Ocean sciences Western Weddell stratification and ice cover

Ocean sciences

Western Weddell stratification and ice cover ARNOLD L. GORDON

and BRUCE A. HuBER, Lamont-Doherty Earth Observatory, Palisades, New York, 10964-8000

ice regimes? Comparison of the data from the 1992 Ice Station he southern oceans' sea-ice extent is characterized by Weddell-1 (Gordon and ISW Group 1993) in the western rim T large seasonal pulsations. The February minimum extent of the Weddell Gyre with the data from the Polarstern 1986 of 3.4 to 4.3 million square kilometers (km 2 ) expands to a Sepcruise in the central gyre region does reveal differences (figure tember maximum of 18.0 to 20.2 million km 2 , with the bulk of 1). The western Weddell is characterized by northward ocean the spring season removal occurring in November and currents, averaging 5 centimeters per second with a net December (Gloersen et al. 1992). The ice cover during the 6 cubic meters per second for 28x10 northward transport of minimum period represents the perennial or multiyear ice. about 200 kilometers seaward of the shelf break (Muench and Winter and spring ocean and atmosphere studies within the Gordon in preparation). The sea ice drifted slightly to the east seasonal sea-ice cover indicate that oceanic heat and salt flux of the ocean currents at a mean rate of 7.6 centimeters per from the deep water to the winter mixed layer is a key factor second (Gordon and ISW Group 1993). The ocean stratificain governing the ice thickness and rapid spring melting (Gortion of the western Weddell has the same basic stratification don and Huber 1990; Martinson 1990). Within the approxiform as the rest of the Weddell region: a cold, low-salinity surmately 4 million km 2 of perennial ice the ocean/ atmosphere face layer separated by a rather weak pycnocline from a thick conditions must be sufficiently different from the seasonal ice layer of relatively warm and salty water, and a cold bottom regions to allow the survival of the ice during the summer layer (Gordon et al. 1993). In the western Weddell, however, months. Much of the perennial ice is concentrated along the depth of the t-max core of the deep water is slightly cooler Antarctica's coast line, with the most extensive cover in the and more than twice as deep than the t-max to the east: 550 western Weddell Sea, the Bellingshausen Sea, and the meters vs. 200 meters. The salinity profile below the mixed Amundsen Sea. In the western Weddell Sea, the nearly 100 percent concentration of perennial ice is per sistent at an extent of 1.2 mill ion km2. Salinity Potential Temperature Perennial ice is formed if the heat-budget-2 .1 0 1 2 34.0 34.2 34.4 34.6 34.8 constituents responsible for the summer 0 removal are absent. Presumably, passage of the winter ice across a region of reduced deep ocean to mixed layer heat flux would allow that 200 ice to survive to a second winter, forming a perennial ice field. The extent of the perennial ice depends on the ice drift. A number of G) 400 testable hypotheses may be put forward to explain why the western Weddell Sea ice cover differs from the seasonal cover to the east: • that the atmosphere is colder along the CL 600 eastern margin of Antarctic Peninsula; • that the oceanic heat flux is reduced because of a more stable pycnocline or a 80( deeper heat source (the temperature maximum layer or t-max); • that ice dynamics are more convergent; or 1 C, • some combination of these three. Figure 1. Potential temperature and salinity vs. depth for the upper 1,000 meters in the Martinson and Gordon (in preparation) are central Weddell Gyre (Po!arstern, 1986) and along the western Weddell (Ice Station Wedinvestigating these ideas. dell, 1992). A "warm regime" station (dashed line, ANT V/2 station 56), a "cold regime" Might differences in the ocean stratifica- station (dotted line, ANT V/2 station 6), and a typical Ice Station Weddell-1 station (solid tion be a significant factor in governing the two line, station 22) are shown. S

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layer displays a density-compensating freshening, suggestive of an isopycnal mixing process. Another obvious stratification feature to inspect is the temperature minimum (t-min) which marks the remnant winter mixed layer of the southern oceans. The salinity of the t-min in the Weddell Gyre is relatively high (plate 44 of Olbers et al. 1992), and somewhat higher in the west. The t-min salinity is an integrated product of ice formation/ melting, net precipitation/ evaporation, and vertical oceanic fluxes. More ice formation is expected closer to Antarctica, with melting occurring nearer to the ice edge. Because salinity is the primary factor in determining density in the cold waters, the tmin of the Weddell is relatively dense, and the Weddell pycnocline is weaker than other regions of the southern oceans. This may be expected to produce large vertical heat fluxes and more ocean overturning. It would also induce seasonal sea ice. The perennial icefields of the Bellingshausen and Amundsen Seas coincide with low salinity t-min, which would be associated with low vertical oceanic heat flux. Why, then, is the western Weddell Sea, with a salty, dense t-min, ice covered all year? Is this permanent ice cover perhaps associated with the pycnocline stability? The data collected during Ice Station Weddell-1 have revealed a layer of relatively low salinity in the upper parts of the pycnocline (figure 2). A narrow band of lower salinity at the -1.7°C isotherm (within the upper pycnocline layer) is observed to flow over the 2,000-meter isobath of the continental slope. This lower

