5.3.5.2. Antarctic Regions and Antarctic Circumpolar Current

The ACC, the longest current system in the world, has a transport through Drake Passage of about 130 Sv, with significant interannual variability. Measurements over 25 years across Drake Passage show no evidence for a systematic trend in total volume transport between the 1970s and the present (Cunningham et al., 2003), although continuous subsurface pressure measurements suggest that trends in seasonality of transport are highly correlated with similar trends in the SAM index (Meredith and King, 2005).

There is growing evidence for the changes in the AABW and intermediate depth waters around Antarctica. In the Weddell Sea, the deep and bottom water properties varied in the 1990s (Robertson et al., 2002; Fahrbach et al., 2004). Changes in bottom water properties have also been observed downstream of these source regions (Hogg, 2001; Andrie et al., 2003) and in the South Atlantic (Section 5.3.2.3). The upper ocean adjacent to the West Antarctic Peninsula warmed by more than 1°C and became more saline by 0.25 psu from 1951 to 1994 (Meredith and King, 2005). The warming is likely to have resulted from large regional atmospheric warming (Vaughan et al., 2003) and reduced winter sea ice observed in this region.

In the Ross Sea and near the Ross Ice Shelf, significant decreases in salinity of 0.003 psu yr^–1 (and density decreases) over the last four decades (Jacobs et al., 2002) have been observed. Downstream of the Ross Ice Shelf in the Australian-Antarctic Basin, AABW has also cooled and freshened (Aoki et al., 2005b). These observed decreases are significantly greater than earlier reports of AABW variability (Whitworth, 2002) and suggest that changes in the antarctic shelf waters can be quite quickly communicated to deep waters. Jacobs et al. (2002) concluded that the freshening appears to have resulted from a combination of factors including increased precipitation, reduced sea ice production and increased melting of the West Antarctic Ice Sheet.