Etapa 5: Formular Mapa Estratégico, Balanced ScoreCard BSC-, plan de comunicaciones

Using the same values for parameters and initial concentrations as the ‘CTRL_awecs’ run in AWECS Exp 1, the model performs relatively poorly to quantitatively, and qualitatively, reproduce macro-nutrient observations (Figure 4.8 and 4.9, black dashed

Figure 4.6 a) Normalized vertical profiles of temperature (°C), salinity (Sp) and brine volume (%) (dark grey lines) and corresponding observations (black crosses) for ARISE IV. The ice thickness is

normalized. b) Contoured temperature (°C), salinity (Sp) and brine volume (%). Horizontal and vertical axes refer to months and ice thickness (m), respectively. Contours plots show the evolution of sea ice from the start of the simulation to the end of the simulation (sampling date).

line). Dissolved macro-nutrients concentrations are generally underestimated in both

stations (except for Si(OH)4

- _{in SIPEX 8), and the general C-shape of their profiles is}

not represented by the model. Both PO4

3- _{and NO}

x are underestimated in ARISE IV

and SIPEX 8, but to a lesser extent in SIPEX 8. Similar results are observed for Chl a

and POC concentrations. In an attempt to get closer to the observed concentration, in the ‘sw_obs’ run, the ice is initialized with the averaged ice concentrations measured

on the respective stations rather than AWECS data. Similarly, seawater is initialized with respective underlying seawater measured on the sampling day. However, the shape of the modelled profile is not affected by this modification in neither ARISE IV

Figure 4.7 a) Normalized vertical profiles of temperature, (°C), salinity (Sp) and brine volume (%) (dark grey lines) and corresponding observations (black crosses) for SIPEX 8, b) contoured

temperature (°C), salinity (Sp) and brine volume (%). Horizontal and vertical axes refer to months and ice thickness (m), respectively. Contours plots show the evolution of sea ice from the start of the simulation to the end of the simulation (sampling date).

nor SIPEX 8 (Figure 4.8 and 4.9, green dotted line). The L-shape of the macro- nutrients (and POC) vertical profiles remains in SIPEX 8. The L-shape is less distinct in ARISE IV.

Simulation of DFe portraits two different profiles between ARISE IV and SIPEX 8 (see also Figure 4.10 for a zoom in). The ‘CTRL_awecs’ run overestimates the DFe concentrations in both ARISE IV and SIPEX 8. The ‘sw_obs’ run led to a better simulation of the DFe profile, but still overestimated DFe in SIPEX 8. Simulated PFe

concentrations remain considerably below observed concentrations at both stations and for each run (Figure 4.8 and 4.9).

In the ‘bott_ice’ run, the initial seawater concentration is set to be equivalent to the bottom ice concentration observed at station ARISE IV (Figure 4.8, dotted blue line) and SIPEX 8 (Figure 4.9, dotted blue line). The initial concentration of ice stays

unchanged compared to the ‘sw_obs’ run. Macro-nutrients, Chl a and POC vertical

profiles are strongly affected by this modification, both qualitatively and quantitatively. This parameterisation results in simulated concentrations that are much

higher than the observed concentrations (except Chl a, which does not increase

strongly in ARISE IV). The general shape of the profiles fluctuates as well. A S-shape is observed for the macro-nutrient profiles in ARISE IV, a Z-shape for POC, and Chl

a stays very low with a slight increase at the bottom of the ice and around 0.6

(normalized ice thickness). The increase of macro-nutrients and Chl a all occur at the

same depth. For station SIPEX 8, the ‘bott_ice’ run leads to the closest match between modelled profiles and observations.

The general shape of Fe profiles is relatively similar for both stations. However, the ‘bott_ice’ run drives large overestimation of the DFe concentrations, e.g., up to 2 orders of magnitude in ARISE IV. Here, the effect of the ‘bott_ice’ run on the PFe is limited to an increase of PFe in the bottom ice section only (Figure 4.8 and 4.9, blue dotted line). Despite the large increase of the PFe concentration in the seawater, profiles in the ice are consistently underestimated by the model. The bottom ice section in SIPEX 8 is the only portion accurately represented by the model using this set-up. Although the profiles need to be refined, these results show that the initialization is crucial to reproduce observed concentrations with the model.

Figure 4.8 Normalized vertical profiles of macro-nutrients (NOx, Si(OH)4- and PO43- in µmol L–1), Chl a (µg L–1_{), POC (µmol L}–1_{) and dissolved and particulate Fe (nmol L}–1_{) in ARISE IV when varying the} initial concentrations. Observed concentrations are shown with the black crosses.

Figure 4.9 Normalized vertical profiles of macro-nutrients (NOx, Si(OH)4- and PO43- in µmol L–1), Chl a (µg L–1_{), POC (µmol L}–1_{) and dissolved and particulate Fe (nmol L}–1_{) in SIPEX 8 when varying the} initial concentrations. Observed concentrations are shown with the black crosses.

