Circumpolar View of the Southern Ocean from 1962 to 1992 by a R N OLD L

SPECIAL ISSUE ON ANTARCTIC O CEAN OGRAPHY IN A C HAN GIN G WORLD

Circumpolar View of the Southern Ocean from 1962 to 1992 BY A R N OLD L . G ORDON

Figure 1. Two views of the Eltanin though photography (December 1971) and in oils, both by the author.

18 Oceanography | Vol. 25, No. 3 ABSTRACT. #e 1962–1992 period spanning the research vessel Eltanin/ months of Ice Station Weddell in 1992 Islas Orcadas expeditions to Ice Station Weddell, and the following World Ocean and the World Ocean Circulation Circulation Experiment, may be viewed as the closing phase of an era of broad-based, Experiment (WOCE) during the early circumpolar surveys of the Southern Ocean. Today’s investigations of the spatial and 1990s. #ese expeditions may be viewed temporal dimensions of this complex ocean and its coupling to the climate system as the closing of an era that began in the integrate observations from ships, aircra&, satellites, tethered instrumentation, and nineteenth century and that extended autonomous vehicles with numerical models. However, the adventure of obtaining the International Geophysical Year Southern Ocean observations remains the same, requiring not just the right people (IGY) of 1957–1958 (Kort, 1969). From and technology, but also special ships capable of working within the harsh Antarctic 1962 to 1992, some time-series observa- environment, where unknowns await discovery. tions were also acquired from moor- BY A R N OLD L . G ORDON ings deployed at various sites around Scienti!c discovery at sea is exhilarating, Besides “adventure,” what makes Antarctica. For example, the array in an adventure that few experience. For the Southern Ocean !eldwork so entic- Drake Passage during the International ocean scientist, the long voyage far away ing is the region’s central e$ects on the Southern Ocean Studies program of from home is rewarded with new views global system. It is a role it took on some the 1970s (Neal and Nowlin, 1979) !rst of how the ocean is strati!ed and circu- 30 million years ago upon the establish- resolved the structure and variability lates, and with insight on the processes ment of a deep, circum-Antarctic oce- of the Antarctic Circumpolar Current that govern its behavior. Collecting anic belt as Drake Passage widened. #is (ACC). But the emphasis on capturing data at sea is especially challenging in geological event has been linked to the the temporal dimension of Southern remote regions such as the Southern thermal isolation of Antarctica and to Ocean variability has become more cen- Ocean where harsh wind and wave con- the chilling of the global climate (Scher tral only in the last two decades, with ditions combined with ice conspire to and Martin, 2006). Without this deep increased attention to the development make observations particularly di"cult. interoceanic link, global climate would of a coordinated, sustained observational Fieldwork in such a hostile environment be sharply di$erent from what we have network within the Southern Ocean requires strong ships to provide scientists today. #e Southern Ocean not only pro- (Rintoul et al., 2009, 2011). with safe at-sea working conditions and motes exchange between ocean basins, Georg Wüst (1964) reviews the great that can to endure long periods at sea, as but it is also the site of major deep-ocean expeditions of the nineteenth century the Southern Ocean is far from ports. overturning, driven by the sinking of up to the 1957–1958 IGY, when oceano- As if the environmental conditions dense upper waters formed at certain graphic progress depended directly on were not harsh enough, to make matters sectors along the continental margins. observations from these expeditions and dynamically more challenging to ocean- Upon reaching great depths and moving their interpretation, usually published in ographers, the Rossby radius drops from northward, this dense water of Antarctic lengthy, detailed monographs. Wüst adds ~ 100 km to ~ 10 km on moving south- origin cools and ventilates the abys- that advances also hinged on technol- ward from the Antarctic Circumpolar sal layer of the world ocean, a$ecting ogy, improvement of instrumentation,

Current (ACC) to the continental mar- oceanic CO2 storage. gins. #is drop means that ship-based #is article focuses on the large-scale Arnold L. Gordon (agordon@ldeo. observations must have higher spatial surveys required to de!ne the basic columbia.edu) is Associate Director, Ocean resolution at higher latitudes to resolve structure of the Southern Ocean, from and Climate Physics, Lamont-Doherty the geostrophic scale, which is sometimes the start of the Eltanin (Figure 1) pro- Earth Observatory of Columbia University, tough to do when ice stands in the way. gram in 1962 to the four remarkable Palisades, NY, USA.

