MERCATOR OCEAN JOURNA: 
SEPTEMBER 2021 
small-scale biogeochemical dynamics, considering that 
an efficient correction of small scales would probably 
require an online coupled physical-biogeochemical DA 
system. In parallel, a climatological relaxation is applied 
to nutrients (NO3, PO4, Si, and Fe), dissolved oxygen and 
variables of the carbonate system (DIC, DOC and Alkalinity). 
This relaxation mitigates the physical DA impact on the 
offline coupled hydrodynamic-biogeochemical system. 
Indeed, this DA caused significant rise of nutrients in the 
Equatorial Belt area and resulted in an unrealistic drift 
of various biogeochemical variables, e.g., Chlorophyll, 
nitrate, phosphate (Gasparin et al., 2020). The time scale 
associated with this climatological damping is set to 1 year 
and enables a smooth constraint that has been shown to 
be efficient enough to reduce the model drift, and to let the 
model developing its own interannual variability. 
In its last - and current - version, this near real time system 
then produces high-frequency daily outputs for Chlorophyll 
concentration, phytoplankton concentration, dissolved 
oxygen, nitrates, phosphates, silicates, iron, as well as 
surface pCO, and pH. 
A global 3D quasi-independent dataset of BGC-Argo 
profiles complemented with profiles reconstructed 
:hrough a neural network (Sauzede et al., 2017) was used 
to statistically measure the impact of these evolutions or 
model’s performance. Figure 2 shows the overall globa 
positive effect of this new system on classical statistica 
metrics (Root Mean Square Error, standard deviation. 
zorrelation), especially in the euphotic layer (0-100 m). Ir 
addition, a set of process-oriented metrics were designed 
in Mignot et et al., (2021) to assess dedicated features of 
acosystem dynamics. Namely: phytoplankton growth via 
photosynthesis, oxygen minimum zones and the carbonate 
system. These metrics have thus been used to compare the 
Jlobal forecasting system with the BGC Argo dataset, and 
highlight the global agreement between both components 
(not shown here). 
Focusing on the spatial distribution of Chlorophyl 
and nutrients, both the extension and amplitude of 
oligotrophic gyres are now better represented. In the 
"ropical band, Chlorophyll and nutrient distribution, as 
well as their interannual variability, are Improved thanks 
to the relaxation toward climatologies. Also note that the 
seasonal cycle has been improved (not shown here but 
available in QUID report, Lamouroux et al., 2019). 
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Normalised standard deviation 
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-igure 2: Taylor diagram comparing the statistical performance of the last two biogeochemical systems, through surface oxygen, oxygen at 
300 m depth, chlorophyll in the 0-100 m surface layer and nitrate in the 0-100 m surface layer. The reference dataset (yellow star) is based 
ın BGC-Aroo database complemented with reconstructions throuch a neural network (see Mianot et al., (2021), in review. for further details)