Madden-Julian oscillation changes drive the long-term decline of Southern Hemisphere tropical cyclones

Tropical cyclones pose disproportionate threats to the least developed countries and small island developing states in the Southern Hemisphere. Long-term trends in tropical cyclone activity in the Northern Hemisphere have usually been linked to seasonal mean state changes, but this conventional explanation does not hold in the Southern Hemisphere. Here, we discover a new pathway: long-term changes in intraseasonal variability modes can significantly influence the long-term trends of tropical cyclone climatology. From 1980 to 2024, the Southern Hemisphere tropical cyclone number decreased by approximately 28%. About 80% of this decline can be attributed to the weakening of Madden-Julian Oscillation circulation during its active phases over the South Indian and South Pacific oceans. Our analysis indicates that the impact of Madden-Julian Oscillation-related lower tropospheric cyclonic rotation exceeds that of the seasonal mean state, resulting in less favorable conditions for tropical cyclone formation. Our findings provide a new perspective on understanding future changes in tropical cyclone activity under anthropogenic warming. The weakening of the Madden-Julian Oscillation has emerged as the primary driver of the declining trend in Southern Hemisphere tropical cyclone occurrence over recent decades—a pathway that outweighs changes in seasonal mean states