Optical%20variability%20and%20optical%20

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Optical variability and optical anomalies in
Mediterranean waters

• André Morel, David Antoine
• and Hervé Claustre
• Laboratoire dOcéanographie de Villefranche
• (CNRS and Univ. P. M. Curie)

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Is desert dust making oligotrophic waters
greener? (H. Claustre et al., GRL, 2002)
Field data (PROSOPE Cruise), versus Remote
Sensing OC algorithms (OC4V4 and OC4Me)
Blue-to-green ratio Anomalously low In Med-Sea
PROSOPE cruise
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Depth of the euphotic layer, as expected from
Surface Chl
Hyperspectral Model (MM01) Sun at 0 or
75 -Uniform profiles (solid lines) or -profiles
including a DCM (dashed lines)
(Morel Gentili, 2004)
Recent data Med-Sea data (anomalously low Zeu)
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Mediterranean Sea A semi-enclosed basin,
Rather well ventilated, however (water residence
time rather short 70 y) Arid climate, and
reduced run off limited
continental shelf A priori, a Case 1 water
domain, with varying trophic status Predominantl
y oligotrophic, sporadically and periodically
mesotrophic (Blooms) SO, why anomalies ?
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Anomalies, or nuances, detectable against a
standard for
Case 1 waters
Therefore, Defining a standard is a
prerequisite, Possibility Consider the
average empirical relationships established
between some optical properties (IOP and AOP)
and (Chl), used as an index of the bio-optical
state.
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Standard for Case 1 waters? Historical
empirical relationships provide such average laws
( SD), (generally non-linear laws of (Chl))
For instance IOP ap(?,
Chl) A(?) Chl B(?)
Bricaud et al, bp(?, Chl)
Bo(?) Chl ß(?) Gordon-Morel,
Loisel-Morel cp(?, Chl)
Co(?) Chl ? (?)
Voss AOP Kd(?,Chl) Kw (?)
?(?) Chl e(?) Morel-Maritorena.
Rrs(?1) / Rrs(?2) Pol (Chl)
OReilly et al.
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COMPATIBILITY BETWEEN THESE EMPiRICAL LAWS ?
has to be checked
before ascertaining Standards, and being able
to identify nuances 1) Closure (IOP) ?

? ap (?, Chl) bp(?,
Chl) cp(?, Chl)
(Bricaud et al, ) (Gordon-Loisel-Morel)
(Voss) 2) Coherency (IOP/AOP) ?
Possible Inversion Kd (?, Chl) ? atot (?,
Chl), (with atot ap ay aw)
? atot (?,
Chl) gt ap (?, Chl) if yes

then ay (?, Chl)
?
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From Bricaud et al., 1998 ap(?) as
f(Chl) Average law and confidence interval
(example for 440nm)
Data Med-Sea
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Loisel-Morel, LO, 1998
slope 0.766
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• Closure c a b
• 
  

Medit. Sea (above average relationship
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(Med-Sea)
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INVERSION (AOP -gt IOP)
Kd 1.0395 (µd) -1 (a bb) R f bb/ (a
bb) a(?) 0.962 Kd (?) µd(?s,
?, Chl ) 1 - R(? ) / f(?s, ?, Chl )
bb(?) 0.962 Kd (?) µd (?s, ?, Chl ) R(? )
/ f(?s, ?, Chl ) Look up Tables for µd and
f (Morel-Gentili, JGR 2004)
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Compatibility between ap and Kd Inversion (Kd ?
atot) through
atot(?,Chl) 0.962 Kd (?,Chl,?s)
µd(?,Chl,?s) 1- R(?,Chl,?s)/f(?,Chl,?s)
R(?,Chl,?s), f(?,Chl,?s), and
µd(?,Chl,?s) in LUTs (from RTE
computations) Then
atot ap ay aw
Is atot coherent with ap ? (i.e., atot gt ap )
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Examples of Kd(?) ? (Chl) empirical relationships
NOMAD Data Black curves Morel-Maritorena Statist
ical relationships
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Example of Inversion Kd(412) ? atot (412) Then
Decomposition atot (ap aw) ay
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ay(412) from the previous figure ay Data
(Pacific) also obtained by inversion using Kd
and R
Conclusion Pacific waters close to standard Case
1 water regarding ay
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ay(412) from the previous figure Data (Med.
Sea) also obtained by inversion
Moroccan upwelling
Conclusion Med-Sea waters above standard
regarding ay
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CONCLUSION

• Empirical relationships are compatible,
• can be used to define standard ,
  Chl-dependent, Case 1 waters
• With respect to this standard, Med-Sea waters
  exhibit notable, and identified nuances (thence
  anomalous reflectances already detected)
• ( Note particulate absorption, as usual )
• 1) excess of Yellow Substance (seasonal?
  regional?
• Blooms in the Northern part? bacterial
  activity?)
• 2) slight excess of particle scattering
  (Saharan dust? Coccoliths?)
• 3) Bio-geo-chemical reasons are not yet
  elucidated