More than meets the eye(ball) for tidally-locked habitability: dependence on atmospheric circulation regime

Rocky planets hosted by M-dwarf stars represent the most abundant and accessible class of potentially habitable targets for atmospheric characterisation with current and planned observatories. Owing to the close proximity of the habitable zone around these cooler stars, such planets are expected to be tidally locked, giving rise to a set of atmospheric circulation regimes determined primarily by planetary rotation rate and incident stellar flux. Previous climate modelling studies have commonly identified a characteristic 'eyeball' habitable climate for these worlds, illustrative of the approximately circular area surrounding the sub-stellar point where surface temperatures rise above freezing. Using an ensemble of three general circulation models (ExoCAM, LFRic, and ROCKE-3D), we examine the influence of circulation regime on surface habitability across the inner edge of the M-dwarf habitable zone, simulating Earth-like aquaplanets with rotation periods spanning the ‘fast’, ‘Rhines’, and ‘slow’ regimes (4.25–44.33 days). We make use of a new metric of surface habitability which has been previously validated against past and present habitability on Earth, and extends beyond the traditionally-used 'liquid water' temperature condition to define two habitable temperature ranges for each microbial and complex life, as well as using surface water fluxes as a proxy for water and nutrient availability. This additional constraint produces spatial patterns of habitability that differ from those defined by temperature alone, whereby large areas surrounding the substellar point with habitable temperatures but negative net precipitation (P - E < 0) are now designated as ‘limited’ habitability. Furthermore, distinct spatial patterns of habitability emerge across the ensemble for each regime, indicating a dependence on the atmospheric circulation and associated transport of heat and moisture. For slower-rotators, habitable area is substantially reduced as surface moisture is largely confined to the day-side, while faster rotators show a more extensive habitable area but greater variation between the models in global habitable fraction.