According to Rintoul & Sokolov 2001 and Rintoul et al. 2001, most of the variability south of Tasmania is due to changes in currents north of the ACC itself. Cunningham et al. 2003 show that the moderate interannual variation in baroclinic transport they observed seem to be coeval with variations in the location of the Polar Front (it moves north or south on a 90km range). But overall the variability seems to be relatively low, and the ACC relatively steady.
White & Peterson, 1996 showed an interesting phenomenon that they called the Antarctic Circumpolar Wave: it is a "wave" in the ACC circling the globe in ca. 8 yrs with a period of 4yrs at any location and it is defined by anomalies in sea-suface temperature, sea-level pressure and sea-ice extent. However no evidence for corresponding anomalies in volume transport was showed, to my knowledge.
So, to summarize, as far as I can see: