Contour plot of predicted K_p−V_sys maps by a WASP-121 b GCM. The model used is a strong drag model (τ_drag = 10⁴ s) based on the work of Tan et al. (2024). The black dashed lines at (ΔK_p, ΔV_sys) = (~−5, 0) km s⁻¹ represent the planet position considering the rotation of the planet and assuming that the observed signal is uniform throughout the visible hemisphere. The grey line denotes the expected K_p without the rotation of the planet. The blue and maroon crosses are the retrieved velocities of H₂O and OH, respectively, along with their 1 σ uncertainties. To get the contour lines, we first cross-correlate noiseless GCMs with molecular opacity templates, followed by a phase folding in the K_p−V_sys space. The contours represent an elevation of 0.99, 0.9 and 0.7 times the map maximum. The left panel is the resulting map from taking both the pre- and post-eclipse phases. The middle and right plot consider only the pre-eclipse and post-eclipse, respectively.