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

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 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)

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 plankta Wehde 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 CZCS 1997-2000 SeaWiFS Differences Difference

Results of numerical studies II Predicted Chlorophyll a evolution 1979-86 CZCS 1997-2000 SeaWiFS Differences Difference

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)) 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!