What Is the Thermohaline Circulation?
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S CIENCE’ S C OMPASS PERSPECTIVES 65 flow thus appear to be driven by thermal 64 PERSPECTIVES: OCEANOGRAPHY and evaporative forcing from the atmo- 63 sphere. The ocean seems to act like a heat 62 engine, in analogy to the atmosphere. 61 What Is the Some authors apparently think of this 60 convective mode of motion as the thermo- 59 haline circulation. But results of the past 58 Thermohaline Circulation? few years suggest that such a convectively 57 Carl Wunsch driven mass flux is impossible. There are 56 several lines of argument. The first goes 55 he discussion of today’s climate and mass affect the movements of all other back to Sandström (4), who pointed out that 54 its past and potential future changes properties, such as heat, salt, oxygen, car- when a fluid is heated and cooled at the 53 Tis often framed in the context of the bon, and so forth (1, 2), all of which differ same pressure (or heated at a lower pres- 52 ocean’s thermohaline circulation. Wide- from each other. For example, the North sure), no significant work can be extracted 51 spread consequences are ascribed to its Atlantic imports heat, but exports oxygen. from the flow, with the region below the 50 shutdown and acceleration—a deus ex It seems most sensible to regard the ther- cold source becoming homogeneous. 49 machina for climate change. mohaline circulation –8 –2 The ocean is both 48 But what is meant by this term? In in- as the circulation of 0 heated and cooled ef- 500 47 terdisciplinary fields such as climate temperature and salt. 10 fectively within about 46 change, terminological clarity is of the However, because the 1000 10 12 100 m of the sea sur- 8 45 essence; otherwise, what everyone thinks three-dimensional 1500 face, but almost every- 14 44 they understand may in fact be a muddle (3D) distributions and 2000 8 where else it has a fi- 43 of mutual misunderstanding. Only if one surface boundary con- 12 nite stable stratifica- 2500 42 can define the circulation, can its control- ditions of temperature 4 0 tion. Returning the 41 ling factors be sensibly discussed. and salt are different, 3000 downwelling mass flux 40 A reading of the literature on climate it should come as no Depth (m) 3500 2 upward across the sta- 39 and the ocean suggests at least seven dif- surprise that one must 4000 0 –2 ble stratification re- 38 ferent, and inconsistent, definitions of the separate the thermal 4500 quires a finite amount 37 term “thermohaline circulation”: circulation from the of work, manifested as 5000 36 1) the circulation of mass, heat, and salt (or freshwater) 0 the turbulent mixing 35 salt; circulation. carrying dense fluid 34 2) the abyssal circulation; What drives the 20°S 020°N40°N60°N across the density gra- Latitude 33 3) the meridional overturning circula- ocean’s mass circula- dient. The only possible 32 tion of mass; tion? The upper layers Meridional overturning circulation sources of this work are 31 4) the global conveyor, that is, the dif- of the ocean are clear- (MOC) in the North Atlantic. This figure tidal stirring and the 6 3 30 fusely defined gross property movements ly wind-driven, involv- shows volume fluxes in units of 10 m /s, wind field (5, 6). 29 in the ocean that together carry heat and ing such major fea- obtained by integrating zonally across the Furthermore, the 28 moisture from low to high latitudes; tures as the Gulf basin in a general circulation model con- work done on the ocean 27 5) the circulation driven by surface Stream and the Cir- strained to observations (3). The north- circulation by the net ward near-surface flow includes the Gulf 26 buoyancy forcing; cumpolar Current. A heating and cooling, Stream and other dominantly wind-con- 25 6) the circulation driven by density large body of observa- trolled elements. Yellow regions are and evaporation and 24 and/or pressure differences in the deep tional, theoretical, and areas of counterclockwise flow; in reddish precipitation, reduces 23 ocean; and modeling literature regions the flow is clockwise. Regions of the system’s potential 22 7) the net export, by the North Atlantic, supports the inference downward motion near 30°N and 60°N energy (7). Paparella 21 of a chemical substance such as the ele- that the mass fluxes in are associated with strong heat losses to and Young (6) have 20 ment protactinium. the top several hun- the atmosphere. The subsequent flows are, shown that a convective 19 These different usages present impor- dred meters of the however, determined largely by the global mode of motion cannot 18 tant conceptual issues. For example, the ocean are