Favorability of hurricane genesis on tidally locked exoplanets orbiting M-dwarf stars

The temperate rocky exoplanets that will be best characterized with future observations orbit M-dwarf stars. Due to their close-in orbits, these planets are expected to be tidally locked to their host stars, with large day-night temperature and moisture contrasts and climates that are significantly different from that of Earth. Hurricanes can cause emergent variability in cloud patterns and atmospheric moisture content, and if present may affect future observations of terrestrial exoplanets. In this work, we explore whether hurricanes can form on tidally locked terrestrial exoplanets. We use Earth-based metrics for the favorability of hurricane genesis (i.e., formation) to study whether the environmental conditions in three-dimensional aquaplanet simulations of terrestrial exoplanets allow for hurricane genesis. We study a wide range of planetary parameters, including rotation period, incident stellar flux, planetary radius, surface gravity, surface pressure, and host star type. We find that hurricanes can form over a wide range of rotation period, surface gravity, surface pressure, and incident stellar flux. Notably, we find that for an incident stellar flux similar to that of Earth, hurricane genesis is most favorable on planets with intermediate rotation periods of 8-10 days. This is because the effect of decreasing absolute vorticity due to decreased planetary spin with increasing rotation period causes hurricane genesis to be less favorable at longer rotation periods. Additionally, increasing mid-level ventilation and wind shear with decreasing rotation period cause hurricane genesis to be less favorable at shorter rotation periods. As a result, we find that hurricane genesis can be favorable on planets within the habitable zones of late-type M dwarf stars. Such planets orbiting small red dwarf stars are some of the most accessible to telescopic observations, which may be able to detect variability in cloud cover or stratospheric water mixing ratio due to hurricanes.