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.