  Ultra-hot Jupiters are among the best targets for atmospheric
characterization at high spectral resolution. Resolving their transmission
spectra as a function of orbital phase offers a unique window into the 3D
nature of these objects. In this work, we present three transits of the
ultra-hot Jupiter WASP-121b observed with Gemini-S/IGRINS. For the first time,
we measure the phase-dependent absorption signals of CO and H$_{\text{2}}$O in
the atmosphere of an exoplanet, and we find that they are different. While the
blueshift of CO increases during the transit, the absorption lines of
H$_{\text{2}}$O become less blueshifted with phase, and even show a redshift in
the second half of the transit. These measurements reveal the distinct spatial
distributions of both molecules across the atmospheres of ultra-hot Jupiters.
Also, we find that the H$_{\text{2}}$O signal is absent in the first quarter of
the transit, potentially hinting at cloud formation on the evening terminator
of WASP-121b. To further interpret the absorption trails of CO and
H$_{\text{2}}$O, as well as the Doppler shifts of Fe previously measured with
VLT/ESPRESSO, we compare the data to simulated transits of WASP-121b. To this
end, we post-processes the outputs of global circulation models with a 3D
Monte-Carlo radiative transfer code. Our analysis shows that the atmosphere of
WASP-121b is subject to atmospheric drag, as previously suggested by small
hotspot offsets inferred from phase-curve observations. Our study highlights
the importance of phase-resolved spectroscopy in unravelling the complex
atmospheric structure of ultra-hot Jupiters and sets the stage for further
investigations into their chemistry and dynamics.
