ALBERT

Description: The WOCE Hydrographic Program (WHP) and repeated hydrographic data were used to document overall property changes of the Antarctic Bottom Water (AABW) in the Australian-Antarctic Basin between the 1990s and 2000s. Strong cooling and freshening is observed on isopycnals for layers denser than γn = 28.30. Changes in average salinity and potential temperature below this isopycnal correspond to basin-wide warming of 1300 ± 200 TW and freshening of 24 ± 3 Gt yr−1. While freshening can be explained by freshening of major source waters, i.e., the High Salinity Shelf Water (HSSW) of the Ross Sea and the dense shelf water formed in the Adélie and George V Land (AGVL) region, extensive warming of the AABW cannot be explained by warming of the source waters. A possible cause of warming of the AABW is a decrease in supply of the Ross Sea Bottom Water (RSBW). Hydrographic profiles between the Drygalski Trough of the Western Ross Sea and 150° E were analyzed in the context of a simple advective-diffusive model to assess the causes of the observed changes. The RSBW has also warmed by a larger amount than its source water (the HSSW). The model suggests that the warming of the RSBW observed between the 1970s and 2000s can be explained by a 21 ± 23% reduction in transport of the RSBW and an enhancement of the vertical diffusion of heat as a result of a 30 ± 7% weakening of the abyssal stratification. Freshening of the HSSW reduced the salinity and density stratification between the bottom water layer and overlying ambient water. Hence, freshening of the HSSW both directly freshened and indirectly warmed the RSBW by enhancing the vertical mixing. A simple box model suggest that changes in property and volume transport (decrease of 6.7% is assumed between the year 1995 and 2005) of the RSBW can explain 51 ± 6% of the warming and 84 ± 10% of the freshening observed in the AABW. These facts demonstrate that changes in both property and volume transport of the RSBW have contributed to the warming and freshening of the AABW in the Australian-Antarctic Basin.

Description: Changes to the properties of Antarctic Bottom Water in the Australian-Antarctic Basin (AA-AABW) between the 1990s and 2000s are documented using data from the WOCE Hydrographic Program (WHP) and repeated hydrographic surveys. Strong cooling and freshening are observed on isopycnal layers denser than γn = 28.30 kg m−3. Changes in the average salinity and potential temperature below this isopycnal correspond to a basin-wide warming of 1300 ± 200 GW and freshening of 24 ± 3 Gt year−1. Recent changes to dense shelf water in the source regions in the Ross Sea and George V Land can explain the freshening of AA-AABW but not its extensive warming. An alternative mechanism for this warming is a decrease in the supply of AABW from the Ross Sea (RSBW). Hydrographic profiles between the western Ross Sea and George V Land (171–158° E) were analyzed with a simple advective-diffusive model to assess the causes of the observed changes. The model suggests that the warming of RSBW observed between the 1970s and 2000s can be explained by a 21 ± 23% reduction in RSBW transport and the enhancement of the vertical diffusion of heat resulting from a 30 ± 7% weakening of the abyssal stratification. The documented freshening of Ross Sea dense shelf water leads to a reduction in both salinity and density stratification. Therefore the direct freshening of RSBW at its source also produces an indirect warming of the RSBW. A simple box model suggests that the changes in RSBW properties and volume transport (a decrease of 6.7% is assumed between the year 1995 and 2005) can explain 51 ± 6% of the warming and 84 ± 10% of the freshening observed in AA-AABW.