Synthetic light curves have been generated from the output of a SPARC/MITgcm simulation of WASP-121b [16, 17], post-processed using Pytmosph3R [2, 3, 53]. The SPARC/MITgcm solves the primitive equations on a cubic-sphere grid. In the simulation, the pressure ranges from 200 bar to 2 µbar over 53 levels, with a horizontal resolution equivalent to 128 longitudes and 64 latitudes. The model includes optical absorbers such as TiO and VO, assumes a solar metallicity and C/O ratio and does not include clouds. As an indication, the temperature on the dayside is ∼2800 K. Pytmosph3R is an open-source Python package that provides tools to compute synthetic observations, transmission and emission spectroscopy, transit light curves and phase curves, from three-dimension atmospheric models such as GCMs.

Its inputs are the planetary, stellar and orbital characteristics, along with the temperature map provided by the GCM, and the abundances of species present in the atmosphere. It computes absorption, Rayleigh and Mie scattering along with continuum absorption (Collision Induced Absorption, CIA) along a set of light rays passing through the atmosphere. The species included are He, H2, H, H2O, CO, TiO, VO, Na, K, SiO, for which we account for the absorption opacity (except for He, H2 and H) and the continuum includes H2-H2 and H2-He. Opacities have been downloaded from Exomol [54]. The orbital period has been set to 1.274925 days, with an orbital radius of 0.025 au, a planet radius of 1.61 Jupiter radii, with a surface gravity of 8.436 m s−2. To compare the simulation results with the observations, we generated light curves for WASP-121 b in NIRSpec G395H’s wavelength range with the star’s limb darkening set to zero. That way, the depth of the model light curve during the full transit (between contact points 2 and 3) can be converted into the planetary-to-stellar radius ratio using Rp/R∗(t) = � 1 −F(t), (5) where F(t) is the model light curve’s flux. To facilitate the comparison between model light curves and the observations of Rp/R∗(t) approximated as a polynomial (see Eq. 1), we fit quadratic polynomials to the model Rp/R∗(t) of each spectral channel to infer the model predictions for R0/R∗, R1/R∗and R2/R∗as functions of wavelength (examples for this approximation are shown in Extended Data Fig. 3). All relative deviations of the fit polynomials to the GCM’s Rp/R∗(t) are smaller than 0.1 % in all simulated time steps and thus, the approximation as quadratic functions for the comparison to the observations is justified. Like the observations, the SPARC/MITgcm shows positive R1/R∗for most and negative R1/R∗with much smaller absolute values for some spectroscopic channels (see Extended Data Fig. 5); however, it systematically underestimates the values for R1/R∗over the entire wavelength range. One limitation that could contribute to the model’s underestimation of R1/R∗is the lack of clouds in the atmosphere that might be present around the morning terminator. Additionally, a previous phase curve analysis of WASP-121 b showed that the SPARC/MITgcm delivers too high nightside temperatures compared to the measurements [21]. With an overestimated nightside temperature, the model underestimates the day- to nightside temperature contrast, leading to underestimated longitudinal temperature gradients across the terminators in line with the observed discrepancy to the observations. Another reason for the mismatch between the observations and the model could be an inaccurate atmospheric chemical composition in the model due to the assumed Solar metallicity and C/O = 0.55, since recent observations have consistently shown that WASP-121 b’s C/O is much higher than 0.55 [22, 29, 30]. These differences in chemistry might also be responsible for the deviations of the model’s transmission spectrum at the point of conjunction from the observations (Fig. 5, blue diamonds in the upper panel), namely, the too steep H2O feature and the underestimation of the CO feature between 4.3 µm and 5.2 µm. To simulate the effect of a colder morning terminator, we modified the ‘nominal’ SPARC/MITgcm by setting the vertical temperature structure from 240◦to 302◦in longitude to its structure at 240◦(see Extended Data Fig.