  Exoplanet atmospheres are inherently three-dimensional systems in which
thermal/chemical variation and winds can strongly influence spectra. Recently,
the ultra-hot Jupiter WASP-76 b has shown evidence for condensation and
asymmetric Fe absorption with time. However, it is currently unclear whether
these asymmetries are driven by chemical or thermal differences between the two
limbs, as precise constraints on variation in these have remained elusive due
to the challenges of modelling these dynamics in a Bayesian framework. To
address this we develop a new model, HyDRA-2D, capable of simultaneously
retrieving morning and evening terminators with day-night winds. We explore
variations in Fe, temperature profile, winds and opacity deck with limb and
orbital phase using VLT/ESPRESSO observations of WASP-76 b. We find Fe is more
prominent on the evening for the last quarter of the transit, with
$\log(X_\mathrm{Fe}) = {-4.03}^{+0.28}_{-0.31}$, but the morning shows a lower
abundance with a wider uncertainty, $\log(X_\mathrm{Fe}) =
{-4.59}^{+0.85}_{-1.0}$, driven by degeneracy with the opacity deck and the
stronger evening signal. We constrain 0.1 mbar temperatures ranging from
$2950^{+111}_{-156}$ K to $2615^{+266}_{-275}$ K, with a trend of higher
temperatures for the more irradiated atmospheric regions. We also constrain a
day-night wind speed of $9.8^{+1.2}_{-1.1}$ km/s for the last quarter, higher
than $5.9^{+1.5}_{-1.1}$ km/s for the first, in line with general circulation
models. We find our new spatially- and phase-resolved treatment is
statistically favoured by 4.9$\sigma$ over traditional 1D-retrievals, and thus
demonstrate the power of such modelling for robust constraints with current and
future facilities.
