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Western boundary currents (WBCs) play an essential role in regulating global climate. In contrast to their widely examined horizontal motions, less attention has been paid to vertical motions associated with WBCs. Here, we examine vertical motions associated with the major WBCs by analyzing vertical velocity from five ocean synthesis products and one eddy-resolving ocean simulation. These data reveal robust and intense subsurface upwelling, which are primarily along isopycnal surfaces, in five major subtropical WBC systems. These upwelling systems are part of basin-scale overturning circulations and are likely driven by meridional pressure gradients along the western boundary. Globally, the WBC upwelling contributes significantly to the vertical transport of water mass and ocean properties and is an essential yet overlooked branch of the global ocean circulation. In addition, the WBC upwelling intersects the oceanic euphotic and mixed layers, and thus likely plays an important role in ocean biological and chemical processes by transporting nutrients, carbon and other tracers vertically inside the ocean. This study calls for more research into the dynamics of the WBC upwelling and their role in the ocean and climate systems.
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1. Introduction
Large-scale vertical motion in the global ocean is generally much weaker than horizontal motions (e.g., Stew-
art,2008). Nevertheless, vertical motions are an important component of the coupled climate system because
they can transport heat, carbon, nutrients, and other tracers into and out of the surface mixed layer, where they can
then be exchanged with the atmosphere. Also, vertical motions are essential for oceanic biological and chemical
processes. For instance, upwelling near the surface brings nutrient-enriched water into the euphotic zone, affect-
ing the ocean primary productivity (Kämpf & Chapman, 2016; Ryther,1969) and, consequently, CO2 uptake
(Ducklow etal.,2001; Pilskaln etal.,1996; Stukel etal.,2013). Downwelling, such as that observed in the South-
ern Ocean, transports heat and tracers sourced at the surface to the deep and abyssal oceans (Gregory,2000; Ito
etal., 2010; Liang etal., 2015), and is therefore essential for the responses of the ocean interior to changes in
climate and human activities.
Based on theoretical understanding (e.g., Ekman dynamics) and the observed distributions of a variety of trac-
ers (e.g., Toggweiler etal.,2019), a number of general patterns of ocean vertical motions have been inferred,
including strong upwelling along most eastern boundaries of the subtropical ocean basins and along the equator
(Huyer,1983; Kämpf & Chapman,2016; Wyrtki,1981), as well as intense vertical motions of both signs in the
Southern Ocean (Huyer,1983; Marshall & Speer,2012; Wyrtki,1981). However, because the weak vertical
velocity associated with the large-scale circulations cannot, in general, be measured directly, quantitative studies
of vertical motions are limited, especially in the subsurface ocean. Over the past decades, several ocean synthe-
sis products became available (Balmaseda etal.,2015; Stammer etal.,2016). Some of these ocean data prod-
ucts synthesize various observations and ocean circulation models, provide vertical velocity among many other
Abstract Western boundary currents (WBCs) play an essential role in regulating global climate. In contrast
to their widely examined horizontal motions, less attention has been paid to vertical motions associated with
WBCs. Here, we examine vertical motions associated with the major WBCs by analyzing vertical velocity
from five ocean synthesis products and one eddy-resolving ocean simulation. These data reveal robust and
intense subsurface upwelling systems, which are primarily along isopycnal surfaces, in five major subtropical
WBC systems. These upwelling systems are part of basin-scale overturning circulations and are likely driven
by meridional pressure gradients along the western boundary. Globally, the WBC upwelling contributes
significantly to the vertical transport of water mass and ocean properties and is an essential yet overlooked
branch of the global ocean circulation. In addition, the WBC upwelling intersects the oceanic euphotic and
mixed layers, and thus likely plays an important role in ocean biological and chemical processes by transporting
nutrients, carbon and other tracers vertically inside the ocean. This study calls for more research into the
dynamics of the WBC upwelling and their role in the ocean and climate systems.
Plain Language Summary This study shows that intense upwelling systems exist along the major
western boundary currents (WBCs) around the global ocean. In contrast to other well-known oceanic upwelling
systems (e.g., equatorial, coastal upwelling), these WBC upwelling systems, which are essential branches
of the global ocean circulation, have been largely unrecognized in the literature. This intense upwelling and
the associated overturning circulation in the subtropical ocean basins can transport nutrients, carbon, and
heat inside the ocean, and consequently act as an important yet unexplored route through which the oceanic
biological, chemical, and physical processes, and consequently the climate system, will be affected. This study
calls for more research into the dynamics of the WBC upwelling and its role in the ocean and climate systems.
LIAO ET AL.
© 2022 The Authors.
