OSOcean ScienceOSOcean Sci.1812-0792Copernicus PublicationsGöttingen, Germany10.5194/os-14-273-2018A study of the variability in the Benguela Current volume transportBenguela current variabilityMajumderSudipsudip.majumder@noaa.govSchmidClaudiahttps://orcid.org/0000-0003-2132-4736Cooperative Institute for Marine and Atmospheric Studies, University of Miami, Miami, Florida, USAAtlantic Oceanographic and Meteorological Laboratory, NOAA, Miami, Florida, USASudip Majumder (sudip.majumder@noaa.gov)5April201814227328314July201716August201718December201718December2017This work is licensed under the Creative Commons Attribution 4.0 International License. To view a copy of this licence, visit https://creativecommons.org/licenses/by/4.0/This article is available from https://os.copernicus.org/articles/14/273/2018/os-14-273-2018.htmlThe full text article is available as a PDF file from https://os.copernicus.org/articles/14/273/2018/os-14-273-2018.pdf
The Benguela
Current forms the eastern limb of the subtropical gyre in the South Atlantic
and transports a blend of relatively fresh and cool Atlantic water and
relatively warm and salty Indian Ocean water northwestward. Therefore, it
plays an important role not only for the local freshwater and heat budgets
but for the overall meridional heat and freshwater
transport in the
South Atlantic. Historically, the Benguela Current region is relatively data
sparse, especially with respect to long-term velocity observations. A new
three-dimensional data set of the horizontal velocity in the upper 2000 m
that covers the years 1993 to 2015 is used to analyze the variability
in the Benguela Current. This data
set was derived using observations from Argo floats, satellite sea surface
height, and wind fields. Since Argo floats do not cover regions shallower
than 1000 m, the data set has gaps inshore. The main features of the
horizontal circulation observed in this data set are in good agreement with
those from earlier studies based on limited observations. Therefore, it can
be used for a more detailed study of the flow pattern as well as the
variability in the circulation in this region. It is found that the mean
meridional transport in the upper 800 m between the continental shelf of
Africa and 3∘ E, decreases from 23 ± 3 Sv
(1 Sv = 106 m3 s-1) at 31∘ S to 11 ± 3 Sv
at 28∘ S.
In terms of variability, the 23-year long time series at 30 and 35∘ S
reveals phases with large energy densities at periods of 3 to 7 months, which
can be attributed to the occurrence of Agulhas rings in this region. The
prevalence of Agulhas rings is also behind the fact that the energy density
at 35∘ S at the annual period is smaller than at 30∘ S because the former latitude is closer to Agulhas Retroflection and therefore
more likely to be impacted by the Agulhas rings. In agreement with this, the
energy density associated with mesoscale variability at 30∘ S is
weaker than at 35∘ S. With respect to the forcing, the Sverdrup
balance and the observed transport at 30∘ S exhibit a strong
correlation of 0.7. No significant correlation between these parameters is
found at 35∘ S.
Introduction
The broad northwestward flow following the west coast of southern Africa from
Cape Agulhas (35∘ S, Fig. ) to Cape Fria (18.4∘ S,
Fig. ) is the Benguela Current which
constitutes the eastern limb of the south Atlantic subtropical gyre. The
Benguela Current transports water masses carried into the Cape Basin by the
South Atlantic Current, the Antarctic Circumpolar Current, and the Agulhas
Current . The contribution from the Agulhas
Current consists of warm, salty Indian Ocean water that enters the Atlantic
in the Agulhas Retroflection region via the shedding of rings and the Agulhas
leakage . Agulhas rings are large (with
diameter 300–400 km) and extremely energetic , in
fact, the ring shedding region is characterized by a significantly higher
level of eddy kinetic energy than observed in the other parts of the world
ocean in the Southern Hemisphere . On average, the
Agulhas rings transfer about 10–15 Sv (1 Sv = 106 m3 s-1)
into the Atlantic in the upper 1000 m . Because of
their water mass characteristics they are important for the heat and
freshwater budget in the South Atlantic.
A map of the Benguela Current region showing local topography and
boundaries.
From its origin in the Cape Cauldron the Benguela
Current flows northwestward along the west coastline of Africa and feeds into
the southern South Equatorial Current , which
flows in a westerly direction between 8 and 22∘ S
. At intermediate depth, the flow towards South
America is more zonal, and once it reaches the boundary in the Santos
Bifurcation , about two-thirds of the intermediate
water contribute to the Brazil Current and one-third to the northward flowing
Intermediate Western Boundary Current
.
