Collapse and reorganisation of the Southern Ocean overturning under global warming in a coupled model

This study investigates the long-term behaviour of the thermohaline circulation (THC) in the CSIRO climate model, under a scenario of transient increase of atmospheric (equivalent) CO2 concentration followed by a perpetual stabilisation at triple the initial level (3 X CO2). The North Atlantic Deep Water Formation (NADWF) declines substantially and the Antarctic Bottom Water Formation (AABWF) essentially ceases by the time of CO2 tripling. During the subsequent millennium of stable 3 X CO2, NADWF recovers slightly but the AABWF shows no sign of returning and the residual deep overturning dies away. Accelerating the convergence to equilibrium of the deep ocean under the 3 X CO2 condition, the global THC eventually reaches a near-stable state with the entire ocean warming by about 7°C, NADWF fully recovered and AABWF partly re-established. This result shows a possible new quasi-equilibrium of the ocean under long-term global warming induced by the anthropogenic CO2 increase.