Reawakening of the bipolar seesaw under CO2 stabilisation

Anti-phased polar thermal and ventilation patterns are a recognised feature of climates of the past, thought to influence climate transitions. Here we initiate a bipolar seesaw in an earth system model by stabilising atmospheric CO2 at year 2020 concentrations. Reduction of Atlantic meridional overturning circulation (AMOC) is succeeded by recovery and overshoot, which is subsequently followed by spontaneous ventilation of the deep Southern Ocean. AMOC is again reduced, and the cycle is repeated more than 1500 years later. Southern Ocean deep convection emits heat and greenhouse gases to the atmosphere and leaves a lasting global imprint of enhanced deep ocean ventilation. Nutrients are mixed to the surface ocean, “resetting” primary production in an otherwise suppressed production regime. Emissions from the Southern Ocean surface initiate abruptly over decades, while imprints on isotopic carbon and nitrogen in the deep sea record these spontaneous events at centennial to millennial timescales. Though idealised, resulting ocean isotopic signatures and reduced meridional temperature gradients are qualitatively and quantitatively supported by paleoproxies from past periods of warmth and warming, indicating the potential for Southern hemisphere climatic transitions to occur much faster than is commonly accepted. These results have broad implications for climate stabilisation at contemporary CO2 levels.