We use a global coupled atmosphere‐ocean sea‐ice model of intermediate complexity to demonstrate that wind‐forcing is a crucial element to sustain meridional overturning flow in the Atlantic. Neglecting wind‐stress in our multi‐century‐long simulations leads to a complete shutdown of the conveyor belt circulation. This result may have tremendous impacts for an assessment of the sensitivity of 2‐d climate models which typically do not capture wind‐driven gyres. It is argued that wind effects may be a key element in determining the fate and length of a collapsed THC state. Possible paleo implications will be discussed

Goosse, H.

Timmermann, Axel

OceanRep

Is the wind stress forcing essential for the meridional overturningcirculation?A. TimmermannLeibniz-Institut fu¨r Meereswissenschaften IFM - GEOMAR Duesternbrooker, Kiel, GermanyH. GoosseInstitut d’Astronomie et de Ge´ophysique G., Lemaitre, Universite´ Catholique de Louvain, Louvain-la-Neuve, BelgiumReceived 6 October 2003; accepted 19 January 2004; published 21 February 2004.[1] We use a global coupled atmosphere-ocean sea-icemodel of intermediate complexity to demonstrate that wind-forcing is a crucial element to sustain meridional overturningflow in the Atlantic. Neglecting wind-stress in our multi-century-long simulations leads to a complete shutdown of theconveyor belt circulation. This result may have tremendousimpacts for an assessment of the sensitivity of 2-d climatemodels which typically do not capture wind-driven gyres. It isargued that wind effects may be a key element in determiningthe fate and length of a collapsed THC state. Possiblepaleo implications will be discussed. INDEX TERMS: 4532Oceanography: Physical: General circulation; 4255 Oceanography:General: Numerical modeling; 4512 Oceanography: Physical:Currents; 4504 Oceanography: Physical: Air/sea interactions(0312); 4267 Oceanography: General: Paleoceanography.Citation: Timmermann, A., and H. Goosse (2004), Is the windstress forcing essential for the meridional overturning circulation?,Geophys. Res. Lett., 31, L04303, doi:10.1029/2003GL018777.1. Introduction[2] The thermohaline circulation (THC) is a key ele-ment of the Northern Hemispheric climate system. Itcarries a substantial amount of heat and saline waterspoleward, thereby altering the atmospheric circulation andsea ice. Despite of the importance of this circulation manyfundamental questions have not been answered satisfacto-rily: What drives the THC? What are the roles ofconvection, turbulent mixing and diffusion? What is therole of the wind-driven circulation? While the secondquestion has been studied extensively [e.g., Welander,1986; Nilsson et al., 2003; Marotzke, 1997, and manyothers] the latter has received only very little scientificattention [e.g., Weaver et al., 1993; Toggweiler andSamuels, 1995; Schiller et al., 1997; Oka et al., 2001].Recent attempts [Oka et al., 2001] to quantify the role ofwind forcing for thermohaline flow have revealed thathorizontal Ekman flow in the subpolar gyre as well asEkman pumping [Schiller et al., 1997] provide surfacesalinity sources which prevent the formation of polar/subpolar haloclines, thereby stabilizing the THC. Further-more, Ekman pumping can be a very important factor inpreconditioning oceanic convection which eventually mayinfluence thermohaline flow, as pointed out by Killworth[1983] and Schiller et al. [1997]. Previous studies [Oka etal., 2001] address this issue systematically. Oka et al.[2001] find that both horizontal and vertical salinity trans-ports in the northern North Atlantic are crucial elementsfor the meridional overturning circulation. Neglectingeither of them leads to a shutdown of the meridionaloverturning. However, this study suffers from severalmodel shortcomings such as a parameterized atmosphere,a lack of sea-ice dynamics and of an arctic ocean. Hence,it is timely to check the general conclusions once more ina physically more complete setup using a 3-d globalatmosphere-ocean model. Our general conclusion is con-sistent with the findings of Oka et al. [2001]. Windforcing is needed to maintain meridional overturning flow.The fundamental question to be addressed here is whathappens to the meridional overturning circulation in theNorth Atlantic when wind-forcing is neglected. We use theglobal coupled atmosphere-ocean-sea ice model ECBilt-Clio which has been described extensively in the literature[Opsteegh et al., 1998; Goosse and Fichefet, 1999;Goosse et al., 2002]. Here we analyse three simulationsperformed with this coupled model: The first experiment(EXP1) is a 700-year long control simulation forced bypre-industrial CO2 concentrations. In EXP2 a persistentfreshwater anomaly of 0.4 Sv is injected into the NorthAtlantic between 50–70N leading to a shut-down of theTHC. The length of this simulation is 280 years. Anothercoupled simulation is performed (EXP3) for 16300 yearsin which no wind-stress forcing is applied to the ocean.2. Main Results[3] Figures 1 and 2 show that in both experiments EXP2and EXP3 the meridional circulation collapses quicklywithin the first few hundred years, whereas it remainsstationary in the control simulation (EXP1).