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BOGCM HAMOCC runs, iron fertilisation

Optimising the model by increasing the Fe-scavenging rate: The comparison of the vertical distribution of dissolved Fe in the standard model HAMOCC5 with observed Fe data revealed, that although the model reflects the deep concentrations of dissolved iron, it strongly overestimates the dissolved iron in the surface ocean layers. Given the model's quite high iron input via dust deposition mainly in the North Atlantic, and the long integration time which distributes the iron within the whole ocean, this of course leads to a weak (if any) iron limitation in the model.

The parameterisation of dissolved iron scavenging in the standard configuration, was based on the paper by Johnson et al. (1997), with a time constant of 0.005 yr-1 for relaxation of excess dissolved iron to values of 0.6nmol L-1. However, Johnson et al. estimated this rate constant from a model that applied an upper (50m) boundary condition of 0.05 nmol/L for dissolved iron. There is no such upper boundary in the model presented here, but surface iron concentrations can become whatever the model suggests, and thus the low scavenging rate constant may be inappropriate in the context of HAMOCC5.1.

Further, this may influence the small effect of Fe-fertilization in the standard scenario. In order to more thoroughly investigate the effect of Fe-fertilization on CO2 gas exchange in an experiment we have increased the scavenging rate by a factor of 100 in year 870 of the standard BGC model simulation, and ran it for 400 years, i.e. both the standard run and the "high Fe-complex" run have the same age. The resulting dissolved iron concentrations in the upper layers much better fit the observations that the standard run does.

The increased iron limitation especially in the Southern Ocean (and also equatorial and northern Pacific) leads to lower Chl concentrations, which also better fits the observations. Reduced production leads to reduce export; this is evident e.g., from the plots of fluxes vs. latitude. Differences between the two scenarios appear mainly in the deep sedimentation in the southern latitudes, where the model with increased Fe-scavenging rate shows a better agreement with observations. Summarizing, it seems that the parameterisation of high Fe-scavenging leads to a better agreement of model surface and deep concentrations and fluxes especially in the Southern Ocean.

The effect of Fe-fertilization: At first, an experiment with the standard model was carried out. Simply simply switching the Fe-limitation for the whole ocean tested the effect of Fe-fertilization. This has been done for ten years of model simulation; after that, Fe-limitation was switched on again, and the model simulated for another 20 years.

Omitting Fe-limitation basically has an effect on the fluxes in the Southern Ocean; here, all fluxes are increased, and the ocean takes up more CO2. There is but little effect in the other oceanic regions. After an initial strong increase of global fluxes (the ocean even turns from source to sink), the model even within the Fe-fertilization period starts to relax back to the initial values (e.g., for air sea gas exchange, the mode in the end only shows a very weak net uptake any more). After the standard model run, an experiment with the high Fe-scavenging rate was performed. Enhancing the effect of Fe-limitation by increasing the Fe-scavenging rate over the 400 years prior to the fertilization, the initial response of the ocean to fertilization is much stronger; but the effect vanished quickly, and in the end (after a relaxation of 20 years), the air sea gas exchange is more or less the same as at the start of the fertilization period.

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