Influence of the Amazon River Runoff on the tropical Atlantic
S. Masson and P. Delecluse
Laboratoire d’Océanologie DYnamique et de Climatologie, Paris, France
Camera-ready Copy for
Physics and Chemistry of the Earth
Manuscript-No. OA29-0002
Offset requests to:
S. Masson
Laboratoire d’Océanologie DYnamique et de Climatologie
Paris, France
Journal: Physics and Chemistry of the Earth
MS No.: OA29-0002
First author: Masson
1
Influence of the Amazon River Runoff on the tropical Atlantic
S. Masson and P. Delecluse
Laboratoire d’Océanologie DYnamique et de Climatologie, Paris, France
Received ??? – Accepted ???
Abstract. The Amazon river has the biggest flow in the
world, 0.2 Sv (1 Sv = 10 6 m3 /s), and is responsible for a
large part of the low Sea Surface Salinity (SSS) in the west
tropical Atlantic ocean. Very few Ocean General Circulation Model (OGCM) include runoff which brings a specific
contribution to the Ocean physics. Comparison of simulations with constant or monthly runoffs shows that the spread
of the Amazon fresh waters offshore of the north Brazilian
coast is controlled by the ocean circulation and not by the Amazon flow. Therefore, in the model like in the observations, SSS minimum is observed in summer three months after
the Amazon flood. In agreement with observations, a thick
(more than 40 m) Barrier Layer (BL) is present every summer north of the Amazon mouth. Because of the strong and
shallow salinity gradient associated with the Amazon freshwater, an important part of the solar radiation is trapped in
the BL and creates an inversion of the vertical gradient of
temperature. However with this forced model, BL does not
seem to have a clear impact on SST and so on the air-sea interaction. Freshwater flux is also able to bend the sea surface.
The geostrophic part of the North Brazilian Current (NBC)
retroflection is then lightly weaker in presence of the Amazon runoff.
1 Introduction
Salt plays an important role on the global ocean thermohaline circulation through its contribution to density (Gordon,
1986). However most of the tropical circulation models neglect salinity effects even the forecast models used for complex phenomena like El Niño.
Recent studies have underlined the importance and the need
to consider salinity in ocean models:
-without salinity, data assimilation in model can degrade the
model performance instead of improving it (Murtugudde and
Busalacchi, 1998).
Correspondence to: S. Masson
-salinity, by creating a “barrier layer” may have a major role
in the physics of the mixed layer -i.e. on Sea Surface Temperature (SST)- and so indirectly in the ocean-atmosphere interaction (Miller, 1976; Lukas and Lindstrom, 1991; Vialard
and Delecluse, 1998a).
Carton (1991) and Murtugudde and Busalacchi (1998) studied the impact of freshwater fluxes on SSS and SST in the
tropical Atlantic ocean. One of their main results is that when
freshwater flux is positive, the decrease in SSS may create a
shallow halocline allowing a SST increase.
In the present paper, we focus our attention on the impact of
the freshwater input by the Amazon river on the tropical Atlantic which is a key region of this ocean. A complex western
boundary current dynamics binds the equatorial waters with
the tropical waters and allows a way between the Northern
hemisphere and the Southern one.
The structure of the paper is as follows. In section 2, a brief
description of the Ocean model is given. In section 3, we will
show how the SSS variation is improved by introducing the
river runoffs. In section 4, we study the effect of freshwater
on the ocean physics: the Barrier Layer and the modulation
of the North Brazilian Current (NBC) retroflection. Finally,
section 5 presents conclusions.
2 Model description
2.1 The TOTEM version of the OPA ocean model
This study was performed with the OPA version 8 Ocean
General Circulation Model (OGCM) developed at Laboratoire d’Ocanographie Dynamique et de Climatologie (LODYC) by the Delecluse’s team (Madec et al., 1999). OPA solves
the primitive equations assuming the Boussinesq and hydrostatic approximations, the incompressibility hypothesis and a
rigid lid boundary as the sea surface. Simulations were performed with TOTEM configuration of the model grid which
has a relatively high resolution grid (0:33 Æ for the maximum
mesh resolution around the equator for zonal and meridian
Journal: Physics and Chemistry of the Earth
MS No.: OA29-0002
First author: Masson
direction) covering the whole tropical oceans between 45 Æ S
and 45Æ N. In the model, there is no restoring in the oceanic
interior in temperature and salinity between 20 Æ S and 20Æ N.
