2 years. Water particles can enter/exit the STUW along the advection route of the STUW. A total of 3.92 Sv of the STUW volume dissipates within the tropical region between 1.5°S and 1°N, implying that STUW contributes to 98% of the total transport of the subtropical cell. This analysis tracks the South Atlantic STUW from formation to dissipation and highlights the importance of the STUW in governing the subtropical cell.
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The North Brazil Undercurrent (NBUC) is a narrow (<1°) northward western boundary current in the tropical South Atlantic Ocean. It carries a large volume of water (>16 Sv) and plays an important role in the Atlantic Meridional Overturning Circulation and the South Atlantic Subtropical Cell. Strong salinity and temperature fronts occur over the NBUC region. The role of temperature and salinity gradients on the genesis of NBUC variability has never been explored. This study uses three high-resolution (< 0.1°) and one low-resolution (=0.25°) model outputs to explore the linear trend of NBUC transport and its variability on annual and interannual time scales. We find that the linear trend and interannual variability of the geostrophic NBUC transport show large discrepancies among the datasets. Thus, the linear trend and variability of the geostrophic NBUC are associated with model configuration. We also find that the relative contributions of salinity and temperature gradients to the geostrophic shear of the NBUC are not model dependent. Salinity-based and temperature-based geostrophic NBUC transports tend to be opposite-signed on all time scales. Thus, variability in the zonal temperature and salinity gradients partially cancel each other out in the NBUC. Despite the limited salinity and temperature profiles, the model results are consistent with the in-situ observations on the annual cycle and interannual time scales. This study shows the relationship of salinity-based and temperature-based geostrophic NBUC variations in the annual and interannual variability and trend among different models and leads to a better understanding of the coupling between temperature/salinity and velocities over the western tropical South Atlantic ocean.
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The Brazil-Malvinas Confluence arises from the frontal encountering of the subtropical Brazil Current and subantarctic Malvinas Current. It displays a complex regional circulation that is accompanied by mesoscale features and thermohaline intrusions. Here we combine altimetry and cruise data to describe the circulation pattern in the upper 2,000 m at two spatial scales encircling the frontal system. The major regional features appear south of the confluence latitude at 39–40°S: (a) a relatively weak Malvinas Current near 41°S, 56°W (28.3 ± 1.4 Sv), followed by its cyclonic retroflection; (b) an intense subtropical anticyclone (59.3 ± 10.7 Sv) that replaces the Brazil Current overshoot; and (c) a very intense subantarctic inflow (78.9 ± 13.7 Sv) near 53°W that is maintained through both an upstream (near 42°S) earlier diversion of the Malvinas Current and the cyclonic recirculation of the flow exiting east along the confluence. North of the confluence, the Brazil Current provides a net input of 30.8 ± 12.0 Sv (29.1 ± 8.3 Sv along the slope). The southern inflow splits nearly equal between barotropic and baroclinic contributions while the entire northern flow is essentially baroclinic. These northern and southern inputs add to an eastward along-front transport of 109.7 ± 15.1 Sv, with significant contribution of highly oxygenated, relatively fresh Subantarctic Mode and Antarctic Intermediate Waters (58.7 ± 5.6 Sv). The regional circulation experiences substantial temporal variability, with southern waters flowing into the Brazil-Malvinas Confluence through along-slope and interior pathways and partly recirculating within the subtropical South Atlantic gyre.
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The meridional heat transport (MHT) of the South Atlantic plays a key role in the global heat budget: it is the only equatorward basin-scale ocean heat transport and it sets the northward direction of the global cross-equatorial transport. Its strength and variability, however, are not well known. The South Atlantic transports are evaluated for four state-of-the-art global ocean reanalyses (ORAs) and two free-running models (FRMs) in the period 1997–2010. All products employ the Nucleus for European Modelling of the Oceans (NEMO) model, and the ORAs share very similar configurations. Very few previous works have looked at ocean circulation patterns in reanalysis products, but here we show that the ORA basin interior transports are consistently improved by the assimilated in situ and satellite observations relative to the FRMs, especially in the Argo period. The ORAs also exhibit systematically higher meridional transports than the FRMs, which is in closer agreement with observational estimates at 35 and 11° S. However, the data assimilation impact on the meridional transports still greatly varies among the ORAs, leading to differences up to ∼ 8 Sv and 0.4 PW in the South Atlantic Meridional Overturning Circulation and the MHTs, respectively. We narrow this down to large inter-product discrepancies in the western boundary currents (WBCs) at both upper and deep levels explaining up to ∼ 85 % of the inter-product differences in MHT. We show that meridional velocity differences, rather than temperature differences, in the WBCs drive ∼ 83 % of this MHT spread. These findings show that the present ocean observation network and data assimilation schemes can be used to consistently constrain the South Atlantic interior circulation but not the overturning component, which is dominated by the narrow western boundary currents. This will likely limit the effectiveness of ORA products for climate or decadal prediction studies.
