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The Great Ocean Conveyor

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The Great Ocean Conveyor REVIEW & COMMENT THE GREAT OCEAN CONVEYOR By Wallace S. Broecker A DIAGRAM DEPICTING the ocean's "conveyor lies in the idea that changes in the Atlantic's ther- belt" has been widely adopted as a logo for the mohaline circulation were responsible for the Global Change Research Initiative. This diagram abrupt and large climatic changes experienced by (Fig. 1) first appeared as an illustration in an article the northern Atlantic basin during the last glacial about the Younger Dryas event that was published period. in the November 1987 issue of Natural History. The objective of this paper is to provide a sum- It was designed as a cartoon to help the largely mary, from my prospective, of the conveyor's op- lay readership of this magazine to comprehend eration present and past. one of the elements of the deep sea's circulation Its Path system. Had I suspected that it would be widely The main problem with the logo is that it im- adopted as a logo, I would have tried to "improve" plies that if one were to inject a tracer substance its accuracy. In hindsight such repairs would likely into one of the conveyor's segments it would travel have ruined the diagram both for the readers of around the loop as a neat package eventually re- Natural History and for use as a logo. turning to its starting point. As we all know this The lure of this logo is that it symbolizes the is hardly the case. Other circulation "loops" exist importance of linkages between realms of the in the ocean and mixing occurs among the waters Earth's climate system. The ocean's conveyor ap- •.. driven by the salt traveling along these intersecting pathways. The pears to be driven by the salt left behind as the logo portrays a far more complex situation. left behind as the result of water-vapor transport through the at- To understand the logo's message, let's start at mosphere from the Atlantic to the Pacific basin. result of water-vapor the point of origin of its lower limb and work our A byproduct of its operation is the heat that way around the ocean. Waters in the vicinity of transport in the maintains the anomolously warm winter air tem- Iceland are cooled through contact with the cold peratures enjoyed by northern Europe. A millen- atmosphere from the winter air masses that sweep in from the Canadian nium of very cold conditions known as the Young Arctic. The cooling increases the density of the Dryas appears to have been the result of a tem- Atlantic to the Pacific surface water to the point where it can sink to the porary shutdown of the conveyor. Thus the con- abyss and flow southward, forming the conveyor's basin. veyor logo portrays the concern that led to the lower limb. In the logo this flow is depicted as a launching of the Global Change Research Initia- ribbon of water that jets its way through the deep tives: that complex interconnections among the Atlantic from the vicinity of Iceland to the tip of elements of our Earth's climate system will greatly Africa. In reality it is a sluggish mass that fills complicate our task of predicting the conse- most of the deep Atlantic. This water mass, known quences of global pollution. to oceanographers as the North Atlantic Deep Most of the concepts involved in this story have Water (NADW), stands out as a tongue of high roots that extend well back in time. The most im- salinity, low nutrient content, and high 14C/12C portant feature of the conveyor is the production ratio water in sections drawn along the Atlantic's of deep water in the northern Atlantic. This aspect length (see Fig. 2). Its only competitor for space of the ocean's thermohaline circulation was thor- oughly described by Wrist (1935) and Wrist and in the deep Atlantic is a wedge of Antarctic Bot- tom Water (AABW), which under-rides the Defant (1936) more than 50 years ago. In 1906 NADW mass. This intruding water is mixed up- Chamberlain explored the importance of fresh- ward into the southward flowing NADW, in- water transport to ocean circulation. He raised the question as to whether changes in the pattern creasing the transport by the conveyor's lower of deep circulation could be responsible for the limb. climate changes of glacial time. My contribution Southward of 30°S the lower limb of the con- veyor joins a rapidly moving deep current that encircles the Antarctic continent. This current w.s. Broecker, Lamont-DohertyGeological Observatory serves as the great mix-master of the world ocean. of Columbia University, Palisades, NY, 10964, USA. It blends the NADW exiting the Atlantic with new