<p>Abrupt climate changes repeatedly occurred during glacial periods, caused by intrinsic instabilities of the Atlantic Meridional Overturning Circulation (AMOC) leading to Dansgaard–Oeschger events, and by the AMOC’s response to massive iceberg discharges in the North Atlantic, known as Heinrich events. This AMOC-driven millennial-scale climate variability is most prominent in the North Atlantic but also propagates to the Southern Ocean, where its imprint is particularly strong during cold (Stadial) phases featuring Heinrich events. Here we use an Earth system model to show that the qualitative differences between Heinrich Stadials and non-Heinrich Stadials seen in proxy records can be explained by a sudden start of convection in the Southern Ocean triggered by a strong weakening of the AMOC during Heinrich events. The sudden convection onset leads to rapid warming and sea ice retreat in the Southern Ocean, and the resulting ventilation of the deep ocean explains the rapid CO₂ increase of ~15 ppm on centennial timescales during some Heinrich Stadials. We propose a general mechanism whereby a shutdown of convection in the North Atlantic triggers convection in the Southern Ocean—a phenomenon we refer to as a bipolar convection seesaw—which could also be activated by a potential future weakening of the AMOC.</p>