Figure 11: Comparison of the RMS of the differences (a) in current modulus and (b) in direction, of the independent drifting buoy velocities and the altimeter geostrophic velocities obtained using MDT solutions CNES-CLS22 and CNES-CLS18, in percent :\(\mathbf{\%}\mathbf{\text{RMSD}}\mathbf{\,=}\frac{\mathbf{\text{RMS}}\left(\mathbf{U}_{\mathbf{\text{ADT}}\mathbf{22}}\right)\mathbf{-}\mathbf{\text{RMS}}\left(\mathbf{U}_{\mathbf{\text{ADT}}\mathbf{18}}\mathbf{-}\mathbf{U}_{\mathbf{d}}\right)}{\mathbf{\text{RMS}}\left(\mathbf{U}_{\mathbf{\text{ADT}}\mathbf{18}}\mathbf{-}\mathbf{U}_{\mathbf{d}}\right)}\mathbf{\,}\). In blue (in red), the bias is reduced (increased) using CNES-CLS22 compared with CNES-CLS18. All these statistics are calculated in 5°X5° boxes, and only boxes with at least 2 drifters difters differents and at least 100 measurement points, are kept.
Comparisons of RMS of the differences (a) in current modulus and (b) in direction, of the independent drifting buoy velocities and the altimeter geostrophic velocities obtained using MDT solutions CNES-CLS22 and CNES-CLS18, in percent, are shown are shown on Figure 11. In blue (in red), the RMSD is reduced (increased) using CNES-CLS22 compared with CNES-CLS18. In terms of current amplitude (Figure 11 a), the majority of boxes show an improvement of between 0 and 2.5% for the CNES-CLS22 solution, with in particular a greater improvement at Kuroshio and in the Caribbean Sea. Areas of degradation in current amplitude RMS are the area around Kerguélen in the ACC and along the southeast coast of Greenland. Comparison of the RMS of differences in current direction (Figure 11 b) shows a more mixed result with degradation boxes compared to the CNES-CLS18 solution: in the ACC in the South Pacific. It can also be noted that some boxes show an improvement in the RMS of differences for the CNES-CLS22 solution in current amplitude but not in direction, and vice versa; this is the case for some boxes south of the Kerguélen Islands and at the extreme south of the Pacific. The areas around the Kerguélen Islands, in the South Pacific between -140°E and -120°E, and around the Fox Islands in the extreme North Pacific remain areas of degradation in RMS of the differences in modulus and current direction for the new solution compared with the previous one. On the other hand, the Kuroshio, the North Atlantic and the Labrador Sea, as well as the Indian Ocean are areas where the RMS of the differences for the new CNES-CLS22 solution has improved compared to the CNES-CLS18 solution.
Globally, the two solutions CNES-CLS22 and CNES-CLS18 remain close with these drifter comparisons, as shown in the following Table 1, which summarizes the drifter comparison statistics over the whole area up to 70°N. Both solutions have a difference RMS of between 10.5 and 11 cm/s for U and V components and current modulus, and a difference RMS of around 34° in direction. The comparison of the two solutions favours the new one, but by less than 1%. It should also be noted that the comparison between the first guess and the CNES-CLS22 MDT shows an improvement in U and V of around 5% in the RMS of differences, and an improvement of around 6% in current amplitude and 2% in direction, which is expected.