A Fresh View of the Asian-Australian Monsoon Complexity: The Intertropical Convective Cell (ITCC) Framework

The monsoon holds great significance in Asian-Australian civilization. Recent studies realized the link between the monsoon onset and the seasonal migration of the Intertropical Convergence Zone (ITCZ). However, no clear ITCZ band is observed in the Asian-Australian sector due to the strong influence of topography. Instead, there exists a large-scale (similar to 1,500 km) tropical convective cell--a perennial system that we hereafter coin as the "intertropical convective cell (ITCC)." Using ERA5 reanalysis and satellite-based outgoing longwave radiation products, here we show by objective detection and tracking that the ITCC exhibits eight phases during its seasonal migration along the Asian-Maritime land bridge. Particularly, its sudden northward jump in boreal spring coincides well with the earliest (abrupt) onset of the Asian rainy season, while its equatorward retreat heralds the overall (gradual) monsoon withdrawal. These findings demonstrate the close link of the ITCC behavior to the spring-fall asymmetry of the monsoon. Dynamically, the off-equatorial ITCC features a monsoon regime with a cross-equatorial overturning circulation, differing markedly from its equatorial regime with two weak overturning cells on each side. Further budget analyses prove our hypothesis that the north-south charging gradient of the moist static energy determines the ITCC's spring-fall asymmetric propagation, illuminating the physical origin of the spring-fall asymmetry in the monsoon. Our results demonstrate the usefulness of the ITCC framework in understanding the Asian-Australian monsoon complexity in a fresh and holistic manner. The framework will facilitate monsoon diagnosis, modeling and subseasonal-to-seasonal forecasting in the Asian-Australian sector. The onset and withdrawal of the monsoonal rains has been central to agriculture, economy and livelihood in the Asian-Australian region. Yet, a holistic and reliable framework to understand the monsoon annual cycle and complexity has been lacking. In this study, we propose a new intertropical convective cell (ITCC) framework to understand the Asian-Australian monsoon annual cycle. The ITCC is a large-scale, tropical deep convective cell that migrates seasonally along Indochina and the Maritime Continent. Over the course of the year, the ITCC's propagation phases are closely tied to regional monsoon onsets. We find that the north-south imbalance of energy determines the ITCC's spring-fall asymmetric propagation, which explains a fast Asian summer monsoon onset with a slow withdrawal. Our results demonstrate the usefulness of the ITCC framework in diagnosing, modeling and forecasting the Asian-Australian monsoon. The intertropical convective cell (ITCC) framework is introduced to understand the Asian-Australian monsoon annual cycle and complexityThe fast springtime jump and slow autumn retreat of the ITCC explain the "spring-fall asymmetry" of the Asian-Australian monsoonThe north-south energy imbalance drives the ITCC's asymmetric propagation, revealing promising monsoon predictability