[4] EXP3 has been continued for 16300 years and showssome ultra-low-frequency oscillations like those discussedby Oka et al. [2001] and Winton [1993]. The prolonged off-state of the THC is accompanied by an overall diffusivewarming of the deep ocean until a threshold temperature isreached. Eventually, the stratification in the ocean becomesunstable and a rapid flush of the THC is induced, whichvents the stored heat of the deep ocean to the surface.Subsequently the flush stops after a few hundred years andthe deep-decoupling phase starts once more. Our modelingresults show that the off-state of the THC is oscillatoryunstable. During the off state the polar regions of bothGEOPHYSICAL RESEARCH LETTERS, VOL. 31, L04303, doi:10.1029/2003GL018777, 2004Copyright 2004 by the American Geophysical Union.0094-8276/04/2003GL018777L04303 1 of 4hemispheres are not able to form a significant amount ofdeep water any longer.[5] Thus, wind forcing is a crucial element to sustain themeridional overturning circulation in the North Atlantic, inaccordance with Oka et al. [2001]. The thermohalinecomponent of the overturning circulation operates on amean zonal and meridional density gradient, which is partlymaintained by the wind-driven salinity and heat transports.This illustrates also the difficulty of separating wind-drivenand thermohaline flow, since the wind-driven flow contrib-utes to thermohaline forcing. Nevertheless an attempt willbe made here using EXP1-3. The final stages of EXP2 andEXP3 are both characterized by a lack of North AtlanticDeep Water formation. Hence their density driven surfaceflow in the North Atlantic is quite similar. The maindifference is that EXP2 still maintains the wind-drivencirculation, although the wind-pattern may have changed alittle between the two simulations. In comparison to EXP3,EXP2 is characterized by a much stronger Goldsbroughcirculation [Goldsbrough, 1933] due to the imposed fresh-water forcing. Neglecting these second order effects thedifference of the surface flow patterns of EXP2 and EXP3(Figure 3) represents the wind-driven circulation.[6] Not surprisingly, the major wind-driven features arethe subtropical and subpolar gyres as well as the AntarcticCircumpolar Current. Assuming the wind-driven circula-tion in EXP1 and EXP2 to be similar the major differencebetween EXP1 and EXP2 is the direct density-drivenmeridional overturning circulation in the Atlantic due tofreshwater and heat fluxes. In fact the atmospheric circu-lation responds to a collapse of the thermohaline circula-tion by an increased surface wind stress. However, thesewind-stress changes are small as compared to the totalmagnitude of the surface winds, which are neglected inEXP 3. The associated surface flow pattern (Figure 3, rightpanel) is quite instructive: One observes the main featuresof the surface conveyor belt flow. A southern Atlanticwarm water branch flowing from the southern Indianocean to the South Atlantic around the southern tip ofAfrica. This flow is also visible in the undifferencedcurrent data of EXP1. Furthermore, we see a strong Brazilcurrent which splits at 25N into two major branches–oneflowing northward parallel to the western boundary cur-rent, the other one opposing the wind-driven subtropicalgyre circulation and flowing northward on the easternboundary. Both current branches merge again in theGreenland-Iceland-Norwegian (GIN) Seas. The diagnosedthermohaline flow tends to reinforce the Gulf Streamtransport whereas it slows down the subpolar gyre. Themodel-based decomposition into wind-driven (EXP2-EXP3) and freshwater and heat flux-driven thermohaline(EXP1-EXP2) flow (similar to Manabe and Stouffer[1988]) is reasonable as can be deduced (not shown) fromFigure 1. Maximum of the meridional streamfunction inthe North Atlantic [Sv] EXP1 (upper panel), EXP2 (lowerpanel). Time axis is in units of