Equatorial superrotation in Held & Suarez-like flows with weak equator-to-pole surface temperature gradient

TL;DR

This study explores how weakening the equator-to-pole surface temperature gradient in a Held & Suarez-like model can lead to equatorial superrotation, driven by reduced Rossby wave breaking and equatorial barotropic instability.

Contribution

It demonstrates a transition to superrotation under specific thermal forcing conditions and identifies the mechanisms behind this transition, including wave interactions and instabilities.

Findings

Superrotation occurs when the equator-to-pole temperature gradient is weakened.

Rossby wave breaking reduction contributes to superrotation.

Barotropic instability and Kelvin waves sustain superrotation.

Abstract

Equatorial superrotation under zonally-symmetric thermal forcing is investigated in a setup close to that of the classic Held & Suarez (1994) setup. In contrast to the behaviour in the classic setup, a transition to equatorial superrotation occurs when the equator-to-pole surface equilibrium entropy gradient is weakened. Two factors contribute to this transition: 1) the reduction of breaking Rossby waves from the mid-latitude that decelerate the equatorial flow and 2) the presence of barotropic instability in the equatorial region, providing stirring to accelerate the equatorial flow. In the latter, Kelvin waves excited by instability near the equator generate and maintain the superrotation. However, the superrotation is unphysically enhanced if simulations are under-resolved and/or over-dissipated.