The Southern Ocean (SO, 60°S–30°S) plays a key role in the global climate system by absorbing and redistributing a large fraction of anthropogenic excess heat. While its long-term warming trend is well documented, the mechanisms controlling decadal variability in SO heat storage require further investigation. In particular, whether and how the Southern Annular Mode (SAM) and the El Niño–Southern Oscillation (ENSO) jointly regulate SO heat uptake has not been systematically clarified. Here we analyze multiple observational ocean temperature datasets (IAPv4.2, Ishii, and EN4) to investigate decadal variability of SO heat storage. We identify three distinct shifts in the warming rate of SO heat storage during 2003-2022. These transitions show strong coherence with concurrent variations in SAM and Niño3.4 indices, suggesting a combined influence of these leading modes of climate variability. Composite analyses of SO heat storage tendency reveal a robust nonlinear response to SAM–ENSO coupling. Positive SAM combined with La Niña conditions lead to enhanced heat accumulation in the SO, whereas negative SAM together with El Niño results in reduced heat storage. The largest anomalies occur when SAM and ENSO act in phase, indicating a synergistic amplification of decadal variability beyond the influence of either mode alone.

The robustness of these relationships is confirmed using the CESM2 Large Ensemble, which successfully reproduces the observed composite structures, demonstrating that the identified signal is not driven by internal noise. Model diagnostics further indicate that the joint SAM–ENSO forcing induces pronounced wind stress curl anomalies. These atmospheric anomalies modify ocean circulation and strengthen meridional ocean heat transport into the SO. In particular, the 30°S boundary acts as the primary gateway through which anomalous heat is imported, and changes in meridional heat transport dominate the variability of SO heat storage tendency. Our results demonstrate that decadal variability of SO heat storage arises from the combined influence of SAM and ENSO through their control on large-scale atmospheric circulation and wind-driven ocean heat transport. These findings highlight the importance of considering SAM–ENSO coupling in interpreting past variability and improving predictions of future changes in SO heat uptake and the global ocean heat budget.