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Variability of Ekman transport and resulting upwelling
Comparison of Southeastern Brazil Coast to Hainan Island This
project will discuss and compare the occurrence of upwelling in two
distinct coastal regions and how different physical processes
contribute to this phenomenon.
1. Introduction and background definitions Alongshore upwelling
fronts are a commonly studied subject in oceanography and can be
observed in a large number of the worlds coastal oceans. The
wind-induced upwelling is the prevailing process but there are
several other mechanisms that can generate and influence upwelling
near coastal regions such as bottom bathymetry, instabilities in
boundary currents, Coriolis effect, Ekman transport, eddies,
seasonal variations, among others. However, this project will
present and discuss only the influence of cyclonic meanders and
topography (or bottom bathymetry) in upwelling systems and, in this
introduction, a simple definition of both processes will be given.
A meander trough consists of a clockwise rotating dome of cold
upwelled water, which lies between a given boundary current and the
continental slope. They tend to be formed after spin off stronger
currents, lasting for weeks to months. One rotation typically takes
place at every 10-30 days in a horizontal scale that ranges from 10
to 100 km, often called meso-scale. According to Gille et al.
(2003), seafloor topography influences ocean circulation and is
relevant because it steers ocean flows, but also because it
inhibits or enhances the mixing and transport of waters from
different regions. Most major currents respond to sea floor
topography and as an example of this we can mention the following
currents: Antarctic Circumpolar Current (ACC); the Gulf Stream and
the Kuroshio Extension all steer around ridges and seamounts. a.
Shelf break upwelling driven by Brazil Current cyclonic meanders
Knowing a little more about the definition of those two ocean
variables we can now introduce two case studies that seek to relate
current meandering and topographic effects with upwelling. Campos
et al., [2000] developed a numerical ocean model for a specific
region of the Brazilian coast that shows the occurrence of
upwelling associated with cyclonic meanders of the Brazil Current,
in the region known as the Southeast Brazil Bight (SBB) or Santos
Bight. It also shows that, in the summer, the South Atlantic
Central Water (SACW) was detected in the SBB as close as 50 Km to
the coast and that, in the wintertime, the SACW retreats to near
the shelf break, being detected only in the middle and outer
shelves (depths greater than 100m). The wind regime can be
designated as the controlling forcing for this seasonal behavior
but it cannot explain why the SACW is found in the outer regions of
the shelf year round, since the
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SACWs core is usually found at depths greater than 200m. Campos
et al., [2000] suggests that during the summer the mechanism
responsible for pumping that water onto the shelf could be a
combination of shelf-break upwelling, induced by the Brazil Current
(BC) cyclonic meanders, and coastal wind-driven upwelling.
By analyzing the horizontal distributions of temperature at a
depth of 100m, the presence of the leading part of cyclonic
meanders can be determined. Vertical sections of temperature and
salinity indicate that the SACW was found climbing the shelf. The
fact that the SACW reached the shallower regions only in the summer
can be justified by additional action of the wind driven Ekman
pumping, schematically represented in Figure 2.
Figure 1. Horizontal distribution of temperature at 100m depths.
Notice the signature of a cyclonic meander in the southeastern
quadrant of the surveyed area.
Figure 2. Schematic of the combined effect of coastal
wind-driven and meander-induced upwelling. During the summer,
coastal upwelling occurs in response to the offshore Ekman
transport near the surface (CW: Coastal Water; SACW: South Atlantic
Central Water; TW: Tropical Water).
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b. Wind and topography influence on an upwelling system at the
eastern Hainan coast The Hainan Island is located along the
northern South China Sea (SCS) coast where upwelling can be
verified by measurements and also by satellite data. In summer,
under the southwesterly monsoon forcing, a northeastward current
flows along the Vietnam coast, the south and east coast of Hainan
Island, and the northern SCS coast, denoted as a western boundary
current. Chu et al. [1999] calculated the transport volume of this
current to be 5.5 Sv. There are some studies concerning the
dynamics and inter-annual variability of upwelling near Hainan
Island but they dont assess or dont explain what is the mechanism
controlling the structure of patchiness and what is the role of
topographic effects. As an effort to further understand this
process, Su, J., and T. Pohlmann (2009) applied a baroclinic free
surface model to an area of the island with dipole structures
(i.e., an upwelling center behind a cape while a downwelling center
exists in front of a cape looking in the direction of the flow).
Whereas in the control run the vertical velocity in shelf areas
indicates the combination of upwelling and downwelling centers
around the capes, only upwelling centers were produced in the
experiment. This indicates that the capes distort the potential
vorticity balance, which leads to localized up- and downwelling
patches. When upwelling favorable winds are strong enough, a sea
surface temperature (SST) anomaly is formed at the downstream of
the cape, causing an upward lift of isopycnals toward the coast.