This is an open access article under
the terms of the Creative Commons
Attribution-NonCommercial License,
which permits use, distribution and
reproduction in any medium, provided the
original work is properly cited and is not
used for commercial purposes.
Hidden Upwelling Systems Associated With Major Western
Boundary Currents
Fanglou Liao1 , Xinfeng Liang1 , Yun Li1, and Michael Spall2
1School of Marine Science and Policy, University of Delaware, Lewes, DE, USA, 2Woods Hole Oceanographic Institution,
Woods Hole, MA, USA
Key Points:
Intense upwelling systems exist along
the major western boundary currents
(WBC) around the global ocean
The WBC upwelling is likely driven
by the meridional pressure gradients
along the western boundary
The WBC upwelling contributes
significantly to the vertical transport
of water mass, ocean properties and
materials around the global ocean
Supporting Information:
Supporting Information may be found in
the online version of this article.
Correspondence to:
X. Liang,
xfliang@udel.edu
Citation:
Liao, F., Liang, X., Li, Y., & Spall,
M. (2022). Hidden upwelling systems
associated with major western boundary
currents. Journal of Geophysical
Research: Oceans, 127, e2021JC017649.
https://doi.org/10.1029/2021JC017649
Received 4 JUN 2021
Accepted 28 FEB 2022
Author Contributions:
Conceptualization: Xinfeng Liang,
Yun Li
Formal analysis: Fanglou Liao
Funding acquisition: Xinfeng Liang,
Michael Spall
Investigation: Fanglou Liao, Xinfeng
Liang, Michael Spall
Methodology: Fanglou Liao, Xinfeng
Liang, Yun Li, Michael Spall
Project Administration: Xinfeng Liang,
Michael Spall
Resources: Xinfeng Liang
Supervision: Xinfeng Liang
Writing – original draft: Fanglou Liao,
Xinfeng Liang
Writing – review & editing: Xinfeng
Liang, Yun Li, Michael Spall
10.1029/2021JC017649
RESEARCH ARTICLE
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variables. Once proven robust, such ocean data products can complement existing observations and advance our
quantitative understanding of oceanic vertical motions as well as large-scale three-dimensional ocean circulation.
Western boundary currents (WBCs) are one of the most important ocean regimes regulating the global climate
(e.g., Wu etal.,2012). While all major WBCs are three-dimensional features, their role in the climate system
has long been studied in terms of lateral transport and air-sea exchange (Hu etal., 2015), generally neglect-
ing the effects of vertical motions. However, there have been several studies indicating that there can be large
vertical transports in western boundary current regions. Interaction of the continental slope and the western
boundary current has been shown to result in upwelling due to large-scale wind-driven gyres (Holland,1972);
upslope Ekman transport below an adiabatic western boundary current (Condie,1995); and onshore flow balanc-
ing an offshore Ekman transport (e.g., Schaeffer etal.,2013). There have also been very idealized studies that
find upwelling along western boundaries for buoyancy-forced flows (e.g., Bire & Wolfe,2018; Pedlosky &
Spall, 2005; Schloesser et al., 2012). Finally, regional observations (e.g., Roughan & Middleton,2004) and
the long-term mean vertical velocity field from a global ocean state estimate (Liang etal.,2017) have revealed
considerable vertical motions associated with the WBCs, even in the absence of local upwelling-favorable wind
stress. Such WBC-associated vertical motions potentially offer a viable and effective mechanism for the exchange
of ocean heat, salt, and other biogeochemical tracers between the mixed layer and underlying layers.
In this study, we examine vertical velocity in the major WBC regions from five ocean synthesis products: Esti-
mating the Circulation and Climate of the Ocean (ECCO), (Forget etal.,2015; Fukumori etal.,2017); European
Centre for Medium-Range Weather Forecasts (ECMWF) ora-s3 (Balmaseda etal., 2008); Global Ocean Data
Assimilation System (GODAS) (Behringer & Xue,2004); Simple Ocean Data Assimilation (SODA) (Carton
etal.,2018); Ensemble Coupled Data Assimilation System (ECDA) (Chang etal.,2013); and one eddy-resolving
ocean simulation-Ocean General Circulation Model For the Earth Simulator (OFES; Sasaki etal., 2008) over
their overlapping period (January 1992 to December 2009). Our primary goals are to describe robust large-scale
features of vertical motions in the WBC regions and to explore their roles in the vertical transport of volume and
ocean properties. In order to demonstrate differences between vertical motions near eastern and western bounda-
ries of ocean basins, we also include the Peruvian upwelling region as a contrasting example.
