Figure 8: (a) ROMS model MDT contours, (b) Mid Atlantic Bight circulation, (c) CNES-CLS18 MDT contours and (d) CNES-CLS22 MDT contours.
CNES-CLS18 MDT (Fig. 8 (c)) shows a slightly more organized circulation on the shelf, although contours on the inner shelf are noisy. Coastal currents in the Gulf of Maine emerge: Coastal currents on the Scotian Shelf or in the Gulf of Maine are present but weak. In addition, there are still MDT contours cutting the coast, and sea-level slope inversions along the shelf.
The CNES-CLS22 MDT (Fig. 8 (d)) is also noisy on the continental shelf, and there are still contours cutting the coast (associated with low geostrophic velocities). Coastal currents on the Scotian Shelf are more organized, but there still flows into the coast of central New Jersey. So CNES-CLS22 MDT is a significant improvement, but that there is still a way to go to bring MDT to the coast on broad shelves.

4.2.2 Quantitative validation with independent T/S profiles

The CNES-CLS22 and CNES-CLS18 MDTs are compared with independent data. Around 5% of the T/S profiles dataset are randomly selected from 2017 and kept for validation (independent data for CNES-CLS22 and CNES-CLS18 MDTs). The dynamic height estimated from T/S profiles (from a reference depth) characterizes a baroclinic component of the dynamic circulation. Whereas altimetry measures a height that is also influenced by baroclinic processes occurring from the reference depth to the bottom of the water column, and by barotropic processes. For validation purposes, we choose to keep only the deepest profiles (with a reference depth of 1900m), thus reducing the number of deep baroclinic processes not taken into account; this leaves us with a validation set of 2% of the database.
As a first step, we compare the CNES-CLS18 and CNES-CLS22 MDTs against these independent dynamic heights, by looking at the correlation of the ADT (SLA+MDT considered) and the independent dynamic heights (figures not shown). Correlations are calculated in boxes of 5° by 5° (with at least 20 data) and are high, mostly between 0.8 and 1 for both MDTs, and it is difficult to differentiate between them.
Secondly, Figure 9 (a) shows the mean bias per 5°X5° box between the ADT estimated from the CNES-CLS22 MDT and the independent dynamic heights. This global mean bias is 1.30 m, with spatial variations in the Norwegian Sea and to the south near Antarctica (equivalent for CNES-CLS18, not shown) and mainly represents the barotropic component not observed by the dynamic heights. This is why it is removed from the estimate of mean synthetic heights for the calculation of the MDT and the following validation diagnosis on Fig. 9 (b) shows a comparison between the variances of the differences (not considering the bias) between the different ADTs and the dynamic heights, in percent. In blue (in red), the variance of differences is reduced (increased) using CNES-CLS22 compared with CNES-CLS18.
Globally, we see an improvement in CNES-CLS22 MDT compared with CNES-CLS18, but this is not true in all regions. South of the Atlantic and the Indian Ocean (as far south as Australia), the variance of differences is reduced for CNES-CLS22 by more than 10% for many boxes (even if boxes of strong reduction are juxtaposed with boxes of increased variance of differences). Areas of degradation are concentrated in the north-western Atlantic, particularly south of Greenland, in the very north of the Pacific (along the Gulf of Alaska to the Fox Islands, and close to Russia) and in the south of the Pacific, where there are degradations of over 10% (also juxtaposed with improvement boxes).