Ross Sea shelf waters in the instrumental period

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This page is part of the topic The Southern Ocean in the instrumental period

The circulation in the Ross Sea is dominated by a wind-driven cyclonic gyre (Treshnikov, 1964[1]) visible as a depression in the steric height transporting Circumpolar Deep Water to the south where by interaction with shelf and slope water Antarctic Bottom Water is produced. It is located south of the mid-ocean ridge between 170° E and 140°W (e.g. Gouretski, 1999[2]) with its centre at about 68°S, 164°W shifting to the southeast with depth. The baroclinic transport of 8.5 Sv is significantly smaller than the one of the Weddell gyre. The eastern boundary is given by a southward deflection of the ACC due to the bottom topography. At the southern limb, westward flow transports water as warm as 1.6°C. From property maps Reid (1986[3]) included the extension up to the Antarctic Peninsula. Antipov et al. (1987[4]), Maslennikov (1987[5]) and Locarnini (1994[6]) locate the eastern boundary at 140°W. The continental shelf area is relatively well sampled due to the normally weak ice cover in summer and the presence of several Antarctic stations (Jacobs and Giulivi, 1999[7]).

‘Shelf waters’ include a variety of low-temperature, ice-modified, high- and low-salinity water masses found below the ocean surface layers on the Antarctic continental shelf (Whitworth et al., 1998[8]). Summer salinity profiles spanning about 20 years in High Salinity Shelf Water (HSSW) near Ross Island displayed gradual salinity increases below 200 m and interannual water column shifts that were several times the measurement accuracy and half the annual cycle in McMurdo Sound (Jacobs, 1985[9]). Atmospheric forcing, sea ice production, HSSW residence time, ice shelf melting and intrusion of Modified Circumpolar Deep Water onto the continental shelf were considered as possible agents of change. Hellmer and Jacobs (1994[10]) noted that salinity decreases could also result from fresher upstream source waters.

Attempts to link observed HSSW changes to multiyear variability in regional sea ice extent, winds and air temperatures revealed the need for longer time series. The data base largely consists of sporadic summer measurements, and both modelers and observers have noted the possibility of aliasing in this shelf water record due to undersampling of a variable inflow. In addition, most measurements were in or near an HSSW eddy, another potential source of variability. Nonetheless, the deep HSSW trend in that area has closely tracked changes at depth along the Ross Ice Shelf and near 500 m throughout the western Ross Sea (Jacobs and Giulivi, 1998[11]; Smethie and Jacobs, 2005[12]).

Record low salinities at the site in February 2000 led to analyses that more strongly implicated changes in ice-ocean interactions upstream in the Amundsen and Bellingshausen Seas (Jacobs et al., 2002[13]). Assmann and Timmermann (2005[14]) successfully modeled averaged HSSW salinity profiles, and inferred that the freshening resulted from a Bellingshausen Sea thermal anomaly. Their periodic signal upwelled in the Amundsen Sea, reduced brine drainage near the sea ice edge and induced a subsurface salinity decrease that was advected into the Ross Sea. Interannual salinity variability is high, but the overall trend has been statistically significant and qualitatively consistent with freshening over a much wider area (Jacobs, 2006[15]).

4.27 Summer temperature and salinity profiles over a 50 year period, some averaged from adjacent profiles, and dashed if more than 15 km from 168º 20’E, 77º 10’S, near Ross Island. Plotted values are 100 m averages/interpolations of CTD or bottle data, edited or extrapolated where shown by open circles. Horizontal lines depict the salinity ranges at six bottle casts in southern McMurdo Sound over 23 December 1960 – 9 February 1961 (Tressler and Ommundsen, 1963[16]). Temperatures are referenced to the surface freezing point, ~-1.91°C at a salinity of 34.8. Tfrs is the surface freezing reference temperature.

