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.