STANLEY S. JACOBS, Lamont-Doherty Earth Observatory of Columbia University, Palisades, New York 10964
Thirty years ago Lamont investigators made the first continuous vertical "STD" (salinity-temperature-depth) profiles in the Ross Sea, casting from the Eltanin with an early model of the now widely used "CTD." The original acronym has long since been abandoned to the public health sector, and the "C" now stands for seawater conductivity, from which salinity is calculated (as is "D" from pressure). From the many "bottle" and CTD casts made in the Ross Sea before and since that time, a rough time series of summer temperature and salinity measurements can be compiled (Jacobs and Giulivi in press-a). Unlike local meteorological observations and satellite-derived information on sea-ice extent, the oceanographic data are highly discontinuous in space and time. In spite of geographical biases (e.g., Giulivi and Jacobs 1997a) and a curious year-round salinity record (from McMurdo Sound), it is apparent that interannual salinity variability is substantial on the Ross Sea continental shelf. The salinity changes are correlated with sea-ice range (figure 1), which has a periodicity of several years. Superimposed on the large short-term variability is a slight decline in shelf water salinity over the past four decades in the southwest Ross Sea (Jacobs and Giulivi in press-b).
Over the past 150 years, even the area of the open Ross Sea continental shelf has changed. Continued monitoring of the position of the Ross Ice Shelf front (Keys, Jacobs, and Brigham in press) reveals that the western portion of the ice front is now more than 75 kilometers (km) north of its location in 1911, around the time that Amundsen and Scott trekked to the South Pole. The "B-9" iceberg released by the eastern ice front in 1987 removed an area larger than the island between Manhattan and Montauk, but steady advance along the entire ice front since then has more than regained the ice shelf real estate lost in that calving event. The growth of the Ross and other ice shelves stands in contrast to well-publicized retreats along the Antarctic Peninsula and suggests that the circumpolar inventory of the shelf ice may be little changed in recent times.
Three years ago, on cruise 9402 of the Nathaniel B. Palmer , we made the first oceanographic measurements in some antarctic coastal regions between the eastern Ross Sea and Marguerite Bay (Giulivi and Jacobs 1997b). Using new Amundsen Sea data for validation, Hellmer, Jacobs, and Jenkins (in press) modeled the flow of circumpolar deep water beneath Pine Island Glacier, where the basal melt rate appears to exceed 10 meters per year (Jacobs, Hellmer, and Jenkins 1996). In combination with a calving rate obtained from radar satellite observations, that melting roughly balances the estimated flow of ice across the deep grounding line (Jacobs, Jenkins, and Hellmer 1996). Meltwater increases the dissolved oxygen content of the deep water that upwells beneath the glacier but lowers its oxygen isotope (d 18 O) content, from which the d 18 O of precipitation on the glacier catchment basin can be inferred (figure 2). Relatively shallow and warmer than the ambient environment, outflows from beneath the glacier are likely to influence local sea-ice formation.
Following up on an earlier finding of higher seabird populations near the Antarctic Slope Front, Ainley et al. (in preparation) evaluated bird distributions observed during Palmer cruise 9402, late enough in the season that summer colonies had been abandoned. They found that ocean thermohaline fronts in the Amundsen and Bellingshausen regions are more diffuse and less related to the continental shelf break than in the Ross Sea. Interpretations were complicated by overly wide ocean station spacing, by the lack of prey data and unavoidable time gaps in the observations, and by large flocks of birds roosting by day and feeding at night. It remains to be determined how deep subsurface features compete with or enhance the ice-edge environment as a magnet for top-gun predators.
From the limited historical ocean data available, it is difficult to determine whether significant temporal changes have occurred in waters on the Southeast Pacific antarctic continental shelf. Nearly 100 years ago, oceanographic measurements were made from the Belgica , beset for more than a year in the close pack of the southern Bellingshausen Sea. Their southernmost temperatures are substantially colder than the Palmer CTD profile at the same location (figure 3): a deep temperature maximum in March approximately 0.3°C below that of our March 1994 data. Ongoing analyses of such comparisons may allow any long-term temperature trend to be separated from the short-term variability.
C. Giulivi and H. Hellmer assisted with data reduction and figures. This research was supported by National Science Foundation (OPP 94-18151), with assistance from the National Aeronautics and Space Administration (NAGW-1344), the Department of Energy (DE-FG02-93ER61716), and Lamont-Doherty Earth Observatory.
Ainley, D.G., S.S. Jacobs, C.A. Ribic, and I. Gaffney. In preparation. Seabirds and oceanic features of the Amundsen and southern Bellings-hausen Seas, late summer-early autumn 1994. Antarctic Science .
Giulivi, C.F., and S.S. Jacobs. 1997a. A zonal oceanographic section in the southern Ross Sea: Oceanographic data taken from the USCGC Polar Sea, February 1994 (Technical Report 97-2). Palisades, New York: Lamont-Doherty Earth Observatory.
Giulivi, C.F., and S.S. Jacobs. 1997b. Oceanographic data in the Amundsen and Bellingshausen Seas: N.B. Palmer cruise 9402, February-March 1994 (Technical Report 97-3). Palisades, New York: Lamont-Doherty Earth Observatory.
Hellmer, H.H., S.S. Jacobs, and A. Jenkins. In press. Oceanic erosion of a floating antarctic glacier in the Amundsen Sea. In S. Jacobs and R. Weiss (Eds.), Ocean, ice, and atmosphere: Interactions at the antarctic continental margin (Antarctic Research Series, Vol. 75). Washington, D.C.: American Geophysical Union.
Jacobs, S.S., and C.F. Giulivi. In press-a. Thermohaline data and ocean circulation on the Ross Sea continental shelf. In G. Spezie and G.M.R Manzella (Eds.), Oceanography of the Ross Sea-Antarctica. Heidelberg: Springer-Verlag.
Jacobs, S.S., and C.F. Giulivi. In press-b. Interannual ocean and sea ice variability in the Ross Sea. In S. Jacobs and R. Weiss (Eds.), Ocean, ice, and atmosphere: Interactions at the antarctic continental margin (Antarctic Research Series, Vol. 75). Washington, D.C.: American Geophysical Union.
Jacobs, S., H. Hellmer, and A. Jenkins. 1996. WAIS underbelly melting at Pine Island Glacier? In H. Oerter (Ed.), Filchner-Ronne Ice Shelf Programme (Report 10). Bremerhaven, Germany: Alfred-Wegener-Institute.
Jacobs, S.S., A. Jenkins, and H.H. Hellmer. 1996. On the mass balance of West Antarctica's Pine Island Glacier. In S. Colbeck (Ed.), Glaciers, ice sheets and volcanoes: A tribute to Mark F. Meier (Special Report 96-27). Hanover, New Hampshire: Cold Regions Research and Engineering Laboratory.
Keys, H.J.R., S.S. Jacobs, and L.W. Brigham. In press. Continued northward expansion of the Ross Ice Shelf. Annals of Glaciology , 27.