**4. Discussion**

The present study analyzed the response of an operational ocean model regional application to changes in its river and coastal freshwater forcing inputs. The study is focused on the European Atlantic margin, the so-called IBI area, and the study domain (IBBIS) has been divided into three subregions of interest: CADIZ, WISHE and BISCA (see geographical domains in Figure 2). It should be emphasized that the two latter regions (both the Western Iberian Shelf (WISHE) and the Gulf of Biscay (BISCA)) are ROFI areas, where circulation patterns are ruled by significant density differences and the baroclinic frontal features existing between the saltier sea waters and the fresher coastal ones (highly influenced by coastal run-off and river freshwater discharges). It is well-known that differences between existing operational modelled salinity products become particularly evident in ROFI areas (mostly due to the diverse usages that operational forecast services apply to coastal and river freshwater forcing). Thus, a non-optimal representation of coastal and river freshwater signal in the ocean model set-ups can certainly lead to biased simulated ocean patterns, jeopardizing the reliability of ocean forecasts in these ROFI areas.

Simultaneously to this need of operational ocean models to count with an optimal land freshwater forcing, there is a steady effort by the hydrology community to enhance monitoring of rivers and runoff rates. Several multidisciplinary R&D projects and initiatives are working on building new improved freshwater river discharge databases (combining hydrological models and observational data sources) that can be useful to improve ocean model forecasting. With the objective of quantifying the impacts that coastal and river freshwater contributions have on IBI regional ocean model simulations, the new LAMBDA river discharge database (LAMBDA Project [27], Campuzano et al. [55]) was tested as part of the IBI ocean model freshwater forcing.

#### *4.1. How Are the Proposed Ocean Model Scenarios Configured?*

Several ocean model simulations were carried out here to evaluate model sensitivity to changes in coastal and river freshwater forcing. The proposed ocean model scenarios were built using the same NEMO model set-up used by the CMEMS IBI-MFC to generate its operational regional forecasts along the year 2018 [56]. This IBI operational model set-up included a freshwater forcing from land composed of two different components: (1) punctual river discharges, imposed as an open boundary condition for the main 33 rivers discharging in the IBI model domain, and (2) an extra monthly climatological runoff applied along all the IBI coastal grid points. This extra coastal freshwater input tends to compensate the identified IBI discharges deficit related to missing rivers and to the use of climatological diminished daily flow rates [69]. The river discharge values originally imposed in this IBI operational model set-up at the 33 rivers result from a combination of different data sources, including: (1) observed inflows from in-situ river discharge stations (daily forcing data applied only at 8 out of the 33 rivers, and only in hindcast and analysis IBI runs), (2) hydrological model estimations (imposed at all the IBI rivers in the forecast horizons, but also applied in IBI hindcast and analysis runs at those rivers where no flow observation is available) and finally, (3) climatological inputs (prepared to be used at any of the 33 IBI rivers as a back-up solution in case of unavailability or failure in the update of any of the aforementioned observed and modelled river data sources). It is pointed out that this combined use of hydrological observations and model outputs to fix the IBI river freshwater forcing was only active until April 2018; afterwards, until the end of the year 2018, the river freshwater input responded to a pure climatological forcing (averaged daily freshwater discharges shown in Figure 3 illustrate this situation).

The four 2018 IBI runs, performed to test model sensitivity to changes in the river and coastal freshwater forcing, were: a control run, the reference (IBI-REF), using a freshwater forcing data identical to the one used in the CMEMS IBI operational forecast service (including both the punctual river contribution and the extra climatological coastal runoff), and a first IBI sensitivity run (defined as IBI-LAM) using the new LAMBDA river discharges at the 33 IBI river inputs and keeping the same extra added climatological coastal run-off. The second model test run (named IBI-NOR) uses the same LAMBDA river discharges, but in this case, no extra coastal run-off contribution is added. Finally, to build the last IBI model scenario (IBI-CLM), a pure climatological river and coastal freshwater forcing was used.