It is proposed that upwelling near the western boundaries of the subtropical gyres is ultimately driven by a
poleward decrease in pressure along the western boundary. This is analogous to the downwelling that is found in
models (Cessi etal.,2010; Katsman etal.,2018; Spall,2010) and inferred from observations (Liang etal.,2017)
in regions of pressure gradients at high latitudes of the North Atlantic. Vertical motions are required in these
boundary regions to maintain a geostrophic balance. The pressure gradient in the high-latitude downwelling
regions is supported by both local buoyancy loss to the atmosphere and lateral eddy fluxes into the basin interior.
In the present study we identify analogous regions of pressure gradients along mid-latitude western boundaries
and discuss potential mechanisms.
2. Data and Methods
2.1. Data
The vertical velocity from six publicly available datasets, including five ocean synthesis products and one eddy-re-
solving ocean simulation, are analyzed in this study. The ocean synthesis products utilize general ocean circu-
lation models to assimilate various observational datasets with varying approaches (e.g., Stammer etal.,2016).
The ocean simulation does not assimilate observational data but is of much higher spatial resolution. Some
basic information on those datasets is provided in the following. The ECCO data utilized in this study are the
ECCOv4r3 monthly products (Forget etal.,2015; Fukumori etal.,2017). ECDA is the Ensemble Coupled Data
Assimilation System developed at the National Oceanic and Atmospheric Administration /Geophysical Fluid
Dynamics Laboratory (Chang etal.,2013). ECMWF product used here is ECMWF ora-s3, an operational ocean
analysis/reanalysis system implemented at the ECMWF (Balmaseda etal.,2008). GODAS is the Global Ocean
Data Assimilation operated at the National Centers for Environmental Prediction(Behringer & Xue,2004). We
use monthly interpolated values from SODA v3, an ocean data assimilation product developed at the University
of Maryland (Carton etal.,2018). Finally, we use the eddy-resolving quasi-global forward ocean model OFES,
developed by the Japan Agency for Marine-Earth Science and Technology (Sasaki etal.,2008). Additional infor-
mation about the data products, such as the grid point numbers and time span, can be found in Table1.
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Monthly vertical velocity data are directly available from all the selected products. The overlapping period
covered by all six products is from January 1992 to December 2009. The vertical velocity data were averaged
over this 18-years period. For consistency, all available vertical velocity fields from the six products were first
transformed to the 1-degree by 1-degree LLC90 (Lat-Lon-Cap 90) grid configuration (Forget etal.,2015) before
further processing. A 3 point × 3 point diffusive smoother provided in the gcmfaces package (https://github.com/
MITgcm/gcmfaces) was applied to obtain robust large-scale patterns. Other configurations of the same smoother
(2×2, 4×4) were also tested, and the results were roughly the same.
2.2. Analyses
As shown in Figure1a, we define a box for each major subtropical WBC region and for the Peruvian upwelling
region. The regions covered by those boxes are as follows: Kuroshio (120°E–150°E, 21°N–40°N); Gulf Stream
(82°W–60°W, 25°N–41°N); Agulhas Current (20°E–38°E, 37°S–27°S); East Australian Current (148°E–158°E,
37°S–20°S); Brazil Current (56°W–30°W, 35°S–10°S); Peruvian upwelling (85°W–70°W, 40°S–8°S).
To examine the vertical distribution of vertical velocity, for each WBC region, we choose a cross section approx-
imately perpendicular to the local coastline and plot the distribution of time-averaged vertical velocity along with
the horizontal velocity perpendicular to the cross section. Details of the selected cross sections are as follows:
Kuroshio, (139°E, 35°N) to (149°E, 25°N); Gulf Stream (74°W, 38°N) to (64°W, 28°N); Agulhas Current (30°E,
31°S) to (40°E, 41°S); East Australian Current (153°E, 30°S) to (163°E, 30°S); and Brazil Current (41°W, 21°S)
to (31°W, 21°S). A contrasting eastern boundary upwelling, the Peruvian upwelling region, was also selected as
(70°W, 23°S) to (80°W, 23°S). The cross sections are marked as thick black lines in Figure1a.
A regional budget analysis is conducted to better understand the dynamics of the WBC upwelling. We choose
the Gulf Stream region as an example and calculate the horizontal time-averaged volume transport into and out
of a control volume and the time-averaged vertical volume transport through several interfaces. Their depths are
subjectively chosen so that the vertical volume transport at the upper interface is just below the Ekman layer and
that the transport through the deep interface is relatively weak. Note that the ECCO product on the native grid
configuration is used for the budget analysis in order to close the volume flux budget.