4.4.2.2.2 Key parameters controlling the biogeochemistry of Fe

This part focuses on refining the representation of Fe in sea ice. Therefore, for the sake of clarity, only DFe and PFe profiles are displayed for the following runs. In this

set of runs, we varied the value of the !!"#!" (equation (4.10)), which is equivalent to

modifying the remineralisation rate of Fe as compared to the remineralisation of carbon. The ‘low_rem_fe’ (Figure 4.10, plain purple line) run and the ‘high_rem_fe’ run (Figure 4.10, plain orange line) were initialized with the same concentrations than the ‘sw_obs’ run (Figure 4.10, dotted green line). For easier comparison, ‘CTRL_awecs’ run and ‘sw_obs’ run are also represented in Figure 4.10. As expected, lower remineralisation increases the concentration of PFe, and slightly decreases the DFe concentrations compared to the ‘sw_obs’ run. Particulate Fe and DFe vertical profiles are mirrored in SIPEX 8: the maximum PFe concentration corresponds to the minimum DFe concentration and vice versa (Figure 4.10 b). The observed high concentration of PFe near the ice surface is not accurately represented in SIPEX 8, in any of the runs. However, this is well represented by the ‘low_rem_fe’ run in ARISE IV.

In the previous runs, aFe (particulate Fe associated to algae) concentration was negligible compared to eFe concentration (eFe >> aFe). In the last run, ‘high_biol’, the initial conditions of aFe are significantly increased in both the ice and the seawater (Table 4.3). Simultaneously, we also increased the initial sea ice and

seawater Chl a concentration. Although DFe concentration remains too low in the

bottom ice, the representation of the profile of DFe is improved in ARISE IV. The shape of the DFe profile in SIPEX 8 stays unchanged, but slightly higher concentrations are observed compared to the previous runs. Particulate Fe is not

slight increase of the PFe at depth > 0.2 (normalized thickness) slightly improved the representation of PFe profile in SIPEX 8 compared to the “high_rem_fe’ and

‘CTRL_awecs’ runs. A similar increase is observed in the Chl a and POC profiles

(result not shown). Parameterization of processes driving Fe dynamics in sea ice has

therefore a bigger impact on PFe and DFe profiles than the increase of Chl a (used as

a proxy for sea ice algae) in the model.

Figure 4.10 Normalized vertical profiles of dissolved and particulate Fe (nmol L–1_{) in a) ARISE IV,} and b) SIPEX 8 when varying parameters affecting the biogeochemistry of Fe. They are compared to the CTRL_awecs run (dashed black line) and the sw_obs run (dotted green line). The black crosses show the observations. In ARISE IV, the observed concentration of PFe being much higher than the simulated PFe concentration, the observations are shown on a different portion of plot using a larger scale.

4.5 Discussion

4.5.1 Sea ice thermodynamic and biogeochemistry

The modelled temperature, salinity and brine volume profiles are in quantitative and qualitative agreement with the observations for both young ice (AWECS) and older

ice (ARISE IV and SIPEX 8). The loss of brine due to the extraction of the core is known to underestimate the bulk salinity of the sample (Notz et al., 2005). This could explain the slight difference between the modelled and the observed salinity (and therefore brine volume) profiles during AWECS simulations.

Although the dissolved macro-nutrients follow the brine dynamic in the model (Vancoppenolle et al., 2010), the model performs poorly at reproducing their vertical profiles in ARISE IV. On the contrary, nutrients profiles are relatively well represented in SIPEX 8, AWECS Exp 1 and Exp 2. It is considered that the ice in AWECS Exp 1, Exp 2 and SIPEX 8 was unaltered by biological activity (van der Merwe et al., 2011b; chapter 2, Janssens et al., 2016), as opposed to a more important biomass in ARISE IV (Lannuzel et al., 2007). Given the difficulty to initialize with the correct nutrients and POC profiles in back trajectory model simulations, and the coarse representation of primary production in the model (Moreau et al., 2015; Vancoppenolle et al., 2010; Vancoppenolle and Tedesco, 2016), the lack of biomass in SIPEX 8, AWECS Exp 1 and AWECS Exp 2 may explain the better representation

by the model of the biologically unaltered in situ nutrients profiles. The poor

representation of Chl a could likely be improved by the addition of a parameter

accounting for the initial physical enrichment of algae observed in newly formed sea ice, similar to what has been introduced in this work for PFe and POC. Gradinger and Ikävalko (1998) and chapter 2 show that algae are highly enriched in the ice at the start of sea ice formation. This process is currently not represented in any model. As the purpose of this chapter is to investigate the representation of Fe dynamic in sea

ice, rather than an investigation of the nutrient, POC and Chl a profiles, and the

sources of error for their misrepresentation, the following discussion will focus on Fe, unless the other parameters provide additional support for elucidating Fe dynamics.

4.5.2 Discussion on the representation of Fe profiles

The analysis of the simulated DFe and PFe profiles provides valuable information on the relative importance of the different processes driving the budget of Fe in the ice.