1] It is generally accepted that surface freshwater anomalies in the Southern Ocean drive increases in North Atlantic Deep Water (NADW) formation via a bipolar density see-saw. We find that a Southern Ocean freshwater pulse of comparable magnitude to meltwater pulse 1A, shuts down, instead of strengthens, NADW in a glacial climate simulation. Unlike the modern-day simulation, the glacial experiment is associated with a more fragile North Atlantic thermohaline circulation, whereby freshwater anomalies that propagate into the North Atlantic are able to dominate the bipolar density see-saw. Meltwater pulses over the North Atlantic and subsequent NADW shut down are often invoked to explain cold 'Heinrich Events' appearing in the paleoclimate record. Our results suggest that triggers for NADW collapse may also originate from the southern hemisphere in glacial epochs. Once NADW collapses, North Pacific Deep Water develops, consistent with a North Pacific/North Atlantic see-saw triggered from the Southern Ocean.