The Benguela Current plays a key role for the Atlantic Meridional Overturning
Circulation (AMOC) by transporting heat and salt from the Indian Ocean
northwards. The AMOC is important for the global energy budget and is
believed to be linked with multiple regional and global climate phenomena as
well as extreme weather events in North America and around the globe
.
Recent model-based studies suggested that heat transfer to the North Atlantic
by the Benguela Current increased due to an increase in the Indian Ocean
inflow through the Agulhas leakage in 1965 to about 1990
. This increase was
attributed to a strengthening of the Agulhas Current
because of a poleward shift of Southern
Hemisphere westerlies as reported in many studies based on climate models
. An
increase in the Indian Ocean inflow could result in increased heat and salt
transports into the South Atlantic causing salinification there, which could
gradually extend into the North Atlantic . From
about 1990 on, no significant change in the Agulhas leakage was detected by
(their Fig. 4). In agreement with this, a recent
observational study by found that the Agulhas
Current has not strengthened during the period 1993 to 2015. Instead, it has
been broadening. observed that intensifying winds
strengthen the eddy kinetic energy of the Agulhas Current but do not
increase its mean transport.
As part of an early effort (Benguela Sources and Transport Experiment, BEST)
based on direct current measurement with a moored current meter array and
inverted echo sounders, derived a northward
transport of about 13 Sv (1 Sv = 106 m3 s-1) across
30∘ S, between the Walvis Ridge in the west and the African coast in
the east (Fig. ). They observed that 50 % of the meridional
transport in the upper 1000 m at this latitude consists of waters from the
South Atlantic and 25 % from the Indian Ocean, and the remaining 25 %
is a mix of water from the Indian and the tropical Atlantic Ocean. In
addition, reported that the Benguela Current at
30∘ S between the African coast and 8∘ E consists of a
relatively steady northwestward flow. Further west, they found that the flow
becomes transient between 8∘ E and the Walvis Ridge at 3∘ E
due to the influence of Agulhas rings. Complementing the BEST program, a
Benguela Current float experiment was conducted during 1997–1999 in an
attempt to directly measure the northward flow of intermediate water using
Lagrangian RAFOS floats in conjunction with moored sound sources and CTD
(conductivity–temperature–depth), O2, and LADCP (lowered acoustic Doppler
current) profiles . These observations
suggested that the Benguela Current extension (also called southern South
Equatorial Current) in the upper 750 m is located between 35 and
20∘ S. They reported that the westward transport of intermediate
water in this current was about 15 Sv between 22 and 35∘ S.
Even though the Benguela Current region features interesting physical
processes, constitutes the eastern limb of the Meridional Overturning
Circulation in the South Atlantic, and has an impact on an important
upwelling region near the African Coast with high biological productivity,
no long-term measurement of this current's flow, transport, and dynamics are
available.
In this study, using extensive observations from Argo and altimetry, we
provide a 23-year long time series as well as the means of the transport of
the Benguela Current. This data set provides horizontal velocities from
observations in the upper 2000 m at a higher resolution in space and time
than was available in earlier studies. Based on this data set, the
variability in the Benguela Current
from seasonal to interannual scales is analyzed. The results from this study
will improve our knowledge of the flow patterns and will be helpful for model
validation in this region. The primary goal of this paper is to improve the
understanding of the variability in the Benguela Current transport between 25 and 35∘ S.
Data and methods
The methodology and the details of the product (called Argo & SSH) are
described in . Improvements were implemented and the time
series was extended in preparation for a study of the Meridional Overturning
Circulation in the South Atlantic. Details about this can be found in
. A short summary of the methodology for
deriving the velocity fields follows:
temperature and salinity profiles from Argo floats measured in the years
2000 to 2015 are used to calculate dynamic height
to improve the monthly
spatial data coverage, fits between sea surface height (SSH) from AVISO
and the dynamic heights are used to derive synthetic
dynamic height profiles on a 0.5∘× 0.5∘ grid
these synthetic dynamic height fields are used to calculate geostrophic
velocities relative to a level of no motion
absolute geostrophic velocity
fields are obtained by adjusting the geostrophic velocity by using velocity
fields obtained from the trajectories of subsurface floats
and, finally, the
Ekman component, estimated from NCEP (National Centers for Environmental Prediction) reanalysis 2 winds
, is added to the derived velocity fields.
