  Using a 3D general circulation model, we demonstrate that a confirmed rocky
exoplanet and a primary observational target, TRAPPIST-1e presents an
interesting case of climate bistability. We find that the atmospheric
circulation on TRAPPIST-1e can exist in two distinct regimes for a 1~bar
nitrogen-dominated atmosphere. One is characterized by a single strong
equatorial prograde jet and a large day-night temperature difference; the other
is characterized by a pair of mid-latitude prograde jets and a relatively small
day-night contrast. The circulation regime appears to be highly sensitive to
the model setup, including initial and surface boundary conditions, as well as
physical parameterizations of convection and cloud radiative effects. We focus
on the emergence of the atmospheric circulation during the early stages of
simulations and show that the regime bistability is associated with a delicate
balance between the zonally asymmetric heating, mean overturning circulation,
and mid-latitude baroclinic instability. The relative strength of these
processes places the GCM simulations on different branches of the evolution of
atmospheric dynamics. The resulting steady states of the two regimes have
consistent differences in the amount of water content and clouds, affecting the
water absorption bands as well as the continuum level in the transmission
spectrum, although they are too small to be detected with current technology.
Nevertheless, this regime bistability affects the surface temperature,
especially on the night side of the planet, and presents an interesting case
for understanding atmospheric dynamics and highlights uncertainty in 3D GCM
results, motivating more multi-model studies.
