How much has the total transport of the Antarctic Circumpolar Current changed over time?
From reading the literature on the subject, my impression is that we don't know that much on the subject, primarily due to a scarcity of measurements. My understanding is that the first and only year-long time-series was made at the Drake Passage in 1979 (e. g. Whitworth &amp; Peterson, 1985) by the ISOS programme (International Southern Ocean Studies). In addition to that NOAA's WOCE (World Ocean Circulation Experiment) added yearly data from 1990 to 2002.

According to Rintoul &amp; 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 &amp; 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:  


not enough data for seasonal trends (only one yearlong time-series);
some interannual variability (possibly as a consequence of variabilities in other water masses); 
not enough data to study decadal variation.