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Optical variability and optical « anomalies » in Mediterranean waters

Optical variability and optical « anomalies » in Mediterranean waters. André Morel, David Antoine and Hervé Claustre Laboratoire d’Océanographie de Villefranche (CNRS and Univ. P. M. Curie). Is desert dust making oligotrophic waters greener? ( H. Claustre et al., GRL, 2002).

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Optical variability and optical « anomalies » in Mediterranean waters

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  1. Optical variability and optical « anomalies » in Mediterranean waters André Morel, David Antoine and Hervé Claustre Laboratoire d’Océanographie de Villefranche (CNRS and Univ. P. M. Curie)

  2. 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

  3. 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)

  4. 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: Predominantly oligotrophic, sporadically and periodically mesotrophic (Blooms) SO, why « anomalies »?

  5. 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.

  6. « Standard » for Case 1 waters? Historical empirical relationshipsprovide such average laws (+ SD), (generally non-linear laws of (Chl)) For instance: IOP ap(λ, [Chl]) = A(λ) [Chl] ^B(λ) Bricaud et al, bp(λ, [Chl]) = Bo(λ) [Chl] ^ β(λ) Gordon-Morel, Loisel-Morel cp(λ, [Chl]) = Co(λ) [Chl] ^ γ (λ) Voss AOP Kd(λ,[Chl]) = Kw (λ) + χ(λ) [Chl] ^e(λ) Morel-Maritorena. Rrs(λ1) / Rrs(λ2) = Pol ([Chl]) O’Reilly et al.

  7. COMPATIBILITY BETWEEN THESE EMPiRICAL LAWS ? has to be checked before ascertaining “Standards”, and being able to identify “nuances” 1) Closure (IOP) ? ? ap (λ, [Chl]) + bp(λ, [Chl]) = cp(λ, [Chl]) (Bricaud et al, ) (Gordon-Loisel-Morel) (Voss) 2) Coherency (IOP/AOP) ? Possible Inversion Kd (λ, [Chl]) → atot (λ, [Chl]), (with atot = ap + ay + aw) ? atot (λ, [Chl]) > ap (λ, [Chl]) if yes then ay (λ, [Chl]) = ? :

  8. From Bricaud et al., 1998: ap(λ) as f(Chl) Average law and confidence interval (example for 440nm) Data Med-Sea

  9. Loisel-Morel, L&O, 1998 slope 0.766

  10. Closure: c = a + b Medit. Sea (above average relationship

  11. (Med-Sea)

  12. INVERSION (AOP -> IOP) Kd = 1.0395 (μd) -1 (a + bb) R = f’ [bb/ (a + bb)] a(λ) = 0.962 Kd (λ) µd(θs, λ, Chl ) [ 1 - R(λ ) / f’(θs, λ, Chl )] bb(λ) = 0.962 Kd (λ) μd (θs, λ, Chl ) [ R(λ ) / f’(θs, λ, Chl )] Look up Tables for μd and f’ (Morel-Gentili, JGR 2004)

  13. Compatibility between ap and Kd Inversion (Kd → atot) through: atot(λ,[Chl]) = 0.962 Kd (λ,[Chl],θs) μd(λ,[Chl],θs) { 1- R(λ,[Chl],θs)/f’(λ,[Chl],θs)} R(λ,[Chl],θs), f’(λ,[Chl],θs), and μd(λ,[Chl],θs) in LUTs (from RTE computations) Then: atot = ap + ay + aw Is atot coherent with ap ? (i.e., atot > ap )

  14. Examples of Kd(λ) ↔ (Chl) empirical relationships NOMAD Data Black curves: Morel-Maritorena Statistical relationships

  15. Example of Inversion Kd(412) atot (412) Then Decomposition atot = (ap + aw) + ay

  16. 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

  17. ay(412) from the previous figure + Data (Med. Sea) also obtained by inversion Moroccan upwelling Conclusion: Med-Sea waters above standard regarding ay

  18. 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

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