Parameterising Primary Production and Convection in a 3D Model

Fabian Große 1 *, Johannes Pätsch 2 and Jan O. Backhaus 2 1 Research Group Scientific Computing, Department of Informatics, University of Hamburg 2 Institute of Oceanography, University of Hamburg * Corresponding author: [email protected]

45 th Liège Colloquium May 13 – 17, 2013

Introduction: ARGO measurements

Source: Quadfasel (unpublished) Source: Quadfasel (unpublished) • convection as driving mechanism (Backhaus et al., 1999)

Introduction: Phytoconvection

• mean spatial aspect ratio of 2.5:1 (horizontal vs. vertical scale) ( Kämpf & Backhaus, 1998) • convective cycle takes 1-2 days ( D’Asaro, 2008) • same probability of residence in the euphotic zone for each phytoplankton particle Source: Backhaus (2003) MLD 2.5:1 MLD Source: Janout (2003)

Phytoconvection in a 3D Model

•

Phytoconvection

= upward and downward displace ment of phytoplankton within a convective cell • hydrostatic approximation requires parameterisation • Steele (1962):

P B

… growth rate • MLD-dependent sliding function between standard and phytoconvection

Model Setup and Simulations

• 3D physical-biogeochemical model ECOHAM4 (Lorkowski et. al., 2012) • 20 km horizontal resolution • 5-1000 m vertical resolution (24 layers) • physics initialised from climatology • initialisation for biochemistry from standard simulation of 1995 • simulation period: 1996 position of 1D analysis •

comparison of 2 simulations:

• Standard • Phytoconvection

Results – Part I: 1D Analysis

MLD sim

• Standard run • low winter concentrations within mixed layer • near-surface bloom in April • high concentrations until autumn within mixed layer chlorophyll-a • Phytoconvection run • high winter concentrations within mixed layer • deep maximum in April • high concentrations until autumn within mixed layer chlorophyll-a

Results – Part I: 1D Analysis

MLD sim

chlorophyll-a

MLD sim MLD obs

chlorophyll-a chlorophyll a [mg m -3 ] Data source: BODC

Results – Part I: 1D Analysis

• Standard run: • significantly lower concentrations throughout whole water column • Phytoconvection run: • upper layer concentrations in good agreement with observations • low chlorophyll-a below mixed layer • depth of chlorophyll-a gradient ≠ MLD

MLD sim MLD obs

chlorophyll a [mg m -3 ] Data source: BODC

Results – Part II: 3D analysis

Primary production April - standard Chlorophyll-a (depth-integrated) April - standard April - phytoconvection April - phytoconvection

Results – Part III: Carbon fluxes

Air-sea flux Export (below 500m)

Summary & Conclusion

• parameterisation of phytoconvection: • observed upper layer chlorophyll-a concentrations reproduced • strong influence of convection on primary production and carbon export production • sliding function allows continuous transition from winter to summer regime • problems during decline of mixed layer in spring • applied MLD criterion (T surf • detect haline stratification – T > 0.4K) not suitable to: • distinguish between convective and frictionional mixing

Outlook

• improvement of sliding function: → include turbulent mixing depth (Taylor & Ferrari, 2011) • replace MLD criterion (T surf – T > 0.4K) • apply parameterisation on model area with more regions of deep winter convection for better data basis • include results from tank experiments investigating phytoplankton adaptation to different dark-light cycles

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Parametrisierung von Primärproduktion und

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Vielen Dank für Ihre Aufmerksamkeit.

[email protected]

45 th Liège Colloquium May 13 – 17, 2013

References

• • • • • • D’Asaro, Eric A.. Convection and the seeding of the North Atlantic bloom.

Journal of Marine Systems, 69:233 –237, 2008.

Backhaus, J., Wehde, H., Hegseth, E., and role of oceanic convection in primary production. Marine Ecology.

Progress Series, 189:77 –92, 1999.

Kämpf, J. ‘Phyto-convection’: the Backhaus, J., Hegseth, E., Wehde, H., Irigoien, X., Hatten, K., and Logemann, K. Convection and primary production in winter. Marine Ecology Progress Series, 251:1 –14, 2003.

Janout, M. Biological parameterization of convection in a mixed layer model.

Pages 1 –87, 2003.

Lorkowski, I., Pätsch, J., Moll, A., and Kühn, W. Interannual variability of carbon fluxes in the North Sea from 1970 to 2006 - Competing effects of abiotic and biotic drivers on the gas-exchange of CO 2 . Estuarine, Coastal and Shelf Science, 2012.

Taylor, J. and Ferrari, R. Shutdown of turbulent convection as a new criterion for the onset of spring phytoplankton blooms. Limnology and Oceanography, 56(6):2293, 2011.