Enhancing MJO Convective Activity in the Twentieth Century

Abstract The responses of the Madden–Julian oscillation (MJO) to greenhouse warming have been widely studied, given its important role in subseasonal to seasonal prediction. However, due to the short satellite observation records, the long-term variability of MJO convective activity in the past century remains underexplored. Thus, this study reconstructed the MJO convection index [outgoing longwave radiation (OLR)-based MJO index (OMI)] since 1900, based on two twentieth century reanalyses. Based on the constructed century-long OMI, we further investigated dominant patterns of long-term changes in MJO convective activity from 1900 to 2009. Results show an overall significant increasing trend in MJO convective activity, while the MJO circulation feature exhibits an insignificant trend. Furthermore, results reveal that the changes in MJO convective activity over the past century are phase dependent, despite the overall enhancement in MJO convection. Empirical orthogonal function (EOF) analyses indicate that the first leading mode of MJO convective days is characterized by an increase across phase 2–3 and 6–7, while that of MJO convection intensity exhibits significant rising trends across all phases. These changes are attributed to the Indo-Pacific warm pool warming and the resulting increase in atmospheric low-level water vapor, which act as the heat and moisture sources for enhancing the moist static energy (MSE) and thus strengthen MJO convection. Meanwhile, the second leading mode of MJO convective days exhibits interdecadal oscillations, primarily modulated by the Pacific decadal oscillation–like sea surface temperature and vertical motion patterns. Finally, the contributions of each EOF mode to the variability of different MJO phases were also quantified.