Outside this free restoring belt, a linear restoring term is applied towards the monthly mean temperature and the seasonal salinity fields of Levitus (1982). A complete description of
this model is available in Maes et al. (1998) and Boulanger
et al. (1997).
2
2.3.3 No runoff: EXP4
In this last sensitivity experiment, no runoff is considered.
3 SSS simulation
2.2 The forcing fluxes: ECMWF ERA-15 and river runoffs
The ocean model is forced by zonal and meridian wind stress,
net surface solar radiation, net surface heat flux and net water flux (evaporation minus precipitation). These fields come
from The European Center for Medium-Range Weather Forecast (ECMWF) reanalysis (1979-1993), named ERA - 15.
They are interpolated on the TOTEM grid. To avoid unrealistic temperature induced by the heat forcing biases, we use
a feedback term on AMIPII SST (Gates, 1992).
We also add to ERA-15 net water flux, the river runoffs.
2.3 Sensitivity experiments
The reference experiment (EXP1) between 1979 and 1993
forced by ERA-15 is performed with a constant Amazon flow
equal to 6300 km 3 /year (0.2 Sv) (Baumgartner and Reichel,
1975). Our work is based on the comparison between this
reference experiment and three other sensitivity experiments
which - because of the computer time constraint - are performed over only 3 years (1980-1982). Sensitivity experiment are used to put the mechanisms found with the reference experiment to the test. 1981 is the most significant year
and we use it to illustrate the figures including sensibility experiments.
2.3.1 Monthly runoffs: EXP2
This experiment differs from EXP1 by the river flow parameterization. We use in this case monthly seasonal runoff values adapting from UNESCO (1996) to obtain the same mean
values as for EXP1 (see thick dotted line in Fig. 2)
2.3.2 Brünt-Visl frequency computed with constant salinity: EXP3
To understand the impact of the Barrier Layer and of the
strong salinity stratification in the vertical physics of the ocean, we run the same experiment as the one made by Vialard
and Delecluse (1998a), where the vertical mixing is computed without considering the salinity stratification. To do this,
we run the same configuration as EXP2 but with a Brünt-Visl
frequency calculated with the temperature of the model and
a constant salinity of 35 psu. The Brünt-Visl frequency is
used to compute the turbulent kinetic energy (TKE) (Blanke
and Delecluse, 1993) which determines the vertical mixing
coefficients. This experiment allows to evaluate the impact
of salinity on the model dynamics and thermodynamics in
regions of the BL.
Fig. 1. SSS map offshore of the Amazon mouth for August 1981. Units are
in psu and contour interval is 0.5 psu. Shaded area for SSS 34 psu. Upper
panel: EXP2 and lower panel: EXP4.
<
One of the main goals for introducing the runoff is to simulate a more realistic SSS, without a restoring feedback term and thus get a better understanding of SSS variations. In
this section we will evaluate the simulated SSS with the climatology (Dessier and Donguy (1994) and its updated version with SSS data until 1997 available on the WEB site
http://www.ifremer.fr/ird/sss/climato.html).
3.1 Annual mean validation
The comparison between the annual mean of the climatology and the fifteen years mean of our reference simulation
is good. In most parts of the ocean, the SSS differences are
less than 0.2 psu. Observations and model results present
both SSS maxima in the northern and south-western part of
the tropical Atlantic ocean (with SSS > 37 psu) separated
by SSS less than 36 psu between the equator and 10 Æ N. SSS
minima (< 35 psu), associated with strong salinity gradient,
Journal: Physics and Chemistry of the Earth
MS No.: OA29-0002
First author: Masson
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are caused by the river runoffs. Two regions are concerned :
the Guinea Gulf and the area located northwest of the Amazon mouth (until 15 Æ N). The region offshore of the Amazon
mouth is nicely simulated. The plume of the fresh water coming from the Amazon is well represented in the model (Fig.
1) even if it does not spread enough north westward.
3.2 Seasonal variations
In the tropical Atlantic ocean the runoffs are responsible for
strong salinity gradients and complex local structures. In particular, seasonal variations occur north of the South American coast, offshore of the Amazon mouth and this will be our
region of interest.
3.2.1 Winter - spring :
This part of the year is not very important for SSS variation in the west tropical Atlantic. The NBC is weak (Bourles
et al., 1999; Johns et al., 1998), steady and flows along the
South American coast beyond the retroflection region to feed
the Guyana current (Richardson and Reverdin, 1987; Johns et al., 1990; Schott et al., 1993, 1995). The fresh water
coming from the Amazon stays near the river mouth and low
salinity area does not spread far away. Spring is a transition
period where the Trande winds increase and summer circulation takes shape.
