AMERIEZ 1988: Mid-winter physical oceanographic observations in ...
AMERIEZ 1988: Mid-winter physical oceanographic observations in the Scotia Sea ROBIN
ing of mixing and circulation processes in the Scotia Sea during winter and to provide supporting physical oceanographic data for other investigators in the AMERIEZ program. The 1988 AMERIEZ study region occupied the southern central Scotia Sea between about 35-48°W and 58-61.5°S and bracketed the marginal ice zone (figure 1). Observations were carried out from the RIV Polar Duke during two separate cruise legs from June through August 1988; leg I took place in early winter (9 June to 5 July) and leg II in mid-winter (18 July to 13 August). The first leg focussed upon processes within the multi-year pack, while the second leg focussed upon watercolumn processes seaward of the marginal ice zone. Watercolumn temperature and salinity observations were obtained on both legs using a SeaBird model SBE 9/11 CTD (conductivity/temperature/depth) profiling system. In this way, 144 CTD casts were obtained during the program, with most casts closely spaced and situated along transects oriented north-south, normal to the mean current (figure 1). During leg I, two ice floes were instrumented with Argos-tracked location buoys to obtain Lagrangian drift tracks for the floes. Early during leg II, six Argos-tracked, surface-drogued Tristar buoys were deployed at the western end of the study region to obtain Lagrangian observations of surface currents. Of these six drifters, three survived long enough to provide useful records.
D. MUENCH and JOHN T. GUNN
Science Applications International Corporation Bellevue, Washington 98005 DAVID
M. HUSBY
Pacific Fisheries Environmental Group National Oceanic and Atmospheric Administration National Marine Fisheries Service Monterey, California 93942
As part of the Antarctic Marine Ecosystem Research at the Ice-Edge Zone (AMERIEZ) program, we measured water-column temperature and salinity and obtained surface Lagrangian water-movement measurements in the Scotia Sea during austral winter 1988. The purpose was to improve our understand-
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Figure 1. Geographical locations of the conductivity/temperature/depth casts made during the austral winter 1988 AMERIEZ program in the Scotia Sea. Leg I casts are shown as solid squares, and leg II casts as solid circles. Light dashed contours represent 1,000 fathom bottom depths from Admiralty Chart 3200 (corrected to 1986). (km denotes kilometer.)
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The study region bracketed most of the Weddell-Scotia frontal system, which includes the Weddell-Scotia Confluence as described by Patterson and Sievers (1980). The Weddell and Scotia fronts define the dynamic boundaries between waters exiting the Weddell Sea, continental margin waters from farther west, and Pacific Ocean water. The Scotia Front was identifiable in winter 1988 as a region of strong temperature gradients, at depths of 300-500 meters, between the water of the WeddellScotia Confluence Zone (temperature equals approximately 0°C) and warmer Pacific Deep Water (1°C) (figure 2). Frontal gradients were strongest at the westernmost (48°W) transect and decreased toward the east, consistent with a western origin for the front and decrease of the gradients away from its source through lateral mixing. Summer observations have suggested that waters in the confluence were less stratified than the surrounding waters (Patterson and Sievers 1980). This locally decreased stratification was hypothesized to be due either to winter vertical convection (Deacon and Moorey 1975) or to boundary layer mixing coupled with meltwater addition (Patterson and Sievers 1980). The winter 1988 data showed salinity (hence, density) stratification within the confluence to be similar to that farther north and south, suggesting that deep winter convection is not a significant regional process. In addition, to the Weddell-Scotia frontal system, whose presence and structure is controlled by regional oceanographic
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conditions, a small number of localized upper layer fronts and lenses were associated with the marginal ice zone. Typically, the mid-winter marginal ice zone is a period of alternate ice advance and retreat which is controlled by local weather conditions. Periods of ice retreat through melting lead to development of low salinity lenses and localized upper layer fronts. Periods of ice advance lead to brine rejection which may locally deepen the mixed layer. June to August 1988 was a period of net ice advance, so that little melting occurred and consequently there were few of the shallow meltwater lenses and frontal structures. A continuous, undulating, east-west oriented frontal structure in the vertically well-mixed upper layer marked the northern limit of water which was at or near the freezing point (about - 1.84°C). This upper