The resulting time series is an extension of the one used by
and covers the years 1993–2015, of which
the first seven fall in the pre-Argo period. This extension of the time
series is based on the assumption that the relationship between SSH and
dynamic height does not change much over time
.
The derived gridded monthly velocity fields are used to estimate volume
transports in the Benguela Current region. Wind stress curl and the Sverdrup
stream functions are estimated using European Reanalysis Interim wind fields
(ERA Interim) . This wind field has a 0.75∘
resolution and is available for the years 1979 to 2016.
ResultsStructure of the Benguela Current
The northwestward flow along the southwestern coast of Africa is called the
Benguela Current. Maps of the adjusted steric height by
show this current east of the Walvis Ridge. Following ,
at 30∘ S derived the transport of this
current for the region between the eastern edge of the Walvis Ridge (at about
3∘ E) and western African coast. For comparability with their
transport estimates, the same region is used herein. The mean flow field in
Fig. indicates that the northwestward flow of this current is east
of 3∘ E between 35 and 25∘ S. Therefore, for transport
estimates, the Benguela current is integrated zonally between 3∘ E
and the African coast within this latitude range.
Climatologies of the flow at 15 m (a) and
transports (b) in the upper 800 m from Argo & SSH. Red and black
arrows indicate flow to the north and the south, respectively, and the
shading represents magnitude. A transport budget is derived for two regions:
ABCD (extending from 3∘ E to the 1000 m isobath near the eastern
boundary) and A′BC′D (extending from 8∘ E to the 1000 m isobath
near the eastern boundary).
Climatologies of currents at 15 m and transports in the upper 800 m in
Fig. clearly visualize the northwestward flow of the Benguela Current
and shows that it is fed by water from the southern Atlantic as well as the
Indian Ocean. The former mostly comes from the South Atlantic Current while
the latter enters the South Atlantic via the Agulhas Retroflection. Between
35 and 30∘ S the meridional velocity within the Benguela Current
weakens as the current turns more westward (Fig. a).
Two main regimes with different flow patterns are identified. The eastern
regime near the African coast is characterized by relatively strong northward
flow, while the western regime has relatively weak alternating flow. For
example, at 30∘ S the flow at 15 m west of 8∘ E is mostly
zonal and meanders slightly. Farther south, the flow in the western regime
does not reveal any preferred direction. In addition to these regimes, south
of 32∘ S reveals several distinct recirculation features in the mean
field. One of them is in the white box with the center between the Walvis
Ridge in the west and the Vema Seamount (at 33∘ S, 6∘ E;
Fig. ) in the east.
The eastern regime with strong northward flow at 30∘ S is broader
than at 35∘ S, extending from the African coast to about
8∘ E (Fig. ). At 35∘ S the mean 15 m velocity
field indicates that the northward flow east of 12∘ E may occur in
two branches. The eastern one, between about 15∘ E and the African
coast, is dominated by meridional flow. In contrast to this, the western one,
between 12 and 15∘ E, has zonal velocity components that are as
large as the meridional velocity. In addition, the magnitude of the velocity
and transport are about twice as large as farther east. A third contributory
branch can also be seen at about 7∘ E. This branch, identified as
part of a meander of the South Atlantic Current, has a relatively weak
contribution in the mean transport at 35∘ S compared to the two
prominent branches in the east.
Climatological mean of the meridional geostrophic velocity across
30∘ S (a) and 35∘ S (b). Black lines are
the contours of zero velocity and the black straight line marks the depth of
800 m.
The vertical structure of the climatological meridional velocity in the upper
1500 m in Fig. suggests that the northward flow east of
12∘ E dominates throughout most of the upper 1500 m, especially at
35∘ S. The exception of this is the southward Benguela undercurrent
near the eastern end of the section at 30∘ S from about 600 m
downward. The third contributory branch is seen between 5 and 7∘ E,
with relatively small northward velocities.
