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Ocean currents are surprisingly resistant to intensifying winds
The Antarctic Circumpolar Current is the current system with the largest volume transport in the world ocean. Between 40 and 60 S strong westerlies move about 140 million cubic metres of water per second around the Antarctic continent (this is about five times the transport of the Gulf Stream). Vertical motions associated with this current have been responsible for transporting a substantial fraction of the anthropogenic carbon dioxide emissions from the atmosphere to the deep ocean, thereby effectively damping the rate of global warming. Investigations in this key region of the world ocean have been hampered by a sparse database due to the logistical challenges for ship based expeditions in the high-latitude Southern Ocean.
'In our study we used data obtained by the international Argo Programme,' explains Prof. Claus Boening from the Leibniz Institute of Marine Sciences (IFM-GEOMAR) in Kiel, Germany. Argo is a system of currently 3000 autonomous free-floating robotic systems which are surveying the world ocean. Every 10 days these buoys measure temperature and salinity profiles over the upper 2000 metres. These measurements are transmitted to land stations via satellite. 'For this study about 52,000 profiles of more than 600 Argo-drifters in the Southern Ocean were used and compared with historic ship measurements,' explains oceanographer Astrid Dispert from IFM-GEOMAR. For this analysis the extensive archives of the Australian marine research centre in Hobart, Tasmania were also used.
As expected, the observations in the subpolar ocean demonstrate an increase of water temperature and a decrease in salinity at the same time. Nevertheless, in contradiction to the simulations of various climate models the data show no significant changes in water transport. 'Our results point to one important thing: Eddies which are currently not resolved in climate models might be the key process in controlling the transport of the ACC,' Prof. Boening explains. Hence, his conclusion is that investigations with high-resolution ocean models are required to test this hypothesis. 'Of course, besides the simulations we also need further observations,' adds Prof. Martin Visbeck (IFM-GEOMAR). 'Thanks to the international Argo observations programme we now have continuously access to data from a worldwide network of more than 3000 profiling-drifters. This is a quantum leap in the field of ocean observations, which, together with high resolution modelling gives us new insights about long-term changes in the ocean.'
Further investigations have to show whether the results are robust. If confirmed, this would in one way be good news: Until now the Southern Ocean is the biggest oceanic sink for anthropogenic carbon dioxide and therefore a crucial regulator for the atmospheric carbon dioxide concentration. Climate models predicted a severe reduction in the southern ocean carbon dioxide uptake due to wind-forced changes in the current fields. Now high-resolution models are needed to assess the role of the hitherto unresolved ocean eddies in the Southern Ocean's response to the progressive changes in the atmospheric conditions.
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