Abstract
Abstract. I study the Hadley circulation of a completely ice-covered Snowball Earth through simulations with a comprehensive atmosphere general circulation model. Because the Snowball Earth atmosphere is an example of a dry atmosphere, these simulations allow me to test to what extent dry theories and idealized models capture the dynamics of realistic dry Hadley circulations. Perpetual off-equatorial as well as seasonally varying insolation is used, extending a previous study for perpetual on-equatorial (equinox) insolation. Vertical diffusion of momentum, representing the momentum transport of dry convection, is fundamental to the momentum budgets of both the winter and summer cells. In the zonal budget, it is the primary process balancing the Coriolis force. In the meridional budget, it mixes meridional momentum between the upper and the lower branch and thereby decelerates the circulation. Because of the latter, the circulation intensifies by a factor of three when vertical diffusion of momentum is suppressed. For seasonally varying insolation, the circulation undergoes rapid transitions from the weak summer into the strong winter regime. Consistent with previous studies in idealized models, these transitions result from a mean-flow feedback, because of which they are insensitive to the treatment of vertical diffusion of momentum. Overall, the results corroborate previous findings for perpetual on-equatorial insolation. They demonstrate that descriptions of realistic dry Hadley circulations, in particular their strength, need to incorporate the vertical momentum transport by dry convection, a process that is neglected in most dry theories and idealized models. An improved estimate of the strength of the Snowball Earth Hadley circulation will also help to better constrain the climate of a possible Neoproterozoic Snowball Earth and its deglaciation threshold.
Highlights
Geological evidence suggests that during the Neoproterozoic era (1000 to 541 million years before present), Earth might have repeatedly experienced global glaciations for many millions of years (Kirschvink, 1992; Hoffman et al, 1998; Hoffman and Schrag, 2000; Pierrehumbert et al, 2011)
Voigt et al (2012) studied the dynamics of the Snowball Earth Hadley circulation using a comprehensive atmosphere general circulation model. Their primary motivation was to test to what extent dry Hadley cell theories are applicable to the Snowball Earth atmosphere and if the assumptions made by these theories are justified in the context of realistic dry atmospheres
I study the dynamics of the Hadley circulation of a Snowball Earth atmosphere
Summary
Geological evidence suggests that during the Neoproterozoic era (1000 to 541 million years before present), Earth might have repeatedly experienced global glaciations for many millions of years (Kirschvink, 1992; Hoffman et al, 1998; Hoffman and Schrag, 2000; Pierrehumbert et al, 2011). Voigt et al (2012) studied the dynamics of the Snowball Earth Hadley circulation using a comprehensive atmosphere general circulation model. Voigt: Dynamics of the Snowball Earth Hadley circulation momentum transport by small-scale (i.e. unresolved) eddies, is fundamental to the Snowball Earth Hadley circulation and weakens the circulation by a factor of 2 to 3 This result is important because dry Hadley cell theories and idealized models usually neglect vertical diffusion of momentum and assume that the flow is governed by its resolved large-scale features. The simulations presented here with seasonally varying insolation connect to recent studies by Schneider and Bordoni (2008) and Bordoni and Schneider (2010) on rapid seasonal changes of the Hadley circulation in an idealized atmosphere general circulation model coupled to a low thermal inertia surface.
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