The coastal shelf region of East Antarctica is hypothesized to be shielded from the offshore heat of Circumpolar Deep Water (CDW) due to the dynamic barrier of the Antarctic Slope Front. Yet modified CDW (mCDW) intrudes into the coastal environment in key locations, with impacts on dense shelf water formation and ocean/ice shelf interaction that remain largely unquantified. Using moored measurements and conductivity‐temperature‐depth‐instrumented seal hydrographic data collected in Prydz Bay, East Antarctica, we find buoyancy‐driven upwelling of mCDW into the subsurface (~50 m) layer of the southeastern embayment. Wintertime convection extends as deep as 300 m, entraining heat of the upwelled mCDW to the surface. Accumulated sensible heat supply to the surface through deep convection during June–July reduces the potential sea ice production by 45% in the Davis Polynya, demonstrating that stronger/warmer mCDW intrusions onto the shelf will likely reduce the shelf water density and threaten Antarctic Bottom Water formation.

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