Abstract
Abstract Various climate states at high obliquity are realized for a range of stellar irradiance using a dynamical atmosphere–ocean–sea ice climate model in an Aquaplanet configuration. Three stable climate states are obtained that differ in the extent of the sea ice cover. For low values of irradiance the model simulates a Cryoplanet that has a perennial global sea ice cover. By increasing stellar irradiance, transitions occur to an Uncapped Cryoplanet with a perennial equatorial sea ice belt, and eventually to an Aquaplanet with no ice. Using an emulator model we find that the Uncapped Cryoplanet is a robust stable state for a range of irradiance and high obliquities and contrast earlier results that high-obliquity climate states with an equatorial ice belt may be unsustainable or unachievable. When the meridional ocean heat flux is strengthened, the parameter range permitting a stable Uncapped Cryoplanet decreases due to melting of equatorial sea ice. Beyond a critical threshold of meridional ocean heat flux, the perennial equatorial ice belt disappears. Therefore, a vigorous ocean circulation may render it unstable. Our results suggest that perennial equatorial ice cover is a viable climate state of a high-obliquity exoplanet. However, due to multiple equilibria, this state is only reached from more glaciated conditions, and not from less glaciated conditions.
Highlights
Orbital configurations such as obliquity and stellar irradiance are key determinants of the surface temperature of a planet (e.g., Pierrehumbert 2010; Ferreira et al 2014; Kaspi & Showman 2015; Kilic et al 2017a)
Kilic et al (2017a), using an atmospheric general circulation model (AGCM) coupled to a slab ocean, found a stable Uncapped Cryoplanet state. This state was found stable for obliquities above 54° and a range of stellar irradiances. They pointed to the importance of initial conditions: in their simulations, an Uncapped Cryoplanet was obtained only when the model was initialized in a Cryoplanet state before increasing stellar irradiance
An important result is that irrespective of Kh the Uncapped Cryoplanet state is not reached when starting from an Aquaplanet. This suggests that the particulars of the ocean model are not the main reason why other studies did not find stable states with an equatorial ice belt at high obliquity (Chandler & Sohl 2000; Jenkins 2000, 2001, 2003; Williams & Pollard 2003; Ferreira et al 2014), but rather the albedo effect was afforded by the initial conditions
Summary
Orbital configurations such as obliquity and stellar irradiance are key determinants of the surface temperature of a planet (e.g., Pierrehumbert 2010; Ferreira et al 2014; Kaspi & Showman 2015; Kilic et al 2017a). Ferreira et al (2014) explored the role of the ocean in controlling the surface temperature under high obliquity and suggested that the climate state with a perennial equatorial ice cover (Uncapped Cryoplanet) is unlikely to be stable due to the meridional structure of the ocean heat flux. On the other hand, Kilic et al (2017a), using an atmospheric general circulation model (AGCM) coupled to a slab ocean, found a stable Uncapped Cryoplanet state This state was found stable for obliquities above 54° and a range of stellar irradiances. The emulator, on which the earlier study of Kilic et al (2017a) was based, permits us to explore here the stability of the Uncapped Cryoplanet at obliquities above 54° with respect to increasing meridional ocean heat flux and to investigate whether such a state may be a likely climate state of an exoplanet.
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