#### *4.2. Is the IBI Model Application (Used in the Scenarios) Suitable to Simulate Salinity Field in the Study Area?*

The CMEMS IBI operational forecasts (generated through the NEMO set-up and forced with the above-mentioned river and coastal freshwater forcing) can reproduce salinity fields with an adequate level of accuracy along the whole water column. The assessment of the IBI operational salinity product performed along the study case year (see results in Section 3.1) confirms a level of accuracy analogous to the one provided by Sotillo et al. [69] for longer time periods, with IBI salinity validation metrics (correlation and RMSE) within the ranges provided in the product quality document ([0.4–0.9] and [0.2–0.8 PSU], respectively). The assessment performed illustrates the important role played by Argo observations in the IBI routine validation. With respect to the use of satellite products, the utility of the new SMOS reprocessed maps should be emphasized. This dataset tends to represent salinity gradients and on-shelf small-scale features in the IBI area more realistically than the SMOS operational products (which are currently far from being fit for model validation purposes in coastal IBI waters). Thus, a higher frequency update of this reprocessed SMOS product (i.e., on a quarterly or monthly basis) can positively impact on the routine validation processes of operational models in the IBI area. Nevertheless, the use of these observational data sources as main references limits the operational model salinity assessments to offshore deep waters (where Argo floats' drifts and SMOS products show their highest representativeness), underrepresenting the model validation on coastal and on-shelf waters.

This limitation in the operational model validation can be partially overcome thanks to the availability of in-situ observations at some buoys moored along the shelf-break, on-shelf and coastal IBI waters. The product quality assessment performed comprises comparison of the IBI operational salinity product with these in-situ surface salinity observations, and some of the buoys used in this study (especially the ones moored along the Iberian shelf break and coast: M4–M13) are also operationally used in NARVAL for a routine near-real-time local assessment of IBI products.

According to some validation analyses done with in-situ observations from a buoy moored in NW Spain (the M12 one), Lorente et al. [17] proved that the IBI operational solution was able to capture shelf dynamics nearby ROFI areas, by better representing the horizontal extent and strength of a river freshwater plume. This validation approach is

extended here to all the in-situ observations available in 2018 at NW Iberian mooring buoys (the M5–M12 ones). This analysis of IBI model and observed salinity timeseries has allowed identifying extreme salinity drop events (associated with across shelf water movements that extend river plumes and coastal fresher water zones to more offshore locations at the shelf-break and beyond). Thus, 35 remarkable surface salinity drops were recorded, with 24 of them being successfully forecasted by the CMEMS IBI service. On the other hand, 11 of these salinity decreases were missed or underestimated by the IBI model, whereas 22 "false alarm" events (salinity drops simulated by IBI, but not observed in-situ) were identified. This kind of observed and forecasted event categorization, summarized in contingency tables (Figure 5a), is quite helpful to evaluate the IBI model capabilities nearby ROFI areas. It gives a measure of how the IBI model realistically reproduces baroclinic frontal structures and their variability, which may be linked to changes in both local dynamical patterns and freshwater coastal and river discharges.

All these validation results of the IBI operational solution in 2018 confirm the IBI model set-up and the approach followed in it to count with the river and coastal freshwater forcing, as a valid approach to achieve realistic simulations of ocean salinity in the IBI area, and they support the decision of using this IBI NEMO model set-up (analogous to the one used in the CMEMS operational forecast service) to build the model scenarios proposed here.

#### *4.3. Model Scenario Validation: Are Available In-Situ Observations Adequate to Evaluate Model Sensitivity?*

The impacts on IBI model salinity related to changes in the river and coastal freshwater forcing are assessed by means of comparing the modelled salinity fields with in-situ salinity observations. To this purpose, a multi-platform observational salinity database (including profiles from CTDs, XBTs and surface observations from operational mooring buoys and a thermo-salinograph campaign) was compiled and used as a reference in the validation of the proposed regional model scenarios. The spatial observational coverage is illustrated in Figure 2. In-situ salinity observations on shelf and along the shelf-break have been prioritized, searching for observational coverage on ROFI areas. This approach showcases that prevailing effects of river plumes and coastal freshwater-induced fronts in such ROFI areas can be analyzed and the response of each model scenario run to simulate them can be assessed.

These IBI model scenario SSS outputs were validated through comparisons with the different in-situ observational data sources available in the study region. The comparison with observed salinity timeseries from moorings, mostly located on-shelf and along the shelf-break, shows that all the IBI simulations globally manage to accurately reproduce the spatial-temporal variability of salinity within the IBBIS domain (statistical metrics are provided for all the stations, and some examples of modelled and observed salinity timeseries at buoys moored in the BISCA, WISH and CADIZ areas are shown as examples in Figures 7, 9 and 12, respectively).