The additional density induced pressure gradient will result in
further movement of the upwelling center. The existence of several
capes along such a short coastline is relatively exceptional, and
thus the resulting number of patches of up- and downwelling is the
dominant feature in these areas. So the main dynamics around these
capes are topographically steered.
Su, J., and T. Pohlmann (2009) affirm that this finding could be
extended to other coastal areas that have similar complex
coastline. This can be confirmed by the work of Tony et al. (2001),
which relates three observed recurrent upwelling centers off the
New Jersey coast with consistent solutions of their oceanic model
and three topographic heights at the locations of Barnegat Inlet,
the Mullica River estuary, and Townsend Hereford Inlet,
respectively (Figure 3). Their research also shows that the
existence of a topographic perturbation component in the upwelling
solution of their theoretical model indicates that the alongshore
topography plays an important role in controlling the formation of
upwelling centers as it enhances upwelling at some locations and
induces downwelling at others.
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Figure 3. Tony et al.s (2001) satellite image of upwelling
events along the southern New Jersey coast. The image indicates the
upwelled surface water has converged into a series three cold
patches. CTD transects through the Mullica upwelling center are
show in the figure.
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2. Discussion It is very important to consider the implications
behind a comparison between different ocean processes happening in
distinct regions, under various circumstances and timelines. To a
certain extent currents and winds formed in one side of the planet
can generate upwelling in the other side and have a significant
impact on the circulation at different latitudes and longitudes.
However, when we focus on smaller scales or specific systems, each
measured property and confirmed process is a result of the
combination of each forcing - that is also valid for global
circulation - consequently, topographic and meandering effects will
be better understood if analyzed as part of the equation that
governs ocean and atmosphere circulation, not as a separate
process. As a result of various oceanographic studies and
researches it is widely known that the major forcing responsible
for coastal upwelling is the Ekman transport and that other
processes play a secondary role on generating upwelling phenomena,
nonetheless, by carefully considering the effects of seafloor
topography and current meandering, among other, we can have a more
thoroughly answer for the unanswered questions that Ekman transport
alone cannot answer. Finally it can be concluded that the Brazil
Current cyclonic meandering inside the SBB causes the presence of
the SACW on the outer regions of the continental shelf. Thus, the
mechanism responsible for pumping the SACW onto the shelf was
mainly due to meander induced shelf break upwelling, during the
winter, and a combination of shelf break and wind-driven upwelling
in the summertime. Highlighting topographic and cyclonic meander
influence is not contradictory to the classical understanding of
Ekman upwelling. The classic Ekman theory considers only
large-scale processes, whereas in a narrow upwelling system the
topography and current meandering can also play an important role.
These smaller-scale processes might have a profound impact on the
biology, chemistry and geology at similar scales.
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3. Summary Table The following table is a summary of the
comparison between the Hainan Island and the Southeast Brazil
Bight, containing the processes that better describe the physical
environment in a concise way.
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4. References Anne F. Sell (2002), An Introduction: Impact on
Marine Food Chains, University of Hamburg Chai, F., H. Xue, and M.
Shi (2001), Upwelling east of Hainan Island, Oceanogr. China, 13,
129137. Edmo J. D. Campos, Denise Velhote and Ilson C.A. Silveira
Geophysical research letters, vol. 27, no. 6, pages 751-754, march
15, 2000 Gille et al. (2003) Seafloor Topography and Ocean
Circulation, Oceanography, Volume 17, Number 1. Guo, F., M. C. Shi,
and Z. W. Xia (1998), Two-dimensional diagnose model to calculate
upwelling on offshore of the east coast of hainan island, Acta
Oceanol. Sinica, 20(6), 109116. Hu, J. Y., H. Kawamura, and D. L.
Tang (2003), Tidal front around the Hainan Island, northwest of the
South China Sea, J. Geophys. Res., 108(C12), 3342,
doi:10.1029/2003JC001883. Sergio R. Signorini (1977), On the
circulation and the volume transport of the Brazil Current between
the Cape of So Tom and Guanabara Bay Su, J., and T. Pohlmann
(2009), Wind and topography influence on an upwelling system at the
eastern Hainan coast, J. Geophys. Res., 114, C06017,
doi:10.1029/2008JC005018. Chu, P. C., N. L. Edmons, and C. Fan
(1999), Dynamical mechanisms for the South China Sea seasonal
circulation and thermohaline variabilities, J. Phys. Oceanogr.,
29(11), 29712989.