With respect to changes in the structure of the Benguela Current on its way
to the north, it can be seen that its strength decreases significantly in the
eastern regime between 35 and 30∘ S. In addition, one can see the
two branches of the northward flow at 35∘ S, distinguished by the
factor of 2 difference in the strength of the meridional velocity that are
separated by the less deep-reaching flow near 15∘ E. West of
12∘ E at 35∘ S, the velocity is mostly much smaller and of
alternating sign when compared with the eastern regime (east of
12∘ E). Also, the Benguela Current completed its westward turn at
30∘ S, which results in relatively weak meridional velocity and a
meander-like pattern of the flow (Fig. ).
Before proceeding to zonal integrals of the transport, it has to be noted that
Argo & SSH does not contain velocities in boxes that are shallower than
1000 m at the center (as can be seen in Fig. ). One way to assess
how much transport occurs in these shallow regions on average is to use the
easternmost near-surface velocity at each latitude to derive an approximate
transport in the regions that are missing. The average northward velocities
of 5 cm s-1 for 30∘ S and 10 cm s-1 for 35∘ S
yield 2.0 and 1.8 Sv, respectively. These transports are about 10 % of
the mean meridional transports at these two latitudes.
Latitude dependence of climatological mean of the meridional
transport in the Benguela Current (between 3∘ E and the eastern
boundary) in the upper 800 m (a) and 1000 m (b). Gray
dots are based on velocity transects derived by
for the purpose of estimating the Meridional Overturning Circulation (MOC)
transports in the South Atlantic. Other gray symbols are based on transport
estimates from three studies: ,
, and . Gray
triangles (circles) indicate that the western integration limit was the
Greenwich Meridian (the western edge of the Benguela Current). All other gray
symbols represent estimates based on a western integration limit at
3∘ E. Gray error bars are shown where an estimate was derived from
multiple transects or a time series. The vertical integration limits for
(shown in b) range from 940 to
1180 m, with the largest values used at 32∘ S and the smaller ones
in 23 to 24∘ S, and they derived each transport for two different reference levels as well as two
different western integration limits. All error bars represent standard
deviations.
For comparison with the estimates
from previous studies e.g.,, this study calculates transport in the meridional
direction; however, transport calculated along a section perpendicular to the
flow is the same. The mean meridional transport of the Benguela Current in
the upper 800 m ranges from 9 ± 3 to 23 ± 3 Sv (Fig. ;
whenever possible transports are represented as time-mean ± standard
deviation, calculated over the entire time series). In the upper 1000 m, the
range is 10 ± 3 to 26 ± 3 Sv. The transports in the upper
1000 m are between 5 and 25 % (1 and 5 %) higher south (north) of
29∘ S than those integrated over the upper 800 m. The agreement
with estimates from previous studies is mostly good if one keeps in mind that
most of them are for synoptic sections and use different vertical integration
limits. In addition, their zonal integration limit in the west varies from
the Greenwich Meridian to 3∘ E. For example
and both use the
Greenwich Meridian as the western edge of the Benguela Current at
32∘ S, whereas integrated to
3∘ E to obtain transport at 30∘ S.
Between 35 and 31∘ S, the transports are relatively stable when
keeping the standard deviations in mind. From 31 to 28∘ S the
transports in the upper 800 m (1000 m) decrease from 23 ± 3 to
11 ± 3 Sv (26 ± 3 to 12 ± 3 Sv). This can be attributed
to the westward turn of the flow as the Benguela Current feeds into the
southern South Equatorial Current. North of 28∘ S the transport is,
once again relatively stable.
Time series of transports in the upper 800 m.
(a) Meridional transport of the Benguela Current at 30∘ S
(black – across line AB; gray – across line A′B; see Fig. ).
(b) Meridional transport of the Benguela Current at 35∘ S
(across line CD in black; across line C′D in gray). (c) Zonal
transport at 3∘ E (across line AC; across A′C′).
(d) Cross-shelf transport at the eastern boundary near the African
coast across 1000 m isobath (across line BD). (e) Transport
imbalance for ABCD (black) and A′BC′D (gray).
Transport budget and its uncertainties
To understand the circulation pattern and its variability in the Cape Basin
region, volume transport budgets in the upper 800 m are derived for the
boxes ABCD and A′BC′D (Fig. ). For the box ABCD, time series of
the volume transport (Fig. ) yield mean northward transports of
18 ± 3 and 19 ± 3 Sv across lines AB (at 30∘ S) and CD
(at 35∘ S). More variable westward transports of 8 ± 4 and
∼ 0.3 ± 3 Sv cross lines AC (at 3∘ E) and BD (parallel
to the shelf break). The latter is derived as the component of the transport
perpendicular to the direction of the 1000 m isobath near the African coast.
