Stability of Antarctic Bottom Water Formation to Freshwater Fluxes and Implications for Global Climate

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Abstract The stability of Antarctic Bottom Water (AABW) to freshwater (FW) perturbations is investigated in a coupled climate model of intermediate complexity. It is found that AABW is stable to surface freshwater fluxes greater in volume and rate to those that permanently “shut down” North Atlantic Deep Water (NADW). Although AABW weakens during FW forcing, it fully recovers within 50 yr of termination of FW input. This is due in part to a concurrent deep warming during AABW suppression that acts to eventually destabilize the water column. In addition, the prevailing upwelling of Circumpolar Deep Water and northward Ekman transport across the Antarctic Circumpolar Current, regulated by the subpolar westerly winds, limits the accumulation of FW at high latitudes and provides a mechanism for resalinizing the surface after the FW forcing has ceased. Enhanced sea ice production in the cooler AABW suppressed state also aids in the resalinization of the surface after FW forcing is stopped. Convection then restarts with AABW properties only slightly colder and fresher compared to the unperturbed control climate state. Further experiments with larger FW perturbations and very slow application rates (0.2 Sv/1000 yr) (1 Sv ≡ 106 m3 s−1) confirm the lack of multiple steady states of AABW in the model. This contrasts with the North Atlantic, wherein classical hysteresis behavior is obtained with similar forcing. The climate response to reduced AABW production is also investigated. During peak FW forcing, Antarctic surface sea and air temperatures decrease by a maximum of 2.5° and 2.2°C, respectively. This is of a similar magnitude to the corresponding response in the North Atlantic. Although in the final steady state, the AABW experiment returns to the original control climate, whereas the North Atlantic case transitions to a different steady state characterized by substantial regional cooling (up to 6.0°C surface air temperature).