Members’ Publications

Roles of shallow convective moistening in the eastward propagation of the Madden-Julian Oscillation

Hirota N., Ogura T., Tatebe H., Shiogama H., Kimoto M., Watanabe M.
J. Climate, 31, 3033–3047

This study examines the roles of shallow convection in the eastward propagation of the Madden–Julian Oscillation (MJO) using a new and old version of the Model for Interdisciplinary Research on Climate (MIROC6 and MIROC5). A major modification of MIROC6 from its previous version, MIROC5, is the implementation of the shallow convective scheme following Park and Bretherton (2009). The MJO representation in MIROC6 is improved compared to MIROC5. The MJO convective envelopes over the Indian Ocean, which decay too early over the western Pacific in MIROC5, propagate farther to the eastern Pacific in MIROC6. In the initial stage of the MJO development, the shallow convection transports boundary layer moisture upward forming an important moisture source for the lower free troposphere in MIROC6. In the mature stage of the MJO, the deep convection becomes increasingly active with the large amount of moisture in the free troposphere. Accordingly, the moisture anomalies associated with the MJO show an upward–westward tilted structure, as in the observations. Conversely, MIROC5 exhibits a dry bias in the lower free troposphere, suggesting that the shallow convective activities are underestimated. A parameter perturbation experiment, modifying the intensity of shallow convection, confirms that enhanced shallow convection reduces the moisture bias in the lower free troposphere and improves the simulation of the MJO in MIROC6.