Presented at the 2015 Aquarius/SAC-D Science Team MeetingMost observational and modeling efforts on the Atlantic meridional overturning circulation (AMOC) have been focused on the North Atlantic and the Southern Oceans. There are comparatively fewer studies characterizing the water mass and circulation variability in the South Atlantic subtropical gyre, or how that variability impacts the AMOC in the South Atlantic. Here we use observations and high-resolution models to examine the interannual to decadal variability of sea level anomalies, water mass properties, and wind stress across the South Atlantic subtropical gyre. We evaluate meridional volume transports by the currents delineating the gyre's zonal boundaries, such as the Brazil Current on the western boundary, and the Benguela Current on the eastern boundary. We highlight that waters transported by these currents, as well as the accumulated effect of Agulhas eddies on the eastern boundary, play an important role in the AMOC. Based on our analysis, variability in the South Atlantic is dominated by sea level rise across the entire basin, while the decadal to multi-decadal subtropical gyre changes are the next largest source of variability. These modes of variability create zonal density gradients, and impact the strength and structure of the boundary currents and AMOC across the basin. Our results hence stress the importance of better characterizing South Atlantic subtropical gyre variability in order to properly monitor the basin AMOC.