Amplifying Variability of the Southern Annular Mode in the Past and Future

Abstract The Southern Annular Mode (SAM) is the dominant mode of atmospheric variability in the Southern Hemisphere extratropics. While its long‐term positive trend is well‐documented, changes in the amplitude of SAM variability remain poorly understood. Based on reanalysis data and surface‐station observations, we demonstrate that the amplitude of SAM variability has substantially increased over the past eight decades, which is associated with more frequent and severe weather extremes. A simple model and idealized climate experiments reveal that this amplification is primarily driven by enhanced stochastic eddy momentum flux, which is linked to a strengthened meridional temperature gradient. State‐of‐the‐art climate model projections indicate that SAM variability will likely continue to intensify throughout the 21st century, driven by a further increase in the meridional temperature gradient. These findings highlight a critical but previously overlooked response of large‐scale circulation to climate change, with important implications for assessing and mitigating future climate risks across the Southern Hemisphere.