Henning Wehde Decadal changes in ocean chlorophyll
Slide 2
Motivation Phytoplankton dynamic Parametrisation of convective
motions Results of numerical model studies Conclusions
Slide 3
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.
Slide 4
CZCS 1979-1986 SeaWiFS 1997-2002 Gregg et al., 2003
Slide 5
Boyce et al, 2010
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Weaker Atmospheric forcing: 2m T air (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
Slide 7
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.
Slide 8
Classical figure of the phytoplankton dynamic Sverdrup (1953)
aus Parsons, Takahashi und Hargrave (1984) Compensation Depth
Critical Depth Net production starting prior the retreat of CML
towards the surface CML Compensation depth Critical depth
Slide 9
Chlorophyll a concentrations [mg m -3 ] and mixed-layer depth
[m] along a quasimeridional transect (57N-75N). Wehde 2001,
2003
Slide 10
Impact of convection on the development - Transport of Plankta
- revisits lead to production enhanced concentration - vertical
motion prevents lost of plankta Wehde 2003
Slide 11
New Compensation depth Backhaus, Nst, Wehde, Irigoien, Hatten
and Logemann, 2003
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Parametrisation of convective motions Aspect ratio Vertical
velocities Convective Mixed Layer (CML) T orb = H CML /0.1 + 2(2.5
H CML )/0.05 + H CML /0.05 Orbital time scale T orb, CML depth H
CML T orb = 1.3 H CML 10 2 T exp ~ 2.5 H CML /0.05 Exposure time
scale T exp T exp ~ 50 H CML
Slide 13
Results of numerical model studies I Predicted temperature
evolution 1979-86 1997-2000 Difference CZCS SeaWiFS
Differences
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Results of numerical studies II Predicted Chlorophyll a
evolution Difference 1979-86 1997-2000 CZCS SeaWiFS
Differences
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Results of numerical studies III Predicted Integrated biomass
Predicted CML Depth Reduction of 6.19 % (6,7 % reduction
observed)
Slide 16
The 2000s Simulated variations in average yearly chlorophyll a
in the North Atlantic for the period 1996-2009 (CHL (mg m -3 ))
OWSM observed (Rey, 2010)
Slide 17
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