Phase Shift of Planetary Waves and Wave--Jet Resonance on Tidally Locked Planets

TL;DR

This paper investigates how atmospheric superrotation influences planetary wave phase shifts on tidally locked planets, revealing nonlinear relationships and wave-jet resonance mechanisms through modeling and simulation.

Contribution

It demonstrates the nonlinear dependence of wave phase shifts on mean flow strength and identifies wave-jet resonance phenomena in tidally locked planetary atmospheres.

Findings

Phase shift is a nonlinear function of mean flow strength.

Resonance occurs when jet speed matches wave phase speed.

Wave-jet resonance observed in both models and simulations.

Abstract

Recent studies found that atmospheric superrotation (i.e., west-to-east winds over the equator) on tidally locked planets can modify the phase of planetary waves. But, a clear relationship between the superrotation and the magnitude of the phase shift was not examined. In this study, we re-investigate this problem using a two-dimensional (2D) linear shallow water model with a specified uniform zonal flow. We find that the degree of the phase shift is a monotonic but nonlinear function of the strength of the mean flow, and the phase shift has two limits of -$\pi$ and +$\pi$. The existence of these limits can be explained using the energy balance of the whole system. We further show that a resonance between the Rossby wave and the mean flow occurs when the speed of an eastward jet approaches to the westward phase speed of the Rossby wave, or a resonance between the Kelvin wave and the mean flow happens when the speed of a westward jet approaches to the eastward phase speed of the Kelvin wave. The resonance mechanism is the same as that found in the previous studies on Earth and hot Jupiters. Moreover, in the spin-up period of a 3D global atmospheric general circulation simulation for tidally locked rocky planet, we also find these two phenomena: phase shift and wave--jet resonance. This study improves the understanding of wave--mean flow interactions on tidally locked planets.