It will be called the cross-shelf transport hereinafter. Combining
transports across the sides of the box ABCD reveals an imbalance of 6 Sv in
the upper 800 m.
The previously discussed recirculation feature at the southwest corner of box
ABCD in Fig. and a meandering flow with a northward component at the
western end of line AB at 30∘ S can impact the budget significantly.
In addition to that, the topography near the Walvis Ridge can have an impact
on the flow near the lower integration limit (800 m).
To understand the causes of this imbalance, the mean transport budget is
estimated in the two boxes mentioned above and the integration depth in the
larger box is varied. It is found that the mean transport imbalance in the
upper 400 m for the box ABCD is about 2 Sv (4 Sv smaller than in the upper
800 m); the individual transports are about 12 Sv (northward) across lines
AB and CD, about 4 Sv across AC (westward), and about 2 Sv across BD
(westward). In the smaller box (A′BC′D), the mean transport in the upper
800 m is balanced, with individual transports of about 14 Sv (northward)
across line A′B, 20 Sv (northward) across line C′D, about 7 Sv
(westward) across A′C′, and about 0.3 Sv (westward) across BD. These
estimates indicate that about 4 Sv of the imbalance in the large box (ABCD)
disappear when the vertical integration limit is reduced from 800 to 400 m,
which indicates that the Walvis Ridge gives rise to a topographic effect that
reduces the chance of closing the budget. Consistent with this, retaining the
full depth and shifting the western boundary eastward (away from the Walvis
Ridge) to 8∘ E (small box A′BC′D) results in a closed budget
within the error bars.
Time variant transports including the budgets for ABCD and A′BC′D are
presented in Fig. . Even though, on an average, transport budget for
the box A′BC′D seems to close, anomalies exist. Some of these exceed
5 Sv. The time series indicates that the most of the imbalances
(Fig. e, gray) occur at times with large anomalous cross-shelf
transports (Fig. d). In the following the characteristics of the three
strongest events – occurring in July 1993, April 2005, and August 2007 –
are explored.
Many studies
e.g.,
showed that Ekman-induced upwelling plays an important role in the coastal
circulation pattern east of the region dominated by the Benguela Current.
Such upwelling typically occurs hand in hand with offshore transports from
the shelf into the open ocean. identified
several upwelling cells in the coastal Benguela regime. The important ones
are the Cape Columbine and the Cape Peninsula cells between 30 and
35∘ S. Upwelling in these cells can modulate cross-shelf transport
and consequently impact the transport budget. Based on the full time series,
it is found that the correlation between the cross-shelf transport and the
along-shelf wind stress from NCEP2 is low (0.14). The wind record is in poor
agreement during strong events like the three listed above. This indicates
that away from the coast at about 1000 m isobath, oceanic processes that are
independent of the wind have to give rise to these events. This is supported
by a Hovmöller diagram of the monthly sea surface anomalies
(Fig. ), which confirms the passage of eddies during each one of
these three events. These eddies, located very close to the shelf break at
35∘ S, therefore, give rise to relatively large cross-shelf
velocities (northeastward or southwestward) north of their center. Maps of sea
surface height anomalies (not shown) during these events also confirm the
strong influence of cyclones and anticyclones in the cross-shelf transport.
Other reasons of the anomalies can be the following:
the integration of the transport to the same depth limit for all sides of the
box
transport at shallow depths near the African coast that is not
represented in the velocity field
a vertical transport into the
box from deeper layers, and
a surface freshwater flux.
These factors are discussed in the following.
It is found that both at 30 and at 35∘ S, 27 kg m-3σ0 isopycnal lies at a depth of 800 m ± 50 m (not shown). This
indicates that the choice of 800 m as the depth limit for all three sides of
the box cannot give rise to a significant cross-isopycnal transport. With
respect to the impact of flow in shallow regions, as mentioned in the
previous section, Argo & SSH misses about 2 Sv near the coast both at 30
and 35∘ S. Since the missed transports at the lines AB and CD are
almost the same, they do not contribute to the imbalance.
