Subtropical gyre expansion causes Southern Ocean salinification contrary to freshening predictions.

The "salty-gets-saltier, fresh-gets-fresher" paradigm predicts that an intensifying hydrological cycle should freshen the climatologically fresh Southern Ocean. Here we show the opposite: sea surface salinity increased at ~0.03 decade-1 across 40-50°S during 2004-2024, most coherently in the Pacific and Atlantic sectors. We attribute this salinification to the poleward expansion of the southern subtropical gyres, which advects saline subtropical water into latitudes of steepest meridional salinity gradient. The Subtropical and Subantarctic Fronts, tracked via the 35 and 34 isohalines, migrate poleward at unequal rates (-0.46° and -0.18° decade-1), narrowing the frontal corridor and sharpening the cross-frontal gradient. A mixed-layer budget shows that horizontal advection dominates the salinity trend, roughly tripling the opposing contribution from surface freshwater flux. Forced by poleward-intensifying westerlies and a positive Southern Annular Mode trend, this circulation-driven salinification demonstrates that ocean dynamics can override freshwater forcing, cautioning against interpreting salinity trends as direct fingerprints of the hydrological cycle.