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Southern Ocean overturning under past and future climate change

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Understanding the response of the Southern Ocean overturning circulation to changes in forcing is pivotal to projecting the future of the Southern Ocean CO_2 sink and developing theories of glacial-interglacial outgassing. Using a series of idealised, eddy-permitting to eddy-resolving numerical simulations of the Southern Ocean, we investigate the sensitivity of the overturning to both wind stress and mid-latitude surface buoyancy forcing.

We find that surface buoyancy forcing in the mid-latitudes is likely to play a significant role in setting the strength of the overturning circulation – providing support for the hypothesis that changes in upwelling during deglaciations may have been driven by changes in heat and freshwater fluxes, instead of or in addition to changes in wind stress.

Previous eddy-permitting studies have shown that the eddy field in the Southern Ocean offsets the impact of strengthening winds on both the overturning circulation and the Antarctic Circumpolar Current (ACC) transport. There is widespread belief that the sensitivities of the overturning and ACC transport are dynamically linked, with limitation of the ACC transport response implying limitation of the overturning response. We show that there are significant differences between the sensitivities and resolution dependence of the overturning and ACC transport, indicating that they are controlled by distinct dynamical mechanisms. The modelled overturning is significantly more sensitive to change than the ACC transport, with the possible implication that the Southern Ocean overturning may increase in response to future wind stress changes without measurable changes in the ACC transport. It is hypothesised that the dynamical distinction between the zonal and meridional transport sensitivities is derived from the depth dependence of the extent of cancellation between the Ekman and eddy-induced transports.

This talk is part of the British Antarctic Survey - Polar Oceans seminar series series.

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