Investigating the contribution due to a vertical transport through the bottom
of the box can be done by approximating the Ekman transport using an
upwelling velocity derived from the mean wind stress curl of
1.5 × 10-7 N m-3 within the box. This velocity is
0.2 × 10-3 cm s-1 which corresponds to a transport of
about 1.5 Sv. Because the Ekman depth is 65 m, the vertical transport at
800 m is likely to be smaller.
An estimate of the transport due to a surface freshwater flux is calculated
using the climatological “evaporation–precipitation” from European
Center for Medium range Weather Forecasting (ECMWF) reanalysis. ECMWF's
ERA-40
(http://www.ecmwf.int/s/ERA-40_Atlas/docs/section_B/parameter_emp.html)
has a climatological mean in the range of 2 to 4 mm day-1 in the box
that does not vary much from season to season. Based on a net surface
freshwater flux of 3 mm day-1 (about 90 mm month-1), the net
transport into the box is 0.02 Sv. This transport is much smaller than the
observed imbalance of 6 Sv. To achieve a gain of 1 Sv transport through the
ocean surface, freshwater input would have to be about 50 times larger than
the typical value in this region (4.5 mm month-1). This indicates that
the surface freshwater input cannot contribute significantly to the
imbalance.
Overall, these estimates lead to the conclusion that the processes (i–iv) discussed
herein may not contribute significantly to the uncertainties in the
transport budget.
Temporal variability in the Benguela Current
The characteristics of the variability in the transports shown in
Fig. are assessed with a wavelet-based spectral analysis
(Fig. ). The spectral energy of transports from the three sections
covers a broad range of periods, mainly within 3 months to 1 year. The
influence of the Agulhas rings is clearly visible, with significant energies
in 3 to 7 months' range in certain years (Fig. a, b).
Wavelet spectral density of meridional transports at
(a) 30∘ S and (b) 35∘ S, and
(c) zonal transport at 3∘ E. (d) Mean wavelet
power spectra at 30∘ S (blue), 35∘ S (red), and
3∘ E (green). The black contours are the 95 % confidence
interval. The values outside the cone of influence (blurred colors) indicate
where the edge errors dominate.
Hovmöller diagram of monthly sea surface height anomalies (in cm)
across 35∘ S (a, b) and 30∘ S (c, d).
Anomalies are stronger west of the black line located at 17.5∘ E for
35∘ S and at 14∘ E for 30∘ S.
Climatologies of (a) eddy kinetic energy in
(cm s-1)2 , (b) mean kinetic energy in (cm s-1)2,
and (c) the ratio of eddy and mean kinetic energy.
In terms of seasonality, a strong annual cycle is visible at 30∘ S
in 2006 to 2011 (Fig. a, d). In most of the other years, the spectral
density for the annual cycle still has a maximum of 3 to
4 Sv2 cycle-1, but it does not reach the level of significance. In
contrast to this, at 35∘ S the level of significance for an annual
cycle is only reached in 1996 to 1998 (Fig. b) and the energy in
other years is mostly lower than at 30∘ S (Fig. d).
The weakness of the annual cycle and the energy at mesoscale periods can be
attributed to the impact of the Agulhas rings. About 5 to 6 Agulhas rings
cross the Cape Basin region annually, and they typically translate into a
northwesterly direction. Overall, the energy at 35∘ S is slightly
higher than at 30∘ S within this frequency band. The differences
between these two latitudes are consistent with the fact that the Agulhas
rings have a larger impact at 35∘ S than at 30∘ S, as can
be seen in the Hovmöller diagrams (Fig. ). In addition to this, the
Hovmöller diagrams reveal why the energy at mesoscale periods (3–7
months), as derived from the time series of the transport in the Benguela
Current, is not always significant. Individual Agulhas rings only have an
impact on a small part of the longitude range used in the transport
computation. Therefore, their signal becomes relatively weak (Fig. ).
Also, it has to be noted that the monthly Argo & SSH data set cannot resolve
periods smaller than 2 months; therefore, transport time series may miss some
of the high-frequency variability due to these rings. Periods with
significant spectral energy at mesoscale frequencies can be attributed to the
presence of more than one Agulhas ring at a given latitude. An example for
this can be seen in 1995 to 1996 at 35∘ S (Fig. 7a).
