The idealized aquaplanet Maritime Continent barrier effect on the MJO predictability

Studies have indicated exaggerated Maritime Continent (MC) barrier effect in simulations of the Madden- Julian oscillation (MJO), a dominant source of subseasonal predictability in the tropics. This issue has plagued the model-ing and operational forecasting communities for decades, while the sensitivity of MC barrier on MJO predictability has not been addressed quantitatively. In this study, perfect-model ensemble forecasts are conducted with an aquaplanet configura-tion of the Community Earth System Model version 2 (CESM2) in which both basic state and tropical modes of variability are reasonably simulated with a warm pool-like SST distribution. When water-covered terrain mimicking MC landmasses is added to the warm pool-like SST framework, the eastward propagation of the MJO is disturbed by the prescribed MC aqua-mountain. The MJO predictability estimate with the perfect-model experiment is about 6 weeks but reduces to about 4 weeks when the MJO is impeded by the MC aqua-mountain. Given that the recent operational forecasts show an average of 3-4 weeks of MJO prediction skill, we can conclude that improving the MJO propagation crossing the MC could im-prove the MJO skill to 5-6 weeks, close to the potential predictability found in this study (6 weeks). Therefore, more effort toward understanding and improving the MJO propagation is needed to enhance the MJO and MJO-related forecasts to improve the subseasonal-to-seasonal prediction.