Excitation of mixed Rossby-gravity waves by wave-mean flow interactions on the sphere

The equatorial mixed Rossby-gravity wave (MRGW) is an important contributor to tropical variability. Its excitation mechanism capable of explaining the observed MRGW variance peak at synoptic scales in the troposphere remains elusive. This study investigates wave-mean flow interactions as a generation process for the MRGWs using the TIGAR model, which employs Hough harmonics as the basis of spectral expansion on the sphere, thereby representing MRGWs as prognostic variables. Idealized numerical simulations reveal the interactions between waves emanating from a symmetric tropical heat source and an asymmetric subtropical zonal jet as an excitation mechanism for the MRGWs. The excited MRGWs have variance spectra resembling the observed MRGWs in the tropical troposphere. The mixed Rossby-gravity energy spectrum has a maximum at zonal wavenumbers k=4$$ k=4 $$-5 also in the case of an asymmetric forcing that generates MRGWs across large scales. Effects of wave-wave interactions appear of little importance for the MRGW growth compared with wave-mean flow interactions. Application of the zonal-mean zonal wind profiles from ERA5 reaffirms the importance of the asymmetry of the zonal mean flow. Our investigation reveals a novel excitation mechanism explaining the synoptic-scale selection of mixed Rossby-gravity waves. Through high-accuracy numerical simulations using a spectral global shallow-water model (TIGAR), we demonstrate that wave-mean flow interactions drive this mechanism. Our modelling framework disentangles various dynamical factors, including wave-wave and wave-mean flow interactions, as well as external forcing that contribute to the spectral tendencies of mixed Rossby-gravity waves, which enables quantification of their influence as depicted in the figure. image