months.Figure 2. Maximum of the meridional streamfunction inthe North Atlantic [Sv] (solid) and global ocean meantemperature [C] (dashed) for EXP3. Time axis is in units ofyears.Figure 3. Circulation splitted into a ‘‘wind-driven’’ and a‘‘thermohaline’’ part.L04303 TIMMERMANN AND GOOSSE : WIND STRESS FORCING OF MERIDIONAL OVERTURNING L043032 of 4a comparison between the sum of these parts with the totalcirculation from EXP1. The role of the wind-drivencirculation in maintaining meridional overturning flow inthe North Atlantic is complex. One important part ishorizontal surface layer salinity advection which preventsthe formation of a polar, subpolar halocline, therebystabilizing thermohaline flow. Figure 4 displays thewind-driven and thermohaline contributions to the meansurface salinity advection in the North Atlantic. The wind-driven circulation contributes up to 0.15 psu/month to thesalinity tendency in the GIN seas and the area of the eastGreenland current, whereas the residual thermohaline flowgenerates a buoyance forcing of similar magnitude butreverse sign. Hence, wind-driven flow prevents the forma-tion of haloclines in the GIN sea. If, however, themeridional overturning circulation is active the meridionaloverturning flow transports a huge amount of salinitynorthward, as can be seen from the large salinity differ-ences in high latitudes between EXP1 and EXP2 (notshown). Similar results were discussed by Manabe andStouffer [1988].[7] A background salinity field is generated by the localfreshwater forcing. The wind-driven and the thermohalinecirculation serve together as coupled horizontal (and verti-cal) advective re-distributors of this inital large-scale surfacesalinity field. If the wind-driven part is neglected, a saltdeficit occurs in high latitudes. Apart from this large-scaleeffect on thermohaline there may be local effects which helpto weaken the meridional overturning flow further. In EXP2and EXP3 a salt deficit in high latitudes leads also to anincrease of the sea ice extent of more than 8  106 km2 inannual mean, i.e. nearly a doubling. This induces anisolating effect on the ocean that prevents further heatrelease from the ocean to the atmosphere, thereby stabilisingthe water column. Furthermore, the presence of sea icemodifies the surface momentum budget. When sea-ice isthin, the wind stress is transmitted to the ocean surface withonly a weak influence of the ice. For a compact and thickice cover on the other hand, the presence of ice can induce areduction of the ocean surface stress by more than 80%compared to the total wind stress [e.g., Goosse and Fichefet,1999]. This reduction of the wind stress influences thesurface currents and thus the oceanic salinity transport.Hence an initial weakening of the winds of sufficientamplitude can trigger a chain reaction of large-scale andlocal processes which involve reduced salinity transport,increased formation of sea-ice and stabilization of the statewithout oceanic convection. Eventually this will affect thelarge-scale meridional overturning circulation.3. Summary and Conclusion[8] Using a fully coupled global atmosphere-ocean sea-ice model we have shown for the first time that wind-forcing plays a substantial role in maintaining large-scalethermohaline flow in the North Atlantic. A model exper-iment without wind-forcing (EXP3) leads to a collapse ofthe meridional overturning circulation. Density gradientsneeded to drive thermohaline flow can not be maintainedwithout wind-forcing. Wind-forcing of the THC has threeimportant contributions: horizontal Ekman-driven salinitytransport due to the wind-driven circulation [Weaver et al.,1993], vertical salinity transport [Oka et al., 2001] and thepreconditioning of oceanic convection [Killworth, 1983]due to Ekman pumping. Furthermore, we have shown forthe first time that deep-decoupling oscillations as describedby Winton [1993] can be simulated using a global 3-dcoupled atmosphere-ocean-sea ice model using full topog-raphy. Our results suggest that the THC off-state is oscil-latory unstable. Our modeling results have importantimplications for the interpretation of paleo records of thelast glacial period. Heinrich events [Heinrich, 1988] haveinfluenced the THC substantially. They were also accom-panied by an equatorward shift of the sea-ice margin in theNorthern Hemisphere. Sea-ice is not only an insulatingfactor for oceanic freshwater and heat forcing, but also aninsulator for the atmosphere-ocean momentum exchange.In particular in the Northern