Solsticial Hadley Cell ascending edge theory from supercriticality

TL;DR

This paper develops an analytical theory predicting the extent of Earth's Hadley circulation's ascending branch during summer, based on supercritical forcing and validated by idealized climate model simulations across various planetary rotation rates.

Contribution

It introduces a novel analytical framework linking supercriticality to the Hadley cell's extent, supported by simulations and a simple temperature profile approximation.

Findings

The supercritical forcing extent scales with the thermal Rossby number to the one-third power.

Simulations confirm the theory's predictions across different planetary rotation rates.

The temperature profile in the RCE state increases linearly with sine of latitude.

Abstract

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 $\sin\varphi$, where $\varphi$ 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.