  Equatorial Kelvin waves can be affected by subtropical Rossby wave dynamics.
Previous research has demonstrated the Kelvin wave growth in response to
subtropical forcing and the resonant growth due to eddy momentum flux
convergence. However, the relative importance of the wave-mean flow and
wave-wave interactions for the Kelvin wave growth compared to the direct wave
excitation by the external forcing has not been made clear. This study
demonstrates the resonant Kelvin wave excitation by interactions of subtropical
Rossby waves and the mean flow using a spherical shallow-water model. The use
of Hough harmonics as basis functions makes Rossby and Kelvin waves prognostic
variables of the model and allows the quantification of terms contributing to
their tendencies in physical and wave space. The simulations show that Kelvin
waves are resonantly excited by interactions of Rossby waves and the balanced
zonal mean flow in the subtropics, provided the Rossby and Kelvin wave
frequencies, which are modified by the mean flow, match. The resonance
mechanism is substantiated by analytical expressions. The Kelvin wave
tendencies are caused by velocity and depth tendencies: The velocity tendencies
due to the meridional advection of zonal mean velocity can be outweighed by the
zonal advection of Rossby wave velocity or by the depth tendencies due to
Rossby wave divergence. Identifying the resonant excitation mechanism in data
should contribute to the quantification of Kelvin wave variability originating
in the subtropics.
