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Precipitation and storm track activity in the Southern Hemisphere feature a remarkable 20 to 30-day periodicity known as baroclinic annular mode. In climate model projections following the representative concentration pathway 8.5 scenario, this work finds a robust increase of intraseasonal 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. For austral winter, the weak 20-30 day periodicity in the current climate becomes a prominent mode of quasiperiodic variability toward the end of the century in these model projections. This work identifies both the increase of diabatic heating and the enhancements of the waveguide effects as candidate mechanisms.

Plain Language Summary This work reports a robust increase of the intraseasonal periodic behavior in the Southern Hemisphere storm track with important implications on midlatitude hydrological extremes. While it is well-documented that the jet stream will shift poleward in a warming climate, this is the first contribution that identifies how the intraseasonal variability of the storm tracks would change in response to global warming in comprehensive climate models (National Center for Atmospheric Research Large Ensemble project and CMIP5 outputs). Evidence suggests that both the increase of moisture and the change of a climatological basic state can contribute to the intensification of the intraseasonal variability, which implies a more predictable baroclinic annular mode in the Southern Hemisphere in a warmer climate. This finding serves as a stimulus for future studies to unveil the role of changing diabatic heating on the intraseasonal variability in the midlatitudes.