Flow of the Antarctic Circumpolar Current: Origins and glacial-interglacial shifts

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• Nick McCave (Dept. of Earth Sciences)
• Monday 16 November 2015, 13:00-14:00
• MR5, Centre for Mathematical Sciences.

If you have a question about this talk, please contact Doris Allen.

The dispute as to whether the ACC was established around 23 Ma ago or at approx 30 Ma is now homing in on the earlier date when the Tasman gap (between Tasmania and Antarctica), which opened at 33 Ma, moved into a position when the westerly winds could drive a flow to the east, reversing the previous flow to the west. This is based on geochemical tracers of water masses. My view (and several others’) had favoured a 23 Ma onset based on a step change in deep flow speed at that location, but that may now be associated with a glacial event. During the last glacial maximum (LGM) ACC flow speed may have been faster, unchanged or slower than present under either stronger or weaker winds and may have shifted to the north (or not). As this is the largest current on the planet (transporting approx. 134×106 m3/s), it deserves to be better understood. I have provided data on the flow of the ACC through the Scotia Sea flow constriction at a past climatic extreme. These data, based on sediment grainsize from 12 cores across the area south of the Subantarctic Front (SAF), indicate essentially no change in the average flow of the ACC through the Drake Passage/Scotia Sea between the LGM and Holocene climatic extremes. However, ACC flow at the LGM was slower in the southern ice-covered portion of the area, and faster in the north, implicating wind stress in spatial current variability. This boundary coincides with the northern limit of winter (i.e. year-round) sea-ice. However two recent works north of the SAF , a region through which approx. 30% of the present ACC flows, suggest significantly reduced glacial flow relative to present, and this may mean reduced flow overall. Slower flow over rough topography and under the ice in the Scotia Sea implies reduced diapycnal mixing in this key region, consistent with a reduced overturning circulation. Added to the constraints that LGM winter sea-ice extent was approx.5° further north and the frontal system was also most likely 5°-7° further north, some areas of speculation are now being restricted by data. However as the recently departed and lamented Yogi Berra said “It ain’t over till it’s over”, and the ACC palaeo-flow disputes ain’t over yet.

This talk is part of the Quantitative Climate and Environmental Science Seminars series.

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