some peculiarities of case 1 waters optical properties in the northwestern mediterranean sea
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ASLO/TOS Ocean Research Conference 2004, Feb. 15-20, 2004, Honolulu, Hawaii1/24
Some Peculiarities of Case 1 Waters Optical Properties
in the Northwestern Mediterranean Sea
(“BOUSSOLE” site, 43°22 ’N; 7°54 ’E)David ANTOINE
André MOREL, Hervé CLAUSTRE
Laboratoire d’Océanographie de Villefranche, 06238 Villefranche sur mer, France
ASLO/TOS Ocean Research Conference 2004, Feb. 15-20, 2004, Honolulu, Hawaii2/24
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 ?
ASLO/TOS Ocean Research Conference 2004, Feb. 15-20, 2004, Honolulu, Hawaii3/24
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 (--> closure)
ASLO/TOS Ocean Research Conference 2004, Feb. 15-20, 2004, Honolulu, Hawaii4/24
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)
ASLO/TOS Ocean Research Conference 2004, Feb. 15-20, 2004, Honolulu, Hawaii5/24
Site characteristics(oligotrophic to eutrophic)
Winter, maximum of the water mixingChl up to ~2-3 mg m-3
mixed layer down to 200 meters
Spring, establishment of the deep chlorophyll maximum around 50 metersChl ~ 0.3 mg m-3
Summer, maximum of the stratification. DCM is maximum, with surface Chl ~ 0.05 mg m-3 (up to 1 in the DCM)
Fall, erosion of the thermocline, the DCM progressively disappearsChl ~ 0.5 mg m-3
ASLO/TOS Ocean Research Conference 2004, Feb. 15-20, 2004, Honolulu, Hawaii6/24
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_region_extracts.pl?TYP=ocean)
ASLO/TOS Ocean Research Conference 2004, Feb. 15-20, 2004, Honolulu, Hawaii7/24
“Anomalies” we already know
in situ TchlaRegional algorithm(Bricaud et al., 2001)
SeaWiFS OC4v4, rep. #4
Chlorophyll time series at the DYFAMED site
Start of the optics time series
ASLO/TOS Ocean Research Conference 2004, Feb. 15-20, 2004, Honolulu, Hawaii8/24
SeaWiFS OC4v4, rep. #4
Regional algorithm(Bricaud et al., 2001)
May 1999 SeaWiFS composite
ASLO/TOS Ocean Research Conference 2004, Feb. 15-20, 2004, Honolulu, Hawaii9/24
Already identifiedanomalous Blue-to-green
ratios Claustre et al., 2002 (Geophys. Res. Letters)(PROSOPE cruise)
And others :(Gitelson et al., 1996 D’Ortenzio 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).
ASLO/TOS Ocean Research Conference 2004, Feb. 15-20, 2004, Honolulu, Hawaii10/24
Which AOPs/IOPs we are looking at
“Blue-to-green reflectance ratios” R(1)/R(2)
Irradiance reflectances at the “0-” level, and normalized for a sun at zenith :
R() = Eu(1)/Ed(2)
Diffuse attenuation coefficients for the downwelling irradiance :
Kd() = -d[ln(Ed()] / dZ
AOPs (using a Satlantic 13-wavelengths “SPMR” radiometer)
IOPs (Wetlabs’ AC9 & particulate absorption on filtered samples)
Total absorption, scattering, attenuation
Particulate absorption (total, phytoplankton, detritus)
ASLO/TOS Ocean Research Conference 2004, Feb. 15-20, 2004, Honolulu, Hawaii11/24
Time series of the blue-to-green ratio
R(443)/R(560)Model Data
in situ Chl
“reconstructed” Chl
ASLO/TOS Ocean Research Conference 2004, Feb. 15-20, 2004, Honolulu, Hawaii12/24
Time series of the blue-to-green ratio
R(490)/R(560)
ASLO/TOS Ocean Research Conference 2004, Feb. 15-20, 2004, Honolulu, Hawaii13/24
Time series of the blue-to-green ratio
R(510)/R(560)
ASLO/TOS Ocean Research Conference 2004, Feb. 15-20, 2004, Honolulu, Hawaii14/24
Time series of the Kd’s
443 nm
490 nm
412 nm
ASLO/TOS Ocean Research Conference 2004, Feb. 15-20, 2004, Honolulu, Hawaii15/24
Time series of the Kd’s (continued)510 nm 560 nm
670 nm
ASLO/TOS Ocean Research Conference 2004, Feb. 15-20, 2004, Honolulu, Hawaii16/24
Time series of the reflectances443 nm
490 nm
412 nm
ASLO/TOS Ocean Research Conference 2004, Feb. 15-20, 2004, Honolulu, Hawaii17/24
Time series of the reflectances (cont’d)560 nm510 nm
670 nm
ASLO/TOS Ocean Research Conference 2004, Feb. 15-20, 2004, Honolulu, Hawaii18/24
Time series of particulate absorption coefficients
Summer : absorption by detritus is at least equal to, and actually greater
than, phytoplankton absorption
Winter : phytoplankton absorption dominates
Year 2003 only
ASLO/TOS Ocean Research Conference 2004, Feb. 15-20, 2004, Honolulu, Hawaii19/24
Particulate absorption spectra
ap is decomposed into a and ad following Bricaud and Stramski, 1990
ap
a
ad
February
JulyMay
March
ASLO/TOS Ocean Research Conference 2004, Feb. 15-20, 2004, Honolulu, Hawaii20/24
Spectra of the total (minus water)
absorption and scattering coefficient
April
JulyMay
Februaryc()
b()
a()
ASLO/TOS Ocean Research Conference 2004, Feb. 15-20, 2004, Honolulu, Hawaii21/24
Particulate scattering coefficient at 550 nm as a function of Chl
0,01
0,10
1,00
10,00
0,010 0,100 1,000 10,000
Chl (mg m-3)
b(5
50
) (m
-1)
from AC9 measurements
(+/- a factor of 2)
« Fresh bloom » ?A lot of detritusin summer
Loisel & Morel (1998)
ASLO/TOS Ocean Research Conference 2004, Feb. 15-20, 2004, Honolulu, Hawaii22/24
In short...
- “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) ?
The “summer anomaly”
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
- It is not a permanent feature
ASLO/TOS Ocean Research Conference 2004, Feb. 15-20, 2004, Honolulu, Hawaii23/24
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
- 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
- Anomalies in the AOPs can be explained in terms of the IOPs, yet fundamental causes remain to be ascertained.
- Any index in the reflectance spectra that may help in a better interpretation of the remotely-sensed observations ?
ASLO/TOS Ocean Research Conference 2004, Feb. 15-20, 2004, Honolulu, Hawaii24/24
AcknowledgementsAlec SCOTT, Chief engineer for the project,
monthly cruises, AOPs collection, data processing
Bernard GENTILI, Data processing codesDavy MERIEN
Joséphine RAS, HPLC and ap measurements
Dominique TAILLIEZ, CTD + IOPs, monthly cruises
R/V Téthys-II Captains & crews
Thank you for your attention
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