It is important to mention that most of the in-situ buoy stations available for the study are quite offshore, moored close to the shelf-break and far away from nearby coastal discharge areas. At these locations, the seasonal variability of salinity is mostly explained by the general circulation and the mesoscale dynamics. River freshwater discharges do not greatly influence the salinity seasonal cycle at the shelf break, but rather act on the higher (daily) variability of salinity, and coastal freshwater influence seems to be mostly linked to major abrupt freshwater intrusions (such as the ones identified, and discussed, by the validation of the operational IBI salinity performed in Section 3.2).

Moreover, it should be emphasized that local validation of model salinity against in-situ observations at specific moored buoys is very challenging since these abrupt lowsalinity water intrusion features, that certainly mark model and observed salinity timeseries at given locations, can be frequently under-represented by the model due to small spatial or temporal feature shifts. Thus, in some of the occasions when the model is unable to reproduce one of these major salinity drops locally measured at a buoy, it is seen how

the model is able to simulate the (usually coastal) fresher water mass that causes the recorded salinity drop, but it is not able to precisely locate the baroclinic frontal structure, not reaching (sometimes just by a matter of model grid points) the buoy location. On the other hand, in some other cases when the simulated frontal structure effectively reaches the buoy location, the fresher water mass arrival can be affected by some temporal shift, and then, abrupt salinity drops are simulated earlier or with some delay with respect to the observed event. Keeping this in mind, the model validation performed with in-situ mooring observations has shown that IBI model configuration is able to capture the level of dynamical activity, reproducing all the sensitivity test simulations of the main events with appropriate spatial-temporal accuracy.

#### *4.4. How Different Are the River Flow Estimations Used as Forcing in the Ocean Model Scenarios?*

Significant differences in terms of total daily river discharge contributions were identified among the different datasets imposed as freshwater forcing in each sensitivity model run. Table 3 shows how the 2018 daily mean flow (averaged over the whole IBBIS study domain) of the new LAMBDA river forcing reaches up to 8910 m3s−1, meaning a 70% higher contribution than the river forcing used in the IBI reference control run (5223 m3s−1) and 128% more than the pure climatological river discharge forcing (3893 m3s−1). Furthermore, the extra climatological coastal runoff, added to compensate lacks in the IBI freshwater signal, represents in the IBBIS region a yearly contribution of 843 m3s−1. It is worth noting that in the first months of 2018, the variability and intensity are equivalent in both the reference and the new LAMBDA datasets, with main peaks of freshwater in January and March. However, from April onwards, the reference data, mainly based on climatology, shows a smooth decrease in total river debit, whereas LAMBDA features a minimum in summer too, associated with a reduced variability. It also shows two important peaks in June and November, that do not exist in the IBI (climatological at that time) reference forcing (Figure 3).

#### *4.5. What Are the Major Impacts in Salinity Associated to the Proposed Changes in Freshwater Forcing?*

The river forcing plays a significant role in the regional simulation of the IBI sea surface salinity, as it is seen when looking at the modelled salinity fields resulting from the different IBI scenario runs (Figure 6). As expected, due to the higher river discharge of LAMDBA (as mentioned above, overall 70% higher than the reference one), the sensitivity runs based on this forcing (IBI\_LAM and IBI\_NOR) are fresher than the control one. Major impacts are found during spring and summer in the BISCA subregion, when IBI simulations forced by LAMBDA feature fresher water masses on-shelf, that extend up to open waters. On the contrary, the simulation forced by a climatology (IBI\_CLM) is saltier than the other simulations (+0.05 PSU over the IBBIS domain compared to the reference in winter). Indeed, using a climatology instead of realistic daily river discharge significatively changes the surface/subsurface salinity budget, not only at the coast, but on the whole shelf and at the shelf-break. The SSS differences between the various simulations are in general more noticeable along the coast and on shelf waters, and especially obvious on the main IBI ROFI areas. Indeed, small SSS changes identified in offshore deeper waters seem more linked to the expected propagation of differences, when modeling structures after 12 months of free simulation, and there are not such model differences in deep-water areas (outside the Gulf of Biscay) driven by variations in the river and coastal freshwater forcing.

#### *4.6. What Are the Regional Impacts in the Three Areas of Interest?*

Each subregion included in the study domain features its proper dynamics and counts with different observational data sources, therefore, the assessment of salinity and frontal structure variability related to the impacts of changes on river discharge inputs has been conducted separately for the three proposed subregions: BISCA, WISHE and CADIZ.

