The Australian sector in the instrumental period

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This page is part of the topic The Southern Ocean in the instrumental period
4.24 Top) Map indicating location of repeat sections along which changes in bottom water properties have been assessed. The Australian Antarctic Basin is supplied by two sources of Antarctic Bottom Water: the Ross Sea and Adélie Land. Lower) Changes in the deep potential temperature – salinity curves along 115°E, over the continental rise (61-63.3°S, left) and further offshore (56.5 – 61°S, right). From Rintoul (2007[1]).

Knowledge of the circulation in the Australian sector of the Southern Ocean has increased significantly in the last decade. Repeat hydrographic sections (Figure 4.24), moorings and satellite altimeter measurements have provided new insights into the structure, variability and dynamics of the ACC, water mass formation and the overturning circulation. The mean baroclinic transport of the ACC south of Australia is 147±10 Sv (Rintoul and Sokolov, 2001[2]), consistent with recent estimates of the flow leaving the Pacific basin through Drake Passage (136±8 Sv, Cunningham et al., 2003[3]) and the Indonesian Throughflow (Meyers et al., 1995[4]). A multi-year time series derived from XBT sections and altimetry shows significant interannual variability (with a standard deviation of 4.3 Sv) but no trend in transport (Rintoul et al., 2002[5]). High resolution hydrographic sections reveal that the ACC fronts consist of multiple jets, aligned with streamlines that can be traced using maps of absolute sea surface height (Sokolov and Rintoul, 2002[6], 2007[7]). Eddy fluxes estimated from current meter moorings confirm that the eddies transport heat poleward and zonal momentum downward (Phillips and Rintoul, 2000[8]). A cyclonic gyre lies between the ACC and the Antarctic continent, closed in the west by a northward boundary current along the edge of the Kerguelen Plateau (McCartney and Donohue, 2007[9]).

The ACC belt in the Australian sector has warmed in recent decades, as found elsewhere in the Southern Ocean (Gille, 2002[10], 2008[11]; Levitus et al., 2000[12], 2005[13]; Willis et al., 2004[14]; Böning et al., 2008[15]). The southward shift of the ACC fronts has caused warming through much of the water column, resulting in a strong increase in sea level south of Australia between 1992 and 2005 (Sokolov and Rintoul, 2003[16]; Morrow et al., 2008[17]). However, there is no observational evidence of the increase in ACC transport also predicted by the models (Böning et al., 2008[15]). Recent studies suggest the ACC transport is insensitive to wind changes because the ACC is in an “eddy-saturated” state, in which an increase in wind forcing causes an increase in eddy activity rather than a change in transport of the current (Hallberg and Gnanadesikan, 2006[18]; Meredith and Hogg, 2006[19]).

Changes have been observed in several water masses in the Australian sector between the 1960s and the present (Aoki et al., 2005[20]). Waters north of the ACC have cooled and freshened on density surfaces corresponding to intermediate waters (neutral densities between 26.8 and 27.2 kg m-3). South of the ACC, waters have warmed and become higher in salinity and lower in oxygen on neutral density surfaces between 27.7 and 28.0 kg m-3 (the Upper Circumpolar Deep Water –UCDW). The changes south of the ACC are consistent with a shoaling of the interface between the warm, salty, low oxygen UCDW and the cold, fresh, high oxygen surface water that overlies it. The pattern of water mass change observed in the Australian sector is consistent with the “fingerprint” of anthropogenic climate change in a coupled climate model (Banks and Bindoff, 2003[21]).

The Antarctic Bottom Water (AABW) in the Australian Antarctic basin has freshened significantly since the early 1970s. Whitworth (2002[22]) detected a shift toward fresher AABW after 1993, concluding that “two modes” of AABW were present in the basin, with the fresher mode becoming more prominent in the 1990s. More recent studies have documented a monotonic trend toward fresher, and in most cases warmer, bottom water between the late 1960s and the present rather than a bi-modal distribution. Aoki et al. (2005[20]) used a hydrographic time series with nearly annual resolution between 1993 and 2002 to show a steady decline in salinity of the bottom water at 140ºE. By using repeat observations from the same location and same season, they could demonstrate that the trend was not the result of aliasing of spatial or temporal variability. Rintoul (2007[1]) showed that the deep potential temperature – salinity relationship of the entire basin had shifted towards lower salinity between the early 1970s and 2005. The average rate of freshening at 115 E is 7 ppm/decade, which can be compared to a mean freshening rate of 12 ppm/decade in the North Atlantic, at similar distances downstream of the source of dense water (Dickson et al., 2002[23]). These results suggest that the sources of dense water in both hemispheres have been responding to changes in high latitude climate.