For the zonal transport across line AC, Fig. c reveals a dominance of
frequencies at periods of 3 to 7 months that is more persistent than at
30∘ S and at 35∘ S. This is because the signal from the
Agulhas rings can be captured more easily in this meridional section (AC)
than in the zonal sections at AB and AC. The meridional section AC is about
3 times shorter than the zonal ones, and its length is close to the
typical diameter of the Agulhas rings. It is noted that the zonal transport
does not exhibit a statistically significant annual peak in any year. A major
reason for this is, again, the prevalence of Agulhas rings. Figure c,
which shows the ratio of eddy kinetic energy (Fig. a) and mean
kinetic energy (Fig. b), reveals that the rings typically cross line
AC. Herein, we used the assumption that powerful Agulhas rings are
characterized by an eddy kinetic energy that is mostly at least 10 times
larger than the mean kinetic energy. The exception to this is the region
where the Benguela Current has the highest velocities (Fig. ).
Wind forcing and the Sverdrup balance
The Sverdrup relation gives a zeroth-order understanding of the wind forcing
and vertically integrated meridional transport in an open ocean. The validity of
the Sverdrup relation has been analyzed in many studies using both observations e.g., and model simulations
e.g.,. The focus of these
studies was mostly to determine whether the Sverdrup balance holds in the
open ocean, but they did not focus on the eastern boundary region.
Nevertheless, stated that the Sverdrup balance cannot
explain the observed transport near the eastern boundary.
, using a regional ocean model, showed that an
approximate Sverdrup balance holds close to the eastern boundary, while it
underestimates the transport in the region where the Benguela Current is
strong. Using the Regional Ocean Modeling System
, also found that
Benguela Current transport is larger than that derived from the Sverdrup
balance (their Fig. 2a, b).
While keeping in mind that the Sverdrup balance has low skill with respect to
reproducing transports from observations or models, it is used herein to
investigate what impact the wind field has on the variability in the
transport. To accomplish this, the curl of the wind stress is calculated
using monthly mean ERA-Interim wind fields for the same time period as the
observations. Figure a shows the climatological mean of the curl of
wind stress as well as the wind stress vectors. The latter reveal, as
expected, that the direction of the winds in the region is similar to the
direction of the transport (Fig. b). The wind-driven transport can be
calculated with the Sverdrup equation
My=∇×τρβ,
where, My is the Sverdrup transport, τ is the wind stress, β=∂f∂y is the meridional gradient of the Coriolis
parameter f, and ρ is density.
Figure b shows the Sverdrup stream function as derived by integrating
My from the African coast to the west. It indicates that the transport
across 30 and 35∘ S is about 2 Sv which, as expected, is much
smaller than the mean meridional transports obtained from Argo & SSH at
these latitudes.
A challenge in this region is the inflow of water from the Indian ocean,
typically about 14.5 Sv in the upper 1000 m ,
via the energetic Agulhas rings and leakage. In addition, the Sverdrup
balance may have a different depth for the wind-forced layer than the 800 m used
for estimating the meridional transport. However, values from a study by
, who estimated the depth of the gyre using a
ventilated thermocline approach by , showed values of
700–800 m for the box ABCD.
To gain further insight into the wind-driven variability in the meridional
transport, the Sverdrup transport across AB and CD is integrated zonally and
its anomalies are compared to the anomalies of the observed transport after
smoothing with an 18-month running mean filter and normalizing
(Fig. ). Across AB (at 30∘ S) the time series exhibits
about the same variability; however, this is not true at 35∘ S.
These results suggest that, for interannual variability, the wind field
forces the circulation at 30∘ S with a 2-month lag. At
35∘ S this is not the case, most likely due to the large impact of
the inflow from the Indian Ocean. A significant portion of the variability,
however, remains unexplained, even at 30∘ S, where the Sverdrup
theory works relatively well. Therefore, processes such as density variation,
eddy shedding, and remote forcing remain important to the overall variability
in the highly dynamic Cape Basin region.
(a) Climatologies of ERA interim wind stress curl (in
color) and the wind stress (arrows). (b) Climatology of the Sverdrup
stream function (color).
Zonally integrated normalized anomalies My (gray) and meridional
transport (tv, black) at 30 and 35∘ S. Both the time series are
smoothed using an 18-month running mean filter.
