Antarctic: Ocean currents defy climate change

Researchers are studying water transport in the circumpolar current

Rough seas and icebergs make measurements in the Southern Ocean a challenge. The photo was taken during an expedition with the US icebreaker Nathaniel B. Palmer, on which Argo Drifters between New Zealand and the Antarctic were exposed. © Martin Visbeck / IFM-GEOMAR
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The westerly winds in the southern ocean are increasing due to global warming. But how does the world's strongest ocean current, the Antarctic circumpolar current, respond? Kiel-based oceanographers have now won a surprising answer to this question with the help of a fleet of free-floating measuring robots.

Although the measurements confirm the warming and salinity decrease of the south polar armor predicted by climate models to water depths of more than 1, 000 meters, the circumpolar current itself does not change. Apparently, the increased wind power is compensated by small-scale vortex, so the scientists in the journal "Nature Geoscience". Climate models have not yet adequately considered this process.

140 million cubic meters of water per second

The Antarctic Circumpolar Current is the ocean current in the world ocean with the largest water transport. The strong west winds between 40 and 60 degrees south latitude drive about 140 million cubic meters of water per second around the Antarctic continent - which is almost five times the amount of the Gulf Stream.

At the same time, deep vertical movements are taking place along this streamline, transporting a significant portion of the carbon dioxide we've put into the atmosphere into the deep sea, thereby reducing global warming. Previous research into the response of this key region to changing atmospheric conditions suffered from a significant lack of measurements due to the enormous demands placed on ship operations under the extreme, inhospitable conditions of the Southern Ocean.

Data from the "Argo" program used

"In our study, we used the data obtained in recent years by the international 'Argo' program, " explains Professor Claus Böning from IFM-GEOMAR in Kiel. Argo consists of a network of measuring robots distributed all over the world's oceans, floating freely in the ocean and autonomously measuring temperature and salinity to depths of 2, 000 meters through regular ascents and descents. The determined values ​​are transmitted by satellite to land stations. display

Suspend an Argo drifter. IFM GEOMAR

"Overall, we were able to use 52, 000 vertical profiles from over 600 Argo drifters in the Southern Ocean for our study, and to compare them with historical ship measurements, " explains oceanographer Astrid Dispert, who works for F Schiff This analysis also included extensive archives from the Australian Oceanographic Research Center in Hobart, Tasmania.

Water transport stable

The improved database has allowed an inventory of changes in circumpolar current over the last four decades. According to the climate model forecasts, an increase in water temperatures with a simultaneous decrease in salinity can be seen in the South Pole. However, in contrast to the climate models, the data show no significant change in water transport.

"This finding and our theoretical work suggest that the role of small-scale oceanic vortexes in the models has not been properly captured yet, " explains B ning, concluding that f Future climate predictions will require simulations with improved, high-resolution ocean models .

Scheme of the circulation in the Antarctic circulation current (ACC). The ACC (red) surrounds the Antarctic continent in an easterly direction, whereby the current is partly severely altered and accompanied by intense eddies (yellow). In addition, along the north flank of the ACC, there is a large drop in surface water to depths of around 1, 000 meters, while on the south flank, water from the depth swells. IFM GEOMAR

Resistance of the circulation current

If the results of the Kiel study are confirmed, this would be good news in one respect: So far, the Sûdpolarmeer has been a major oceanic sink for anthropogenic carbon dioxide and has been attenuating This is the main reason for the increase in greenhouse gas concentration in the atmosphere. The results of climate models made it probable that the ocean sink will decrease as a result of changes in circulation due to westerly westerly winds.

Now higher-resolution model studies are in demand: they should clarify whether the small-scale processes that were insufficiently taken into account in the previous simulations are in fact of surprising resistance responsible for the circulation flow.

(idw - Leibniz Institute of Marine Sciences, 28.11.2008 - DLO)