Toward a unified theory for the Hadley cell descending and ascending edges

TL;DR

This paper develops a unified theoretical framework to predict the latitudinal edges of Hadley cells throughout the annual cycle, validated by idealized GCM simulations and accounting for planetary rotation effects.

Contribution

It combines recent scaling laws and baroclinic instability theory to analytically predict both edges of Hadley cells, advancing understanding of their seasonal and planetary variations.

Findings

The theory accurately captures the annual cycle of Hadley cell edges in idealized models.

The descending edge scales approximately as Ω^{-1/2} with planetary rotation rate.

The ascending edge scales approximately as Ω^{-2/3}, consistent with simulations.

Abstract

We present theories for the latitudinal extents of both Hadley cells throughout the annual cycle by combining our recent scaling for the ascending edge latitude (Hill et al. 2021) with the uniform Rossby number (Ro), baroclinic instability-based theory for the poleward, descending edge latitudes of Kang and Lu 2012. The resulting analytic expressions for all three Hadley cell edges are predictive except for diagnosed values of Ro and two proportionality constants. The theory captures the climatological annual cycle of the ascending and descending edges in an Earth-like simulation in an idealized aquaplanet general circulation model (GCM), provided the descending edge prediction is lagged by one month. In simulations in this and two other idealized GCMs with varied planetary rotation rate ($\Omega$), the winter, descending edge of the solsticial, cross-equatorial Hadley cell scales approximately as $\Omega^{-1/2}$ and the summer, ascending edge as $\Omega^{-2/3}$, both in accordance with our theory.