9.5.1.2 Water Mass Properties

Interior water masses, which are directly ventilated at the ocean surface, act to integrate highly variable surface changes in heat and freshwater, and could therefore provide indicators of global change (Stark et al., 2006). Some studies have attempted to investigate changes in three-dimensional water mass properties (Section 5.3). Sub-Antarctic Mode Water (SAMW) and the subtropical gyres have warmed in the Indian and Pacific basins since the 1960s, waters at high latitudes have freshened in the upper 500 m and salinity has increased in some of the subtropical gyres. These changes are consistent with an increase in meridional moisture flux over the oceans over the last 50 years leading to increased precipitation at high latitudes (Section 5.2.3; Wong et al., 1999) and a reduction in the difference between precipitation and evaporation at mid-latitudes (Section 5.6). This suggests that the ocean might integrate rainfall changes to produce detectable salinity changes. Boyer et al. (2005) estimated linear trends in salinity for the global ocean from 1955 to 1998 that indicate salinification in the Antarctic Polar Frontal Zone around 40°S and in the subtropical North Atlantic, and freshening in the sub-polar Atlantic (Figures 5.5 and 5.7). However, variations in other terms (e.g., ocean freshwater transport) may be contributing substantially to the observed salinity changes and have not been quantified.

An observed freshening of SAMW in the South Indian Ocean between the 1960s and 1990s has been shown to be consistent with anthropogenically forced simulations from HadCM3 (Banks et al., 2000) but care should be taken in interpreting sparse hydrographic data, since apparent trends could reflect natural variability or the aliased effect of changing observational coverage. Although SAMW was fresher along isopycnals in 1987 than in the 1960s, in 2002 the salinity was again near the 1960s values (Bindoff and McDougall, 2000; Bryden et al., 2003). An analysis of an ocean model forced by observed atmospheric fluxes and SSTs indicates that this is likely associated with natural variability (Murray et al., 2007), a result supported by an analysis of 20th-century simulations with HadCM3 which shows that it is not possible to reject the null hypothesis that the observed differences are due to internal variability (Stark et al., 2006), although this model does project a long-term freshening trend in the 21st century due to the large-scale response to surface heating and hydrological changes (Banks et al., 2000).