PT Unknown
AU Rohr, I
TI A model-based assessment of observational methods to estimate anthropogenic CO2
PY 2008
PU Christian-Albrechts-Universität zu Kiel
WP https://macau.uni-kiel.de/receive/diss_mods_00003108
LA en
DE anthropogenic CO2; North Atlantic; Cstar; TTD; anthropogenes CO2; Nordatlantik; Cstern
AB Three methods to estimate anthropogenic CO2 in the ocean (deltaC* method, TTD (transit time
distribution) method and taking the difference of two DIC equilibrium concentrations), which
are widely used in observations, were adapted to a coarse resolution (4/3°) ocean general
circulation model of the North Atlantic. This is done for testing the methods and comparing
them to anthropogenic CO2 modelled directly in this model.
Furthermore two methods for modelling anthropogenic CO2 are compared in a climatological
forced model run. Modelling anthropogenic CO2 as a passive tracer with a surface flux
containing the information of anthropogenic pCO2 perturbation leads to 15% to 20% lower
concentrations than the “true” anthropogenic CO2 of the model. This is computed as the
difference of DIC concentrations from a model run with realistically prescribed atmospheric
pCO2 concentrations minus a model run with a constant preindustrial atmospheric pCO2
concentration, where in both cases a NPZD (nutrient, phytoplankton, zooplankton, detritus)
model is coupled to the physical model. For correcting this underestimation by the passive
anthropogenic CO2 tracer an improvement to model this tracer is suggested.
Adapting a method which uses an age-spectrum (TTD method) or directly modelled water
age (difference of two DIC equilibrium concentrations) of a water patch for estimating anthropogenic
CO2, leads to concentrations, which are close to the “true” modelled anthropogenic
CO2 concentrations. In contrast to that, estimating anthropogenic CO2 by using a method
which needs a single chlorofluorocarbon (CFC) derived water age (deltaC* method), leads to high
overestimation of the “true” anthropogenic CO2 of the model, especially in the deep ocean.
One assumption of the deltaC* method is that anthropogenic CO2 is transported predominantly
along isopycnal surfaces. To test this, a sensitivity model run excluding isopycnal mixing
was performed. Especially estimating anthropogenic CO2 with the deltaC* method using the
output of this model run should fail. However, the results in comparison to the modelled
anthropogenic CO2 are differing in nearly the same magnitude as for the model run including
isopycnal mixing. The same is the case for the other two methods (TTD method and taking
the difference of two DIC equilibrium concentrations) for estimating anthropogenic CO2.
Another sensitivity run forced by inter-annual variable surface fluxes was performed in order
to determine the influence of inter-annual variability on the estimates of anthropogenic CO2
by using the different methods. In this case the inter-annual variability of anthropogenic CO2
by using the deltaC* method is of the same magnitude as the difference to the “true” modelled
anthropogenic CO2 itself. The inter-annual variability of the total amount of anthropogenic
CO2 deviates by just about 1% from the climatological reference experiment. Locally variations
of up to 10% compared to the climatological mean occur.
PI Kiel
ER