The Xiamen Symposium on Marine Environmental Sciences (XMAS 2027)

Mesoscale eddies in the Southern Ocean can extend from the surface to the deep ocean and play a crucial role in both regional and global heat budgets. However, sparse in situ observations have limited prior estimates of eddy meridional heat transport (EMHT) to the near surface, leaving deep ocean processes poorly understood. Here, we present the first observational quantification of deep eddy kinetic energy (EKE) and EMHT using Argo data. Our results show that deep EMHT reaches at least the same order of magnitude as the surface transport, despite deep EKE being only one-fifth of surface levels. Float observations and sensitivity experiments reveal that, in contrast to the relatively short-range cross-frontal transport observed near the surface, deep EMHT extends the meridional movement of subtropical warm waters by over ten degrees of latitude closer to Antarctica than that driven by mean flows alone, with about 60% occurring in eddy hotspot regions of the Indian Ocean sector. These findings provide new observational evidence for the role of mesoscale eddies in modulating deep Southern Ocean heat redistribution and their potential impact on Antarctic climate.