Oceanography | September 2012 19 and data analysis closely linked to the other parameters from more than Paci!c sector, at a location very remote development of new theoretical concepts 1,500 “hydro stations” (Figure 2a), from ports. I combined two casts to (dynamical oceanography). He points along with many thousands of bathy- draw a total of 24 water samples from out that in oceanographic history, ship thermograph casts and photographs Nansen bottles with reversing thermom- names have o&en been better remem- of the sea'oor. In the period between eters (for more on Nansen bottles, see bered than those of expedition leaders. 1962 and 1975, the Eltanin logged Warren, 2008). At the sampling location, In this sense, the Eltanin circumpolar data along 420,000 nautical miles of the ACC squeezes in a jetlike stream survey is well remembered. track, extending from the Scotia and through the deep Eltanin Fracture Zone #e few data points available in the Weddell Seas westward into the cen- System (50°S, 146°W). Gauging the Southern Ocean before the modern era tral Indian Ocean, and between 40°S Eltanin’s rapid eastward dri&, we lowered led scientists to conveniently assume and Antarctica. Transferred to the the Nansen cast into the sea upstream the ocean was in steady state. For them, Argentine Navy in 1975 and renamed of the fracture zone so that hours later it was an assumption of necessity (and Islas Orcadas, the ship continued it would be dipping exactly into the perhaps a way not to excuse every devi- to cruise the Atlantic sector of the fracture zone’s depths—a great coordi- ant data point as a sign of change). But Southern Ocean until 1978. #e survey nated e$ort to get a few data points. #e oceanographers of the early to mid- of the Southwest Indian Ocean sector ACC then broadens over the Southeast twentieth century began to appreciate was !lled in by R/V Conrad cruise 17 Paci!c Basin before reorganizing to pass the ocean’s temporal variability and to of 1974 (Figure 2a; Jacobs and Georgi, into Drake Passage and then turning admit that it was not “steady” at all. 1976), and the perennially ice-covered northward in the Scotia Sea to cross the western Weddell Sea was surveyed in South Atlantic. Although some sectors ELTANIN AND ISLAS ORCADAS 1992 with the US-Russian Ice Station of the ACC and its frontal zones have It was in the Southern Ocean, with the Weddell (Figure 2b; Gordon and Ice been studied in detail since the Eltanin Eltanin, that I began my oceanographic Station Weddell Group of Principal days, particularly at the “choke points” career in the early 1960s. It was still the Investigators and Chief Scientists, 1993). between Antarctica and the warmer time of the great ocean-scale surveys that An advantage of participating in a Southern Hemisphere continents, there de!ned the ocean’s spatial characteris- broad survey as a young scientist was are many other sectors that deserve a tics. #e Eltanin (Figure 1), supported that it e$ectively introduced me to the closer look. #ese would include a revisit by the National Science Foundation’s Southern Ocean, slowly, data bit by data to that most dramatic choke point at the O"ce of Polar Programs, was launched bit, from which emerged broad views of Eltanin Fracture Zone System and other in 1957 as a noncommissioned Navy the Antarctic margins and of the ACC, encounters of the ACC with speci!c cargo ship with a special feature that with its assemblage of jets and fronts. topographic features, such as the various would prove essential to Southern Ocean ACC characteristics and frontal zona- plateaus of the Indian Ocean. scientists—an ice-capable, double hull. tion are not the same at all longitudes, #ere are two processes by which #us, this vessel was o"cially classi!ed and they respond to bottom topography. Southern Ocean surface waters may as an Icebreaking Cargo Ship. In August For example, the ACC is a broad current reach the deep ocean: gravity cur- of 1962, the ship was re!tted to perform with rather ill-de!ned fronts in the abys- rents over the continental slope and research in the Southern Ocean and sal plain of the Southeast Paci!c sector, convection in the open ocean similar reclassi!ed an Oceanographic Research but it is narrow with sharply de!ned to what occurs within the Greenland Vessel (T-AGOR-8). fronts over the zonal mid-ocean ridge and Labrador Seas (Gordon, 1991). In Eltanin’s research mission included south of Australia and in areas where February 1977, the Islas Orcadas found physical, biological, and geological- the current encounters other major clear evidence of the latter process when geophysical oceanography. Researchers topographic features. scientists aboard the ship observed the obtained vertical pro!les of tempera- I remember attempting to obtain remnants of a convective “chimney” ture, salinity, dissolved oxygen, and very deepwater samples in the Central near Maud Rise in the Weddell Sea. #is

20 Oceanography | Vol. 25, No. 3 observation was key in linking deep- period of negative or neutral Southern ICE STATION WEDDELL ocean convective processes to the “Great Annular Mode (SAM) index, which #e Islas Orcadas work led to two Weddell Polynya,” a 250 x 103 km2 also occurred in the leading decade of decades of expeditions in the central area virtually free of sea ice during the the twentieth century (Gordon et al., Weddell Gyre. In the 1980s, a series of winters of 1974–1976, in the midst of a 2007). Might the Weddell Polynya have winter and spring expeditions (Somov, region that is normally covered with sea- occurred in that earlier period? Did 1981; PoIarstern, 1986, 1989; Fedorov sonal sea ice (Gordon, 1978). the 1911–1912 Deutschland expedition 1989; see Gordon and Huber, 1990) #e Weddell Polynya was observed into the Weddell Sea region (Brennecke, provided a more precise view of the shortly a&er satellites began to gather 1921) record deep-reaching convec- deepwater coupling with the winter images of the Southern Ocean sea ice tion, as Wüst (1928) interpreted the mixed layer and its control of seasonal !elds, but a persistent, winter-long Deutschland data? A series of papers by sea ice distribution and thickness. Over polynya hasn’t formed again since 1976 Wüst (1933, 1935) and Mosby (1934) the far western limb of the Weddell Gyre, within the Weddell Gyre. Archived turned the tide of opinion to the con- however, high concentration of peren- observations of surface-layer salinity tinental margin origin of Antarctic nial sea ice had hindered even the basic within that larger area suggest that the Bottom Water (AABW). Perhaps exploration of the northward-'owing Great Weddell Polynya of 1974–1976 the nonsteady state issue needed to western boundary current carrying the may have been induced by a prolonged be considered then? densest forms of AABW. #is region is

