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Interactions of the Eastern and Western Boundary Systems Off South America and South Africa With the Large-Scale Circulation in the Southern Ocean
Figure 1



Authors:


P. Ted Strub
College of Oceanic and Atmospheric Sciences,
Oregon State University, U.S.A.

Ricardo P. Matano
College of Oceanic and Atmospheric Sciences,
Oregon State University, U.S.A.

Corresponding Author:
Ted Strub
postal address: College of Oceanic and Atmospheric Sciences,
Oregon State University,
Ocean Admin Bldg 104
Corvallis, OR 97331-5503
USA
E-mail: tstrub@oce.orst.edu




Abstract:

The goal of this project is to investigate linkages between
basin-scale circulation and the eastern and western boundary currents
next to South America and South Africa. We are attempting to identify
the primary causes of variability in those boundary currents. The
specific regions of interest are the two eastern boundary currents
(the Peru-Chile Current System and the Benguela Current System)
and the two extremely energetic confluence regions for the western
boundary currents (the Brazil-Malvinas Confluence and the Agulhas
Retroflection Region).

Objectives:

Our overall scientific goal is to quantify the contribution of upstream
and downstream features to the variability of the regional boundary
current systems.

Our specific objectives include:

Analyzing model output, and altimeter and other satellite data in the Southern
Ocean Peni-Chile and Bengula eastern boundary currents (EBC), and exploring
their connections to basin scale currents. These basin-scale currents are affected
by the equatorial currents on their northern boundaries and the eastward flowing
west wind drift currents and Antarctic Circumpolar Current (ACC) to their south.

We will also extend our analyses to the western boundary currents (WBCs) off
the eastern coasts of South America and South Africa, and to the
Cape Horn Current off the southwest coast of South America.
Off South America, the strongest interactions are presently thought
to be between the ACC, Malvinas and Brazil Currents. Off South Africa,
the Agulhas and Benguela Currents interact with the South Atlantic
Current and the ACC, providing a connection between the boundary
currents from both sides of the continent. Our recent analyses of both
altimeter and model data suggests that the upstream region of the
Agulhas Current is affected by eddies that originate north of Madagascar.
Similarly, the Brazil Current is thought to be impacted by upstream eddies
that originate in the Agulhas Retroflection Area, after crossing the South
Atlantic.

Our final objective is to conduct regional model studies around the southern ``cones"
of both continents, in order to further examine the dynamics of the currents
and their interactions with upstream and downstream influences.

Figure 2
Figure 1 shows our large-scale regions of interest. Our present efforts
are primarily directed toward understanding the causes of annual and
interannual variability in the Peru-Chile Current and in examining
the sources of annual variability in the Agulhas Current, in both cases
extending work started under the TOPEX/Poseidon Extended Mission (TPEM).
On interannual scales, the work in the Peru-Chile Current has traced the
Equatorial El Niño signal into both hemispheres. This work is continuing,
primarily with funding from the U.S. GLOBEC Northeast Pacific project.
The first EOF of SSH in the eastern Pacific (Figure 2) is the clear El Niño
signal, which explains 20% of the variance and shows the strong connection
between the equator and coastal regions to approximately 20N and 20S. Weaker
connections continue to mid- and high-latitudes. Figure 3 shows the second
EOF of SSH in the eastern Pacific, which explains 10% of the variance
and picks out the seasonal cycle, including the strong connection between
the North Equatorial Counter Current (NECC), the Costa Rica Current along
Central America and the California Current along North America (out of
phase with the signal off Central America). The striking feature in
Figure 3
Figure 3 is the relative weakness in the seasonal cycle off South America
in comparison to Central/North America. We hypothesize that this is due
to the lack of a strong ITCZ and NECC in the southeast Pacific [Strub and
James, 2001a, 2001b, 2001c].

Figure 4
Off South Africa, analysis of regional models and altimeter data have
documented the annual cycle of transport in the Agulhas Current and
determined that this variability is not propagating into the Agulhas
region from the mid-latitude South Indian Ocean to its east [Matano et
al., 1998; Matano et al., 1999]. Further analysis using global numerical
model output has traced this variability to the Mozambique and North
Madagascar Currents (Figure 4) where the variability appears to be driven by
the large-scale winds in the western part of the South Indian Ocean north
of Madagascar [Matano et al., 2001]. The same analysis determined that
this variability does not originate farther east. Both altimeter data and
the global model output suggest that the eastern and central Indian Ocean
regions are decoupled from the circulation in the western region by the
topographic control of the Mid-Indian Ridge. A similar analysis of the
Brazil-Malvinas Confluence is in its early stages.

References

Matano, R.P., C.G. Simionato, W.P. de Ruijter, P.J. van Leeuwen, P.T. Strub,
D.B. Chelton, and M.G. Schlax, 1998: Seasonal variability in the Agulhas
Retroflection region, Geophys. Res. Letters, 25, 4361-4365.

Matano, R.P., C.G. Simionato, and P.T. Strub,
1999: Modeling the wind-driven variability of the South Indian Ocean,
J. Phys. Oceanogr., 29, 217-230.

Matano, R.P., E.J. Beier, P.T. Strub and R. Tokmakian, 2000: Large-scale
forcing of the Agulhas variability: The seasonal cycle, J. Phys. Oceanogr.,
(submitted).

Strub, P.T., and C. James, 2001a: Altimeter-derived surface circulation variability
in the large-scale NE Pacific Gyres: Part 1. Annual variability, Prog.
Oceanogr., (accepted, 1-01).

Strub, P.T., and C. James, 2001b: Altimeter-derived surface circulation variability
in the large-scale NE Pacific Gyres: Part 2. 1997-1998 El Niño anomalies,
Prog. Oceanogr., (accepted, 1-01).

Strub, P.T., and C. James, 2001c: The 1997-1998 El Niño signal along the SE
and NE Pacific boundaries - an altimetric view. Prog. Oceangr. (submitted


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