Summary
The role played by Southern Hemisphere sea-ice in global climate is explored using an Earth system climate model of intermediate complexity. An ensemble of experiments is analysed where freshwater forcing equivalent to a complete 100 year meltback of Southern Hemisphere sea-ice is applied to a model run that simulates the present climate. This freshwater forcing acts to mildy subdue Southern Ocean deep overturning, reducing mean AABW export by 0.5 Sv in the ensemble average. The decreased convective overturning cools the surface waters thereby increasing sea-ice volume, and thus forming a negative feedback that stabilises Antarctic sea-ice. In contrast, the reduced convective overturn warms sub-surface waters in the Southern Ocean which, combined with the imposed freshening, results in a reduction in the meridional steric height gradient and hence a slow-down of the Antarctic Circumpolar Current (ACC). The reduction in ACC strength is, however, only modest at 1.5 Sv. These responses are thus of only weak magnitude, and the system recovers to its original state over time-scales of decades. An extreme scenario experiment with essentially instantaneous addition of this meltwater load shows similar results, indicating the limited response of the climate system to the freshening implied by Antarctic sea-ice melt. An additional experiment in which a much larger freshwater forcing of approximately 0.4 Sv is applied over 100 years confirms the relatively weak response of the model’s climate state to such forcing, relative to the well-documented climatic effects of freshwater forcing added to the North Atlantic.