Changes in the Atlantic Sector of the Southern Ocean estimated from the CESM Last Millennium Ensemble

The changes in the Antarctic Circumpolar Current system associated with the Polar, sub-Antarctic and Subtropical Fronts in the Atlantic are examined in a ten-member ensemble using the Community Earth System Model. Results for the ensemble average mean show that the Polar Front at 25 degrees W shifts to the south by 0.8 degrees during 1970-2000 compared to its mean latitude over the period 1050-1950. This shift is significant because it is more than twice the standard deviation of the mean latitude time series during 1050-1950. The shift is caused by a slight southward displacement of the Antarctic Circumpolar Current, which in turn is caused by a southward shift in the latitude of the maximum zonal wind stress. The sub-Antarctic Front also shows a small southward shift after 1970, with a maximum latitudinal displacement of 0.2 degrees. However, this shift is not significant compared to the standard deviation of the time series during 1050-1950. The Subtropical Front does not change its latitude during 1970-2000 compared to 1050-2000 because there is very little change in the wind-stress curl in the subtropics. Differences in temperature and salinity throughout the water column at 25 degrees W reveal that during 1970-2000 there is freshening of Antarctic Intermediate Water, whereas the Circumpolar Deep Water becomes saltier.