View source for The Amundsen and Bellingshausen seas in the instrumental period

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The southeast Pacific Ocean (70&deg;W &ndash; 150&deg;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&deg;C at the Antarctic Peninsula (King, 1994<ref name="King, 1994">King, J.C. 1994. Recent variability in the Antarctic Peninsula, ''International Journal of Climatology'', '''14'''(4), 357-369.</ref>; Turner et al., 2005a<ref name="Turner et al, 2005a">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.</ref>) which can be linked to changes in the Bellingshausen Sea, like sea ice retreat (Jacobs and Comiso, 1993<ref name="Jacobs and Comiso, 1993">Jacobs, S.S. and Comiso, J.C. 1993. A recent sea-ice retreat west of the Antarctic Peninsula, ''Geophys. Res. Lett.'', '''20''', 1171-1174.</ref>), increased ocean surface summer temperatures of more than 1&deg;C, enhanced upper-layer salinification (Meredith and King, 2005<ref name="Meredith and King, 2005">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 20<sup>th</sup> century, Geophys. Res. Lett., 32, L19604. (doi: 10.1029/2005GL024042)</ref>), the disintegration of smaller ice shelves (Doake and Vaughan 1991<ref name="Doake and Vaughan, 1991">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.</ref>), and accelerated retreat of glaciers (Cook et al., 2005<ref name="Cook et al, 2005">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.</ref>). The changes can be related to atmospheric variability including the Antarctic Circumpolar Wave (White and Peterson, 1996<ref name="White and Peterson, 1996">White, W.B. and Peterson, R. 1996. An Antarctic Circumpolar Wave in surface pressure, wind, temperature, and sea ice extent, ''Nature'', '''380''', 699-702.</ref>) or the SAM (Hall and Visbeck, 2002<ref name="Hall and Visbeck, 2002">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.</ref>; Lefebvre et al., 2004<ref name="Lefebvre et al, 2004">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.</ref>), 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<ref name="Siegel, 2005">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.</ref>). A comprehensive field study on Antarctic krill in the Amundsen Sea has yet to be conducted.

[[File:Figure 4.25 - Amundsen Sea bathymetric chart and hydrographic stations of different cruises.png|thumb|'''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<ref name="Nitsche et al, 2007">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.</ref>). The CTD station identifiers refer to the Nathaniel B. Palmer (NBP), the James Clark Ross (JCR) and the Polarstern (ANT).]]
[[File:Figure 4.26 - Cross-section showing warm water penetrating the Pine Island Bay sub-ice shelf cavity.png|thumb|'''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&deg;W and 105&ordm;W 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&rsquo;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&deg;C isotherm on the continental shelf and slope is marked in red (modified from Hellmer et al., 1998<ref name="Hellmer et al, 1998">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.</ref>). 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&deg;W &ndash; 150&deg;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<ref name="Rowley et al, 2007">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.</ref>), but is not likely in the next 100 years. The southernmost position of the ACC southern front (Orsi et al., 1995<ref name="Orsi et al, 1995">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.</ref>) together with a relative narrow continental shelf crisscrossed by numerous channels ([[:File:Figure 4.25 - Amundsen Sea bathymetric chart and hydrographic stations of different cruises.png|Figure 4.25]]) allows Upper Circumpolar Deep Water (UCDW) with temperatures near 1&deg;C to reach the ice shelf edges in the Amundsen and Bellingshausen Seas ([[:File:Figure 4.26 - Cross-section showing warm water penetrating the Pine Island Bay sub-ice shelf cavity.png|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&deg;C ocean warming derived from observations of 23 glaciers (Rignot and Jacobs, 2002<ref name="Rignot and Jacobs, 2002">Rignot, E.J., and Jacobs, S.S. 2002. Rapid Bottom Melting Widespread near Antarctic Ice Sheet Grounding Lines, ''Science'', '''296''', 2020-2023.</ref>). 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<ref name="Olbers and Hellmer, 2009">Olbers, D. and Hellmer, H. 2009. A box model of circulation and melting in ice shelf caverns, Ocean Dynamics, submitted.</ref>) 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<ref name="Payne et al, 2007">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.</ref>) concludes that the observed thinning at a rate of 3.9 &plusmn; 0.5 m/yr between 1992 and 2001 (Shepherd et al., 2002<ref name="Shepherd et al, 2002">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.</ref>) would correspond to a different rate of ~ 0.25&deg;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&deg;C (Jacobs et al., 2002<ref name="Jacobs et al, 2002">Jacobs, S.S., Giulivi, C.F. and Mele, P.A. 2002. Freshening of the Ross Sea during the late 20<sup>th</sup> century. Science, 297(5580), 386-389, doi:10.1126/science.1069574.</ref>). 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<ref name="Gille, 2002">Gille, S.T. 2002. Warming of the Southern Ocean since the 1950s, ''Science'', '''295'''(5558), 1275-1277, doi:10.1126/science.1065863.</ref>; B&ouml;ning et al., 2008<ref name="B&ouml;ning et al, 2008">B&ouml;ning, 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.</ref>), but the few oceanographic snapshots (e.g., Hofmann and Klinck, 1998<ref name="Hofmann and Klinck, 1998">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.</ref>; Walker et al., 2007<ref name="Walker et al, 2007">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.</ref>) from the Amundsen/Bellingshausen Sea continental margin are insufficient to identify the time scales and strengths of variability or any trends.
==References==
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[[Category:The instrumental period]]
[[Category:The Southern Ocean]]