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 (3xCO(2)). 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 3xCO(2), 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 3XCO2 condition, the global THC eventually reaches a near-stable state with the entire ocean warming by about 7 degreesC, NADWF fully recovered and AABWF partly re-established. This result shows a possible new quasiequilibrium of the ocean under long-term global warming induced by the anthropogenic CO2 increase.