Temporal changes in shelf water of the southern Ross Sea
Temporal changes in shelf water of the southern Ross Sea HARTMUT H. HELLMER
and STANLEY S. JACOBS, Lamont-Doherty Earth Observatory of Columbia University, Palisades, New York 10964
ver much of the circumpolar antarctic continental shelf, O offshore and prevailing easterly winds drive the sea ice and surface currents toward the west. These currents are well known in the southern Ross Sea, where some of the waters may be derived from the upstream Amundsen shelf. To investigate temporal changes in the Ross Sea, we completed an 800-kilometer (km) transect, parallel to the Ross Ice Shelf edge, on the USCGC Polar Sea from 4 to 10 February 1994 (figure 1). On 42 stations, most begun less than 1 km from the ice front, conductivity-temperature-depth (CTD) measurements were made to within 10 meters (m) of the bottom. At several locations, the ship drifted rapidly in the coastal current, resulting in tow-yo vertical profiles. The casts were accompanied by sampling for salinity, dissolved oxygen, total carbon dioxide (TCO2), chlorofluorocarbons, helium, tritium, isotopic oxygen-18 (180), and nutrients. On alternate stations water samples were obtained from the upper 150 m for two other projects. Those samples are to be processed for particulate organic carbon and nitrogen, pigments, biogenic silica, bacteria, dissolved inorganic carbon, carbon-13 ( 13 C), and carbon-14 ( 14 C) isotope uptake rates on incubated samples. Portions of this transect repeat earlier sections along the Ross Ice Shelf edge, some taken more than 25 years ago. A comparison of preliminary results with earlier observations between 173 0W and 1740W reveals that the 1994 data are cooler by approximately 0.4°C and fresher by approximately 0.06 practical salinity units (psu) in the salinity range from 34.40 to
34.55 psu (figure 2). In this region, Modified Circumpolar Deep Water (MCDW, the "warm core" of Pillsbury and Jacobs 1985, pp. 87-107), diluted by mixing with cooler and fresher water as it penetrates onto the continental shelf, appears to feed a shallow sub-ice shelf circulation confined to the outer reaches of the Ross Ice cavity (Jacobs, Fairbanks, and Horibe 1985, pp. 59-85; MacAyeal 1985, pp. 133-143). Similar differences were measured between 1994 and historical salinity profiles near a site north of Ross Island, suggesting that such salinity decreases occur simultaneously along the whole ice shelf edge (figure 3). This freshening may be related to weaker on-shelf flow of MCDW, less sea-ice formation, and related brine rejection caused by a weaker or less variable atmospheric circulation, less precipitation incorpo rated into the shelf waters, or fresher source waters from upstream. Determining whether the changes represent a trend that can be related to other environmental parameters or the larger scale shelf-water circulation will require a detailed study of all the available data. The impact of seasonal (measured in 1983-1984) and longer term changes in shelf water characteristics on ocean/ice shelf interactions is being studied by numerical modeling of the circulation underneath the eastern Ross Ice Shelf (Helimer and Jacobs in press). When the 0.06-psu salinity change observed over the last decade is applied to the eastern Ross Ice Shelf edge, model results differ only slightly from those obtained for the 1983-1984 data set. Over a century's
Figure 1. Transect of oceanographic stations (circled stars) occupied along the Ross Ice Shelf front from the (JSCGC Polar Sea 4-10 February 1994. The track has been superimposed on a 1643Z, 17FEB94 F-iC DMSP (Defense Meteorological Satellite Program) image obtained aboard ship by Marine Science Technician E. Fusco. The data were subsequently processed by R. Whritner on SeaspacefreraScan equipment at the Arctic and Antarctic Research Center, Scripps Institution of Oceanography. The stations extend from Edward VII Land at right to Victoria Land at left. The latitude and longitude grid is 2 0 x5°, respectively. The profiles shown in figure 2 are centrally located in the region where the ocean stations are closely spaced. The profiles shown in figure 3 lie directly north of Mount Erebus and Mount Terror, which appear prominently on Ross Island, across McMurdo Sound from Victoria Land. The latitude lines are 2 0 apart. ANTARCTIC JOURNAL 123
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REVIEW 1994
salinity 34.4 34.5 34.6 34.7 34.8 34.9 0
1.0 0.5
ic
0.0 -0.5
20
-1.0
30
-1.5
40
4.,
E
-2.0
50
34.0 34.1 34.2 34.3 34.4 34.5 34.6
salinity
60
Figure 2. Temperature/salinity characteristics (°C/psu) of the Modified Circumpolar Deep Water between 173 0W and 1740W for the years 1968 (stations 727 and 729; diamonds), 1984 (stations 30 and 31; gray dots), and 1994 (stations 15 and 16; black dots). The T f line indicates the sea surface freezing temperature over this salinity range. All observations were made on 26 January or 6 February of the corresponding years.