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Figure 2. (a) Circumpolar Survey 1962–1992: Eltanin (black stars), Islas Orcadas (green circles), Conrad 17 (orange boxes), Ice Station Weddell stations (blue symbols). Bathymetry is from Smith and Sandwell (1997). Maximum and minimum ice edge (February and September; gray lines), 1979–2007 average, from the National Snow and Ice Data center. "e black lines denote the 0/1500 dynamic height from the World Ocean Atlas 2009 (http://www.nodc.noaa.gov/OC5/WOA09/pr_woa09.html). (b) Expanded map showing Ice Station Weddell conductivity-temperature-depth (CTD) stations. Inverted triangles mark the sites of the CTDs obtained from the drifting ice floe, upright triangles indicate CTDs obtained from helicopter flights off the ice floe, and solid circles denote ship-based CTD casts.

Oceanography | September 2012 21 precisely where, on January 18, 1915, the (Gordon and Ice Station Weddell Group studies. We now have repeats of the Endurance and Sir Ernest Shackleton and of Principal Investigators and Chief WOCE sections within the CLIVAR his crew were stranded in the Weddell Scientists, 1993). Observations were also (Climate Variability and Predictability) Sea ice pack and began the most famous made from remote instrumented dri&ers, program, as well as sustained measure- example of survival in polar exploration helicopters, and ships associated with ments from mooring arrays, Lagrangian (Shackleton, 1919). the various phases of the ISW !eldwork dri&ers, and pro!ling 'oats. #e Argo An e$ective and successful method (Figure 2b). #e US and Russian science array provides circumpolar snapshot for gathering extensive observations programs complemented each other views, enabling study of shi&ing fronts in the ice-cluttered western Weddell to yield a more complete picture of the in association with a changing SAM Sea was borrowed from the Arctic: environment. Measurements included index, although not yet of the Rossby deployment of a scienti!c station on a thermohaline and tracer !elds, currents, radius series of observations across dri&ing ice 'oe. On February 12, 1992, and turbulent 'uxes within oceanic and the circumpolar zones as brought out Ice Station Weddell (ISW) was estab- atmospheric planetary boundary layers. by the survey sections. #e surveys lished at 71.4°S by a joint e$ort of the #ese data were used in studies of sea of the past have enabled a view of the United States and Russia (Gordon and ice dynamics, sea ice physical, chemi- slow temporal changes that the cli- Ice Station Weddell Group of Principal cal, and biological characteristics, and mate system is presently undergoing Investigators and Chief Scientists, 1993) water column biology. (Purkey and Johnson, 2010). In today’s world, as measurements from multiple instruments and platforms work together with a suite of numerical models to leverage the still-limited …THE ADVENTURE OF OBTAINING observations into a broader picture, the SOUTHERN OCEAN OBSERVATIONS REMAINS right combination of capable scientists and ships are needed to sustain Southern THE SAME, REQUIRING NOT JUST THE RIGHT Ocean adventure and discovery. PEOPLE“ AND TECHNOLOGY, BUT ALSO SPECIAL SHIPS CAPABLE OF WORKING WITHIN THE ACKNOWLEDGEMENTS HARSH ANTARCTIC ENVIRONMENT, WHERE My years of Southern Ocean research were funded by the National Science UNKNOWNS AWAIT DISCOVERY. Foundation, O"ce of Polar Programs. I appreciate the support of a string of chief scientists and program manag- ers, among them Albert Crary, Louis Quam, John Kelley, Victor Neal, during which participating researchers CO”NCLUDING REMARKS Peter Wilkniss, Bernard Lettau, and dri&ed until June 4 with the perennial From 1989 to the mid-1990s, a series of most recently Peter Milne. Lamont- ice, roughly from 53°W to 65.8°S along oceanographic sections were obtained Doherty Earth Observatory contribu- the western boundary of the Weddell in the Southern Ocean as part of WOCE tion number 7562. Gyre (Figure 2b). (see the WOCE Southern Ocean Atlas During ISW, the 'oe dri&ed at a mean by Orsi and Whitworth, 2005; http:// speed of 6.6 km per day. Scientists col- woceatlas.tamu.edu). Gradually, the lected an impressive array of data at the broad spatial survey e$ort yielded camp along a 750 km path that closely to emphasis on temporal scales and followed the track of the Endurance mesoscale to submesoscale process

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