Western Weddell Sea hydrography
Western Weddell Sea hydrography HARTMUT H. HELLMER, ARNOLD L. GORDON, JAVIER I. ALBARRACIN, and BRUCE A. HUBER, Lamont-Doherty Earth Observatory, Columbia University, Palisades, New York 10964
vertical mixing and further inflow from the continental margin. The newly formed bottom water leaving the western rim of the Weddell Gyre is a blend of low- and high-salinity water types. The structure of the shelf breakfront is best resolved in the temperature and salinity distributions of helicopter-section III (figure 3). Maximum changes of 1.3°C and 0.08 practical salinity units, respectively, occur over a distance of 10 kilometers (km). Mixing at this V-shaped front, as described by Gill (1973) for a section further to the south, might influence the characteristics of Weddell Deep Water, because the base of the front is connected with the lower levels via isopycnals.
he U.S.-Russian Ice Station Weddell 1 (ISW-1) provided T the first comprehensive data set for the perennially icecovered western Weddell Sea (Gordon et al. 1993a). As a region of potentially significant formation of cold bottom water, the western Weddell is of key importance in understanding the southern oceans' role in global heat fluxes and ventilation of the world's deep ocean. Oceanographic observations confirmed that the region covered by 15W-1 contributes to newly formed bottom water that by virtue of its cold temperature (less than -0.8°C) may be referred to as Weddell Sea Bottom Water (Foster and Carmack 1976). Two helicopter sections normal to the drift track (figure 1) revealed a thin [less than 300 meters (m)], highly oxygenated benthic layer (Gordon et al. 1993b). As the composition of isopycnals relative to 500 decibars (dbar) (sigma 0.5) and 2,000 dbar (sigma 2) shows, cold slope water masses (less than -1.0°C) are connected to the bottom layer of the Continental Shelf (figures 2 and 3). The thin benthic layer with its complex structure, a low-salinity component overlying a high-salinity component (Gordon et al. 1993b), disappears near 66 0S due to
lW 1 i^
P11, 0
Figure 2. Temperature and salinity distributions along helicopter section II at 68040S traversing the Continental Shelf, slope, and deep ocean of the western Weddell Sea. Superimposed are the isopycnals relative to 500 dbar (sigma 0.5) and 2,000 dbar (sigma 2); the dashed line at 1,000-rn depth marks the change in reference pressure for density calculation. Upper panel numbers identify helicopter stations and the corresponding ice floe station.
Figure 1. Ice Station Weddell 1 station map showing conductivitytemperature-depth (CTD) profile locations from the ships (diamond) and helicopter (+), and at the ice station (shaded circles). Because existing bathymetric charts are unavailable for the 15W-1 area, the 500-, 1,000-, 2,000-, 3,000-, and 4,000-rn isobaths shown were constructed from recent aerogravity and magnetics survey data (Labrecque and Ghidella in press).
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REVIEW 1993
Evidence for sinking along this path is given by intrusions of cold water and freshwater observed at 500-rn (section II) and 1,400-rn (section III) depths (figures 2 and 3). The temperature and salinity maximum over the continental slope characterizes the core of Weddell Sea's coastal current, which, as the southern limb of the cyclonic circulation, provides to the Continental Shelf the salt required for bottom-water production and the heat lost by air-ice-sea interaction. Comparison with temperatures at the inflow region (Greenwich Meridian) reveals that the core cools by approximately 0.6°C as it follows the continental margin toward the 15W-i area (Gordon et al. 1993b). The dilution with underlying water masses produces Antarctic Bottom Water, which carries the characteristics of Weddell Sea Bottom Water into the world oceans' abyss. We thank J. Ardai, R. Guerrero, G. Mathieu, S. O'Hara, and R. Wepperriig for their help in collecting the conductivitytemperature-depth data and W. Haines and P. Mele for data processing. The research is funded by National Science Foundation grant OPP 90-24577.
