The Southern Ocean freshwater content (FWC) shapes global climate, yet its variability beneath Antarctic sea ice remains poorly constrained. We establish "Southern Ocean Ice Gauge": an interannual FWC budget and reconstruction (2005–2024) using hydrographic profiles with Data Interpolating Empirical Orthogonal Functions (DINEOF) which preserves spatiotemporal and thermohaline relationships. We identify significant FWC increase of up to 2,000 Gt in the Freezing Zone (74–63°S) concurrent with Antarctic ice mass loss over 20 years, and distinct interannual FWC variability of ±2,000 Gt in the Melting Zone (63–54°S) consistent with sea ice volume fluctuations. We also find fresh anomalies propagating from the Weddell to the Amundsen Sea in ~6 years along the Antarctic Circumpolar Current, preceding the 2016/2017 sea ice extreme. This reconstruction provides an observational benchmark linking cryospheric changes and Southern Ocean dynamics, suggesting an Antarctic alternative to "fresh-get-fresher" pattern driven by its land and sea ice melting.