The contribution of WBC upwelling to the vertical transport in the subtropical ocean basin is also calculated. At
each depth, we select the grid cell with positive mean vertical velocity and calculate the upward vertical volume
flux in the domain bounded by each box in Figure1a. We define the results as the vertical volume transport
related to the WBC upwelling. We also calculate the volume transport within all the other grid cells in the same
latitude band across the whole ocean basin (domain shown in Figure1b). The sum of these two terms is the net
volume transport within the corresponding latitude band across the ocean basin.
Moreover, we compare the vertical volume transport induced by the WBC upwelling with those associated with
other well-known upwelling regimes. We calculate the vertical volume transport at each grid cell where the
time-averaged vertical velocity is positive in these four different regimes as marked in Figures1a and1c: WBCs,
Eastern Boundary Upwelling, Equator (within 8° of the equator) and the Southern Ocean (south to 40°S). Note
ECCO ECDA ECMWF GODAS SODA OFES
Version v4r3 ora-s3 3.4.2
Model MITgcm MOM4 HOPE MOM3 MOM5 MOM3
Lon grids 720 360 360 360 720 3,600
Lat grids 360 200 179 418 330 1,500
Vertical grids 50 (z) 50 (z) 29 (z) 40 (z) 50 (z) 54 (z)
Time span 1992–2015 1961–2016 1959–2009 1980–2019 1980–2018 1950–2016
Note. ECCO, Estimating the Circulation and Climate of the Ocean; ECDA, Ensemble Coupled Data Assimilation; ECMWF,
European Centre for Medium-Range Weather Forecasts; GODAS, Global Ocean Data Assimilation; SODA, Simple Ocean
Data Assimilation; OFES, Ocean General Circulation Model For the Earth Simulator.
Table 1
Summary of the Six Ocean Data Products
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that the Angola Dome is classified into the eastern boundary current region in this paper. These upward volume
transports are then summed over the corresponding region and compared with each other.
3. Results
3.1. Vertical Velocity Associated With the Major WBCs
Time-averaged vertical velocity
̄
near 300m from six selected ocean products is displayed in Figure1. While
there are differences in the detailed regional patterns, intense vertical motions in the Southern Ocean, along the
Equator, and in the WBC regions are observed in all of the examined data products. The strong upwelling in the
Southern Ocean (Anderson etal.,2009) and in the equatorial regions (Yoshida,1959) are well known and mainly
induced by Ekman dynamics (i.e., wind pumping and suction). To our knowledge, the strong and robust upwelling
(∼1m/day) apparent in all major WBC regions in all six products has not been explicitly identified and its dynam-
ics are not well understood. Also, both the strength and vertical extent of the upwelling in the WBC regions are
distinctly different from those in the eastern boundary upwelling systems, the latter of which are barely detectable
at this depth. Strong upwelling can also be seen at 1,000m and deeper in WBC regions, especially near the Gulf
Stream and the Kuroshio (Figures S1 and S2 in Supporting InformationS1). Apart from the boundary current
systems, the vast area of the subtropical oceans at this depth is dominated by weak downwelling.
Figure 1. Time-averaged vertical velocity
̄
near 300m between January 1992 and December 2009.
̄
from six selected
products: (a) Estimating the Circulation and Climate of the Ocean (ECCO). (b) Ensemble Coupled Data Assimilation
(ECDA). (c) European Centre for Medium-Range Weather Forecasts (ECMWF). (d) Global Ocean Data Assimilation
(GODAS). (e) Simple Ocean Data Assimilation (SODA). (f) Ocean General Circulation Model For the Earth Simulator
(OFES). The black boxes in (a) show the domains of the five western boundary and one eastern boundary systems
investigated in this study. The thick black lines represent the cross sections shown in Figure2. The black boxes in (b) (at the
same latitude band of the corresponding boxes in a) represent the domains where vertical volume flux was calculated. The
dashed lines split the domains into western boundary currents regions and the rest of the subtropical ocean basins. Boxes in
(c) mark three other well-known upwelling regimes (equatorial, eastern boundary and the Southern Ocean).
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There is also strong downwelling along the boundaries at high latitudes. This is the downwelling limb of the
Eulerian meridional overturning circulation (Katsman etal.,2018; Spall,2010). As discussed further below, we
argue that the WBC upwelling regions are analogous to these downwelling cells at high latitudes.
