The northern Antarctic Circumpolar Current’s flow speed in the Drake Passage was reduced by 40 % during the last glacial in comparison with the present interglacial. This is one result of a study by Dr. Frank Lamy from the Alfred-Wegener-Institute, Helmholtz Centre for Polar and Marine Research (AWI) and colleagues in this week’s “Proceedings of the Natural Academy of Sciences of the United States of America”.

The Drake Passage is about 1000 kilometers wide and a is a constriction for the Antarctic Circumpolar Current.
The Drake Passage is about 1000 kilometers wide and a is a constriction for the Antarctic Circumpolar Current.

The Drake Passage represents the most important oceanic gateway along the pathway of the world’s largest current, the Antarctic Circumpolar Current. Resolving changes in the flow of circumpolar water masses through the Drake Passage is crucial for advancing our understanding of the Southern Ocean role in affecting ocean and climate change on a global scale. Despite this importance and corresponding current scientific efforts, the hydrographical measurements cover no more than 20 years. These instrumental time series are too short to record the natural fluctuations and develop possible future predictions.

For the reconstruction of climatic and oceanographic changes on longer geological time scales, scientists use sediment cores. Their analyses allow recording the natural variability between warm and cold climate conditions in the area of the Antarctic Circumpolar Current between the southern tip of South America and the Antarctic Peninsula. AWI geologist Lamy analysed the sedimentological parameters and the geochemical composition of two marine sediment cores covering the last 65,000 years that is from the last glacial maximum to the current interglacial.

In order to reconstruct climatic and oceanographic changes on longer geological time scales, hence scientists use sediment cores.
In order to reconstruct climatic and oceanographic changes on longer geological time scales, hence scientists use sediment cores.

Changes of the flow velocity can be derived from the sediments’ grain-sizes: fine grain-sizes stand for reduced velocity, because only under these circumstances can small material be deposited. Vice versa strong currents wash out the small particles and only coarse particles are deposited under these circumstances.

“Our data indicate reduced glacial wind forces in the core of the westerly wind zone for the first time,” Lamy explains a potential cause of the varied flow pattern. On the other hand the data suggest a stronger circumpolar current, stronger westerly winds and a reduced export of cold water into the southern Pacific Ocean during the interglacial. “This also has consequences for possible future scenarios: instrumental measurements already show an increase and southerly shift of the Southern West Wind Belt, while the Antarctic Circumpolar Current’s flow velocity does not show a clear trend yet, maybe because the time series are too short and the inertia of ocean currents”, says the geologist.

The new data object the prevailing opinion that the Antarctic Circumpolar Current had been unchanged or even stronger in glacial periods. This has far-reaching consequences for the global circulation patterns of the atmosphere and the oceans. “A reduced flow velocity through the Drake Passage during the last glacial increased the export of cold water masses into the Humboldt current system along the South American continental shelf up to the tropics. This is clearly documented by the available estimations of surface water temperatures. At the same time less water was exported into the South Atlantic, influencing the oceanography up to the Northern Atlantic region”, Lamy explains the importance of his study for future predictions.

Source: AWI, Bremerhaven