5
Conclusions
The main improvement of this new CNES-CLS22 MDT over the previous
CNES-CLS18 MDT is in the Arctic, with better coverage, no artifacts and
a more realistic solution. Globally, the new CNES-CLS22 solution is
close to the CNES-CLS18 solution, both have better resolution of small
scales than previous CNES-CLS MDTs but are potentially polluted by noise
at very short scales. The CNES-CLS22 MDT has been evaluated against
independent height and velocity data in comparison with the previous
version, the CNES-CLS18. The new solution presents slightly better
results, although not identical in all regions of the globe. In
particular, the results are better in the Antarctic Circumpolar Current
in terms of height, better off Japan and particularly in the Arctic in
terms of geostrophic velocities. For geostrophic currents, those of the
new CNES-CLS22 MDT in the Antarctic Circumpolar Current are slightly
worse than those of the CNES-CLS18 in comparison with drifters.
Improvements to this new MDT include a new first-guess with the
CNES-CLS22 MSS and the GOCO06s geoid to which optimal filtering has been
applied, as well as Lagrangian filtering at the coast to reduce the
intensity of normal currents at the coast, drifting buoy and T/S profile
databases have been updated, as have updated processing to obtain
synthetic mean geostrophic velocities and synthetic mean heights. In
addition, a new type of data, HF radar data, was processed to extract
physical content consistent with MDT in the Mid Atlantic Bight region.
The study of this region in particular, showed the improvements of
CNES-CLS22 MDT, but that there is still work to be done to obtain a more
physical solution on the continental shelf. Indeed, on these continental
shelves in general, the first guess is not always good because it’s
close to the coast and today we only process T/S profiles with a depth
greater than 200m; we therefore lack data on these shelves. HF radar
data can provide some velocities information, as there are few drifting
buoy data in these coastal regions, but processing is difficult, as
complex and non-negligible ageostrophic currents have to be removed. In
the case of the Mid Atlantic Bight, these HF radar data do not allow us
to obtain a MDT with a geostrophic flow not crossing the coastline.
Thanks to the new first-guess and in particular the new CNES-CLS22 MSS,
the Arctic area is covered in this MDT and the artifacts of CNES-CLS18
have disappeared. Looking more specifically at the European Arctic
region, the new CNES-CLS22 solution presents structures in agreement
with the literature in the Yermak Plateau area and in the St Anna
Through region.
With the arrival of new swath observations from the SWOT (Surface Water
and Ocean Topography) satellite launched in December 2022 (Fu et al.
2012), the continuous improvement of MDT accuracy and resolution is
necessary. The inclusion of new coastal data, such as HF radar and SAR
data, and shallower T/S profiles not currently taken into account,
requires a clear separation of geostrophic and ageostrophic processes,
and access to physical content consistent with MDT. These treatments
call for new methods in order to best process continental shelf areas
with varied oceanographic phenomena.