Summary and conclusions
The objective of this study is to increase the knowledge of the structure and
variability in the flow in the
eastern limb of the Atlantic Meridional Overturning Circulation in the
subtropical South Atlantic by deriving and studying a 23-year long time
series of transport estimates in the Benguela Current region from
observations. Historically, the coverage with observations of this nature has
been relatively sparse in this region. The relatively long time series of
velocities and integrated transports as well as the derived results will be
valuable for validating models in this dynamic region. This study provides
mean volume transports every 0.5∘ between 35 and 25∘ S and
thus greatly expands the knowledge on the latitude dependence of the
integrated meridional transport of the Benguela Current. In addition to this,
it allows a more detailed analysis of its temporal variability than was
possible previously.
The mean volume transports are mostly in good agreement with previous
estimates when comparing those with similar integration limits. It is found
that the meridional transports of the Benguela Current are relatively higher
south of 31∘ S, and relatively lower values are typically seen north of
28∘ S where the zonal component of this current is stronger.
A recirculation cell is observed in the climatologies of 15 m velocity and
integrated transport in the upper 800 m between the Walvis Ridge and Vema
Seamount. The recirculation cell is centered at 6∘ E and 33∘ S and might
be formed due to the interaction of eddies with the bathymetry in this
region.
In terms of the variability, neither 30 nor 35∘ S reveal a strong
annual cycle. However, there are periods where the energy density reaches the
level of significance at both latitudes. Overall, the energy density at
30∘ S has more energy in this frequency band than at 35∘ S.
With respect to mesoscale variability, the time series at 30 and
35∘ S exhibit large energies in 3-
to 7-month periods due to the influence of the Agulhas rings. However, this
energy does not reach the level of significance in many years because it is
based on a zonal integral of the transport as explained in Sect. .
At 30∘ S, the normalized anomalies of smoothed meridional transports
from the observations and from the Sverdrup balance exhibit strong
correlations (0.7), with the latter leading the former by 2 months. Smoothed
time series of these two transports show similar interannual variability at
30∘ S. In contrast to this, variability in the observed transport across 35∘ S is
significantly different than the Sverdrup balance. Therefore, this leads to
the conclusion that the variability in the meridional transport at 30∘ S is significantly
impacted by the local wind forcing, while this is not the case at
35∘ S. Discrepancies at 35∘ S can be explained by the
impact of the water flowing from the Indian Ocean into the South Atlantic.
To understand the flow pattern in the Cape Basin region, transport budgets are
estimated in two different boxes between 30 and 35∘ S, and
3∘ E (8∘ E) and the eastern boundary near the African
Coast. It is found that the local topography near the Walvis Ridge and a
recirculation cell centered at 33∘ S, 6∘ E can
significantly influence the volume transport budget in the upper 800 m.
Cyclonic and anticyclonic eddies can also cause large anomalies in the
budget. However, away from the coast at the ∼ 1000 m isobath, coastal
upwelling events do not seem to influence the transport budget.
Recent studies with climate models suggest an intensification of the
westerlies in the Southern Ocean
; it has also been shown that
the increasing westerlies in the Southern Ocean resulted in an intensification
of the Agulhas Current and its leakage since 1965 to about 1990
. Both
and did not find a
positive trend in the Agulhas transport from 1993 on. Consistent with this,
the time series presented herein do not indicate that there is a trend in the
transport of the Benguela Current or the transport across 3∘ E
between 30 and 35∘ S.
The 3-D observation-based velocity field used in this study is available from the corresponding author upon request.
The authors declare that they have no conflict of
interest.
Acknowledgements
This paper was funded by the Climate Observation Division, Climate Program
Office, Climate Monitoring Program, National Oceanic and Atmospheric
Administration, US Department of Commerce, and the Atlantic Oceanographic and
Meteorological Laboratory of the National Oceanic and Atmospheric
Administration. This research was also carried out in part under the auspices
of the Cooperative Institute for Marine and Atmospheric Studies (CIMAS), a
cooperative institute of the University of Miami and the National Oceanic and
Atmospheric Administration (NOAA) (cooperative agreement NA'0OAR432013). The
authors would like to thank the researchers and technicians involved in the
Argo project for their contributions to generating a high-quality global
subsurface data set. Argo data were obtained from the Global Data Assembly Centre (Argo GDAC;
10.12770/71b7b0ed-1e3a-4ebc-8e3b-b5b363112f2a).
Altimeter products were produced by Ssalto/Duacs and distributed by AVISO, with support from Cnes
(http://www.aviso.altimetry.fr/ducas/).
Edited by: Piers
Chapman Reviewed by: Jochen Kämpf and one anonymous
referee
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