The abyssal layers of the Australian Antarctic Basin are supplied by the two primary sources of bottom water that lie outside of the Weddell Sea: a fresh variety formed along the Adélie Land coast (144º E) and a salty variety produced in the Ross Sea (Rintoul, 1998[24]). The changes observed in the Australian Antarctic Basin therefore reflect freshening of the AABW formed in the Indian and Pacific sectors of the Southern Ocean, which accounts for about 40% of the total production of AABW (Orsi et al., 1999[25]).

The cause of the freshening of AABW in the Australian sector is not yet fully understood. Changes in precipitation, sea ice formation and melt, ocean circulation patterns, and melt of floating glacial ice around the Antarctic margin could all influence the salinity where dense water is formed. Oxygen isotope measurements in the Ross Sea indicate that an increase in the supply of glacial melt-water has contributed to the large freshening of shelf waters observed there in recent decades (Jacobs et al., 2002[26]). The most likely source of the increased supply of melt-water is the rapidly thinning glaciers and ice shelves of the Amundsen Sea, including the Pine Island Glacier (Jacobs et al., 2002[26]), where enhanced basal melt has been linked to warmer ocean temperatures (Rignot and Jacobs, 2002[27]; Shepherd et al., 2004[28]). While most of the ice sheet in East Antarctica appears to be gaining mass, glaciers draining parts of the Wilkes Land coast where the Adélie Land bottom water is formed have decreased in elevation (Shepherd and Wingham, 2007[29]), and the floating ice in this sector thinned between 1992 and 2002 (Zwally et al., 2005[30]). Therefore increased supply of glacial melt-water may have played a role in the freshening of both the Adélie Land and Ross Sea Bottom Water.

