Decrease in MJO Predictability Following Indo–Pacific Warm Pool Expansion.

The characteristics of the Madden–Julian oscillation (MJO) have changed and are projected to continue changing with the expansion of the Indo–Pacific warm pool, which is the Earth's largest region of warm sea surface temperatures (SSTs). However, the likelihood of a change in MJO predictability following warm pool expansion remains unaddressed. Therefore, this study investigated the effect of warm pool expansion on MJO variability and predictability using the highly idealized aquaplanet configuration of Community Earth System Model 2 (CESM2). By expanding the warm pool in the Indo–Pacific, MJO‐like waves become more regionally confined, short‐lived convective events with weaker magnitude and less robust eastward propagating signals, possibly due to stronger zonal SST gradients and wider meridional widths of the warm pool. Perfect‐model ensemble forecast experiments revealed that the MJO predictability decreased by approximately 5 days, the forecast error proliferated, and the signal rapidly reduced following warm pool expansion. Plain Language Summary: The Madden–Julian oscillation (MJO) is a prominent mode of tropical intra‐seasonal variability in which its convective anomalies drive anomalous circulation within the tropics and remotely in the extratropics, thus providing a "forecast of opportunity" for global weather and climate events. Over the decades, MJO characteristics have changed and are projected to continue changing with the expansion of the Indo–Pacific warm pool—the largest pool with the warmest ocean temperature on Earth. If warm pool expansion changes the MJO characteristics, it is likely that its predictability has changed and will continue to do so in the future. In this study, the change in MJO predictability by expanding the Indo–Pacific warm pool was explored using CESM2 aquaplanet model experiments. The results show that, with warm pool expansion, MJO predictability decreased by approximately 5 days. This decrease is primarily attributed to the change of the MJO‐like waves into more regionally confined, short‐lived convective events with weaker magnitude and less robust eastward propagating signals, owing to the change in SST and circulations. Key Points: A sensitivity experiment was performed using Community Earth System Model 2 aquaplanet with different prescribed sea surface temperaturesExpanding the Indo–Pacific warm pool makes the Madden–Julian oscillation‐like waves less organizedThe warm pool expansion quickens the forecast error increase and predictability decrease [ABSTRACT FROM AUTHOR]