References Foster, T.D., and E. Carmack. 1976. Frontal zone mixing and Antarctic Bottom Water formation in the southern Weddell Sea. Deep-Sea Research, 23(4), 301-317. Gill, A.E. 1973. Circulation and bottom water production in the Weddell Sea. Deep-Sea Research, 20(2), 111-440. Gordon, A.L., and Ice Station Weddell Group. 1993a. Weddell Sea
exploration from ice station. EOS, Transactions of the American Geophysical Union, 74(11), 124-126. Gordon, A.L., B.A. Huber, H.H. Heilmer, and A. Field. 1993b. Deep and bottom water of Weddell Sea's western rim. Science, 262, 95--97. Labrecque, J., and M. Ghidella. In press. Bathymetry, depth to magnetic basement and sediment thickness estimates from aerogeophysical data over the western Weddell. Journal of Geophysical
Figure 3. Same as figure 2 for helicopter section III at 67040'S. Comparison between helicopter sections II and Ill shows the importance of close station spacing. The smooth V-shape near the continental break in the salinity distribution of section II might be only the result of lower resolution.
Research.
Salinity variations in Weddell Sea pack ice S.F. ACKLEY and A.J. Gow, U.S. Army Cold Regions Research and Engineering Laboratory, Hanover, New Hampshire 03755 V.I. LYTLE, Antarctic Division, Co-operative Research Centre, University of Tasmania, Hobart, Tasmania, 7001 Australia
ice salinity data from cruises in the eastern and western Weddell Sea and provided interpretation of the processes of salinity transformation in the region's sea ice. A variety of these processes are active in the Weddell region, including winter thermodynamic growth, rafting and ridging, surface flooding, bottom melting, summer decay, and autumn freezeup of second-year ice. The salinity is an integral response to these processes, and these processes are difficult to determine or deconvolve without knowledge of the sequence and nature of events (Eicken 1992). These events could only be inferred previously from the one-time samples obtained in transects.
he formation, growth, and decay of sea ice leads to transT port processes for the liquid phase (brine) within the ice that are unique to earth materials. Sea ice probably represents the only natural system where the liquid and solid phases are composed of the same material and, therefore, undergo complex thermodynamic transformations that feed back into the transport of the fluid phase. The liquid phase is determined by the salinity and temperature of the ice and controls electromagnetic, mechanical, thermal, and more indirectly, its biological properties (Ackley and Sullivan in press; Weeks and Ackley 1986). In a previous study, Eicken (1992) compiled sea-
ANTARCTIC JOURNAL - REVIEW 1993 79
vertical mixing and further inflow from the continental margin. The newly formed bottom water leaving the western rim of the Weddell Gyre is a blend of low- and high-salinity water types. The structure of the shelf breakfront is best resolved in the temperature and salinity distributions of helicopter-section III (figure 3). Maximum changes of 1.3°C and 0.08 practical salinity units, respectively, occur over a distance of 10 kilometers (km). Mixing at this V-shaped front, as described by Gill (1973) for a section further to the south, might influence the characteristics of Weddell Deep Water, because the base of the front is connected with the lower levels via isopycnals.
he U.S.-Russian Ice Station Weddell 1 (ISW-1) provided T the first comprehensive data set for the perennially icecovered western Weddell Sea (Gordon et al. 1993a). As a region of potentially significant formation of cold bottom water, the western Weddell is of key importance in understanding the southern oceans' role in global heat fluxes and ventilation of the world's deep ocean. Oceanographic observations confirmed that the region covered by 15W-1 contributes to newly formed bottom water that by virtue of its cold temperature (less than -0.8°C) may be referred to as Weddell Sea Bottom Water (Foster and Carmack 1976). Two helicopter sections normal to the drift track (figure 1) revealed a thin [less than 300 meters (m)], highly oxygenated benthic layer (Gordon et al. 1993b). As the composition of isopycnals relative to 500 decibars (dbar) (sigma 0.5) and 2,000 dbar (sigma 2) shows, cold slope water masses (less than -1.0°C) are connected to the bottom layer of the Continental Shelf (figures 2 and 3). The thin benthic layer with its complex structure, a low-salinity component overlying a high-salinity component (Gordon et al. 1993b), disappears near 66 0S due to
lW 1 i^
P11, 0
Figure 2. Temperature and salinity distributions along helicopter section II at 68040S traversing the Continental Shelf, slope, and deep ocean of the western Weddell Sea. Superimposed are the isopycnals relative to 500 dbar (sigma 0.5) and 2,000 dbar (sigma 2); the dashed line at 1,000-rn depth marks the change in reference pressure for density calculation. Upper panel numbers identify helicopter stations and the corresponding ice floe station.