70 80 90'
time, the annual average basal melting increases substantially if shelf water temperature is allowed to rise by 0.01°C per year, but less so if that warming is accompanied by simultaneous freshening. This occurs because the strength of the thermohaline (i.e, density-driven) sub-ice shelf circulation depends mainly on salinity contrasts between the open continental shelf and the sub-ice cavity. We thank Captain L. Brigham, N. Thayer, W. Roberts, and the officers and crew of the USCGC Polar Sea for their outstanding support, and R. Guerrero, S. O'Hara, A. Jenkins, M. Noonan, and R. Weppernig for essential help in obtaining the CTD and chemistry data. The fieldwork was supported by National Science Foundation grant OPP 92-20009 and the sub-ice modeling by Department of Energy grant DE-FG0293ER61716.
1000
I I I I
Figure 3. A comparison of the salinity distribution (psu) below 100 m at stations taken over a 36-year period from approximately the same location north of Ross Island. Measurements since 1967 were made with CTD instruments. The 1984 and 1994 profiles are averages from two adjacent stations. Jacobs, S.S., R.G. Fairbanks, and Y. Horibe. 1985. Origin and evolution of water masses near the Antarctic continental margin: Evidence from H 2 180/H2 16 0 ratios in sea water. In S.S. Jacobs (Ed.), Oceanology of the Antarctic Continental Shelf (Antarctic Research Series, Vol. 43). Washington, D.C.: American Geophysical Union. MacAyeal, D.R. 1985. Evolution of tidally triggered meltwater plumes below ice shelves. In S.S. Jacobs (Ed.), Oceanology of the Antarctic Continental Shelf (Antarctic Research Series, Vol. 43). Washington, D.C.: American Geophysical Union. Pillsbury, R.D., and S.S. Jacobs. 1985. Preliminary observations from long-term current meter moorings near the Ross Ice Shelf, Antarc-
References Heilmer, H.H., and S.S. Jacobs. In press. Seasonal circulation under the eastern Ross Ice Shelf, Antarctica. Journal of Geophysical
tica. In S.S. Jacobs (Ed.), Oceanology of the Antarctic Continental Shelf (Antarctic Research Series, Vol. 43). Washington, D.C.: Amer-
Research.
ican Geophysical Union.
ANTARCTIC JOURNAL - REVIEW 1994 124
and STANLEY S. JACOBS, Lamont-Doherty Earth Observatory of Columbia University, Palisades, New York 10964
ver much of the circumpolar antarctic continental shelf, O offshore and prevailing easterly winds drive the sea ice and surface currents toward the west. These currents are well known in the southern Ross Sea, where some of the waters may be derived from the upstream Amundsen shelf. To investigate temporal changes in the Ross Sea, we completed an 800-kilometer (km) transect, parallel to the Ross Ice Shelf edge, on the USCGC Polar Sea from 4 to 10 February 1994 (figure 1). On 42 stations, most begun less than 1 km from the ice front, conductivity-temperature-depth (CTD) measurements were made to within 10 meters (m) of the bottom. At several locations, the ship drifted rapidly in the coastal current, resulting in tow-yo vertical profiles. The casts were accompanied by sampling for salinity, dissolved oxygen, total carbon dioxide (TCO2), chlorofluorocarbons, helium, tritium, isotopic oxygen-18 (180), and nutrients. On alternate stations water samples were obtained from the upper 150 m for two other projects. Those samples are to be processed for particulate organic carbon and nitrogen, pigments, biogenic silica, bacteria, dissolved inorganic carbon, carbon-13 ( 13 C), and carbon-14 ( 14 C) isotope uptake rates on incubated samples. Portions of this transect repeat earlier sections along the Ross Ice Shelf edge, some taken more than 25 years ago. A comparison of preliminary results with earlier observations between 173 0W and 1740W reveals that the 1994 data are cooler by approximately 0.4°C and fresher by approximately 0.06 practical salinity units (psu) in the salinity range from 34.40 to
34.55 psu (figure 2). In this region, Modified Circumpolar Deep Water (MCDW, the "warm core" of Pillsbury and Jacobs 1985, pp. 87-107), diluted by mixing with cooler and fresher water as it penetrates onto the continental shelf, appears to feed a shallow sub-ice shelf circulation confined to the outer reaches of the Ross Ice cavity (Jacobs, Fairbanks, and Horibe 1985, pp. 59-85; MacAyeal 1985, pp. 133-143). Similar differences were measured between 1994 and historical salinity profiles near a site north of Ross Island, suggesting that such salinity decreases occur simultaneously along the whole ice shelf edge (figure 3). This freshening may be related to weaker on-shelf flow of MCDW, less sea-ice formation, and