45th Liège ColloquiumMay 13 – 17, 2013
Fabian Große1*, Johannes Pätsch2 and Jan O. Backhaus2
1 Research Group Scientific Computing, Department of Informatics, University of Hamburg2 Institute of Oceanography, University of Hamburg
* Corresponding author: [email protected]
Parameterising Primary Production and Convection in a 3D Model
Source: Quadfasel (unpublished)
Introduction: ARGO measurements
• convection as driving mechanism (Backhaus et al., 1999)
Source: Quadfasel (unpublished)
Introduction: Phytoconvection
Source: Backhaus (2003)
• 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: Janout (2003)
2.5:1MLD
MLD
Phytoconvection in a 3D Model
• hydrostatic approximation requires parameterisation
• Steele (1962):
• MLD-dependent sliding function between standard and phytoconvection
• Phytoconvection = upward and downward displace-ment of phytoplankton within a convective cell
PB … growth rate
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
• comparison of 2 simulations:
• Standard• Phytoconvection
position of 1D analysis
Results – Part I: 1D Analysis• Standard run
• low winter concentrations within mixed layer
• near-surface bloom in April• high concentrations until
autumn within mixed layer
• Phytoconvection run• high winter concentrations
within mixed layer• deep maximum in April• high concentrations until
autumn within mixed layer
dept
h [m
]
mg
chl-a
m-3
dept
h [m
]
mg
chl-a
m-3
MLDsim
chlorophyll-a
chlorophyll-a
dept
h [m
]
mg
chl-a
m-3
dept
h [m
]
mg
chl-a
m-3
Results – Part I: 1D Analysis
Data source: BODC
dept
h [m
]
chlorophyll a [mg m-3]
MLDsim
MLDobschlorophyll-a
chlorophyll-a
MLDsim
Results – Part I: 1D Analysis
MLDsim
MLDobs
Data source: BODC
dept
h [m
]
chlorophyll a [mg m-3]
MLDsim
MLDobs
• 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
Results – Part II: 3D analysisPrimary production
April - standard
g C
m-2
mon
th-1
g C
m-2
mon
th-1
April - phytoconvection
Chlorophyll-a (depth-integrated)April - standard
mg
chl-a
m-2
mg
chl-a
m-2
April - phytoconvection
Results – Part III: Carbon fluxes Air-sea flux
Export (below 500m)
mm
ol C
m-2
mon
th-1
mm
ol C
m-2
mon
th-1
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 (Tsurf – T > 0.4K) not suitable to:
• detect haline stratification
• distinguish between convective and frictionional mixing
Outlook
• improvement of sliding function:
→ include turbulent mixing depth (Taylor & Ferrari, 2011)
• replace MLD criterion (Tsurf – 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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Vielen Dank für Ihre Aufmerksamkeit.
Parametrisierung von Primärproduktion und winterlicher Konvektion in einem 3D Modell
Thank you for your attention.
45th Liège ColloquiumMay 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 Kämpf, J. ‘Phyto-convection’: the role of oceanic convection in primary production. Marine Ecology. Progress Series, 189:77–92, 1999.
• 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 CO2. 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.