# Ocean Currents Reconstruction from a Combination of
## 3 Results
### Qualitative Assessment  
The optimally reconstructed currents are presented in three test cases on 30 March 2017, sketched in Figure 3. In our numerical study, the late-March time window is a favourable one for the OPC reconstruction. In this period, higher sea surface Chl concentrations result from the blooming activity [37; 38], facilitating the extraction of dynamical information from the surface Chl patterns. We chose three well-known dynamically active areas of the Mediterranean Sea [36]: the Northwestern Mediterranean in correspondence with the Liguro-Provencal Current, the area in proximity of the Alboran Sea and the Sicily Channel; the last region was also investigated by past studies based on a similar approach [29; 30]. Figure 3A-C allow for inspecting simultaneously the surface circulation field described by the SAC and OPC datasets, with the CMEMS PHY model providing the ground truth. In general, the SAC underestimates the maximum current intensities, providing a smoother description of the main meanders, filaments, and eddy-like features. Some examples of the circulation features not correctly captured by the SAC dataset in the three analysed areas are provided below,  
* Figure 3A: the anticyclonic meander found at 43\({}^{\circ}\)N-7\({}^{\circ}\)E, the two eddy system located at 41.3\({}^{\circ}\)N-4\({}^{\circ}\)E and the meandering tongue flowing from 40\({}^{\circ}\)N-5\({}^{\circ}\)E towards the northeastern section of area A;
* Figure 3B: the current system flowing off the western tip of Sicily, perpendicular to the coastline (at the approximate location of 37.5\({}^{\circ}\)N-12.5\({}^{\circ}\)E), the eddy located at 38.5\({}^{\circ}\)N-12\({}^{\circ}\)E and the western boundary of the the Messina Rise Vortex (37\({}^{\circ}\)N-15.5\({}^{\circ}\)E);
* Figure 3C: the Alboran gyre and multiple eddies system found north of the 36.5\({}^{\circ}\)N parallel, particularly the one circulating at 37\({}^{\circ}\)N-1.5\({}^{\circ}\)W.  
The aforementioned features are almost fully recovered by the OPC datasets (Figure 3). Recalling the basics of Equation (4), the improvements must come from the surface tracer gradients field: they enable to build the correction terms for the SAC dataset and carry out the optimal reconstruction. An additional qualitative analysis, depicted by Figures 4 and 5, indicates the validity of this assumption. The figures show the patterns of the surface Chl spatial gradient magnitude, \(|\
abla\text{Chl}|=\sqrt{(\partial_{\text{x}}\text{Chl})^{2}+(\partial_{ \text{y}}\text{Chl})^{2}}\), and the corrections factors (u\({}_{\text{corr}}\), v\({}_{\text{corr}}\)) computed with (4). Comparing Figures 3-5 allows for confirming that the main corrections are found in areas where the \(|\
abla\text{Chl}|\)  is larger than \(\simeq\)2\(\cdot\)10\({}^{-6}\) mg\(\cdot\)m\({}^{-4}\) and exhibits patterns related to the advection of the horizontal currents. Elsewhere, the optimal reconstruction cannot extract dynamical information from the surface tracer patterns. We also notice that the case \((\partial_{x}Chl)^{2}+(\partial_{y}Chl)^{2}=0\), following Equation (4), is not accepted for the optimal reconstruction. In our numerical study, this condition only happened in 0.1% of the analysed cases and mostly during summertime (late June to early July), i.e., when the biological productivity is damped compared to the late-winter/early-spring months [37]. If we exclude the coastal areas, the summertime surface Chl fields are mostly \(\mathcal{O}(10^{-2})\) mg\(\cdot\)m\({}^{-3}\) against the \(\mathcal{O}(1)\) mg\(\cdot\)m\({}^{-3}\) found during winter. Moreover, the summertime Chl surface fields tend to be more uniform throughout the basin, hindering the appearance of surface Chl gradients related to the horizontal currents advection. Figure 3: **(A)** Northwestern Mediterranean: left, middle and right panels, respectively, indicate the Synthetic Altimeter Currents (SAC), Optimal Currents (OPC) and CMES PHY modelled currents. (B) same as **(A)**, with focus on the Sicily Channel.