1. Decadal changes in ocean chlorophyll
Henning Wehde

2. Motivation
Phytoplankton dynamic
Parametrisation of convective motions
Results of numerical model studies
Conclusions

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.

4. CZCS
SeaWiFS
Gregg et al., 2003

5. Boyce et al, 2010

6. 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 5 -5
2000s-1980s
2000s-1970s 5 5 -5 -5
2000s-1960s
2000s-1950s 5 5 -5 -5

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.

8. Classical figure of the phytoplankton dynamic Sverdrup (1953)
Compensation Depth
Critical Depth
Net production starting prior the retreat of CML towards the surface
CML
Compensation
depth
Critical depth
aus Parsons, Takahashi und Hargrave (1984)

9. Chlorophyll a concentrations [mg m-3] and mixed-layer depth [m] along a quasimeridional transect (57°N-75°N).
Wehde 2001, 2003

10. Impact of convection on the development
Transport of Plankta
revisits lead to production  enhanced concentration
vertical motion prevents lost of plankta
Wehde 2003

11. New Compensation depth
Backhaus, Nøst, Wehde, Irigoien, Hatten and Logemann, 2003

12. Parametrisation of convective motions
Aspect ratio
Vertical velocities
Convective Mixed
Layer (CML)
Torb = HCML/ (2.5 HCML)/ 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

13. Results of numerical model studies I
Predicted temperature
evolution
CZCS
SeaWiFS
Differences
Difference

14. Results of numerical studies II
Predicted Chlorophyll a
evolution
CZCS
SeaWiFS
Differences
Difference

15. Results of numerical studies III
Predicted Integrated biomass
Predicted CML Depth
Reduction of 6.19 %
(6,7 % reduction observed)

16. The 2000s
Simulated variations in average yearly chlorophyll a in the North Atlantic for the period (CHL (mg m-3))
OWSM observed
(Rey, 2010)

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

18. Thank you!