3.2.2 Summer - autumn :
The main effects of the Amazon freshwater input on the SSS
are observed in summer. According to fig.1, in EXP4, SSS
minima is 36 psu in August, whereas in observations as well
as in EXP1 and in EXP2, SSS is less than 33 psu on a large
area north of the Amazon mouth during summer. This very
low SSS is possible owing to the presence of the huge value
of the Amazon river flow. Thus we focus our interest on EXP1 and EXP2.
Unlike in winter, the local currents are now accelerated by
the Trade winds which are stronger during this time of the
year (Schott et al., 1993; Johns et al., 1998) . Model and
observations are now perfectly in agreement. The NBC velocity is larger than 1m/s (Stramma and Schott, 1996; Johns
et al., 1998; Bourles et al., 1999) and mostly feeds the NECC (Katz, 1993; Molinari and Johns, 1994; Limeburner et al.,
1995; Stramma and Schott, 1996; Johns et al., 1998; Bourles
et al., 1999). This strong current system advects the Amazon waters which pour in the ocean in the retroflection area.
Therefore we observe a SSS minimum along the NBC/NECC
system, exactly where the Amazon waters are spreading. The
shape of the simulated SSS (Lower panel in fig. 1) is close to
the CZCS image for September 24-26 1979 plate 1 in Johns et al. (1990) (see also Lentz (1995); Muller-Karger et al.
(1988) their figure 2). During these three months, SSS comparison between EXP1 (solid line) and EXP2 (dashed line)
gives interesting results (Fig. 2):
-When the monthly mean cycle is taken into account, the
Fig. 2. SSS time series averaged in the box 55Æ W-45Æ W, 0-10Æ N for the
year 1981. EXP1 in solid line, EXP2 in dashed line. Units are in psu on the
left axis. Amazon river flow used in EXP2 is represented by the thick dotted
line. Units are in Sv (106 m/s) on the right axis
amount of the fresh Amazon water in July which will be advected northeastward is greater than for EXP1. Therefore the
summer SSS decrease in EXP2 is greater than in EXP1 (in
EXP2, 1.5 psu decrease instead of 0.5 psu in EXP1 in August
1981 on Fig. 2). Monthly runoffs improve the amplitude of
the model SSS variations.
-With or without the Amazon flow monthly variations, there
is always a delay between the Amazon flood in May and the
SSS decrease in July (Fig. 2). In this Figure the equal phase
of the two curves in EXP1 and EXP2 proves that the SSS
variation northward of the Amazon mouth is not controlled
by the Amazon flow but by the currents dynamics : the increase of the NBC and NECC in summer.
3.2.3 Conclusion :
The amplitude of the seasonal variation of the SSS is controlled by the amount of the river flow but the phase depends
only on the ocean dynamics and not on the river flood.
4 Impact of runoff
4.1 The Barrier Layer
In most cases, it is possible to schematize the vertical structure of the ocean in two layers : a mixed layer in temperature
and salinity -thus in density- separated from the deep ocean
by a strong vertical temperature gradient, the thermocline.
In presence of a Barrier Layer (BL) (Lukas and Lindstrom,
1991), the halocline is shallower than the thermocline and
the vertical structure of the ocean is divided in three layers.
In this case, the upper pycnocline is determined by the salinity gradient and not by the temperature gradient. The BL is
the layer between the pycnocline and the thermocline, it iso-
Journal: Physics and Chemistry of the Earth
MS No.: OA29-0002
First author: Masson
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lates the warm surface waters from the cold deep ocean. We
define also the barrier layer thickness (BLT) by the distance
between the pycnocline and the thermocline.
ed with the thicker BL spreads on a bigger area. Therefore
the mean BLT simulated in the second experiment is slightly
larger than EXP1.
