Solsticial Hadley Cell ascending edge theory from supercriticality
How far the Hadley circulation's ascending branch extends into the summer hemisphere is a fundamental but incompletely understood characteristic of Earth's climate. Here, we present a predictive, analytical theory for this ascending edge latitude based on the extent of supercritical forcing. Supercriticality sets the minimum extent of a large-scale circulation based on the angular momentum and absolute vorticity distributions of the hypothetical state were the circulation absent. We explicitly simulate this latitude-by-latitude radiative–convective equilibrium (RCE) state. Its depth-averaged temperature profile is suitably captured by a simple analytical approximation that increases linearly with sinφ, where φ is latitude, from the winter to the summer pole. This, in turn, yields a one-third power-law scaling of the supercritical forcing extent with the thermal Rossby number. In moist and dry idealized GCM simulations under solsticial forcing performed with a wide range of planetary rotation rates, the ascending edge latitudes largely behave according to this scaling.
Additional Information© 2021 American Meteorological Society. Manuscript received 11 November 2020, in final form 22 March 2021. Published-online: 10 Jun 2021. We are very grateful to Sean Faulk and Martin Singh for sharing the data from their simulations and for many valuable discussions. S.A.H. acknowledges financial support from NSF Award 1624740 and from the Monsoon Mission, Earth System Science Organization, Ministry of Earth Sciences, Government of India. J.L.M. acknowledges funding from the Climate and Large-scale Dynamics program of the NSF, Award 1912673. We thank Martin Singh and two anonymous reviewers for helpful comments.