Figure 5: Zoom in on the Artic
zone of (a) the CNES-CLS22 MDT (in cm), (b) the amplitude of the
associated geostrophic currents (in cm/s), (c) the CNES-CLS18 MDT (in
cm) and (d) the amplitude of the geostrophic currents associated with
the CNES-CLS18 MDT (in cm/s).
To look at the energy content of MDT, different MDT solutions were
compared through a spectral analysis: the first guess (called
FirstGuess22), the previous CNES-CLS13 MDT, the previous CNES-CLS18 MDT
(Mulet et al. 2021), the DTUUH22 MDT (Knudsen et al. 2022), the new
CNES-CLS22 MDT and the Glorys12 numerical model MDT (1/12° numerical
model from Mercator-Ocean and distributed within CMEMS: product
GLOBAL_REANALYSIS_PHY_001_030,https://resources.marine.copernicus.eu/?option=com_csw&view=details&product_id=GLOBAL_REANALYSIS_PHY_001_030).
Figure 6 shows spectral analysis for a 10° box in the Antarctic
Circumpolar Current area, between 38°E and 48°E and between 44°S and
54°S. In this zone, the CNES-CLS22 MDT (in black) has significantly more
energy than the first guess (noted FirstGuess22, in dashed blue) on
scales from 600 km to the smallest, and it is the in-situ data that
increase the energy on these scales. The three CNES-CLS MDTs (13 in
yellow, 18 in red and 22 in black) follow the energy level and descent
of the GLORYS12 model (in green), up to 100km for the CNES-CLS13 MDT,
and up to around 60km for the CNES-CLS18 (small offset on the energy
level with GLORYS12, but same decrease slope) and the CNES-CLS22 MDT. At
the smallest scales, there is still an energy decay towards the smallest
scales, but this is less marked than at scales above 60km, probably due
to the noise that must be starting to affect these scales. For the
CNES-CLS22 MDT, residual scales below 60km, below 30km and below 15km
were observed (figures not shown). The MDT fields and associated
geostrophic currents show coherent physical structures, but the more we
look at the smaller scales the more noise becomes visible; the residual
scales below 15km are noisy. Figure 6 also shows the spectrum of the
DTUUH22 MDT (in dashed pink), whose energy curve decays more rapidly
than GLORYS12 below 300km scales, reaching a plateau around 60-70km, at
which scales the noise should start to affect the structures, and then
decays more slowly.