The Amundsen and Bellingshausen seas in the instrumental period

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

The southeast Pacific Ocean (70°W – 150°W) deserves enhanced scientific interest because of the significant alterations it faces or is supposed to face in a changing climate. Since 1951 annual mean atmospheric temperatures rose by almost 3°C at the Antarctic Peninsula (King, 1994[1]; Turner et al., 2005a[2]) which can be linked to changes in the Bellingshausen Sea, like sea ice retreat (Jacobs and Comiso, 1993[3]), increased ocean surface summer temperatures of more than 1°C, enhanced upper-layer salinification (Meredith and King, 2005[4]), the disintegration of smaller ice shelves (Doake and Vaughan 1991[5]), and accelerated retreat of glaciers (Cook et al., 2005[6]). The changes can be related to atmospheric variability including the Antarctic Circumpolar Wave (White and Peterson, 1996[7]) or the SAM (Hall and Visbeck, 2002[8]; Lefebvre et al., 2004[9]), which both exhibit extreme values in the southeast Pacific Ocean. Different hydrographic conditions have a severe impact on marine species (e.g., the Antarctic krill) which use the Bellingshausen Sea for breeding and nursery before the larvae mainly drift eastward to the southern Scotia Sea/northwestern Weddell Sea (Siegel, 2005[10]). A comprehensive field study on Antarctic krill in the Amundsen Sea has yet to be conducted.

4.25 Bathymetric chart of the Amundsen Sea continental shelf and adjacent deep ocean spotted with the distribution of hydrographic stations of different cruises (colour coded). Fringing ice shelves and glaciers draining the West Antarctic Ice Sheet in light blue (Nitsche et al., 2007[11]). The CTD station identifiers refer to the Nathaniel B. Palmer (NBP), the James Clark Ross (JCR) and the Polarstern (ANT).
4.26 A cross-section showing penetration of warm water to the sub-ice shelf cavity in Pine Island Bay. Potential temperature of the upper 1,200 m along a band between 100°W and 105ºW projected on a strait transect from the open ocean (left) to Pine Island Bay (right) measured during NBP9402 (see Figure 4.25 for station locations). Due to the ice conditions the stations could not be done along a straight line. The sea floor depth was extracted from the ship’s 3.5 kHz echosounder data. Right figure depicts the temperature field in front of Pine Island Glacier with its draft shaded in gray. This short line is along the line of pink dots (sample stations) shown in Figure 4.25. The 1°C isotherm on the continental shelf and slope is marked in red (modified from Hellmer et al., 1998[12]). The solid black area indicates the sea bed.

Connected via the westward flowing, and in this part, weak coastal current, changes in the Bellingshausen Sea also influence the Amundsen Sea (100°W – 150°W) which is fringed to the south by the outlets of major ice streams draining the West Antarctic Ice Sheet. A possible collapse of the latter would result in a 5-6 m global sea level rise threatening many low-lying coastal areas around the globe including millions of their residents (Rowley et al., 2007[13]), but is not likely in the next 100 years. The southernmost position of the ACC southern front (Orsi et al., 1995[14]) together with a relative narrow continental shelf crisscrossed by numerous channels (Figure 4.25) allows Upper Circumpolar Deep Water (UCDW) with temperatures near 1°C to reach the ice shelf edges in the Amundsen and Bellingshausen Seas (Figure 4.26). This ocean heat could fuel melting of up to tens of metres per year at deep ice shelf bases. A linear relationship between melt rates beneath Antarctic ice shelves and ocean temperature is roughly linear at 1 m/yr per 0.1°C ocean warming derived from observations of 23 glaciers (Rignot and Jacobs, 2002[15]). The change of ice melt rate to a change in ocean temperature may not follow this same relationship. In a simple box model Olbers and Hellmer (2009[16]) confirm this rate to be consistent with the involved physical processes and investigate the sensitivity. Numerical modeling of ice-ocean interaction beneath Pine Island Glacier (Payne et al., 2007[17]) concludes that the observed thinning at a rate of 3.9 ± 0.5 m/yr between 1992 and 2001 (Shepherd et al., 2002[18]) would correspond to a different rate of ~ 0.25°C warming of the UCDW underneath Pine Island Glacier. Such warming has not been observed on the Amundsen Sea continental shelf, but a 40-year long temperature time series from the nearby Ross Sea exhibits a warming of the off-shore temperature maximum (190-440 m depth) of ~ 0.3°C (Jacobs et al., 2002[19]). Temperature variability correlated with changing freshwater fluxes due to basal melting is likely at the fringe of the West Antarctic Ice Sheet. Upper Southern Ocean temperatures increased since the 1950's (Gille, 2002[20]; Böning et al., 2008[21]), but the few oceanographic snapshots (e.g., Hofmann and Klinck, 1998[22]; Walker et al., 2007[23]) from the Amundsen/Bellingshausen Sea continental margin are insufficient to identify the time scales and strengths of variability or any trends.

