Impact of transient freshwater releases in the Southern Ocean on the AMOC and climate
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The bipolar ocean seesaw is a process that explains the competition between deep waters formed in the North Atlantic (NA) and in the Southern Ocean (SO). In this picture, an increase in the rate of formation of one of these water masses is made at the expense of the other. However, recent studies have questioned the effectiveness of this process. Namely, they show that adding freshwater in the SO can reduce deep water formation in the SO as well as in the NA. In this study, we explore the mechanisms and time scales excited by such a SO freshwater release by performing sensitivity experiments where a freshwater input is added abruptly in the ocean, south of 60°S, with different rates and durations. For this purpose, we evaluate the separate effects of wind, temperature and salinity changes, and we put the emphasis on the time evolution of the system. We find three main processes that respond to these freshwater inputs and affect the NA Deep Water (NADW) production: (i) the deep water adjustment, which enhances the NADW cell, (ii) the salinity anomaly spread from the SO, which weakens the NADW cell, and (iii) the increase in the Southern Hemisphere wind stress, which enhances the NADW cell. We show that process (i) affects the Atlantic in a few years, due to an adjustment of the pycnocline depth through oceanic waves in response to the buoyancy perturbation in the SO. The salinity anomalies responsible for the NADW production decrease [process (ii)] invades the NA in around 30 years, while the wind stress from process (iii) increases in around 20 years after the beginning of the freshwater perturbation. Finally, by testing the response of the ocean to a large range of freshwater release fluxes, we show that for fluxes larger than 0.2 Sv, process (ii) dominates over the others and limits NADW production after a few centuries, while for fluxes lower than 0.2 Sv, process (ii) hardly affects the NADW production. On the opposite, the NADW export is increased by processes (i) and (iii) even for fluxes smaller than 0.1 Sv. The climatic impact of the freshwater release in the SO is mainly a cooling of the Southern Hemisphere, of up to 10°C regionally, which increases with freshwater release fluxes for a large range of values.
KeywordsBipolar ocean seesaw Thermohaline circulation Southern Ocean Ocean–climate interactions Climate variability Glacial meltwater Climate modelling
We are grateful to two anonymous reviewers for helpful advice and comments. We thank the European Marie Curie Research Training Network for Ice sheet and Climate Evolution (NICE) for its support. We also acknowledge the ASTER (Assessment of modelling uncertainties in long-term climate and sea level change projections) and BELCANTO (BELgian research on Carbon uptake in the ANTarctic Ocean) projects funded by the Belgian Science Policy for their support. The authors wish to acknowledge use of the Ferret program for analysis and graphics in this paper and the help of Patrick Brockmann for the use of this program.
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