#### *4.7. Impacts in the Gulf of Biscay*

The IBI scenario simulations are all coherent in the BISCA region with the local seasonal patterns. The use of climatological river data as forcing degrades the model solution in this region, changing surface/subsurface salinity budgets (not only at the coast, but on the whole shelf and at the shelf-break), and especially in terms of variability (featuring the climatological run in some mooring locations' (e.g., M6) unrealistic variability pattern, especially in wintertime). The model variability is enhanced when daily updated river data is included in the model forcing. This emphasizes the fact that using climatological instead of more realistic, higher frequency (daily) freshwater discharge inputs significantly changes the salinity budget on the shelf. The most important salinity differences between the model scenario runs are found in this BISCA region, and particularly on the French Shelf. In its northern side, discharges linked to the Vilaine and Loire river system impacts on surface and subsurface layers all year-long. LAMBDA freshwater inflows are bigger than the IBI reference ones and model simulations forced with LAMBDA in the region feature on-shelf fresher water masses that extend up to the open ocean (particularly in spring and summer). 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 least at the RECOPESCA campaign time (Figure 8 and Table 5). However, by removing the extra coastal runoff (IBI\_NOR), the solution is improved. This extra coastal runoff is not needed anymore on the southern shelf of Biscay, as the freshwater discharge is more realistic, even though it can be still needed in other zones in the northern BISCA region, such as Brittany, where rivers are not parametrized. In that sense, the BISCA southernmost area, especially along the northern Iberian shelf, arises as the most sensitive zone to the use of an extra coastal runoff (confirmed by the higher salinity simulated in the area by the run without such extra coastal climatological forcing: the IBI\_NOR run). In this region, there are no major rivers (at least they are not included in the IBI river forcing as punctual freshwater sources), but many small freshwater inputs whose cumulated flow is not negligible. Then, here, using a monthly coastal runoff contribution added to the daily river freshwater forcing can locally improve simulated coastal salinity. However, this extra climatological coastal freshwater input must be tuned to avoid overestimations of total freshwater inputs that may result in unrealistic simulation of coastal freshwater fronts or across-shelf intrusions. Unfortunately, the lack of a network of coastal salinity observations is a limitation in the area to adequately tune available coastal freshwater inputs.

#### *4.8. Impacts in the Western Iberian Shelf*

Salinity in the WISHE zone is steadier than in the BISCA area and, as documented in the literature, it is defined by a coastal fresher water mass, usually limited to the narrow shelf, but occasionally extended offshore. The location of the baroclinic front is mainly controlled by the Iberian poleward slope current and its interactions with the shelf waters, resulting in complex front dynamics with occasional episodes marked by strong acrossshelf-break advection of lower salinity water masses coming from coastal areas. These intrusions of waters, highly modified by the river and coastal freshwater influence, are clearly identified in the salinity records at mooring sites and they are captured to some extent by the IBI model application (as previously proven through the analysis of the IBI operational performance during the main 2018 observed salinity drops). The IBI model application is adequate to simulate shelf dynamics in ROFI areas of the WISHE zone and the different IBI model scenarios generally reproduce these observed freshwater intrusions, with small variations in timing and/or intensity (with the IBI-NOR and IBI-CLM runs showing better statistical metrics, depending on the buoy). Along the western Galician coast (monitored through the very coastal periodic INTECMAR CTD stations located in front of the Rias), it is the IBI-NOR run, forced with the new LAMBDA river data but without extra coastal runoff, the model scenario that better performs statistically (showing lower RMSE and bias when compared to the INTECMAR 2018 periodic salinity profiles). Emphasize that present IBI model set-up does not include any riverine inflows along

this Galician coast, except the Minho and the major, but southern, regional freshwater contributors: the Douro and Mondego rivers. This would mean that salinity budget at this coastal zone off the Rias is rather controlled by the extension of the western Iberian buoyancy plume (mostly fed by freshwater contribution from these three major rivers) than by the local runoff from the nearby Rias itself. Further south, the model scenario assessment performed with the IPMA SSS campaign along the Portuguese shelf shows how all runs feature a similar salinity pattern. This suggests that, in this zone, variations in the imposed river discharge data may not be the main cause of salinity model errors on the shelf freshwater budget. This is so even nearby the Tejo river mouth, where all runs show a similar pattern of salinity differences, pointing out other dynamical factors as more determinant to explain this consistent model behavior.

#### *4.9. Impacts in the Gulf of Cadiz*

This consistency between model scenarios is also seen in the Gulf of Cadiz. Here, a similar salinity pattern is identified, with main differences between model runs located on-shelf, and especially on coastal areas close to the two rivers considered by IBI (the Guadalquivir and Guadiana) in the CADIZ zone. The occasional comparison with the SSS IPMA campaign (limited to a 9-day campaign in May 2018) indicates an error pattern marked by too-salty modelled waters in the western shelf, getting fresher as moving eastwards closer to the 2 IBI rivers' mouths. The model run that uses climatological discharges at these two rivers shows lower RMSE and bias values, whereas the 2 runs using LAMBDA inputs (IBI-LAM and IBI-NOR) show higher negative biases. This general anomalously lower model salinity seen in the area for all the runs is directly linked to either the excess of river freshwater contribution imposed at the 2 rivers, especially when using LAMBDA data, or an unrealistic accumulation of freshwater at the river mouths due to the model dynamics. All the nearby on-shelf areas seem affected, as confirmed at the single buoy available in the region (M13, moored offshore at deep waters, but not far from the Guadalquivir mouth).

#### **5. Conclusions and Future Research Directions**

In conclusion, the effect of varying imposed river outflows in the IBI operational ocean model system was investigated, showcasing the potential impacts that a new river freshwater model database (such as the LAMBDA one) can have on regional operational forecasts. Some enhancements in model capabilities to better represent salinity and especially baroclinic frontal structures linked to coastal and river freshwater buoyancy plumes have been demonstrated.

Although major impacts were identified on ROFI areas associated with bigger river discharges (i.e., the French shelf in the Gulf of Biscay or the Northwestern Iberian coast), it is found that in some other regions (such as the Portuguese shelf) or in areas with lower riverine freshwater contribution along the study year (such as the Gulf of Cadiz), these impacts related to changes in the imposed river inflows are lower, with other dynamical factors playing a more important role in governing the modelled salinity field.

The CMEMS IBI operational model set-up can reproduce salinity fields with an adequate level of accuracy along the whole water column, including remarkable salinity drop events linked to across shelf water movements that extend river plumes and coastal fresher water zones to more offshore locations at the self-break and beyond. The validation of the 2018 IBI operational solution confirms that the IBI model set-up (and the approach used to include in it the river and coastal fresh water forcing) is valid to achieve realistic simulations of ocean salinity in the IBI area, and they support the decision of using this IBI NEMO model set-up as a base to build the model scenarios proposed here.

The river forcing plays a significant role in the regional simulation of the IBI sea surface salinity, as it is seen when looking at the modelled salinity fields resulting from the different IBI scenario runs. Major impacts are found during spring and summer in the BISCA subregion, when IBI simulations forced by LAMBDA feature fresher water masses on shelf, that extend up to open waters. On the contrary, the simulation forced by a climatology is saltier than the other simulations. The SSS differences between the various simulations are in general more noticeable along the coast and on shelf waters, and especially obvious on the main IBI ROFI areas.

Significant differences in terms of total daily river discharge contributions were identified among the different river flow datasets imposed as freshwater forcing in each sensitivity model run. The use of "realistic" daily updated model-derived river inputs can benefit operational ocean models, especially improving their ability to capture salinity variability, and as demonstrated through the model sensitivity tests performed, their uses can be an option to avoid, or at least to minimize, the use of more static climatological approaches (such as the coastal climatological correction currently applied in the IBI model set-up). Furthermore, some regions, identified here as sensitive to changes in the freshwater forcing data, are of special interest to upgrade their river freshwater contribution, replacing the current freshwater climatological approach. The most noticeable case is the northern Iberian coast, where the current IBI operational model set-up does not count with any river source (with all the regional freshwater contribution coming from the coastal runoff correction applied), despite the existence of numerous small rivers in the area whose cumulated freshwater contribution into the ocean is not negligible (all year round).

A full network of coastal salinity observations would be optimal to evaluate and tune the contribution of freshwater forcing data in ocean simulations. This need of an enhanced in-situ observational coverage should be met for on-shelf and coastal zones (and especially in ROFI areas). Recovered ancillary databases or specific campaigns' data (such as the RECOPESCA or IPMA data used here) can be useful to study cases, but sustained operational monitoring (i.e., the moored buoys) or routine periodic repetition of salinity observations at specific locations (such as the INTECMAR data) make it possible to analyze the salinity variability. Improvements of satellite-based products are also desired. the with the SMOS products available for coastal uses being mostly reprocessed ones (with an intense use of in-situ observations for calibration and filling gap purposes), again, the in-situ component arises as critical. It should be highlighted that enhancement of the river hydrological observation component is also a key issue: without river flow observations, calibration of hydrological models is limited, decreasing the accuracy of freshwater river flow estimations available to force ocean models.