We also present selected sections of the time-averaged vertical velocity
̄
across the major WBCs from ECCO
(Figure2). Despite differences in resolutions, numerical configurations, and assimilated data, all of the examined
products show similar spatial patterns and magnitudes (Figures S3–S8 in Supporting InformationS1). Intense
subsurface upwelling in the WBC regions is co-located with the strong boundary currents, suggesting a dynam-
ical connection between them. Also, the strong time-averaged upwelling (∼1m/day) in WBC regions generally
extends from near the surface down to 1,000m or even deeper. The strong vertical motion is, however, located well
above the bottom topography, indicating that it does not result from direct interaction with the sloping bottom. In
contrast to the WBC sections, upwelling near the Peruvian coast, an example eastern boundary upwelling region,
is confined to a shallower layer and is also much weaker. In addition, weak downwelling resulting from Ekman
pumping occurs to the east of the WBC upwelling. Note that the water in the Ekman layer is transported into the
subtropical gyre from the north and south and pumped down by Ekman convergence. It is not simply connected
to the western boundary upwelling in a local overturning gyre. This downwelling of course forces the anticyclonic
subtropical gyres, which do recirculate water through the western boundary current.
We then examine the relationship between the current vectors and the background (zonal) density structure
(Figure3). The meridional averages of the time-averaged current vectors in the WBC regions are approximately
aligned with sloping isopyncal surfaces associated with the WBCs, suggesting that the strong upwelling in the
WBC regions is primarily along rather than across isopycnals. A decomposition of the vertical velocity from
ECCO into diapycnal and isopycnal contributions following Bennett(1986) confirms that the WBC upwelling
is mainly associated with along-isopycnal flow (Figure4). Therefore, the strong WBC upwelling is unlikely to
Figure 2. Time-averaged vertical velocity
̄
(color) and horizontal velocity (contour lines, unit: cm/s) in selected cross sections from Estimating the Circulation and
Climate of the Ocean. The other datasets show similar spatial patterns (Figures S3–S8 in Supporting InformationS1). The cross sections are marked with thick black
lines in Figure1a. (a) Kuroshio. (b) Gulf Stream. (c) Agulhas Current. (d) East Australian Current. (e) Brazil Current. (f) Peruvian upwelling. The contour lines show
the horizontal velocity (cm/s) perpendicular to the cross sections, indicative of the strength of adjacent western boundary currents. Note that the depth axis is stretched
for better visualization.
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be related to local mixing, by which vertical velocity will be primarily in diapycnal direction instead of along
isopycnals.
3.2. Boundary Pressure and WBC Upwelling
The difference in the pressure between the eastern and western boundaries drives a meridional geostrophic flow,
which is the dominant component of the meridional overturning circulation. Due to buoyancy loss to the atmos-
phere, the upper ocean pressure decreases from low to high latitudes along the eastern boundary of the subtropical
and subpolar gyres and, in the North Atlantic, cyclonically around the Nordic Seas. The high pressure at low lati-
tudes results from relatively high sea surface height. The poleward decrease in pressure is mitigated with depth by
the increasing density along the boundary. These regions of increasing density have been shown to be where the
Eulerian downwelling limb of the meridional overturning circulation are located (Katsman etal.,2018; Marotzke
& Scott,1999; Spall,2004,2010).
The potential density in the upper 1,000m in the ECCO product increases in the poleward direction along all
WBCs (Figure5). The regions of upwelling along the western boundaries of the subtropical gyres are thus also
regions of negative meridional pressure gradient along the boundary. As a result, the change in pressure from
the eastern boundary to the western boundary is likely larger to the north of the upwelling region than it is to the
south. This then requires upwelling at mid-latitudes to provide the required increased poleward geostrophic flow
in the upper ocean. Support for such a mid-latitude upwelling cell is provided by the mid-latitude maximum in
the meridional overturning circulation in depth coordinates often found in high resolution numerical models (e.g.,
Hirschi etal.,2020).
Note that this upwelling is different from the well-known upwelling driven by numerical diffusion of density
across sloping isopycnals (the so called “Veronis Effect,” Veronis,1975). For numerical models in which diffu-
sion of density is along horizontal surfaces, in regions of sloping isopycnals, such as in western boundary currents,
Figure 3. Meridional averages of the time-averaged current vector (arrows, normalized in each region individually for better visualization) and potential density
anomaly (contours) in selected regions from Estimating the Circulation and Climate of the Ocean. The other datasets show similar spatial patterns. The averaged
regions, which are marked with black boxes in Figure1b, correspond to: (a) Kuroshio. (b) Gulf Stream. (c) Agulhas Current. (d) East Australian Current. (e) Brazil
Current. (f) Peruvian upwelling.
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diffusion of density is balanced by the vertical advection of the mean stratification, leading to strong upwelling
and diapycnal mass transport. This effect is eliminated by rotating the mixing tensor to be along isopycnals
(Danabasoglu & McWilliams,1995), leading to better representations of the meridional overturning circulation
and meridional heat transport. All of the non-eddy-resolving data products we have diagnosed use a rotating
mixing tensor and the Gent and McWilliams(1990) eddy tracer flux parameterization. The eddy-resolving OFES
model (Sasaki etal.,2008) also shows similar upwelling patterns and magnitudes.