OCEANOGRAPHY-Vol. 4. No. 2-1991 79 Great Ocean Conveyor Belt Q \ ~ °' oo~ OCt: PA CtYit~ Fig. 1." The great ocean conveyor logo (Broecker, 1987). (Illustration by Joe Le Monnier, Natural History Magazine.) deep water generated along the perimeter of the have much lower PO~ values than those formed Antarctic continent and also with old deep waters in the southern Ocean. Further, the range of recirculated back into the Antarctic from the deep PO~ values for the northern source waters (0.73 Pacific and Indian Oceans. So efficient is this _ 0.03) and for southern source waters (1.67 + blending that the NADW entering from the At- 0.10) is small compared with the difference be- lantic loses its identity before it passes even one tween the means for these end member values half of a revolution around the Antarctic! (1.67 - 0.73 = 0.94). A rough quantification of the contribution of As can be seen in the map in Fig. 3, at a depth NADW to the deep waters of world ocean is pro- of 3 km the contribution of NADW to the deep- vided by a property called PO$ (Broecker et al., water mix remains strong throughout the Atlantic, 199 la), which is defined as follows but after the conveyor's lower limb passes around the southern tip of Africa into the Antarctic it •.. NADW... loses its 02 rapidly becomes blended with the high PO~ deep PO* = PO4 + ~ - 1.95 #m/kg identity before it water generated along the edge of the Antarctic continent. In this way an ambient deep water mix passes even one half where PO4 and 02 are the measured phosphate with a PO~ value of 1.37 is produced (see histo- of a revolution around and dissolved oxygen gas concentrations in a given grams in Fig. 3). This blend, which consists of one water sample. The coefficient 175 is the global part deep water produced in the northern Atlantic the Antarctic! Redfield coefficient relating 02 consumption to with about two parts of deep water produced in PO4 release during respiration (Broecker et al., the Antarctic, floods the deep Pacific and Indian 1985), and the coefficient 1.95 is arbitrarily intro- Oceans. duced in order to bring the values of PO$ into the As depicted in the logo, the lower limb water range of deep water PO4 concentrations. To the returns to the surface in the northern Indian and extent that the respiration coefficient is a constant, Pacific Oceans. In reality this upwelling is widely PO* constitutes a conservative property of any spread with a large amount taking place in the given deep water parcel; the increase in PO4 due Antarctic. The logo also suggests that the major to the oxidation of organic material is exactly bal- route for return flow to the Atlantic (i.e., the con- anced by the decrease in 02/175. PO~' is attractive veyor's upper limb) is through the Indonesian ar- as an indicator of the contribution of NADW be- chipelago and around the tip of Africa. This view cause deep waters formed in the northern Atlantic was impressed on me through enthusiastic pre- 80 OCEANOGRAPHY-Vol. 4, No. 2.199! culations. Fortunately the two approaches yield similar answers. The radiocarbon-based estimate of the flux for NADW into the deep Atlantic is obtained by di- viding the volume of water contained in the deep Atlantic by the radiodecay-based mean residence ::2 time. This estimate must be corrected for the con- tribution made by AABW to the conveyor's lower w c~ limb. It must also be corrected for the impact of temporal changes in the 14C/~2C ratio for atmo- spheric CO2. The major obstacle to calculation from radio- carbon measurements of residence times for water in the deep Atlantic is the determination of the 60 40 20 0 20 40 60 S LATITUDE N initial 14C/12C ratio for each parcel. The reason is that all waters in the deep Atlantic are mixtures of northern component water with a compara- tively high 14C/12C ratio (AI4C = -68%0) and of southern component water with a comparatively low 14C/12C ratio (A14C = -158%o). Because of The radiocarbon the large difference in the AI4C values for these end members, much of the variation in 14C/12C deficiency for the entire ratio within the deep Atlantic is created by differ- deep Atlantic... ences in the end-member blend. As shown by Broecker et al. (1991a), PO* provides a quite ac- corresponds to a curate means of establishing the proportions of northern and southern component water in the residence time of sample analyzed for radiocarbon. The measured about 180 y. radiocarbon concentration is then subtracted from 60 50 40 50 20 I0 0 I0 20 50 40 50 60 S LATITUDE N the initial concentration calculated for the mix- ture, yielding the deficiency attributable to radi- Fig.
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