Influence of subtropical westerlies on the equatorial wave spectrum: implications for future changes in Kelvin wave variance

Abstract Models from phase 6 of the Coupled Model Intercomparison Project (CMIP6) simulate an intensification of equatorial Kelvin Waves (KW) with global warming. In contrast, the power spectrum is projected to weaken for most other wavenumber-frequency combinations, including higher wavenumber Equatorial Rossby waves (ER). These changes are attributed to the greenhouse gas forcing, while aerosols partially mitigate this effect. The qualitatively different projected responses of KW and ER suggest that dynamical forcings play an important role in the physical mechanism of the changes. This hypothesis is tested using targeted simulations of the Model of an Idealized Moist Atmosphere in which we impose perturbations to upper-tropospheric zonal winds that mimic projected end-of-century changes. These simulations demonstrate that future changes in KW depend on changes in the Southern Hemisphere subtropical jet. A similar dependence is also evident in CMIP6 models. These results have implications for future projections of KW activity in models with biased subtropical westerlies.