The underlying relationship between vertical stratification, horizontal transport, and upwelling is illustrated
through a sample volume budget analysis for the Gulf Stream (Figure6). The surface area of the control volume
is triangular and marked in the inset, and the depth range is between 55m and 2,000 m. The budget analysis
(Figure6a) reveals large horizontal divergences/convergences in different layers, requiring vertical transport to
conserve mass. The density structure along the two sections (BA, BC in Figure6b) provides the dynamical
framework for the existence of horizontal convergence. Because the density increases poleward along the western
boundary (Figure5), the density change from the western boundary to the interior point (B) is larger along the
northern section (BC) than it is along the southern section (AB). Thermal wind balance thus requires a larger
vertical shear in the horizontal velocity along BC. But mass conservation requires that the flow through each
section is the same (except for the small transport into the upper 55m). The only way to close the mass budget
is for water to upwell within the control volume. In other words, the WBC upwelling can be explained through
mass conservation and geostrophy. In this sense, these upwelling regions are analogous to the high latitude
downwelling found in regions of horizontal pressure gradients on the boundary. The requirement that there be
upwelling near the western boundary is not dependent on the details of the numerical model, subgrid-scale
mixing, or bottom topography. Possible mechanisms for maintenance of this pressure gradient will be discussed
in Section4.
Figure 4. Along- and across-isopycnal components of the time-mean vertical velocity
̄
at around 300m. (a) Isopyncal
vertical velocity
̄
. (b) Diapycnal vertical velocity
̄
.
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Figure 5. Potential density anomaly along the western boundary currents (WBCs) and Peruvian upwelling. The potential density anomalies were zonally averaged
within the WBC regions marked as boxes in Figure1a. (a) Kuroshio. (b) Gulf Stream. (c) Agulhas Current. (d) East Australian Current. (e) Brazil Current. (f) Peruvian
upwelling.
Figure 6. Time-averaged vertical velocity
̄
, potential density anomaly
 𝐴
, and volume flux in a triangle-shape domain in the
Gulf Stream region. (a) Time-averaged vertical velocity at four depths (colors) and lateral and vertical volume fluxes. The
black and blue arrows represent the lateral volume fluxes in Sv, and the purple arrows show the vertical volume fluxes in Sv.
(b) Time-averaged potential density anomaly along the southern section and northern section of the triangle-shaped domain
between 55 and 2,000m (shown in the inset). The gray curve in the 55m section along AC represents the coastline. The
results are based on ECCO product on the native grids.
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3.3. Vertical Transport Associated With the WBC Upwelling
We now quantify the contribution of the WBC upwelling to the vertical transport of mass/volume in the subtrop-
ical ocean basins using ECCO (Figure7), with the other products generally showing similar results (Figures
S3–S8 in Supporting InformationS1). Although the WBC regions occupy only a minor portion of the subtropical
ocean basins with respect to the ocean surface area, as shown in Figure1b, the vertical volume transport induced
by upwelling in the WBCs is generally of the same order of magnitude as and is almost always opposite in the
direction to the vertical volume transport in the rest of the subtropical basin within the same latitudinal band.
We also calculate the vertical transport of heat and salt using ECCO (Figures S9 and S10 in Supporting Infor-
mationS1), and the results are consistent with the volume transport, that the WBC regions dominate subsurface
vertical transport of salt and heat in the subtropical ocean basins within certain depth ranges.
Specifically, upwelling in the Kuroshio, Gulf Stream, and Brazil Current regions dominates the net volume
transport in the corresponding subtropical ocean basins within the depth ranges between a few hundred and about
2,000m. As a contrasting example, vertical volume transport in the Peruvian upwelling region is much weaker
and shallower compared to the WBC upwelling. The net volume transport in the subtropical basin is in general
downward near the surface and changes to upward beneath, reflecting the fact that the upward volume transport
in the WBCs generally reaches its maximum around 200–500m. In contrast, the downward transport in the rest
of the subtropical basins has its maximum downward volume transport near the surface. The surface intensified
downwelling is due to the Ekman pumping occurring inside the subtropical ocean basins and the maximum
impact of the Ekman pumping generally appears around 100m and then decreases significantly with increasing
depth, as expected from Sverdrup dynamics. Also, the finding that the overturning circulation is not closed within
these latitude bands (i.e., non-zero net vertical volume transport) emphasizes that the WBC upwelling is part of
a basin-scale three-dimensional overturning circulation (Talley,2003) and part of this upwelling is balanced by
downwelling at higher latitudes.