related brine rejection caused by a weaker or less variable atmospheric circulation, less precipitation incorpo rated into the shelf waters, or fresher source waters from upstream. Determining whether the changes represent a trend that can be related to other environmental parameters or the larger scale shelf-water circulation will require a detailed study of all the available data. The impact of seasonal (measured in 1983-1984) and longer term changes in shelf water characteristics on ocean/ice shelf interactions is being studied by numerical modeling of the circulation underneath the eastern Ross Ice Shelf (Helimer and Jacobs in press). When the 0.06-psu salinity change observed over the last decade is applied to the eastern Ross Ice Shelf edge, model results differ only slightly from those obtained for the 1983-1984 data set. Over a century's
Figure 1. Transect of oceanographic stations (circled stars) occupied along the Ross Ice Shelf front from the (JSCGC Polar Sea 4-10 February 1994. The track has been superimposed on a 1643Z, 17FEB94 F-iC DMSP (Defense Meteorological Satellite Program) image obtained aboard ship by Marine Science Technician E. Fusco. The data were subsequently processed by R. Whritner on SeaspacefreraScan equipment at the Arctic and Antarctic Research Center, Scripps Institution of Oceanography. The stations extend from Edward VII Land at right to Victoria Land at left. The latitude and longitude grid is 2 0 x5°, respectively. The profiles shown in figure 2 are centrally located in the region where the ocean stations are closely spaced. The profiles shown in figure 3 lie directly north of Mount Erebus and Mount Terror, which appear prominently on Ross Island, across McMurdo Sound from Victoria Land. The latitude lines are 2 0 apart. ANTARCTIC JOURNAL 123
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REVIEW 1994
salinity 34.4 34.5 34.6 34.7 34.8 34.9 0
1.0 0.5
ic
0.0 -0.5
20
-1.0
30
-1.5
40
4.,
E
-2.0
50
34.0 34.1 34.2 34.3 34.4 34.5 34.6
salinity
60
Figure 2. Temperature/salinity characteristics (°C/psu) of the Modified Circumpolar Deep Water between 173 0W and 1740W for the years 1968 (stations 727 and 729; diamonds), 1984 (stations 30 and 31; gray dots), and 1994 (stations 15 and 16; black dots). The T f line indicates the sea surface freezing temperature over this salinity range. All observations were made on 26 January or 6 February of the corresponding years.
70 80 90'
time, the annual average basal melting increases substantially if shelf water temperature is allowed to rise by 0.01°C per year, but less so if that warming is accompanied by simultaneous freshening. This occurs because the strength of the thermohaline (i.e, density-driven) sub-ice shelf circulation depends mainly on salinity contrasts between the open continental shelf and the sub-ice cavity. We thank Captain L. Brigham, N. Thayer, W. Roberts, and the officers and crew of the USCGC Polar Sea for their outstanding support, and R. Guerrero, S. O'Hara, A. Jenkins, M. Noonan, and R. Weppernig for essential help in obtaining the CTD and chemistry data. The fieldwork was supported by National Science Foundation grant OPP 92-20009 and the sub-ice modeling by Department of Energy grant DE-FG0293ER61716.
1000
I I I I
Figure 3. A comparison of the salinity distribution (psu) below 100 m at stations taken over a 36-year period from approximately the same location north of Ross Island. Measurements since 1967 were made with CTD instruments. The 1984 and 1994 profiles are averages from two adjacent stations. Jacobs, S.S., R.G. Fairbanks, and Y. Horibe. 1985. Origin and evolution of water masses near the Antarctic continental margin: Evidence from H 2 180/H2 16 0 ratios in sea water. In S.S. Jacobs (Ed.), Oceanology of the Antarctic Continental Shelf (Antarctic Research Series, Vol. 43). Washington, D.C.: American Geophysical Union. MacAyeal, D.R. 1985. Evolution of tidally triggered meltwater plumes below ice shelves. In S.S. Jacobs (Ed.), Oceanology of the Antarctic Continental Shelf (Antarctic Research Series, Vol. 43). Washington, D.C.: American Geophysical Union. Pillsbury, R.D., and S.S. Jacobs. 1985. Preliminary observations from long-term current meter moorings near the Ross Ice Shelf, Antarc-
References Heilmer, H.H., and S.S. Jacobs. In press. Seasonal circulation under the eastern Ross Ice Shelf, Antarctica. Journal of Geophysical
tica. In S.S. Jacobs (Ed.), Oceanology of the Antarctic Continental Shelf (Antarctic Research Series, Vol. 43). Washington, D.C.: Amer-
Research.
ican Geophysical Union.
ANTARCTIC JOURNAL - REVIEW 1994 124