References

  1. ↑ 1.0 1.1 Rintoul, S.R. 2007. Rapid freshening of Antarctic Bottom Water formed in the Indian and Pacific Oceans, Geophys. Res. Lett., 34, L06606, doi:10.1029/2006GL028550.
  2. ↑ Rintoul, S.R. and Sokolov, S. 2001. Baroclinic transport variability of the Antarctic Circumpolar Current south of Australia (WOCE repeat section SR3), Journal of Geophysical Research, 106, 2795-2814.
  3. ↑ Cunningham, S.A., Alderson, S.G., King, B.A. and Brandon, M.A. 2003. Transport and variability of the Antarctic Circumpolar Current in Drake Passage. J. Geophys. Res., 108 (C5), 8084, doi:10.1029/2001JC001147.
  4. ↑ Meyers, G., Bailey, R.J. and Worby, A.P. 1995. Geostrophic transport of Indonesian Throughflow, Deep-Sea Res., I, 42, 1163-1174.
  5. ↑ Rintoul, S.R., Sokolov, S. and Church, J. 2002. A six year record of baroclinic transport variability of the Antarctic Circumpolar Current at 140CE, derived from XBT and altimeter measurements, Journal of Geophysical Research – Oceans, 107 (C10): art.no. 3155.
  6. ↑ Sokolov, S. and Rintoul, S.R. 2002. The structure of Southern Ocean fronts at 140E, Journal of Marine Systems, 37, 151-184.
  7. ↑ Sokolov, S. and Rintoul, S.R. 2007. Multiple jets of the Antarctic Circumpolar Current south of Australia, Journal of Physical Oceanography, 37, 1394-1412.
  8. ↑ Phillips, H.E. and Rintoul, S.R. 2000. Eddy variability and energetics from direct current measurements in the Antarctic Circumpolar Current south of Australia, Journal of Physical Oceanography, 30, 3050-3076.
  9. ↑ McCartney, M.S. and Donohue, K.A. 2007. A deep cyclonic gyre in the Australian-Antarctic Basin, Progress in Oceanography, 75, 675-750.
  10. ↑ Gille, S.T. 2002. Warming of the Southern Ocean since the 1950s, Science, 295(5558), 1275-1277, doi:10.1126/science.1065863.
  11. ↑ Gille, S.T. 2008. Decadal-scale temperature trends in the Southern Hemisphere ocean, J. Clim., 21(18), 4749-4765.
  12. ↑ Levitus, S., Antanov, J.I., Boyer, T.P. and Stephens, C. 2000. Warming of the world ocean, Science, 287 (5461), 2225-2229.
  13. ↑ Levitus, S., Antonov, J. and Boyer, T. 2005. Warming of the world ocean, Geophysical Res. Letters, 32, L02604, doi:10.1029/2004GL021592.
  14. ↑ Willis, J.K., Roemmich, D. and Cornuelle, B. 2004. Interannual variability in upper ocean heat content, temperature, and thermosteric expansion on global scales, J. Geophys. Res., 109, doi:10.1029/2003JC002260.
  15. ↑ 15.0 15.1 Böning, C. W., A. Dispert, M. Visbeck, S. R. Rintoul, and F. Schwarzkopf, 2008. Observed multi-decadal ocean warming and density trends across the Antarctic Circumpolar Current, Submitted.
  16. ↑ Sokolov, S. and Rintoul, S.R. 2003. The subsurface structure of interannual temperature anomalies in the Australian sector of the Southern Ocean, Journal of Geophysical Research, 108 (C9): Art. No. 3285.
  17. ↑ Morrow, R., Valladeau, G., and Sallee, J. B. 2008. Observed subsurface signature of Southern Ocean sea level rise. Progress in Oceanography, 77(4), 351-366.
  18. ↑ Hallberg, R. and Gnanadesikan, A. 2006. The role of eddies in determining the structure and response of the wind-driven Southern Hemisphere overturning: Initial results from the Modelling Eddies in the Southern Ocean project, J. Phys. Oceanogr., 36, 3312-3330.
  19. ↑ Meredith, M.P. and Hogg, A.M. 2006. Circumpolar response of Southern Ocean eddy activity to a change in the Southern Annular Mode, Geophys. Res. Letters, 33, doi: 10.1029/2006GL026499.
  20. ↑ 20.0 20.1 Aoki, S., Rintoul, S.R., Ushio, S., Watanabe, S. and Bindoff, N.L. 2005. Freshening of the Adelie Land Bottom Water near 140°E, Geophys. Res. Lett., 32, L23601, doi10.1029/2005GL024246.
  21. ↑ Banks, H. T. and Bindoff, N.L. 2003. Comparison of observed temperature and salinity changes in the Indo-Pacific with results from the coupled climate model HadCM3: processes and mechanisms, J. Climate, 16, 156-166.
  22. ↑ Whitworth, T. 2002. Two modes of bottom water in the Australian-Antarctic Basin, Geophys. Res. Lett., 29(5), 1073, doi:10.1029/2001GL014282.
  23. ↑ Dickson, B., I. Yashayaev, J. Meincke, B. Turrell, S. Dye and J. Holfort. 2002. Rapid freshening of the deep North Atlantic Ocean over the past four decades, Nature, 416, 832-837.
  24. ↑ Rintoul, S.R. 1998. On the origin and influence of Adelie Land Bottom Water. In: Ocean, Ice and Atmosphere: Interactions at the Antarctic Continental Margin, S. Jacobs and R. Weiss (eds.), Antarctic Research Series, 75, 151-171, American Geophysical Union, Washington.
  25. ↑ Orsi, A.H., Johnson, G.C. and Bullister, J.B. 1999. Circulation, mixing and production of Antarctic Bottom Water, Prog. Oceanog., 43, 55-109.
  26. ↑ 26.0 26.1 Jacobs, S.S., Giulivi, C.F. and Mele, P.A. 2002. Freshening of the Ross Sea during the late 20th century. Science, 297(5580), 386-389, doi:10.1126/science.1069574.
  27. ↑ Rignot, E.J., and Jacobs, S.S. 2002. Rapid Bottom Melting Widespread near Antarctic Ice Sheet Grounding Lines, Science, 296, 2020-2023.
  28. ↑ Shepherd, A., Wingham, D. and Rignot, E. 2004. Warm ocean is eroding West Antarctic ice sheet, Geophys. Res. Lett., 31, L23402, doi:10.1029/2004GL021106.
  29. ↑ Shepherd, A. and Wingham, D. 2007. Recent sea-level contributions of the Antarctic and Greenland ice sheets, Science, 315 (5818), 1529-1532.
  30. ↑ Zwally, H.J., Giovinetto, M., Li, J., Cornejo, H., Beckley, M., Brenner, A., Saba, J. and Yi, D. 2005. Mass changes of the Greenland and Antarctic ice sheets and shelves and contributions to sea-level rise: 1992-2002, Journal of Glaciology, 51(175), 509-527.
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