Most of BL studies (Lukas and Lindstrom, 1991; Vialard
and Delecluse, 1998a,b) focus on the Pacific ocean. In that
ocean, the BL is mainly caused by the maximum of precipitation in the western Pacific which feeds the fresh waters in
the warm pool. In the Atlantic ocean, the low precipitation
amplitude is not able to create a large low SSS area like in the
Pacific. Therefore in the Atlantic ocean, the BL mechanism
must be different. Defant (1961) and observations used in
Miller (1976) showed in the tropical Atlantic ocean a mixed
layer in salinity shallower than the mixed layer in temperature. They explained the presence of this -not at this time
called- BL by the presence, just over the thermocline, of the
high salinity waters formed in the central subtropical gyres
of both hemispheres. Sprintall and Tomczak (1992) -with
the low resolution salinity data set of Levitus (1982)- reached
the same conclusions. Pailler et al. (1999) made the first study of the BL in the west tropical Atlantic with high vertical
resolution data. They used 350 Conductivity Temperature
Depth (CTD) profiles and proved that, in the west tropical
Atlantic, a BL mostly created by the fresh waters of the Amazon, extends over a large part of the equatorial basin in
boreal summer-fall.
4.1.2 Formation mechanisms
Fig. 3. 15 year average of BLT, for August, in EXP1. Units are in m.
Contour interval is 10m. Shaded area are for BLT 30 m. Surface currents
are also draw. Maximum speed value is 1.2 m/s
>
4.1.1 Seasonal variation and validation
In the model, the BL appears every year in the 15 year reference experiment, from July to October. The BL is present
when the SSS is less than 35.5 psu (see Fig. 3). Thus it does
not exist in EXP4. The maximum BLT is more than 40 m
thick and appears where the SSS is minimum. The BL simulated by the model is very close to the BL map proposed by
Pailler et al. (1999), their figure 5. The sensitivity of the BL
to the Amazon flow is not very important. Like for SSS, the
phase of the BLT variation is always the same with or without
monthly runoffs. However in EXP2, very low SSS associat-
The BL needs a shallow pycnocline and a deeper thermocline. To obtain this situation :
-the pycnocline must be created by a strong salinity gradient
and thus confounded with the halocline.
-a heat contribution in the subsurface must allow a thermocline deeper than the turbulent mixed layer. In the next two
sections, we will see why such a situation can exist in the
west tropical Atlantic from July to October.
A shallow pycnocline :
We have already seen in section 2.2.3. how the fresh water from the Amazon river is advected northward by the NBC/NECC system. This very low SSS patch is associated
with a shallow pycnocline. However the subsurface circulation (under the thermocline) has also a significant role on
the halocline formation. Once again, the change from the
winter / spring to the summer / autumn season in the west
Atlantic dynamics is worth considering. In winter / spring,
the NBUC, which advects salty water from the south Atlantic
(da Silveira et al., 1994; Stramma et al., 1995), is mainly
retroflected on the Equator to feed the EUC (Johns et al.,
1990; Stramma and Schott, 1996; Johns et al., 1998). So,
during these months, little salty water crosses the Equator to
the north Atlantic. In summer / autumn, with the north position of the ITCZ, the SEC and the NBUC increase (Schott et al., 1995; Johns et al., 1998). The retroflection of the
NBUC is now farther north near 6 Æ N and feeds the NEUC
(Bourles et al., 1999). Therefore the NBUC advects salty
water just under the fresh water of the Amazon river and reinforces the vertical salinity gradient between the two water
masses. The inflow of fresh water at the surface and salty
water in subsurface raises the pycnocline from 80 - 100 m to
10 - 20 m.
A deeper thermocline :
In summer, in addition to the seasonal heating, the NBC
/ NECC system advects warmer water from the South and
helps to maintain a deep thermocline (see the gray part of the
lower panel in fig 4 corresponding to the current maximum).
We will see in the next section that in presence of the BL an
important part of the penetrative solar radiation is trapped in
the BL and reinforces the possibility to maintain a thermocline deeper than the pycnocline.
4.1.3 Consequences for the vertical physics :
The works of Lukas and Lindstrom (1991) and Vialard and
Delecluse (1998a) underline the role of the BL on the ocean
vertical physics by trapping the wind momentum in the thin
mixed layer or by modifying the heat exchange between atmosphere and ocean. To explore the BL consequences for the
ocean physics, we proceed to EXP3. As explained in section
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MS No.: OA29-0002
First author: Masson
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mocline is slightly shallower.
4.2
Modification of the circulation in presence of runoffs
One interesting difference between EXP1 and EXP4 is the
possibility to change the amplitude of the NBC retroflection
only by introducing Amazon runoff. The area north of the
Amazon mouth is far enough from the Equator to consider
that the currents are mainly geostrophic. Therefore the NBC
retroflection is characterized by a bump in sea level around
which turns the geostrophic part of the currents. In EXP1 and
EXP2, the presence of the light fresh waters of the Amazon
bends the sea level and decreases the height of the bump. As
the geostrophic currents are proportional to the value of the
slope in sea level, the norm of the surface currents in the NBC
retroflection is 10 to 20 cm/s smaller in the experiments with
runoff than in EXP4 (Fig. 5).