The progressive replacement of static climatological river inputs in ocean models is likely to become a key research line, needed to upgrade operational ocean circulation models, with a broadening of scope, especially from coastal model services, to increase the number of rivers to be considered as part of the land boundary contribution. This would not be only a matter of including freshwater contribution from major rivers in ocean models (currently the most common approach in operational systems), but also to count with relatively minor ones, which can play a significant role in local coastal dynamics, enhancing its influence under extreme weather event conditions. In that sense, Ruiz-Parrado et al. [70] and Sotillo et al. [71] show how a lack of adequate real-time updated river inputs is a major limitation for the performance of ocean circulation models, including the IBI one, during specific storm events.

Future research lines could include the application of these updated river freshwater model estimations into ocean models, not only in hindcast or analysis mode, but also in forecast runs. The objective would be to substitute the present common approach, mostly based on the use of persistence of the last available river discharge value, by nearreal-time updated forecasted river estimations. This upgrade, that can certainly enhance ocean forecast skill, may lead to more integrated multi-disciplinary approaches based on combined near-real-time forecast runs of ocean and hydrological models (both models using the same atmospheric forcing to keep consistency).

**Author Contributions:** Conceptualization, M.G.S.; methodology, K.G., A.M. and M.G.S.; software, K.G., A.M. and M.A.A.-B.; validation, P.L., K.G. and A.M.; resources, F.C., F.S., E.O., A.N. and M.G.S.; writing—original draft preparation, M.G.S., K.G., P.L. and A.M.; writing—review and editing, F.C., F.S., E.O. and A.N.; project administration, M.G.S., F.C. and A.N. All authors have read and agreed to the published version of the manuscript.

**Funding:** Part of this research work was conducted in the framework of the following 2 projects: The EU Interreg Atlantic Area MyCoast Project EAPA\_285/2016 (F.C., A.M. and M.G.S.) and the CMEMS Service Evolution Project LAMBDA (F.C., F.S., E.O. and A.N.). It was also supported by activity from the CMEMS IBI-MFC (K.G., P.L. and A.A.B.).

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** The following publicly available datasets were analyzed in this study: Model data: The CMEMS IBI MFC operational forecast product can be found in the CMEMS catalogue (IBI-MFC forecast analysis product: https://resources.marine.copernicus.eu/?option=com\_csw& view=details&product\_id=IBI\_ANALYSISFORECAST\_PHY\_005\_001 (accessed on 8 April 2021)); LAMBDA river data can be found in: http://www.cmems-lambda.eu/#data-portal (accessed on 8 April 2021); data from the different ocean model scenarios analyzed in the study can be available on request from the corresponding author. Observational data: The data from in-situ buoys, Argo floats and Recopesca can be found in the CMEMS catalogue (Insitu-TAC Near-Real-Time observational product: https://resources.marine.copernicus.eu/?option=com\_csw&view=details&product\_id= INSITU\_IBI\_NRT\_OBSERVATIONS\_013\_033 (accessed on 8 April 2021)). The salinity data from SMOS (the low-resolution level 3 SSS product computed with smoothening spatial window of 50- km radius) can be found in the BEC public ftp catalogue: http://bec.icm.csic.es/bec-ftp-service/ (accessed on 8 April 2021); Finally, the INTECMAR and IPMA observational salinity datasets used in this study are 3rd Party Data, and restrictions are applied to their availability (contact with the institutions owner of these datasets would be required for access permission).

**Acknowledgments:** The authors are grateful to the following experts (and institutions) for their support to this research: Pedro Montero (INTECMAR) for supporting with the INTECMAR CTD data, Guillaume Charria (Ifremer) for supporting with the RECOPESCA data, Diogo André Reis de Sousa and Paulo Oliveira (both from the Instituto Português do Mar e da Atmosfera) for the IPMA campaign data. Other MyCoast ocean modelers: Juan Taboada (MeteoGalicia), Joao Sobrino (IST), Tomasz Dabrowski (IMI). This study has been conducted using E.U. Copernicus Marine Service Information. The authors especially thank the CMEMS Insitu-TAC (for the moored buoys and Argo profilers) and the CMEMS IBI-MFC (for their IBI operational model products and validation assessments).

**Conflicts of Interest:** The authors declare no conflict of interest.
