Antarctic sea ice extent (SIE) has experienced unprecedented variability in recent decades, with record expansion through 2015, followed by an abrupt transition to sustained decline. Using over two decades of under-ice Argo float observations, we show that changes in ocean heat ventilation have modulated these extreme sea ice variations on interannual timescales. Between 2007 and 2015, the ocean thermocline warmed and shoaled within the Weddell Sea and off East Antarctica, with the former accounting for most of the interannual variability in Antarctic SIE. After 2016, as Antarctic SIE declined, surface salinity increased, enhancing exchange between the sharpened thermocline and surface waters. Idealized modeling of the Weddell Sea indicates that these upper ocean trends were due to concurrent variations in wind-driven Ekman upwelling and precipitation. During the sea ice expansion phase, increased precipitation enhanced ocean stratification, suppressing the upward flux of subsurface heat while promoting sea ice growth. However, between 2014 and 2016, a nearly three-fold increase in upwelling rates weakened the upper ocean stratification, releasing the accumulated subsurface heat. Though a similar sequence of events occurred along the East Antarctic margin, distinct upper-ocean trends and surface forcing in the Pacific sector of the Southern Ocean imply alternative drivers of recent sea ice loss in that region. Nevertheless, these results suggest that future multiyear Antarctic SIE variability will depend on the competing influences of wind-driven upwelling and surface freshwater fluxes.