  Recent observations of ultra-hot Jupiters with dayside temperatures in excess
of $2500~\mathrm{K}$ have found evidence for new physical processes at play in
their atmospheres. In this work, we investigate the effects of the dissociation
of molecular hydrogen and recombination of atomic hydrogen on the atmospheric
circulation of ultra-hot Jupiters. To do so, we incorporate these effects into
a general circulation model (GCM) for hot Jupiter atmospheres, and run a large
suite of models varying the incident stellar flux, rotation period, and
strength of frictional drag. We find that including hydrogen dissociation and
recombination reduces the fractional day-to-night temperature contrast of
ultra-hot Jupiter atmospheres and causes the speed of the equatorial jet to
decrease in simulations with fixed rotation. This is because the large energy
input required for hydrogen dissociation cools the dayside of the planet, and
the energy released due to hydrogen recombination warms the nightside. The
resulting decrease in the day-to-night temperature contrast reduces the
day-to-night pressure gradient that drives the circulation, resulting in weaker
wind speeds. The results from our GCM experiments qualitatively agree with
previous theory which found that the fractional day-night temperature contrast
of ultra-hot Jupiters should decrease with increasing equilibrium temperature
due to hydrogen dissociation and recombination. Lastly, we compute full-phase
light curves from our suite of GCM experiments, finding that the reduced
day-to-night temperature contrast in ultra-hot Jupiter atmospheres causes a
smaller phase curve amplitude. The reduction in phase curve amplitude due to
hydrogen dissociation and recombination could explain the relatively small
phase curve amplitudes of observed ultra-hot Jupiters.
