Illustrating isovelocity contours and the presence of coherent velocity substructures in the disc of AS 209 as probed by ¹²CO and ¹³CO J = 2−1 line emission. Black solid and dashed lines show isovelocity contours from the data and the best-fit Keplerian model, respectively. For reference, the isovelocity levels correspond to the same velocity channels of the intensity maps displayed around the main panels. Coherent sub- and super Keplerian substructures identified by FILFINDER (Koch & Rosolowsky 2015) in velocity residuals (Figs. 5 and B.7) are overlaid as blue and red thick lines. The green circle marks the location of the CPD candidate proposed by Bae et al. (2022b) in the disc reference frame. We note that the intensity kinks most prominently seen in the south of the sky in ¹²CO channels, between 3.5 and 5.5 km s⁻¹, are not driven by localised velocity perturbations, but instead are the consequence of a large-scale blue-shifted flow that spans over the entire azimuth of the disc at around ~200 au, and appears at both scale-heights traced by ¹²CO and ¹³CO. In Sect. 4.1, we show that such a coherent signature is the result of upward vertical flows.