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Combining SMOS SSS with altimeter-derived geostrophic currents and wind-driven
(Ekman) estimated motions (Lagerloef et al.
1999
), the advection of the spatial patterns of
low salinity discharged from the major river mouths can now be analyzed systematically
with an unprecedented resolution.
As illustrated by the Fig.
3
and by the animation available at
http://www.ifremer.fr/
naiad/salinityremotesensing.ifremer.fr/altimetry_amazon_atl.gif
,
a very good visual con-
sistency is found between the geostrophic and Ekman surface current pattern estimates and
the SMOS SSS spatiotemporal distribution along the year.
Mignot et al. (
2007
) show a long-term seasonal to monthly climatology that highlights
two freshwater offshore pathways—the north passage to the warm pool and eastward
entrainment into the North Equatorial Counter Current (NECC)—but they cannot clearly
confirm or track this laterally with time in a given year.
SMOS SSS data combined with altimetry and surface wind information now enable to
follow the spatiotemporal evolution of the plume along these two freshwater offshore
pathways.
As illustrated in Fig.
3
(top), the surface freshwater dispersal patterns of the Amazon
River plume are closely connected to the surface current topology derived from the merged
altimeter and wind field product. As also evidenced earlier from several hydrographic
surveys (e.g., Hellweger and Gordon
2002
), it is clearly apparent in the satellite imagery
that the NBC rings are key factors in modulating the freshwater pathways of the Amazon
plume from the river mouth at the equator toward higher latitudes up to 20-22N.
Eastward entrainment of low-salinity water from the mouth of the Amazon River into
the NECC is also evident in the SMOS data for the second half of the year 2010 (see Fig.
3
,
bottom). During that period, freshwater dispersal structure exhibits a zonal wavy pattern
centered around * 8N induced by current instability waves shed near the NBC retro-
flection region (52W, 8N). To analyze the freshwater plume transport and the evolution
of salinity along Lagrangian paths following such wavy patterns, hypothetical drifters were
dropped around the mouth of the river at the beginning of June and temporally advected
with the surface currents deduced from merged altimeter and wind products. The evolution
of SSS from SMOS L-band and AMSR-E C-band sensors (see Reul et al.
2009
for details
on the AMSR-E SSS product), sea surface temperature analysis products and merged
MERIS-MODIS colored dissolved organic matter (CDOM) absorption coefficient was
estimated by interpolating the data in space and time along the path of such drifters.
As further illustrated by the example shown in Fig.
4
, it takes approximately 6 months
to cover a distance of 3,700 km for a freshwater particle (SSS * 26-28) in the proximity
of the Amazon mouth to relax to an open ocean surface salinity of *36. At the beginning
of the period, the low SSS of water particles is modulated by mixing processes with saltier
waters transported westward by the NBC rings shed at the NBC retroflection. The particle-
following SSS signal modulation observed here is clearly consistent with the ocean color
signal (anti-correlated with SSS), fresher water being systematically associated with col-
ored waters showing high CDOM values, typical of the brackish plume waters. The drifter
is then advected eastward along the NECC, remixed with ''younger'' advected plume
waters in August and reached an eastern position slightly north of 8N-38W with an SSS
of about 32 at the beginning of October. The SSS change along the drifter pathway is
progressively and quasi-linearly relaxing to the open ocean values during the next 3-month
period.
The link between the SSS and ocean color properties moreover enables investigations of
the interactions between bio-optical and bio-chemical properties of the ocean and hydro-
logical fluxes of terrestrial origin. Along with the freshwater, the Amazon provides the
