In AR4, Chapter 5, Antarctic Intermediate Water (AAIW) was reported to have warmed and freshened since the 1960s (Figure 3.9). In most recent studies, usually—but not always—a dipole pattern was found: on isopycnals denser than the AAIW salinity minimum, a warming and salinification was observed and on isopycnals lighter than the AAIW salinity minimum, a cooling and freshening trend (Böning et al., 2008; Durack and Wijffels, 2010; Helm et al., 2010; McCarthy et al., 2011). The salinity minimum core of the AAIW also underwent changes consistent with these patterns on isopycnals: In 1970–2009, south of 30°S, the AAIW salinity minimum core showed a strong, large-scale shoaling (30 to 50 dbar per decade) and warming (0.05°C to 0.15°C per decade), leading to lighter densities (up to 0.03 kg m–3 per decade), while the salinity trends varied regionally. A long-term freshening of the AAIW core is found in the southwest Atlantic, southeast Pacific, and south-central Indian oceans, with salinification south of Africa and Australia. All trends were strongest close to the AAIW formation latitude just north of the Antarctic Circumpolar Current (Schmidtko and Johnson, 2012).
Both an increase in precipitation—evaporation and poleward migration of density surfaces caused by warming have likely contributed to the observed trends (Section 3.3; Böning et al., 2008; Durack and Wijffels, 2010; Helm et al., 2010; McCarthy et al., 2011). Changes in AAIW properties in particular locations have also been linked to other processes, including exchange between the Indian and Atlantic basins (McCarthy et al., 2011) and changes in surface forcing related to modes of climate variability like ENSO and the SAM (Garabato et al., 2009). Whether these changes in properties also affected the formation rates of AAIW cannot be assessed from the available observations.