Abstract
Abstract. 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.
Highlights
The abyssal layer of the world ocean is occupied by Antarctic Bottom Water (AABW), which originates from cold dense shelf water formed by large volumes of sea ice produced in Antarctic coastal polynyas (e.g. Orsi et al, 1999; Jacobs, 2004)
The temporal change in uH ∗ from the 1970s to the 2000s is estimated as a decrease of 21 ± 23 %, it is subjected to a large uncertainty. These results suggest a reduction in the volume transport of the RSBW is associated with enhanced vertical diffusion, as a result of weakening in the stratification
The RSBW warmed, and at a faster rate than reported for its source waters. We infer that this warming is driven by a 21 ± 23 % reduction in the volume transport of the RSBW and enhanced vertical diffusion of heat as a result of a 30 ± 7 % decrease in density stratification
Summary
The abyssal layer of the world ocean is occupied by Antarctic Bottom Water (AABW), which originates from cold dense shelf water formed by large volumes of sea ice produced in Antarctic coastal polynyas (e.g. Orsi et al, 1999; Jacobs, 2004). AA-AABW warmed over much of the basin between the years 1936–1993 and 1994–1996 (Whitworth, 2002), but cooled in the region between 140 and 150◦ E from the 1950 to 2001 (Jacobs, 2004, 2006) and at 140◦ E between the 1970s and 2002 (Aoki et al, 2005), the magnitude of the trend may be aliased by higher-frequency signals These overall changes in temperature with space and time suggest that the relative contribution of different source waters may have played a role in the observed changes of AA-AABW. The influences of changes in both RSBW properties and volume transport on AA-AABW are assessed
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