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The South Atlantic meridional transports are evaluated for four state-of-the-art global Ocean Reanalyses (ORAs) and two Free-Running Models (FRMs) in the period 1997–2010. All products employ the Nucleus for European Modelling of the Oceans model, and the ORAs share very similar configurations. The ORA basin interior transports are consistently modified relative to the FRMs, especially in the Argo period, with an improved representation of the south equatorial currents. The ORAs also exhibit systematically higher meridional transports than the FRMs, in closer agreement with large-scale observational estimates at 35° S and western boundary measurements at 11° S. However, the transport impacts by data assimilation still greatly vary between the ORAs, leading to differences up to ~ 8 Sv and 0.4 PW in the South Atlantic Meridional Overturning Circulation and the Meridional Heat Transports (MHTs), respectively. Large inter-product discrepancies arise in the ORA western boundary currents at both upper and deep levels explaining up to ~ 85 % of the inter-product differences in their total MHTs, and meridional velocity differences, rather than temperatures differences, drive ~ 83 % of this spread. Further analysis shows that only very confined temperature differences right against the western boundary geostrophically explain the large boundary current velocity differences. These findings suggest that the current data assimilation schemes, even with Argo data, can consistently constrain the basin interior circulation in the ORAs, but not the overturning transport component dominated by the narrow western boundary currents as in the South Atlantic.
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The Copernicus Marine Environment Monitoring Service (CMEMS) Ocean State Report (OSR) provides an annual report of the state of the global ocean and European regional seas for policy and decision-makers with the additional aim of increasing general public awareness about the status of, and changes in, the marine environment. The CMEMS OSR draws on expert analysis and provides a 3-D view (through reanalysis systems), a view from above (through remote-sensing data) and a direct view of the interior (through in situ measurements) of the global ocean and the European regional seas. The report is based on the unique CMEMS monitoring capabilities of the blue (hydrography, currents), white (sea ice) and green (e.g. Chlorophyll) marine environment. This first issue of the CMEMS OSR provides guidance on Essential Variables, large-scale changes and specific events related to the physical ocean state over the period 1993-2015. Principal findings of this first CMEMS OSR show a significant increase in global and regional sea levels, thermosteric expansion, ocean heat content, sea surface temperature and Antarctic sea ice extent and conversely a decrease in Arctic sea ice extent during the 1993-2015 period. During the year 2015 exceptionally strong large-scale changes were monitored such as, for example, a strong El Nino Southern Oscillation, a high frequency of extreme storms and sea level events in specific regions in addition to areas of high sea level and harmful algae blooms. At the same time, some areas in the Arctic Ocean experienced exceptionally low sea ice extent and temperatures below average were observed in the North Atlantic Ocean.
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The Copernicus Marine Environment Monitoring Service (CMEMS) Ocean State Report (OSR) provides an annual report of the state of the global ocean and European regional seas for policy and decision-makers with the additional aim of increasing general public awareness about the status of, and changes in, the marine environment. The CMEMS OSR draws on expert analysis and provides a 3-D view (through reanalysis systems), a view from above (through remote-sensing data) and a direct view of the interior (through in situ measurements) of the global ocean and the European regional seas. The report is based on the unique CMEMS monitoring capabilities of the blue (hydrography, currents), white (sea ice) and green (e.g. Chlorophyll) marine environment. This first issue of the CMEMS OSR provides guidance on Essential Variables, large-scale changes and specific events related to the physical ocean state over the period 1993–2015. Principal findings of this first CMEMS OSR show a significant increase in global and regional sea levels, thermosteric expansion, ocean heat content, sea surface temperature and Antarctic sea ice extent and conversely a decrease in Arctic sea ice extent during the 1993–2015 period. During the year 2015 exceptionally strong large-scale changes were monitored such as, for example, a strong El Niño Southern Oscillation, a high frequency of extreme storms and sea level events in specific regions in addition to areas of high sea level and harmful algae blooms. At the same time, some areas in the Arctic Ocean experienced exceptionally low sea ice extent and temperatures below average were observed in the North Atlantic Ocean.
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