Future changes in North Atlantic tropical cyclone intraseasonal variability associated with the Madden-Julian Oscillation

Abstract A better understanding of how the influence of the Madden–Julian Oscillation (MJO) on Atlantic tropical cyclones (TCs) will change in the future has important implications for emergency planning. In this study, we examine how future North Atlantic TC activity may be modulated by the MJO using the Database for Policy Decision-Making for Future Climate Change (d4PDF), a TC-permitting large-ensemble dataset. Consistent with observations, d4PDF simulates significant changes in basin-wide TC activity between phases 2–3 and phases 6–7 during the historical period. Future ensemble-mean projections indicate that phase-to-phase swings in basin-wide Atlantic TC activity may diminish, implying a weakened MJO influence. Nevertheless, projected changes in the MJO–TC relationship show pronounced uncertainty. Variations in intraseasonal TC variability are significantly correlated with changes in MJO convection over the Atlantic main development region (MDR). Compared with future simulations characterized by slightly enhanced MJO convection, projections with strongly suppressed MJO convection in the MDR exhibit weaker phase-to-phase fluctuations in lower-tropospheric diabatic potential vorticity production. Consequently, intraseasonal variability in African easterly waves (i.e., TC seeds) is reduced in these projections. Furthermore, relative to projections characterized by stronger MJO convection, those with muted MJO convection exhibit a thermodynamic mean state less favorable for sustained intraseasonal convection, as indicated by lower advective moistening efficiency (α), which is strongly modulated by local SST warming. Finally, changes in α alone cannot explain the response of MJO convection over the Atlantic MDR across different warming scenarios, whereas changes in α and mean-state saturation deficit together offer greater explanatory power.