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 depthaveraged temperature profile is suitably captured by a simple analytical approximation that increases linearly with sinu, where u 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.