Notwithstanding significant progress made in the past decades on understanding the Madden-Julian oscillation (MJO), many fundamental questions regarding its observed behaviors over the Maritime Continent (MC) remain unanswered. One intriguing aspect is the so-called “MJO detours”, which describe a southward shift of the MJO while propagating across the Maritime Continent during boreal winter. While previous studies have proposed some convincing hypotheses regarding the underlying mechanisms, the factors that govern the intensity of this southward detour, particularly the potential role of remote extratropical forcing, remain unknown. Yet this understanding is crucial for extreme weather and sub-seasonal forecasts over the MC and other regions.

In this study, we find that the cross-equatorial surge (CES), a large-scale meridional circulation associated with cold air outbreaks from the high latitudes, amplifies MJOs southward detour during the active Australian monsoon season (December to March). The CES increases northwesterly and westerly winds, which in turn enhances zonal moisture convergence and strengthens positive wind-evaporation feedback onto MJO convection. Further analysis indicates that the zonal convergence by intraseasonal zonal wind anomalies acting upon the background moisture associated with the monsoon system, is intensified in the presence of CES events, leading to stronger convective activity in the southern MC regions. A better understanding of the relationship between the CES and the MJO can lead to a better understanding of extreme climate and natural disasters along the periphery of the southern Indian Ocean and southern Pacific Ocean, such as catastrophic flooding in Indonesia and recent devastating wildfires in Australia, while also aiding in the investigation of bias in simulating MJO propagation in global climate models