layer front generally, but not always, coincided with the deeper signature of the Scotia Front (see figure 2). Net currents in the southern Scotia Sea are eastward, reflecting eastward flow in the Antarctic Circumpolar Current and northeastward flow from the Weddell Sea. The study region was characterized in winter 1988 by eastward baroclinic currents and mesoscale eddies imbedded in these currents. One of the instrumented ice floes, and the three drogued drifters, each moved toward the east-northeast in response to the regional mean currents. One of the drogued drifters became entrained in a mesoscale eddy centered on about 47°W at 57.75'S, circled about the eddy several times and documented eddy speeds of 35-40 centimeters per second. A second possible eddy was present, based on temperature and salinity structure, at the northern end of the transect along 40°W. Both of these features were anticyclonic (counterclockwise) and had cores warmer than the surrounding water down to depths exceeding 1,000 meters. Similar mesoscale eddies were reported in the same region, during summer, by Foster and Middleton (1984). The meanders and eddies in the flow field are reflected in the 50-meter temperature distribution (figure 3). Mid-winter 1988 temperature, salinity, and Lagrangian drifter observations have provided the first winter information on the Scotia Front and the Weddell-Scotia Confluence. Eastward mean flow was confirmed, and energetic mesoscale eddies were detected in this flow. Vertical stratification within the WeddellScotia Confluence was significantly greater than had been supposed on the basis of previous summer data, and the region did not appear to be a site for deep convective mixing. This work was supported primarily by National Science Foundation grants DPP 87-15979 and DPP 84-20646 to Science Applications International Corporation (for Muench and Gunn) and DPP 85-13098 to National Oceanic and Atmospheric Administration/National Marine Fisheries Service (for Husby). Some support was also provided by the National Marine Fisheries Service of the National Oceanic and Atmospheric Administration (for Husby).
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Figure 2. Vertical distribution of temperature (°C) along the northsouth transect at 440W on 31 July to 2 August 1988 (see figure 1). Contour interval is 0.5°C. Depth scale change at 500 meters is denoted by horizontal dashed line. Heavy black line at the surface denotes pack-ice cover. (m denotes meter. km denotes meter.)
1989 REVIEW
References Deacon, G.E.R., and J. A. Moorey. 1975. The boundary region between currents from the Weddell Sea and Drake Passage. Deep-Sea Research, 22, 265-268. Foster, T.E., and J.H. Middleton. 1984. The oceanographic structure of the eastern Scotia Sea—I. Physical Oceanography. Deep-Sea Research, 31, 529-550. Patterson, S.L., and H.A. Sievers. 1980. The Weddell-Scotia Confluence. Journal ot Pluisical Oceanography, 10, 1,384-1,610.
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Figure 3. Distribution of temperature (°C) at 50 meters in the southern Scotia Sea during 18 July to 13 August 1988. Contour interval is 0.5°C. Light dashed contours represent 1,000 fathom bottom depths from Admiralty Chart 3200 (corrected 1986). Note surface traces of warm core anticyclonic (clockwise) eddies indicated as elevated temperatures at northern ends of 40°W and 48 0W north-south transects. (T denotes temperature. m denotes meter. km denotes kilometer.)
AMERIEZ 1988: Nutrient distributions and variability along the Weddell-Scotia confluence and marginal ice zone during austral winter Louis I. GORDON and DAVID M. NELSON College of Oceanography Oregon State University Corvallis, Oregon 97331-5503
During the 1988 Antarctic Marine Ecosystem Research at the Ice-Edge Zone (AMERIEZ) field program, we measured nu150
trient distributions along several north-south sections through the ice-edge region. Ainley and Sullivan (Antarctic Journal, this issue) have presented a general overview of the AMERIEZ program and the 1988 field work in particular, and we will not repeat that discussion here. Our focus in this brief presentation will be upon the nutrient distributions we observed. We used a Technicon AutoAnalyzer II system to measure the concentrations of phosphate, silicic acid, nitrate plus nitrite, nitrite, and ammonium. For all but ammonium, we used the methods of Atlas et al. (1972). To analyze for ammonium on leg I, we followed the method given by Whitledge et al. (1981) and on leg lithe method given by Head (1971). Within the precision of the two methods, we are confident that the results are comparable. We obtained our nutrient samples from the conductivity/temperature/depth casts (Muench, Gunn, and Husby, Antarctic Journal, this issue), sampling from almost all casts, from the surface to the maximum depths. We arranged the sample depths to emphasize the upper 200 meters on leg I and the upper 150 meters on leg II. ANTARCTIC JOURNAL
