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Influence of Ross Sea Bottom Water changes on the warming and freshening of the Antarctic Bottom Water in the Australian-Antarctic Basin

Ocean Science (2012) 8(4) 419-432
DOI: 10.5194/os-8-419-2012
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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 Combining double low line 28.30 kg mĝ̂'3. Changes in the average salinity and potential temperature below this isopycnal correspond to a basin-wide warming of 1300 A± 200 GW and freshening of 24 A± 3 Gt yearĝ̂'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 (171-158A°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 A± 23% reduction in RSBW transport and the enhancement of the vertical diffusion of heat resulting from a 30 A± 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 A± 6% of the warming and 84 A± 10% of the freshening observed in AA-AABW. © Author(s) 2012.

Figures

  • Fig. 1. Locations of the WOCE WHP (black circles) and their repeat (red and green circles) meridional hydrographic sections crossing the Australian-Antarctic Basin with schematic view of spreading path of the AABW. Red circles on SR3 show locations of repeat occupations in the year 2001 (50–61◦ S), 2002 (61–66◦ S), 2003 (61–66◦ S) and green circles show those in 2008, respectively. All the red circles on both I9S and I8S show repeat occupations in the 2000s. Red and blue arrows indicate the RSBW from the Ross Sea and ALBW from the AGVL region respectively and the AA-AABW is indicated by purple arrows.
  • Fig. 2. Meridional variations of the water properties averaged over the 100 m-thick layer at the bottom for (a) I8S, (b) I9S and (c) SR3. Blue circles and lines are from the WHP occupations in the 1990s. Red circles and lines in (c) are from repeat occupations in the year 2001 (50–61◦ S), 2002 (61–66◦ S), 2003 (61–66◦ S) and green circles and lines in (c) are from those in 2008. All the red circles and lines in both (a) and (b) are from repeat occupations in the 2000s.
  • Table 1. Summary of the WOCE WHP and repeat occupations. The WOCE WHP section designation, years and months and latitudinal range of the WHP and repeat occupations, cruise code names, vessels used for repeat occupations and time intervals.
  • Fig. 3. Variations of θ -S curves for (a) I8S (46–60◦ S, the Australian-Antarctic Basin), (b) I8S (60–67◦ S, the PET), (c) I9S (50–61◦ S), (d) I9S (61–66◦ S) and (e) SR3. Data are averaged on the isopycnal surfaces. Means are shown by thick lines and one standard deviation envelopes are shown by thin lines (following Johnson et al., 2008). Blue lines are from the WHP occupations in the 1990s. Red lines in (e) are from repeat occupations in the year 2001, 2002, 2003 and green lines in (e) are from in the year 2008. All the red lines in other panels are from repeat occupations in the 2000s. Labeled black lines are contour of neutral density (kg m−3).
  • Table 2. Basin and section averaged trends in property of the AA-AABW. (a) Total volume of layers below γ n = 28.30 kg m−3 isopycnal, area of the Australian-Antarctic basin, mean potential temperature trend, required heat flux, mean salinity trend, required freshwater input in the Australian-Antarctic Basin. (b) Section averaged thickness of layers below γ n = 28.30 kg m−3 isopycnals, potential temperature trends, required heat fluxes, salinity trends, required freshwater inputs for respective sections. Error ranges for SR3 were derived from 95 % confidence limit of linear regression coefficient.
  • Fig. 4. Difference in isobaric potential temperature between the WHP and repeat occupations. Red areas indicate warming and blue areas indicate cooling. (a) I8S (the Australian-Antarctic Basin), (b) I8S (the PET), (c) I9S, (d) SR3 (obtained from subtracting the WHP from combined repeat occupations in the year 2001, 2002, and 2003), (e) SR3 (obtained from subtracting the WHP from repeat occupations in the year 2008). Mean γ n = 28.30 kg m−3 isopycnals are shown by solid lines.
  • Fig. 5. Difference in isobaric salinity between the WHP and repeat occupations. Red areas indicate increase in salinity and blue areas indicate freshening. (a) I8S (the Australian-Antarctic Basin), (b) I8S (the PET), (c) I9S, (d) SR3 (obtained from subtracting the WHP from combined repeat occupations in the year 2001, 2002, and 2003), (e) SR3 (obtained from subtracting the WHP from repeat occupations in the year 2008). Mean γ n = 28.30 kg m−3 isopycnals are shown by solid lines.
  • Fig. 6. θ -S diagrams of region between the Drygalski Trough and 150◦ E. (a) around 150◦ E, (b) around 160◦ E, (c) immediately downstream of the Ross source region, (d): locations of stations. Blue circles are from occupations in the 1970s, green circles are from 1990s and red circles are from 2000s. Labeled black lines in (a), (b), and (c) are contour of neutral density (kg m−3). Dashed arrow in (d) indicates approximate position of flow path of the RSBW and white circles are marked on 100 km interval.

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CITATION STYLE

APA

Shimada, K., Aoki, S., Ohshima, K. I., & Rintoul, S. R. (2012). Influence of Ross Sea Bottom Water changes on the warming and freshening of the Antarctic Bottom Water in the Australian-Antarctic Basin. Ocean Science, 8(4), 419–432. https://doi.org/10.5194/os-8-419-2012

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