5 Conclusion
Fig. 4. Vertical section at 49Æ W between 0 and 12Æ N of the temperature in
August 1981. Units are in Æ C. Contour interval is 0.25Æ C and shaded area
27 Æ C. The dashed line represents the pycnocline
are for temperature
>
depth. Upper panel: EXP2, lower panel EXP3
2.3.2., comparison between EXP3 and EXP2 allows to quantify the role of salinity on the ocean vertical stratification.
The impact of salinity on the mixed layer depth is important.
For example in 1981, the mixed layer is 30 meter deeper in
EXP3 in the area where the BL is present in EXP1. What are
the consequences for the ocean-atmosphere exchanges ?
During a westerly wind burst, Vialard and Delecluse (1998a)
prove that the thinning of the mixed layer in presence of BL
traps the wind momentum and reinforces the Ekman currents. In summer, north of the Amazon mouth, local winds
are weak because of the ITCZ. The currents in this area are
indeed mainly remotely forced and their amplitude is not influenced by the thickness of the local mixed layer. Therefore
differences in surface currents between EXP2 and EXP3 do
not show significant patterns (not showed here).
Vialard and Delecluse (1998a) have shown that in presence
of the BL, a significant part of the solar radiation penetrates
under the mixed layer and heats the BL which is isolated
from the surface mixing by the pycnocline. This warming of
the BL is able to create a negative vertical temperature gradient. In the Atlantic ocean, observations used in Miller (1976)
and collected during the March 1989 STACS cruise (Wilburn
et al., 1990) indicate some negative vertical temperature gradient in presence of BL. This feature is also observed in the
model simulation. Fig. 4 illustrates the trapping of heat in
the BL :
-in EXP2, the temperature is 0.25 C higher than in EXP3,
just under the pycnocline at 10 m deep.
-in EXP2, the stratification in salinity prevents vertical mixing from penetrating under 50 m like in EXP3, and the ther-
The present paper is the first one, to the knowledge of the authors, to study the formation of a Barrier Layer in the tropical
Atlantic ocean with an OGCM. It shows in particular that the
formation of a BL is strongly related to the influence of the
Amazon river runoff on the west tropical Atlantic Ocean.
Our first task was to evaluate the impact of adding a runoff
on the simulated SSS. We proved that introducing Amazon
runoff allow us to simulate a SSS very close to observations.
The large amplitude of the seasonal variations of the Amazon
flow is important to obtain the good SSS minima during the
summer. However the SSS time evolution is controlled by
the dynamics : in summer the NBC/NECC system advects
the fresh waters north and northeastward and so creates the
large Amazon plume observed for example in the the CZCS
images three months after the Amazon flood. EXP4 allows
us to underline the major role of the Amazon river runoff in
comparison with the precipitation, even in summer when the
ITCZ is over the Amazon mouth.
The seasonal fresh water input creates a thick BL from July
to September. The robust formation mechanism and the CTD observations prove that this BL is not a model artefact and
does really exist.
The impact of the BL on the air-sea interaction is difficult
to estimate with a forced model. A clear temperature inversion is observed in presence of BL (Fig. 4) but SST remains
the same in EXP2 and EXP3 (the restoring term to AMIPII SST does not show significant differences between EXP2
and EXP3).
The action of the Amazon river on the ocean circulation is an
interesting feature. It modifies the surface salinity distribution and the associated local dynamics. Nevertheless some
more work is needed to improve the regional simulation of
currents and to test whether this local process could affect
the whole equatorial response.
Acknowledgements. The authors thank M. Ioualalen, C. Levy and G. Madec
for the finalizing of the of the OPA-TOTEM / ERA - 15 experiment. J.
Journal: Physics and Chemistry of the Earth
MS No.: OA29-0002
First author: Masson
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Fig. 5. August 1981 surface current norm for EXP2 in left panel and EXP4 in right panel. Units are in m/s, contour interval is 0.2 m/s and shaded area are for
current norm 0.6 m/s
>
Vialard, B. Bourles and K. pailler provided very useful discussions on the
Barrier Layer. We are grateful to the anonymous referees for their valuable
suggestion
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