Figure 7. Vertical volume fluxes in the subtropical ocean basins from Estimating the Circulation and Climate of the Ocean. Vertical volume transport due to the
western boundary currents upwelling is shown in red, vertical volume transport integrated across the rest of the corresponding ocean basin within the same latitude
band is shown in blue, and the net vertical volume transport is displayed as the magenta dashed line. The six regions, which are marked in Figure1b, correspond to (a)
Kuroshio. (b) Gulf Stream. (c) Agulhas Current. (d) East Australian Current. (e) Brazil Current. (f) Peruvian upwelling. Note that the depth axis is divided into two
parts for better visualization.
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We also calculate and compare the vertical volume transport associated with the four major upwelling regimes
(WBCs, Eastern Boundary Upwelling, Equator and Southern Ocean) around the global ocean with ECCO
(Figure8). Near the surface, equatorial upwelling is the dominant process for the global oceanic vertical volume
transport, with a maximum value around 100Sv. But in the subsurface, the strongest upward transport is asso-
ciated with the Southern Ocean and the WBC regions. Between 200 and 1,000 m, the WBC-related upward
volume transport is generally more than 1/3 of the value in the Southern Ocean, with the maximum value around
25Sv appearing near 400m. Below 2,000m the pressure gradient along the western boundary is weak and thus
a reduced contribution to the upward transport is expected. Again, this comparison demonstrates the overlooked
role of the WBC upwelling in the subsurface vertical exchanges of ocean properties and materials.
4. Discussion
This study provides evidence for the existence of as well as a dynamical framework for intense subsurface
upwelling associated with the major subtropical WBCs around the global ocean. Vertical motions in many
regions of the global ocean, such as in most eastern boundary currents, along the Equator, and in the Southern
Ocean, show evident upwelling signals in surface temperature and/or chlorophyll fields (Huyer,1983; Kämpf &
Chapman,2016; Marshall & Speer,2012; Naveira Garabato etal.,2017; Toggweiler etal.,2019; Wyrtki,1981)
and have been known and studied for a long time. In contrast, vertical motions in WBC regions are generally weak
close to the surface and become strong below the surface. Also, the strong horizontal transport and eddies associ-
ated with WBCs make direct detection of surface signals of WBC upwelling challenging. The intense subsurface
upwelling in WBC regions, therefore, have long been unrecognized in the literature.
Although in this study subsurface upwelling in the WBC regions is not directly observed, a variety of ocean data
products provide evidence supporting the inference that WBC upwelling is likely a real phenomenon in the global
ocean. The primary reason we believe that the WBC upwelling is real is that in order for the western boundary
currents to remain in geostrophic balance to leading order, the observed density gradient along the western
boundary requires that there be upwelling. Second, the WBC subsurface upwelling appears in all the examined
products (Figures S3–S8 in Supporting InformationS1), including coarse-resolution ocean synthesis products
and a high-resolution ocean model simulation. Those products differ in many aspects, including ocean model
Figure 8. Comparisons of vertical volume transport in four different upwelling regimes. The four regimes are western
boundary currents (WBC), the Eastern Boundary Currents (EBC), the Equatorial region and the Southern Ocean. The vertical
volume transport is calculated within the corresponding upwelling regions marked in Figure1c.
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numerics, external forcing, mixing parameterizations and assimilated observational data. The apparent robust-
ness of WBC upwelling suggests that it is likely controlled by processes that are well represented in all the prod-
ucts, like geostrophy and the large-scale density field. Third, vertical motions in other regions of the global ocean
(e.g., at low latitudes, and in eastern boundary currents) in the examined data products are generally consistent
with previous theoretical and observational studies, further increasing our confidence in their representation of
the large-scale vertical motions.
The presence of pressure gradients along the boundary implies strong vertical transports, regardless of how those
pressure gradients are maintained. In the high-latitude downwelling regions the pressure gradients are supported
by a combination of lateral eddy fluxes and local surface buoyancy loss. These processes might also be important
along the western boundary of the subtropical gyres, but other dynamics might also be at play. The pressure signal
of buoyancy loss at high latitudes will propagate equatorward along the western boundary via coastal/boundary
waves, so remote buoyancy forcing might also be important. Purely wind-forced circulations might result in
upwelling along the western boundary, as implied by the inertial models of Parsons(1969), Veronis(1966), and
Stommel(1965). In these models, conservation of potential vorticity requires a rising and eventual outcropping
of isopycnals as the WBC flows poleward. This would produce a pressure gradient on the boundary and subse-
quent upwelling. Note that, unlike on eastern boundaries, such a pressure gradient on the western boundary can
be balanced in a viscous boundary layer and does not require diapycnal mixing. In any case, there must be some
ageostrophic process active along the western boundary to balance the pressure gradient and satisfy the no-nor-
mal flow boundary condition.