Figure 1. Ice Station Weddell 1 station map showing conductivitytemperature-depth (CTD) profile locations from the ships (diamond) and helicopter (+), and at the ice station (shaded circles). Because existing bathymetric charts are unavailable for the 15W-1 area, the 500-, 1,000-, 2,000-, 3,000-, and 4,000-rn isobaths shown were constructed from recent aerogravity and magnetics survey data (Labrecque and Ghidella in press).
ANTARCTIC JOURNAL -
78
REVIEW 1993
Evidence for sinking along this path is given by intrusions of cold water and freshwater observed at 500-rn (section II) and 1,400-rn (section III) depths (figures 2 and 3). The temperature and salinity maximum over the continental slope characterizes the core of Weddell Sea's coastal current, which, as the southern limb of the cyclonic circulation, provides to the Continental Shelf the salt required for bottom-water production and the heat lost by air-ice-sea interaction. Comparison with temperatures at the inflow region (Greenwich Meridian) reveals that the core cools by approximately 0.6°C as it follows the continental margin toward the 15W-i area (Gordon et al. 1993b). The dilution with underlying water masses produces Antarctic Bottom Water, which carries the characteristics of Weddell Sea Bottom Water into the world oceans' abyss. We thank J. Ardai, R. Guerrero, G. Mathieu, S. O'Hara, and R. Wepperriig for their help in collecting the conductivitytemperature-depth data and W. Haines and P. Mele for data processing. The research is funded by National Science Foundation grant OPP 90-24577.
References Foster, T.D., and E. Carmack. 1976. Frontal zone mixing and Antarctic Bottom Water formation in the southern Weddell Sea. Deep-Sea Research, 23(4), 301-317. Gill, A.E. 1973. Circulation and bottom water production in the Weddell Sea. Deep-Sea Research, 20(2), 111-440. Gordon, A.L., and Ice Station Weddell Group. 1993a. Weddell Sea
exploration from ice station. EOS, Transactions of the American Geophysical Union, 74(11), 124-126. Gordon, A.L., B.A. Huber, H.H. Heilmer, and A. Field. 1993b. Deep and bottom water of Weddell Sea's western rim. Science, 262, 95--97. Labrecque, J., and M. Ghidella. In press. Bathymetry, depth to magnetic basement and sediment thickness estimates from aerogeophysical data over the western Weddell. Journal of Geophysical
Figure 3. Same as figure 2 for helicopter section III at 67040'S. Comparison between helicopter sections II and Ill shows the importance of close station spacing. The smooth V-shape near the continental break in the salinity distribution of section II might be only the result of lower resolution.
Research.
Salinity variations in Weddell Sea pack ice S.F. ACKLEY and A.J. Gow, U.S. Army Cold Regions Research and Engineering Laboratory, Hanover, New Hampshire 03755 V.I. LYTLE, Antarctic Division, Co-operative Research Centre, University of Tasmania, Hobart, Tasmania, 7001 Australia
ice salinity data from cruises in the eastern and western Weddell Sea and provided interpretation of the processes of salinity transformation in the region's sea ice. A variety of these processes are active in the Weddell region, including winter thermodynamic growth, rafting and ridging, surface flooding, bottom melting, summer decay, and autumn freezeup of second-year ice. The salinity is an integral response to these processes, and these processes are difficult to determine or deconvolve without knowledge of the sequence and nature of events (Eicken 1992). These events could only be inferred previously from the one-time samples obtained in transects.
he formation, growth, and decay of sea ice leads to transT port processes for the liquid phase (brine) within the ice that are unique to earth materials. Sea ice probably represents the only natural system where the liquid and solid phases are composed of the same material and, therefore, undergo complex thermodynamic transformations that feed back into the transport of the fluid phase. The liquid phase is determined by the salinity and temperature of the ice and controls electromagnetic, mechanical, thermal, and more indirectly, its biological properties (Ackley and Sullivan in press; Weeks and Ackley 1986). In a previous study, Eicken (1992) compiled sea-
ANTARCTIC JOURNAL - REVIEW 1993 79