directly con- wind distribution. generate the turbulence 17 deep ocean is in a near-equilibrium state, trolled by the wind required to carry the 16 and it is not possible, without an intricate stress (the force per unit area exerted by MOC across the stable stratification. Labora- 15 calculation, to determine if the density/ the wind on the ocean). tory-scale theories indicate that in the absence 14 pressure differences drive the flow field, If the flow is integrated zonally in the of intense turbulence at depth, the deep ocean 13 or the reverse. Some authors claim to be ocean (see the figure), one notices what is would be unstratified (8)—in accord with 12 able to separate the fraction of the flow de- best called a meridional overturning circu- more elaborate oceanographic models (9) and 11 rived from density field gradients from lation (MOC) (3). Features such as the in conflict with what is observed. 10 that caused by the wind field (definition Gulf Stream are not evident, but the Gulf The conclusion from this and other 9 6). But the density gradients are set up pri- Stream dominates the mass flux in the up- lines of evidence is that the ocean’s mass 8 marily by the wind. per ocean and is clearly part of the MOC flux is sustained primarily by the wind, 7 For present purposes, I define the ocean (1). Circulations at high latitudes generally and secondarily by tidal forcing. Both in 6 circulation as that of its mass. The fluxes of contain a downward mass flux at high lati- models and the real ocean, surface buoy- 5 tudes that is associated, at least loosely, ancy boundary conditions strongly influ- 4 with regions of severe heat loss to the at- ence the transport of heat and salt, because The author is in the Department of Earth, Atmospheric 3 and Planetary Sciences, Massachusetts Institute of mosphere. In these regions, the fluid be- the fluid must become dense enough to 2 Technology, Cambridge, MA 01239, USA. E-mail: comes dense and convectively unstable; sink, but these boundary conditions do not 1 [email protected] the downward flux and subsequent lateral actually drive the circulation. www.sciencemag.org SCIENCE VOL 298 8 NOVEMBER 2002 1179 S CIENCE’ S C OMPASS 65 The ocean is thus best viewed as a me- large part, determine the regions of convec- the mass circulation without knowledge of 64 chanically driven fluid engine, capable of tive sinking and of the resulting 3D water the corresponding property distribution. 63 importing, exporting, and transporting vast properties. Fluxes and net exports of proper- 62 quantities of heat and freshwater. Although ties such as heat and carbon are determined References and Notes 61 of very great climate influence, this trans- by both the mass flux and spatial distribu- 1. A. Ganachaud, C. Wunsch, Global Biogeochem. Cycles 60 port is a nearly passive consequence of the tion of the property, and not by either alone. 16, 1057 (2002). 2. Flux here denotes the movement of a property with- 59 mechanical machinery. When Stommel Tidal motions were different in the past in the ocean, both vertically and laterally. 58 (10) first introduced the term “thermoha- than they are today, owing to lower sea 3. D. Stammer et al., J. Geophys. Res., in press; published 57 line circulation” in a box model, he explic- level during glacial epochs, and moving online 5 September 2002 (10.1029/2001JC000888). 56 itly provided a source of mechanical energy continental geometry in the more remote 4. J. W. Sandström, Annal. Hydrogr. Marit. Meteorol. 36, 6 (1908). 55 in the form of mixing devices. These de- past. The consequent shifts in tidal flow 5. W. Munk, C.Wunsch, Deep-Sea Res. 45, 1976 (1998). 54 vices disappeared in subsequent discussions can result in qualitative changes in the 6. F. Paparella, W. R. Young, J. Fluid. Mech. 466, 205 53 and extensions of this influential model. oceanic mixing rates, and hence in the (2002). 52 For past or future climates, the quantity mass and consequent property fluxes. 7. R. X. Huang,W.Wang, in preparation. 8. W. D. Baines, A. F. Corriveau, T. J. Reedman, J. Fluid 51 of first-order importance is the nature of the The term “thermohaline circulation” Mech. 255, 621 (1993). 50 wind field. It not only shifts the near-surface should be reserved for the separate circu- 9. R. M. Samelson, G. K. Vallis, J. Mar. Res. 55, 223 49 wind-driven components of the mass flux, lations of heat and salt, and not conflated (1997). 48 but also changes the turbulence at depth; into one vague circulation with unknown 10. H. Stommel, Tellus 13, 131 (1961). 11. Supported in part by the ECCO (Estimating the Cli- 47 this turbulence appears to control the deep or impossible energetics.