  Our recent work shows how M-Earth climates and transmission spectra depend on
the amount of ice-free ocean on the planet's dayside and the mass of N2 in its
atmosphere. M-Earths with more ice-free ocean and thicker atmospheres are
hotter and more humid, and have larger water vapour features in their
transmission spectra. In this paper, we describe a climate transition in
high-pN2 simulations from the traditional ``eyeball" M-Earth climate, in which
only the substellar region is temperate, to a ``temperate nightside" regime in
which both the dayside and the nightside are entirely ice-free. Between these
two states, there is a ``transition" regime with partial nightside ice cover.
We use 3D climate simulations to describe the climate transition from frozen to
deglaciated nightsides. We attribute this transition to increased advection and
heat transport by water vapour in thicker atmospheres. We find that the
nightside transitions smoothly back and forth between frozen and ice-free when
the instellation or pCO2 is perturbed, with no hysteresis. We also find an
analogous transition in colder planets: those with thin atmospheres can have a
dayside hotspot when the instellation is low, whereas those with more massive
atmospheres are more likely to be in the ``snowball" regime, featuring a
completely frozen dayside, due to the increased advection of heat away from the
substellar point. We show how both of these climate transitions are sensitive
to instellation, land cover, and atmosphere mass. We generate synthetic
transmission spectra and phase curves for the range of climates in our
simulations.