The record of summer shelf water thermohaline properties has recently been extended to 50 years (Figure 4.27), and the study area widened to include profiles in McMurdo Sound. The 50 year salinity trend continues to be near -0.03/decade, while slightly warming temperatures have remained consistent with HSSW formation by surface freezing in winter. HSSW near Ross Island thus serves as an index site to monitor change occurring in the Ross Sea and upstream (eastward) in the Antarctic Coastal Current. The salinity decline appears to derive mainly from increasing continental ice meltwater, and will subsequently change the properties if not the production rates of deep and bottom waters. Over regional areas the lower salinity has raised sea level via the halosteric component of seawater density.

References

  1. ↑ Treshnikov, A.F. 1964. Surface water circulation in the Antarctic Ocean, Information Bulletin of the Soviet Antarctic Expedition, 5, 81-83 (English translation)
  2. ↑ Gouretski, V. 1999. The large-scale thermohaline structure of the Ross Gyre, In: Eds G. Spezie and G.M.R. Manzella, Oceanography of the Ross Sea Antarctica, Springer-Verlag Italia, Milano, 77-100.
  3. ↑ Reid, J.L. 1986. On the total geostrophic circulation of the South Pacific Ocean: flow patterns, tracers and transports, Progress in Oceanography, 16, 1-61.
  4. ↑ Antipov, N.N., Maslennikov, V.V. and Pryamikov, S.M. 1987. Location and structure of the Polar Frontal zone in the western part of the Pacific sector of the Southern Ocean (in Russian). Biological oceanographic investigations in the Pacific sector of Antarctica, VNIRO, Moscow, 19-32.
  5. ↑ Maslennikov, V.V. 1987. Secondary frontal discontinuities in the western part of the Antarctic Pacific sector (in Russian), Biological oceanographic investigations in the Pacific sector of Antarctica, VNIRO, Moscow, 32-41.
  6. ↑ Locarnini, R.A. 1994. Water masses and circulation in the Ross Gyre and environments. PhD thesis, Texas A and M University, College Station.
  7. ↑ Jacobs, S.S. and Giulivi, C.F. 1999. Thermohaline Data and Ocean Circulation on the Ross Sea Continental Shelf. In: Eds G. Spezie and G.M.R. Manzella, Oceanography of the Ross Sea Antarctica, Springer-Verlag Italia, Milano, 3-16.
  8. ↑ Whitworth, T., Orsi, A.H., Kim, S.J., Nowlin, W.D. and Locarnini, R.A. 1998. Water masses and mixing near the Antarctic Slope Front, Antarct. Res. Ser., 75, 1-27.
  9. ↑ Jacobs, S.S. 1985. Oceanographic evidence for land ice/ocean interactions in the Southern ocean. In: Glaciers, Ice Sheets and Sea Level: Effect of a CO2-Induced Climatic Change, Report of a Workshop, Seattle, 13-15 Sep 1984, DOE/ER/60235-1, 116-128.
  10. ↑ Hellmer, H.H. and S.S. Jacobs (1994) Temporal changes in shelf water of the southern Ross Sea, Antarct. J. of the U.S., 29(5), 123-124.
  11. ↑ Jacobs, S.S. and Giulivi, C.F. 1998. Interannual ocean and sea ice variability in the Ross Sea, Antarct. Res. Ser., 75, 135-150.
  12. ↑ Smethie, W.M. and Jacobs, S.S. 2005. Circulation and melting under the Ross Ice Shelf: Estimates from evolving CFC, salinity and temperature fields in the Ross Sea, Deep-Sea Res. I, 52, 959-978.
  13. ↑ 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.
  14. ↑ Assmann, K.M. and Timmermann, D.R. 2005. Variability of dense water formation in the Ross Sea, Ocean Dynamics, 55, 68-87.
  15. ↑ Jacobs, S.S. 2006. Observations of change in the Southern Ocean, Phil. Trans. Roy. Soc. A, 364, 1657-1681.
  16. ↑ Tressler, W.L. and Ommundsen, A.M. 1963. Oceanographic studies in McMurdo Sound, Antarctica, IG Bull. 67, Trans. Am. Geophys. Un. 44(1), 217-225.
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