Influence of Subtropical Jets on the Equatorial Wave Spectrum: implications for future changes in Kelvin waves and MJO variance

Abstract Models from phase 6 of the Coupled Model Intercomparison Project (CMIP6), simulate an intensification in equatorial Kelvin Waves (KW) and the Madden-Julian Oscillation (MJO) 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). The qualitatively different projected response of KW and ER suggest that dynamical forcings have an important role in the physical mechanism of the changes. This hypothesis is tested using targeted simulations of the Model of an Idealised Moist Atmosphere (MiMA) in which we impose perturbations in upper-tropospheric zonal winds that mimic projected end-of-century changes. These simulations demonstrate that future changes in KW and the MJO strongly depend on changes in the South Pacific subtropical jet. A similar dependence is also evident in CMIP6 models. These results have implications for future projections of KW and MJO activity in models with biased subtropical jets.