Figure 2. Salinity on the -1.7°C surface determined from Ice Station Weddell-1 stations. Station positions are shown as dots. The 500meter and 2,000-meter isobaths are shown. salinity band falls within a deepening of the t-max layer and perhaps is the same feature presented by Gill (1973) within the "V-shaped" form of the pycnodine over the continental slope. Several profiles exhibit step structures in the region of the t-max. Similar features have been observed near the northwestern corner of the Weddell Gyre (Muench, Fernando, and Stegen 1990). Upon inspection of the stratifi cation changes across the Weddell Gyre, several concentrations of step structures are found (figure 3), primarily to the west of Maud Rise, in the northwest corner of the Weddell, and in the Ice Station Weddell-1 region. In most cases, the steps occur in zones of transition between regions of warm and cold temperature maximum characteristics. The research reported on in this article was funded by National Science Foundation grants OPP 90-24755 and

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00 -2.0 0.0 0.1 0.2 0.3 0.4 0.6 0.8 0.9 1.0 1.8 Figure 3. Distribution of stations in the Weddell, with those exhibiting steps shown as diamonds. The station locations are superimposed on a map of the temperature of the t-max, to demonstrate the concentration of step structures in regions of transition between warm and cold t-max characteristics. The t-max map and station locations were determined from conductivity-temperature-depth data collected since 1980.

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OPP 93-13700.

References Gill, A. 1973. Circulation and bottom water

production in the Weddell Sea. Deep-Sea Research, 20(2), 111-140.

Martinson, D. 1990. Evolution of the southern ocean winter mixed layer and sea ice: Open ocean deepwater formation and ventilation. Journal of Geophysical Research, 95(C7), 11641-11654. Martinson, D., and A.L Gordon. In preparation. Oceanic origin for Weddell Sea perennial ice. Muench, R.D., and A.L. Gordon. In preparation. Circulation and transport of water along the western Weddell Sea margin. Muench, R., H. Fernando, and G. Stegen. 1990. Temperature and salinity staircases in the northwestern Weddell Sea. Journal of Physical Oceanography, 20(2), 295-306. Olbers, D., V. Gouretski, G. Seiss, and J. Schröter. 1992. Hydrographic atlas the southern ocean. Bremerhaven: Alfred-Wegener-Institut.

Gloersen, P., W. Campbell, D. Cavalieri, J. Comiso, C. Parkinson, and

H. Zwally. 1992. Arctic and antarctic sea ice, 1978-1987. Satellite passive-microwave observations and analysis. (Scientific and Technical Information Program of the National Aeronautics and Space Administration.) Washington, DC: NASA. Gordon, A., and B. Huber. 1990. Southern ocean winter mixed layer. Journal of Geophysical Research, 95(C7), 11655-11672. Gordon, A., B. Huber, H. Hellmer, and A. Ffield. 1993. Deep and bottom water of the Weddell Sea's western rim. Science, 262(5 130), 95-97. Gordon, A., and ISW Group. 1993. Ice Station Weddell 1 explores the western edge of the Weddell Sea. EOS, Transactions of the American Geophysical Union, 74(11), 121 and 124-126.

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Large, steplike temperature and salinity structures observed in the central Weddell Sea THEODORE D. FOSTER, Institute of Marine Sciences, University of California, Santa Cruz, California 95064

As part of a project to study the formation of Antarctic Bot-

did not observe these structures, possibly since our investigation that year never reached the central part of the Weddell Sea. In 1991 and 1992, we did observe the large, steplike structures and carried out a series of 64 CTD stations at 30-minute intervals from about 50 to 650 meters depth in 1991 and a series of 32 CTD stations in 1992. After we had analyzed these so-called yo-yo CTD series and corrected all the salinity profiles, we found that the series showed a surprising amount of

tom Water in the Weddell Sea, three oceanographic expeditions have been carried out in the northwestern sector of the Weddell Sea. On each of these expeditions, we carried out time series of CTD (conductivity-temperature-depth) stations to investigate the very large, up to about 500 meters thick, steplike structures in the temperature and salinity vertical profiles we have observed in the central regions of the Weddeli Sea since 1973 (Foster and Carmack 1976). In 1987, we

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Time series of temperature (C) and salinity (psu) vs. pressure (decibars) at 30-minute intervals in the central Weddell Sea (approximately at 650S 470 W) on 24 to 25 February 1991. Each temperature profile is displaced successively to the right by 0.2°C and each salinity by 0.02 psu. ANTARCTIC JOURNAL - REVIEW 1994

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