The mesosphere and lower thermosphere (MLT, 60–110 km) are a critical atmosphere-space transition layer, yet its density variability remains poorly quantified. Using 24 years (2002–2025) of the Sounding of the Atmosphere using Broadband Emission Radiometry (SABER) temperature measurements, we demonstrate that the T80 index—a temperature-based proxy at 80 km for the strength of austral summer polar upwelling—dominates MLT density fluctuations. A distinct vertical density dipole emerges over southern high latitudes: stronger upwelling (lower T80) increases density below 77 km via adiabatic cooling, yet substantially reduces density from 83 to 105 km. This dipole peaks near 95 km with a correlation of R = 0.98 and a sensitivity of 3.1 % K^-1, jointly shaped by the column adjustment effect and the local thermal effect. Mirroring this high-latitude response, a weaker, negative correlation (peak R = -0.8, -0.5 % K^-1) appears over low latitudes (55° S–35° N, 80–90 km), establishing an interhemispherically anti-phase density pattern driven by the same upwelling. The T80-density relationship remains robust on daily timescales, with a mean correlation of 0.78 (1.7 % K^-1) at 80° S, 95 km. Owing to the strong month-to-month persistence of polar upwelling, the T80 index enables one-month-ahead MLT density predictions with a peak correlation of 0.90. Our results indicate that internal dynamical forcing from summer polar upwelling dominates interannual MLT density variability, while signals from the 11-year solar cycle and long-term CO₂ cooling cannot be reliably isolated from the 24-year observations.