Influence of Rossby vortex zonal scale on MJO crossing Maritime Continent barrier

The Madden-Julian Oscillation (MJO) is a prominent source of subseasonal-to-seasonal predictability in the tropics. Both observations and numerical simulations indicate that the MJO typically weakens when it propagates across the Maritime Continent, a phenomenon known as “the barrier effect”. However, numerical models often overestimate this effect, resulting in reduced prediction skill for MJO crossing events. Previous research has emphasized the influence of moisture structure in this deficiency. However, the dynamical structure of the MJO, which is essential for vorticity advection and generation, has received less attention.

This study investigates the dynamical characteristics of MJO events crossing the Maritime Continent using CESM1. Based on the criterion that the Real-time Multivariable MJO index (RMM) amplitude remains above 1 throughout phases 4 and 5, we identified 23 MJO events between 1997 and 2012 that successfully crossed the Maritime Continent. For each case, hindcasts were initialized with ERA-Interim during RMM phases 1 or 2, amounting to 197 dates. The model was also initialized with YOTC (Year of Tropical Convection) analysis during October 2008 and April 2010 for comparison. The model successfully reproduced the Maritime Continent crossing in 16 cases for at least one initialization date, amounting to 50 dates.

Comparison between successful crossing and blocked hindcasts reveals distinct differences in the vorticity field. Specifically, successful crossing events are characterized by a Rossby vortex to the west of the MJO convective center that exhibits a more compact zonal scale, despite having similar intensity to blocked events. In the lower troposphere, when the MJO center is over the Indian Ocean, the zonal scale of the Rossby vortex is approximately 3000 km, compared to 5000 km in blocked events. With a more compact structure, the low-level cyclonic vorticity generation, caused by the leading convective heating, pulls the Rossby vortex eastward more efficiently. Consequently, the MJO is less prone to the reduction of oceanic convection over the Maritime Continent, thereby increasing the probability of a successful crossing. Furthermore, compared with ERA-Interim, YOTC analysis exhibits a more compact dynamical structure as well as a higher fraction of crossing events.

These findings indicate that the horizontal spatial scale of the Rossby vortex is critical for MJO crossing the Maritime Continent barrier. Better initialized dynamical field can improve the model ability to capture MJO propagation across this complex region.