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!