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Some Peculiarities of Case 1 Waters Optical
Properties in the Northwestern Mediterranean
Sea (BOUSSOLE site, 4322 N 754 E)
David ANTOINE André MOREL, Hervé
CLAUSTRE Laboratoire dOcéanographie de
Villefranche, 06238 Villefranche sur mer, France
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The problem we aim at
Bricaud et al. (1998)
Morel and Maritorena (2001)
Natural variability of Case 1 waters optical
properties is known Can we explain this
variability in terms of the AOPs versus IOPs
relationships ?
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Our approach to understand the natural
variability of the AOPs and IOPs
Combination of - A time series with monthly
resolution (ship operations) - A high-frequency
(i.e., 15 min.) permanent sampling near the
surface with a new type of optical mooring -
Collection of a full set of IOPs, needed to
understand the AOPs and their anomalies with
respect to standard, global, models (--gt closure)
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The site where we collect data BOUSSOLE site
program Buoy for the acquisition of a
long-term (bio)optical series
Monthly cruises (started July 2001) a new type
of optical buoy (since Sept. 2003) Marine
optics, Bio-optics, Ocean color calibration /
validation program (MERIS, SeaWiFS, POLDER)
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Site characteristics (oligotrophic to eutrophic)
Winter, maximum of the water mixing Chl up to
2-3 mg m-3 mixed layer down to 200 meters
Summer, maximum of the stratification. DCM is
maximum, with surface Chl 0.05 mg m-3 (up to 1
in the DCM)
Spring, establishment of the deep chlorophyll
maximum around 50 meters Chl 0.3 mg m-3
Fall, erosion of the thermocline, the DCM
progressively disappears Chl 0.5 mg m-3
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SeaWiFS chlorophyll 2001-2004
Feb March Apr May June Jul Sept Oct Nov Dec
2001
2002
2003
2004
SeaWiFS/SIMBIOS diagnostic data
sets (http//seawifs.gsfc.nasa.gov/cgi/seawifs_reg
ion_extracts.pl?TYPocean)
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Anomalies we already know
Chlorophyll time series at the DYFAMED site
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SeaWiFS OC4v4, rep. 4
Regional algorithm (Bricaud et al., 2001)
May 1999 SeaWiFS composite
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Already identified anomalous Blue-to-green ratios
Claustre et al., 2002 (Geophys. Res.
Letters) (PROSOPE cruise) And others (Gitelson
et al., 1996 DOrtenzio et al., 2001, 2002
Corsini et al., 2002) Possible cause
deposition of small dust particles coming from
Sahara, reinforcing absorption in the blue and
scattering in the green
Morel Maritorena (2001).
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Which AOPs/IOPs we are looking at
AOPs (using a Satlantic 13-wavelengths SPMR
radiometer)
Irradiance reflectances at the 0- level, and
normalized for a sun at zenith R(l)
Eu(l1)/Ed(l2)
Blue-to-green reflectance ratios R(l1)/R(l2)
Diffuse attenuation coefficients for the
downwelling irradiance Kd(l) -dln(Ed(l) /
dZ
IOPs (Wetlabs AC9 particulate absorption on
filtered samples)
Total absorption, scattering, attenuation
Particulate absorption (total, phytoplankton,
detritus)
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Time series of the blue-to-green ratio
R(443)/R(560)
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Time series of the blue-to-green ratio
R(490)/R(560)
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Time series of the blue-to-green ratio
R(510)/R(560)
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Time series of the Kds
443 nm
412 nm
490 nm
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Time series of the Kds (continued)
510 nm
560 nm
670 nm
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Time series of the reflectances
443 nm
412 nm
490 nm
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Time series of the reflectances (contd)
560 nm
510 nm
670 nm
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Time series of particulate absorption coefficients
Year 2003 only
Winter phytoplankton absorption dominates
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Particulate absorption spectra
February
March
ap
af
ad
May
July
ap is decomposed into af and ad following Bricaud
and Stramski, 1990
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Spectra of the total (minus water) absorption
and scattering coefficient
April
February
c(l)
b(l)
a(l)
July
May
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Particulate scattering coefficient at 550 nm as a
function of Chl
from AC9 measurements
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In short...
The summer anomaly
- Lower-than-expected reflectances in the blue
are due to high absorption several causes are
possibly intermingled, such as Saharan dust,
detritus, CDOM...
- Greater-than-expected reflectances in the
green Saharan dust, detritus, others
(coccolithophorids) ?
- It is not a permanent feature
The winter anomaly
- Greater-than-expected reflectances in the
blue might be due to a lower absorption fresh
phytoplankton bloom with a lower proportion of
detritus
- Lower-than-expected reflectances in the green
could be due to large proportion of big cells
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General conclusions, perspectives
- The preliminary analysis of the AOPs and IOPs
time series has confirmed some trends already
observed in the Med. Sea (although not
permanent), and revealed others
- Anomalies in the AOPs can be explained in terms
of the IOPs, yet fundamental causes remain to be
ascertained.
- Understanding of the causes requires further
analysis of the data, and may require as well
some additional parameters (in particular CDOM
absorption, backscattering coefficient, AOPs in
the UV), as well as inversion of the AOPs into
IOPs (e.g., Loisel and Stramski 2001)
- Exploitation of the buoy time series (AOPs,
c(660), bb(443 550)) should help in this respect
- Any index in the reflectance spectra that may
help in a better interpretation of the
remotely-sensed observations ?
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Acknowledgements
Alec SCOTT, Chief engineer for the project,
monthly cruises, AOPs collection,
data processing Bernard GENTILI, Data
processing codes Davy MERIEN Joséphine RAS,
HPLC and ap measurements Dominique TAILLIEZ,
CTD IOPs, monthly cruises R/V Téthys-II
Captains crews
Thank you for your attention