3.2.3. CADIZ Region

At the northern shelf-break of the Bay of Cádiz, we know from the data at mooring M13 that the salinity is steady around 36.5 PSU, except in summer when some small freshwater intrusions occur (±0.2 PSU, Figure 12b). The simulations have a correct average salinity here (less than 0.2 PSU of bias, Table 7), but they reproduce punctual spurious intrusions, associated with peaks of river discharge. For instance, the strong river discharge of the reference forcing in February (Figure 3) has induced an extension of the ROFI up to the station, which is not realistic and only reproduced by IBI\_REF. The same kind of behavior is observed in IBI\_LAM in November.

**Figure 12.** (**a**) Surface salinity during the IPMA campaign in late May 2018 in the CADIZ area. The left panel shows the observed salinity measured (by means of a thermo-salinometer), and the other panels (from left to right) show differences between modelled salinities (from IBI\_REF, IBI\_LAM, IBI\_NOR, IBI\_CLM model scenarios) and the observations. The purple dot depicts the location of the mooring buoy M13. (**b**) Timeseries of salinity at station M13, at the shelf-break of the Gulf of Cadiz. Observed salinity is represented by black dots and the different IBI model scenarios by solid lines (IBI\_REF in blue, IBI\_CLM in light blue, IBI\_LAM in red and IBI\_NOR in orange color). The grey rectangle highlights the period of the IPMA campaign in the Gulf of Cadiz.

**Table 7.** Mean difference (Bias) and Root Mean Squared Error (RMSE) between observed salinity (from the M13 mooring buoy and the IPMA campaign) and simulated one (from the IBI\_REF, IBI\_LAM, IBI\_CLM, IBI\_NOR model scenarios), over the respective length of the simulations, in the CADIZ area. The smallest model bias and RMSE for each dataset are in bold. The mooring data have hourly frequency and IPMA data was measured every 10 min. N is the number of measurements.


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

While the salinity at the M13 shelf-break station features a higher variability from May to August (Figure 12), the model scenario simulations reproduce instabilities over a longer period (March to July) and much fresher waters (up to 5 PSU of bias with respect to the measured salinity). The most significant error occurs in March, where all simulations except the one forced by the climatology reproduce a significative drop of salinity at the shelf-break. The IPMA campaign in CADIZ area took place in late May on the shelf. The comparison against the simulated salinities shows that the bias of salinity observed at the shelf-break mooring for this period of the year extends to the whole shelf, from 8.2◦ W to the strait of Gibraltar. In this regard, Table 7 shows how the waters simulated by the proposed model scenarios are 0.5 to 0.87 PSU fresher than the ones observed in the IPMA campaign. This bias is especially strong in the simulations forced by LAMBDA, which has the biggest river discharge at this time of the year.

Even though the comparison of IBI model scenarios against surface salinity shelf transects shows a systematic bias in the IBI simulations in May, this assessment cannot be extended to the whole period of study, as there is no salinity measurement available in winter on the shelf to support or invalidate the assumption. In this case, observations from the only mooring buoy available in the region (M13, located at the shelf-break and far away from the coast) show much lower variability than in the mooring buoy cases shown for the other study subregions. Indeed, the existence of salinity drop events, controlled by ROFIs intrusions, are not so evident at this M13 station. The climatological forcing seems to be the most adequate in this CADIZ region, showing lower errors and bias.
