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Ocean Surface Topography from Space
SCIENCE
Study of seasonal-to-decadal variability in the Tropical Atlantic using altimetry, models and in situ observations


Figure 1

Authors:


S. Arnault
LODYC, France

CORRESPONDING AUTHOR:
Sabine Arnault
LODYC
Tour 14-2 case 100
4, place Jussieu
75252 Paris Cedex 05 - France
sa@lodyc.jussieu.fr



Abstract:

It is now well recognized that tropical oceanic regions have a big influence on Earth's climate
variability. Of particular interest is the tropical Atlantic Ocean. Relative to the Pacific,
the Atlantic extends to much higher northern latitudes and the cooling of relatively saline
waters of southern origin leads to deep mixing in the vicinity of Greenland. Surface waters
become increasingly saline as they flow in from the southern ocean, and are compensated by
the outflow of deep relatively fresh water. This process has been dubbed the Atlantic conveyor
belt [Gordon, 1982]. The thermohaline circulation of the Atlantic Ocean drives a pattern of
warm upper layer flow northward from the subtropical South Atlantic into the North Atlantic.
Cross-equatorial heat and salt flux is then required to produce this climatic Atlantic signal,
which lasts over decades.

Low-frequency variability

If a general picture of the seasonal cycle of upper layer variability in the tropical Atlantic
has been obtained in recent years, the low-frequency variability has been much less studied.
It is often regarded as being dwarfed by the powerful influence of the annual cycle. Recently
a number of studies have examined decadal tropical and midlatitude Atlantic climate variability
[Rajagopolan et al., 1998; Tourre et al., 1999; Robertson et al., 2000; Ruiz Barradas et al.,
2000] but the degree of correlation is not high. These studies are handicapped by the lack of
a reliable time series over a long period: in situ data are not homogeneous and numerical
coupled models are prone to significant bias. Integrated Sea Level Anomalies (SLAs) obtained
by altimetry will provide a unique data set covering these long-period fluctuations at a global
geographical scale. The nearly 10-year series of TOPEX/POSEIDON data already offers an insight
into the year-to-year SLA variability in the tropical Atlantic domain (figure 1). Clear deviations
from the seasonal cycle appear, peaking in boreal 1995 and 1996 winters, and during the 1998 summer.

It is therefore crucial that Jason and TOPEX/POSEIDON measurements be merged to obtain a
homogeneous data set.

Western boundary dynamics


Figure 2

Numerical studies of the tropical Atlantic ocean also reveal that most of the complex mechanisms
of these mass and heat transports should be concentrated on the western boundary, a complex area
with eddies, current retroflection, undercurrents and through-flows. Early modeling studies
indicated that the advection of South Atlantic waters into the subtropical gyre of the North
Atlantic was the result of a considerable continuous western boundary transport [Philander et
Pacanowski, 1986]. However, results from the WOCE community modeling effort do not show such
a large direct transport along this route [Schott and Boning, 1991], and observations indicate
that some transport occurs along the western boundary via intermittent shallow boundary currents
and eddies that peak off from the North Brazil Current (NBC) retroflection [Richardson et al.,
1994]. Recently, a joint investigation involving in situ measurements and TOPEX/POSEIDON altimetry
during the ETAMBOT experiment has improved our understanding of the surface circulation in
that area [Arnault et al., 1999]. For instance, altimetry has evidenced the presence in
September 1995 of a large eddy structure at about 51°30'W, 8°30'N, that was only partly
sampled during the campaign (figure 2).
It also showed the NBC retroflecting between 45
and 50°W, 4 and 8°N towards the North Equatorial Counter Current (NECC). But the NBC was
not the only current feeding the NECC at that period, as the North Equatorial Current also
contributed. Southward, around 3-4°S, the NBC also retroflected eastwards. Altimetry thus
gave an explanation for in situ measurements showing a surface eastward flow in that area
during boreal falls [Schott et al, 1998; Bourlès et al., 1999].

These results imply a high degree of accuracy, attained thanks to high-quality altimetric
measurements.

