# Ocean Currents Reconstruction from a Combination of
## 4 Discussion and Conclusions  
The OSSE has showed encouraging results to improve the altimeter-derived surface currents in the Mediterranean Area by exploiting the combination of SSH-derived and surface Chl data with the PIT09 method. This study relied on numerical hydrodynamical and biogeochemical models distributed within the Copernicus Marine Service CMEMS, providing gap-free data that can be manipulated to simulate satellite-derived observations and, at the same time, to rely on gap-free fields that can be used as a validation benchmark. Specifically, the CMEMS PHY hydrodynamical model has been used to simulate the altimeter-derived currents and to extract total surface currents, e.g., our ground truth. The CMEMS MedBFM model was used to simulate the satellite surface Chl fields. In the simulated scenario, the PIT09 method allowed for computing a one year long time series of optimal currents by merging the altimeter-derived currents (\(1/8^{\circ}\) horizontal resolution) with the surface Chl data (\(1/24^{\circ}\) horizontal resolution). Analysing the optimally reconstructed surface currents, we could mainly determine that:  
* the effective temporal resolution of the optimal currents is enhanced compared to the altimeter estimates. This was obtained computing the temporal standard deviations (STD) of the Synthetic Altimeter-derived and OPTimal surface Currents (SAC and OPC, respectively). The difference between the OPC and SAC STDs, computed on both annual and weekly timescales, is positive in 80% of the Mediterranean, demonstrating an enhanced temporal variability;
* the spectral analyses of the SAC, OPC and CMEMS PHY Kinetic Energy fields suggest that the OPC fully recovers the surface dynamics until scales of 30 km that we defined as the OPC effective spatial resolution. Following the same spectral analysis, the SAC dataset fully describes larger mesoscale features around 100 km;
* the optimal currents improve the surface circulation estimates provided by satellite altimetry by about 30 to 50% at the basin scale for both the zonal and meridional currents. This was determined checking simultaneously the RMSD values of the SAC and OPC with respect to the total surface currents estimates provided by the CMEMS PHY model. Such improvements can be found throughout the year. However, the enhanced biological activity during late winter/early spring [36; 37] may give rise to significant changes in surface Chl gradients. Such gradients are not strictly related to the horizontal advection and can thus slightly reduce the OPC maximum improvements with respect to the altimeter system. The summer period (mostly late June/early July) can also give rise to issues in the optimal reconstruction: in this period, the Mediterranean sea surface Chl gradients reach their minimum value (evaluated via the CMEMS BIO model and shown in Figure 14 as a basin-scale average), preventing the optimal reconstruction (PIT09 method) to extract dynamical information from the surface tracer patterns;
* the OPC built from Chl concentrations, despite Chl being obtained integrating contributions in the first tenths of meters of the water column, instead represent the surface circulation in the Mediterranean area. The present-day Altimeter-derived gridded products in the Mediterranean Area fully resolve dynamical features at scales around 100 km and 10 days [21]. Therefore, extracting dynamical information from the patterns of the high-resolution Chl is advantageous to correct and improve the effective spatial-temporal resolution of such products. Our study, however, has been carried out in a very idealized context. Relying on hydrodynamical and biogeochemical 3D models enables accessing full information on the Chl evolution that is used to constrain and correct the synthetic altimeter-derived currents.