Henning Wehde
description
Transcript of Henning Wehde
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Henning Wehde
Decadal changes in ocean chlorophyll
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• Motivation
• Phytoplankton dynamic
• Parametrisation of convective motions
• Results of numerical model studies
• Conclusions
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• A decreasing trend within ocean chlorophyll was observed in most of the world oceans during the last decades.
• Simultaneously a decrease of penetration depths of oceanic convection in higher latitudes in winter was observed.
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CZCS
1979-1986
SeaWiFS
1997-2002
Gregg et al., 2003
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Boyce et al, 2010
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Weaker Atmospheric forcing:
2m Tair (deg K)(NCEP re-analysis)
2000s warmest
decade in ICES
waters in the last 60 yrs
but warming trend decreases in the 2000s
strongest increase in the Arctic >5 deg
2000s
2000s-1990s
2000s-1970s
2000s-1980s
2000s-1960s
2000s-1950s
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• The main assumption:– it is the decrease of the strength of oceanic
convection in the North Atlantic that contributes significantly to the observed decrease in ocean chlorophyll.
• The assumption is based on the hypothesis on the strong relationship between oceanic convection and primary production
• To support this, a coupled convection-phytoplankton model was used to provide parameterisations of the impact of convection for MLMs and GCMs
• A phytoplankton mixed layer model was applied for the North Atlantic region.
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Classical figure of the phytoplankton dynamicSverdrup (1953)
aus Parsons, Takahashi und Hargrave (1984)
Compensation Depth
Critical Depth
Net production starting prior the retreat of CML towards the surface
CMLCompensationdepthCritical depth
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Chlorophyll a concentrations [mg m-3] and mixed-layer depth [m] along a
quasimeridional transect (57°N-75°N).
Wehde 2001, 2003
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Impact of convection on the development
- Transport of Plankta
- revisits lead to production enhanced concentration
- vertical motion prevents lost of planktaWehde 2003
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New Compensation depth
Backhaus, Nøst, Wehde, Irigoien, Hatten and Logemann, 2003
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Parametrisation of convective motions
• Aspect ratio• Vertical velocities• Convective Mixed
Layer (CML)
Torb = HCML/0.1 + 2(2.5 HCML)/0.05 + HCML/0.05
Orbital time scale Torb, CML depth HCML
Torb = 1.3 HCML 102
Texp ~ 2.5 HCML/0.05
Exposure time scale Texp
Texp ~ 50 HCML
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Results of numerical model studies I
Predicted temperature evolution
1979-86
1997-2000
Difference
CZCS
SeaWiFSDifferences
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Results of numerical studies II
Predicted Chlorophyll a evolution
Difference
1979-86
1997-2000
CZCS
SeaWiFSDifferences
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Results of numerical studies III
Predicted Integrated biomass
Predicted CML Depth
Reduction of 6.19 %
(6,7 % reduction observed)
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The 2000s Simulated variations in average yearly chlorophyll a in the North Atlantic for the period 1996-2009 (CHL (mg m-3))
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OWSM observed
(Rey, 2010)
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Summary
• Impact of oceanic convection in primary production was investigated
• Parametrisation of convective motions in Mixed Layer Models
• Application of the modified model to CZCS and SeaWiFS period late 1970s –late 1990s
decrease of Chl
• Application for the 2000s No significant changes in Chl
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Thank you!