Subsurface dynamics

Looking at subsurface dynamics from altimetry is more challenging, because satellite data only
concern the ocean surface. Assimilation of satellite data in numerical models is a way to
project the 2D-altimetric information in deeper oceanic layers. In the tropical Pacific,
Carton et al. [1996] have demonstrated that altimetric data assimilation has a greater
impact than temperature profile or mooring assimilation on the SLA and surface current
variability. We conducted several Geosat and TOPEX/POSEIDON data assimilation experiments
in the tropical Atlantic using a variational approach [Greiner et al., 1998a and b; Greiner
and Arnault, 2000; Arnault and Greiner, 2001]. The results emphasized the impact of assimilation
not only on the SLA but also on the current structure, and not only at the surface.


Figure 3

For instance, the figure above reveals that assimilation plays an important role in terms of kinetic
energy for the oceanic currents located at about a depth of 50 meters along the Equator and
along the western American coast. This is the mean location for the eastward South Equatorial
Current, a key factor influencing heat and mass redistribution in the Tropical Atlantic basin.

In conclusion, high-quality satellite missions such as TOPEX/POSEIDON and, in the future,
Jason to obtain measurements spanning several years, combined with modeling and in situ
data collection, will provide a unique tool to learn more about the oceanic contribution
to climate variability.

References

Arnault S., B. Bourlès, Y. Gouriou, R. Chuchla, 1999: Intercomparison of the upper layer circulation of the western equatorial Atlantic Ocean: in situ and satellite data. J. Geophys. Res., 104, C9, 21171-21194.

Arnault S., E. Greiner, 2001: Upper layer circulation and transport in the tropical Atlantic ocean during 1993-1994 from a 4D-variational assimilation of satellite and in situ data. Progr. Oceanogr. (submitted).

Bourlès B., R.L. Molinari, E. Johns, W.D. Wilson, K.D. Leaman, 1999: Upper layer currents in the western tropical North Atlantic (1989-1991). J. Geophys. Res., 104, 1361-1376.

Carton J.A., B.S. Giese, X. Cao, L. Miller, 1996: Impact of altimeter, thermistor, and expendable bathythermograph data on retrospective analyses of the tropical Pacific Ocean. J. Geophys. Res., 101, C6, 14147-14159.

Gordon A.L., 1986: Interocean exchange of thermocline water. J. Geophys. Res., 91, 5037-5046.

Greiner E., S. Arnault, A. Morlière, 1998a: Twelve-monthly experiments of 4D-variational assimilation in the tropical Atlantic during 1987: Part 1: Method and statistical results. Progr. Oceanogr., 41, 2, 141-202.

Greiner E., S. Arnault, A. Morlière, 1998b: Twelve-monthly experiments of 4D-variational assimilation in the tropical Atlantic during 1987: Part 2: Oceanographic interpretation. Progr. Oceanogr., 41, 2, 203-247.

Greiner E., S. Arnault, 2000: Comparing the results of a 4D-variational assimilation of satellite and in situ data with WOCE CITHER 1 hydrographic measurements in the tropical Atlantic. Progr. Oceanogr., 47, 1-68.

Philander S.G.H., R.C. Pacanowski, 1986: A model of the seasonal cycle in the tropical Atlantic Ocean. J. Geophys. Res., 91, 14192-14206.

Rajagopalan B., Y. Kushnir, Y.M. Tourre, 1998: Observed decadal midlatitude and tropical Atlantic climate variability. Geophys. Res. Lett., 25, 3967-3970.
Richardson P.L., G. Hufford, R. Limeburner, W.S. Brown, 1994: North Brazil Current retroflection eddies. J. Geophys. Res., 99, 5081-5093.

Robertson A.W., C.R. Mechoso, Y.J. Kim, 2000: The influence of Atlantic sea surface temperature anomalies on the North Atlantic Oscillation. J. Climate, 13, 122-138.

Ruiz-Barradas A., J.A. Carton, S. Nigam, 2000: Structure of interannual-to-decadal climate variability in the tropical Atlantic sector. J. of Climate, 13, 3285-3297.

Schott F.A., C.W. Boning, 1991: The WOCE model in the western Atlantic upper layer circulation. J. Geophys. Res., 96, 6993-7004.

Schott F.A., L. Stramma, J. Fischer, 1998: Transports and pathways of the upper layer circulation in the western tropical Atlantic. J. Phys. Oceanogr., 28, 1904-1928.

Tourre Y., B. Rajagopalan, Y. Kushnir, 1999: Dominant patterns of climate variability in the Atlantic Ocean during the last 136 years. J. Climate, 12, 2285-2299.


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