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Decadal changes in ocean chlorophyll. Henning Wehde. Motivation Phytoplankton dynamic Parametrisation of convective motions Results of numerical model studies Conclusions. A decreasing trend within ocean chlorophyll was observed in most of the world oceans during the last decades. - PowerPoint PPT Presentation
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Henning Wehde
Decadal changes in ocean chlorophyll
• Motivation
• Phytoplankton dynamic
• Parametrisation of convective motions
• Results of numerical model studies
• Conclusions
• 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.
CZCS
1979-1986
SeaWiFS
1997-2002
Gregg et al., 2003
Boyce et al, 2010
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
5
-5
0
5
-5
0
5
-5
0
5
-5
0
5
-5
0
• 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.
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
Chlorophyll a concentrations [mg m-3] and mixed-layer depth [m] along a
quasimeridional transect (57°N-75°N).
Wehde 2001, 2003
Impact of convection on the development
- Transport of Plankta
- revisits lead to production enhanced concentration
- vertical motion prevents lost of planktaWehde 2003
New Compensation depth
Backhaus, Nøst, Wehde, Irigoien, Hatten and Logemann, 2003
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
Results of numerical model studies I
Predicted temperature evolution
1979-86
1997-2000
Difference
CZCS
SeaWiFSDifferences
Results of numerical studies II
Predicted Chlorophyll a evolution
Difference
1979-86
1997-2000
CZCS
SeaWiFSDifferences
Results of numerical studies III
Predicted Integrated biomass
Predicted CML Depth
Reduction of 6.19 %
(6,7 % reduction observed)
The 2000s Simulated variations in average yearly chlorophyll a in the North Atlantic for the period 1996-2009 (CHL (mg m-3))
19
90
19
92
19
94
19
96
19
98
20
00
20
02
20
04
20
06
20
08
20
10
Y ear
0
0.2
0.4
0.6
0.8
1
Yea
rly
ave
rag
e c
hlo
roph
yll a
(m
g m
-3)
OWSM observed
(Rey, 2010)
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
Thank you!
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