Precipitation and storm track activity in the Southern Hemisphere feature a remarkable 20‐30 day periodicity known as Baroclinic Annular Mode. While it is well‐documented that the jet stream will shift poleward in a warming climate, the change of coherent modes of Rossby waves remains unknown. We finds a robust increase of intra‐seasonal variability of the precipitation and density‐weighted eddy kinetic energy at the 20‐30 day frequency range by 25% and 20% respectively in austral summer toward the end of the century, despite small changes in seasonal‐mean quantities at corresponding latitudes. These results suggest that a warming climate can feature a stronger dynamical organization of the 20‐30 day periodicity by the moist baroclinic waves. This work identifies that both the increase of diabatic heating and the enhancements of the waveguide effects as candidate mechanisms. This finding serves as a stimulus for future studies to unveil the role of changing diabatic heating on the intra‐seasonal variability in the mid‐latitudes.