  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.