Vertical motions in the WBC regions can reach much deeper than in equatorial and Eastern Boundary upwelling.
In addition, while the WBC upwelling is primarily along the isopycnal surfaces, it extends upward into the
surface mixed layer. Consequently, they can play an important role in the subsurface exchange of ocean properties
and materials and air-sea exchange in the subtropical regions. Given the consistent and strong vertical motions,
the vertical transport of heat and carbon in the WBCs may be significant in regulating the heat and carbon content
in both the upper ocean and atmosphere over longer timescales. Moreover, the basin-wide overturning circula-
tions spanning the subtropical and subpolar gyres could exchange ocean properties and tracers between the ocean
interior and western boundaries, as well as playing a role in the climate system.
Although point-wise values of ocean vertical velocity from models and synthesis products are generally weak
and noisy, spatial filtering reveals interesting and robust large-scale patterns that are not readily apparent in other
variables. We consider it particularly surprising that we have been able to determine a novel aspect of WBCs,
one of the most widely studied ocean processes, simply by examining the time-averaged vertical velocity from
available ocean synthesis and modeling products. At present, few ocean synthesis products and climate models
provide output of ocean vertical velocity, which we suggest should be archived and examined routinely.
Data Availability Statement
All the data used in this study are publicly available. The ECCOv4r3 data are available at https://ecco.jpl.nasa.gov/
products/all/. The ECDA data are available at ftp://nomads.gfdl.noaa.gov/2/ECDA/ecda/GFDL-CM2.1-ECDA/
CM2.1R-ECDA-v3.1-1960/mon/ocean/dc_Omon/r1i1p1/v20110601/. The ECMWF data are available at http://
apdrc.soest.hawaii.edu:80/dods/public_data/Reanalysis_Data/ORA-S3/1x1_grid. The GODAS data are availa-
ble at http://apdrc.soest.hawaii.edu:80/dods/public_data/Reanalysis_Data/GODAS/monthly. The SODA data are
available at https://www.atmos.umd.edu/ocean/index_files/soda3.4.2_mn_download_b.htm. The OFES data
are available at https://www.jamstec.go.jp/esc/fes/dods/OFES/OFES_NCEP_RUN. The bathymetry data are
available at https://www.gebco.net/data_and_products/gridded_bathymetry_data/.
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Acknowledgments
X. Liang is supported by the National
Science Foundation through Grants
OCE-2021274, OCE-2122507, and the
Alfred P. Sloan Foundation through Grant
FG-2019-12536. M. Spall is supported
through the National Science Foundation
Grants OCE-1947290 and OCE-2122633.
We greatly appreciate comments and
edits from Andreas Thurnherr on various
versions of this paper. Comments and
suggestions from three reviewers help
improve this paper.
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A theory is given to account for the CROMWELL Current-the Equatorial Undercurrent. The equatorial upwelling is closely related to the Current, and a remarkable similarity between the equatorial circulation and the circulation in the coastal upwelling regions. The widths of the equatorial upwelling as well as of the CROMWELL Current the maximum speeds of the upwelling and of the CROMWELL Current, the depth of the core and the thickness of the CROMWELL Current, are all found to be explained quantitatively. The rate of change in the CORIOLIS parameter with latitude and the mean vertical stability of the waters are essential to determine those length scales of the phenomena. The easterly wind stress over the equatorial region is responsible for the processes.
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An eastern boundary current (EBC) system driven by a large-scale meridional buoyancy gradient is simulated using an idealized eddy-resolving model. The EBC system consists of a pair of stacked meridional currents that flow poleward near the surface and equatorward at intermediate depths. Buoyancy advection in the EBC is primarily balanced by the shedding of eddies, with anticyclonic, warm-core eddies dominating near the surface and cyclonic, cold-core eddies found at intermediate depths. These boundary eddies play a significant role in both the eastern boundary circulation-by helping to trap the EBC near the coast-and the large-scale circulation through their effect on the downwelling limb of the overturning circulation. Momentum and thickness budgets analyzed using the thickness-weighted average framework highlight the role of eddy form drag in shaping and maintaining the EBC. The efficiency of the form drag increases dramatically at the offshore flank of the EBC. This zonal variation of the formdrag is essential formaintaining a swift, narrow EBC. The essential physics of the EBC are illustrated using a simple, semianalytical model.