References

  1. ↑ King, J.C. 1994. Recent variability in the Antarctic Peninsula, International Journal of Climatology, 14(4), 357-369.
  2. ↑ Turner, J., Colwell, S.R., Marshall, G.J., Lachlan-Cope, T.A., Carleton, A.M., Jones, P.D., Lagun, V., Reid, P.A. and Iagovkina, S. 2005a. Antarctic climate change during the last 50 years, International Journal of Climatology, 25, 279-294.
  3. ↑ Jacobs, S.S. and Comiso, J.C. 1993. A recent sea-ice retreat west of the Antarctic Peninsula, Geophys. Res. Lett., 20, 1171-1174.
  4. ↑ Meredith, M.P. and King, J.C. 2005. Rapid climate change in the ocean west of the Antarctic Peninsula during the second half of the 20th century, Geophys. Res. Lett., 32, L19604. (doi: 10.1029/2005GL024042)
  5. ↑ Doake, C.S.M. and Vaughan, D.G. 1991. Rapid disintegration of the Wordie Ice shelf in response to atmospheric warming, Nature, 350, 328-330.
  6. ↑ Cook, A., Fox, A., Vaughan, D. and Ferrigno, J. 2005, Retreating glacier fronts on the Antarctic Peninsula over the past half-century, Science, 308, 541-544.
  7. ↑ White, W.B. and Peterson, R. 1996. An Antarctic Circumpolar Wave in surface pressure, wind, temperature, and sea ice extent, Nature, 380, 699-702.
  8. ↑ Hall, A. and Visbeck, M. 2002. Synchronous variability in the southern hemisphere atmosphere, sea ice, and ocean resulting from the annular mode, Journal of Climate, 15(21), 3043-3057.
  9. ↑ Lefebvre, W., Goosse, H., Timmermann, R. and Fichefet, T. 2004. Influence of the Southern Annular Mode on the sea ice-ocean system, J. Geophys. Res., 109, C09005, doi:10.1029/2004JC002403.
  10. ↑ Siegel, V. 2005. Distribution and population dynamics of Euphausia superba: summary of recent findings, Pol. Biol., 29, 1-22, doi:10.1007/s00300-005-0058-5.
  11. ↑ Nitsche, F.O., Jacobs, S.S., Larter, R.D. and Gohl, K. 2007. Bathymetry of the Amundsen Sea continental shelf: Implications for geology, oceanography, and glaciology, Geochem. Geophys. Geosyst., 8, Q10009, doi:10.1029/2007GC001694.
  12. ↑ Hellmer, H.H., Jacobs, S.S. and Jenkins, A. 1998. Oceanic erosion of a floating Antarctic glacier in the Amundsen Sea, in: Ocean, Ice, and Atmosphere: Interactions at the Antarctic continental margin, S S Jacobs and R F Weiss (eds), Antarc. Res. Ser., 75, 83-99.
  13. ↑ Rowley, R.J, Kostelnick, J.C., Braaten, D., Li, X. and Meisel, J. 2007. Risk of rising sea level to population and land area, EOS Transactions, 88, 105, 107.
  14. ↑ Orsi, A.H., Whitworth III, T.W. and Nowlin Jr.,W.D. 1995. On the meridional extent and fronts of the Antarctic Circumpolar Current, Deep-Sea Res., 42, 641-673.
  15. ↑ Rignot, E.J., and Jacobs, S.S. 2002. Rapid Bottom Melting Widespread near Antarctic Ice Sheet Grounding Lines, Science, 296, 2020-2023.
  16. ↑ Olbers, D. and Hellmer, H. 2009. A box model of circulation and melting in ice shelf caverns, Ocean Dynamics, submitted.
  17. ↑ Payne, A.J., Holland, P.R., Shepherd, A.P., Rutt, I.C., Jenkins, A. and Joughin, I. 2007. Numerical modeling of ocean-ice interactions under Pine Island Bay's ice shelf, J. Geophys. Res., 112, doi:10.1029/2006JC003733.
  18. ↑ Shepherd, A.P., Wingham, D.J. and Mansley, J.A.D. 2002. Inland thinning of the Amundsen Sea sector, West Antarctica, Geophys. Res. Lett., 29, 1364, doi:10.1029/2001GL014183.
  19. ↑ 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.
  20. ↑ Gille, S.T. 2002. Warming of the Southern Ocean since the 1950s, Science, 295(5558), 1275-1277, doi:10.1126/science.1065863.
  21. ↑ 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.
  22. ↑ Hofmann, E.E. and Klinck, J.M. 1998. Thermohaline variability of the waters overlying the West Antarctic Peninsula continental shelf, in: Ocean, Ice, and Atmosphere: Interactions at the Antarctic continental margin, S S Jacobs and R F Weiss (eds), Antarc. Res. Ser., 75, 67-81.
  23. ↑ Walker, D.P., Brandon, M.A., Jenkins, A., Allen, J.T., Dowdeswell, J.A. and Evans, J. 2007. Oceanic heat transport onto the Amundsen Sea shelf through a submarine glacial trough, Geophys. Res. Lett., 34, doi:10.1029/2006GL028154.
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