3.2.1. BISCA Region

The BISCA area is sensible to the open-ocean dynamics in its Southern part, where the shelf is narrower and the poleward slope-current is close to the coast. Where the shelf is wider, in the French shelf area, the tides and the freshwater runoff are the dominant factors. This local variability shows in the mooring's measurements: in the northern part (M1 and M2, in Bretagne), the salinity is steady in summer and autumn, when stratification occurs. In the southern section (M5 to M7), the variability is, on the contrary, strong in spring/summer, when the mesoscale activity on the shelf is at its maximum (such as M6, Figure 7b). The simulations are all coherent with these local seasonal patterns. In the case of the station M6, IBI\_CLM features an unrealistic variability in winter, that tends to support the hypothesis that a climatological river forcing degrades the solution in terms of variability, compared to a diurnal one.

**Figure 6.** (**a**) Seasonal surface salinity in winter 2018 from IBI\_REF (left panel) and difference of seasonal surface salinity between IBI\_LAM, IBI\_NOR, IBI\_CLM (respectively in panels from left to right) and IBI\_REF. (**b**) Same as (**a**) but in spring. (**c**) Same as (**a**) but in summer. (**d**) Same as (**a**) but in autumn. Red (blue) shading in the panels (**a**) to (**c**) means that the simulation is saltier (fresher) than IBI\_REF. Salinity mean values and statistical metrics (root mean square error and bias computed when comparing each scenario with the reference one) are shown in each panel. Grey dots show the river mouth locations of the rivers considered in the IBI model set-up.

**Figure 7.** (**a**) Taylor diagrams showing observed-modelled salinity comparisons at moorings in the BISCA, WISHE and CADIZ areas. Each location is represented by a different symbol, whereas colors represent the model scenario simulation (IBI\_REF in blue, IBI\_CLM in light blue, IBI\_LAM in red and IBI\_NOR in orange). M3, M4 and M10, because of the important bias between observations and simulations, are not represented here. (**b**) Timeseries of salinity at station M6, at the shelf-break of Santander, Northern Spain, in BISCA. The color code is the same as in panel (**a**).

The RECOPESCA campaign took place from 24 April to 30 May 2018. Even though its temporal coverage is limited to one month, it covers the whole French shelf and allows to estimate the extension of the ROFI (Figure 8). The low-salinity front does not reach the shelf break on the wider part of the shelf (north of 46◦ N). All the simulations overestimate the offshore extension of the ROFI, but the run forced by a climatology which, as seen from the in-situ timeseries, degrades the variability of salinity, features a smaller bias at the time of the campaign (1.55 PSU at the surface) than the other simulations. We can hypothesize that all the "realistic" river forcing data introduce too much discharged water on the shelf and/or do not manage to evacuate it. Nevertheless, validation of the salinity field is difficult because of the grea<sup>t</sup> variability of salinity on the shelf in the BISCA area due to large inputs of fresh water and strong variability.

**Figure 8.** Sea surface salinity (SSS) measured with XBTs from the RECOPESCA campaign for May 2018 in the BISCA area. The right panel shows observed (OBS) salinity, whereas panels on the left show the respective differences between the simulated salinity by each model scenario (i.e., IBI\_REF, IBI\_LAM, IBI\_CLM, IBI\_NOR) and the RECOPESCA observations.

The combined input from the extra coastal runoff and LAMBDA (IBI\_LAM) creates a significant fresh pool of water on the shelf, particularly in the South, which degrades the solution at the time of the RECOPESCA campaign (Figure 8 and Table 5). However, by removing the extra coastal runoff (IBI\_NOR), the solution is improved. The same observation can be made from the moorings' timeseries: the simulation run with LAMBDA but without extra coastal runoff locally improves the solution, meaning that the salinity bias does not come from the LAMBDA forcing itself. In fact, the extra coastal runoff is not needed anymore on the southern shelf of Biscay, as the freshwater discharge is more realistic, even though it is still needed in other areas, where rivers are not parametrized (such as Brittany in the northern BISCA). A full network of coastal salinity observations would be optimal to tune this extra monthly runoff.


**Table 5.** Mean difference (Bias) and Root Mean Squared Error (RMSE) between observed salinity (from mooring buoys and RECOPESCA XBT profiles) and simulated one (from the IBI\_REF, IBI\_LAM, IBI\_CLM, IBI\_NOR model scenarios), over the respective length of the simulations, in the BISCA area. The smallest model bias and RMSE for each dataset are in bold. The datahaveandRECOPESCAdataNisthenumberof

> \* IBI\_CLM timeseries are shorter than the others.
