  We have developed a pseudo two-dimensional model of a planetary atmosphere,
which takes into account thermochemical kinetics, photochemistry, vertical
mixing, and horizontal transport, the latter being modeled as a uniform zonal
wind. We have applied the model to the atmospheres of the hot Jupiters HD
209458b and HD 189733b. The adopted eddy diffusion coefficients are calculated
by following the behaviour of passive tracers in three-dimensional general
circulation models, which results in eddy values significantly below previous
estimates. We find that the distribution of molecules with altitude and
longitude in the atmospheres of these two hot Jupiters is complex because of
the interplay of the various physical and chemical processes at work. Much of
the distribution of molecules is driven by the strong zonal wind and the
limited extent of vertical transport, resulting in an important homogenisation
of the chemical composition with longitude. In general, molecular abundances
are quenched horizontally to values typical of the hottest dayside regions, and
thus the composition in the cooler nightside regions is highly contaminated by
that of warmer dayside regions. As a consequence, the abundance of methane
remains low, even below the predictions of previous one-dimensional models,
which is likely to be in conflict with the high CH4 content inferred from
observations of the dayside of HD 209458b. Another consequence of the important
longitudinal homogenisation of the abundances is that the variability of the
chemical composition has little effect on the way the emission spectrum is
modified with phase and on the changes in the transmission spectrum from the
transit ingress to the egress, these variations in the spectra being mainly due
to changes in the temperature rather than in the composition between the
different sides of the planet.