Hemisphere, a larger sea-icearea will be associated with a strong modification of wind-driven salinity transports with the potential to weaken theTHC and to increase the sea-ice cover further. Some of thepresent climate models of intermediate complexity [e.g.,Crucifix et al., 2002] employ a 2-D ocean model thatcaptures the wind-driven ocean circulation in a parameter-ized manner. In contrast to our 3-d simulations and those ofOka et al. [2001] the 2-d model of Crucifix et al. [2002]does not simulate a collapse of the overturning circulation(Michel Crucifix, personal communication), in response toweakened winds. This indicates that 2-d models of theTHC miss an important feedback. Overall, our studysuggests that the wind-driven circulation is a pre-requisitefor the set-up of meridional overturning flow in the NorthAtlantic. Further studies using coupled general circulationmodels are needed to confirm this far-reaching conclusion.State-of-the art models do not agree on the prediction ofthe future evolution of the intensity of the thermohalinecirculation. Several mechanisms have been suggested toexplain the disagreement: such as a different response ofthe meridional freshwater transport in the various modelsand an influence of tropical dynamics. Our experimentssuggest that the influence of the wind stress must be takeninto account in an assessment of the different behaviour ofthe models.Figure 4. Left: Time averaged surface salinity field ofEXP1 and surface salinity transport (shading) due to thewind driven circulation ([~Uwind] [rSEXP1]). Right: As leftbut for the ‘‘thermohaline’’ circulation.L04303 TIMMERMANN AND GOOSSE : WIND STRESS FORCING OF MERIDIONAL OVERTURNING L043033 of 4[9] Acknowledgments. A. Timmermann is supported by the Deut-sche Forschungs Gemeinschaft. H. Goosse is Research associate with theBelgian National Fund for Scientific Research.ReferencesCrucifix, M., M. F. Loutre, and P. Tulkens (2002), Climate evolution duringthe Holocene: A study with an Earth system model of intermediate com-plexity, Clim. Dyn., 19, 43–60.Goldsbrough, G. R. (1933), Ocean currents produced by evaporation andprecipitation, Proc. R. Soc. London Ser. A, 141, 241–248.Goosse, H., and T. Fichefet (1999), Importance of ice-ocean interactions forthe global ocean circulation: A model study, J. Geophys. Res., 104(C10),23,337–23,356.Goosse, H., F. M. Selten, R. J. Haarsma, and J. D. Opsteegh (2002), Largesea-ice volume anomalies simulated in a coupled climate model, Clim.Dyn., 10, doi:10.1007/s0382-002-0290-4.Heinrich, H. (1988), Origin and consequences of cyclic ice rafting in theNorth Wast Atlantic Ocean during the past 130,000 years, Quat. Res., 29,143–152.Killworth, P. D. (1983), Deep convection in the world ocean, Rev. Geophys.Space Phys., 21, 1–26.Manabe, S., and R. J. Stouffer (1988), Two Stable Equilibria of a CoupledOcean-Atmosphere Model, J. Clim., 1, 841–866.Marotzke, J. (1997), Boundary mixing and the dynamics of the three-dimen-sional thermohaline circulations, J. Phys. Oceanogr., 27, 1713–1728.Nilsson, J., G. Brostro¨m, and Go¨sta Walin (2003), The thermohalinecirculation and vertical mixing: Does weaker density stratification givestronger overturning, J. Phys. Oceanogr., 33, 2781–2795.Oka, A., H. Hasumi, and N. Suginohara (2001), Stabilization of thermoha-line circulation by wind-driven and vertical diffusive salt transport, Clim.Dyn., 18, 71–83.Opsteegh, J. D., R. J. Haarsma, F. M. Selten, and A. Kattenberg (1998),ECBILT: A dynamic alternative to mixed boundary conditions in oceanmodels, Tellus, 50A, 348–367.Schiller, A., U. Mikolajewicz, and R. Voss (1997), The stability of theNorth Atlantic thermohaline circulation in a coupled ocea-atmospheregeneral circulation model, Clim. Dyn., 13, 325–347.Toggweiler, T. R., and B. Samuels (1995), Effect of Drake Passage on theglobal thermohaline circulation, Deep Sea Res., 42, 477–500.Weaver, A. J., J. Marotzke, P. F. Cummins, and E. S. Sarachik (1993),Stability and variability of the thermohaline circulation, J. Phys. Ocean-ogr., 23, 39–60.Welander, P. (1986), Thermohaline effects in the ocean circulation andrelated simple models, in Large-scale transport processes in the oceansand atmosphere, edited by J. Willebrand and D. L. T. Anderson,D. Reidel, Norwell, Mass. (imprint of Kluwer Acad.).Winton, M. (1993), Deep decoupling oscillations of the oceanic thermoha-line circulation, in Ice in the climate system, edited by W. R. Peltier,pp. 417–432, Springer Verlag, Berlin.H. Goosse, Institut d’Astronomie et de Ge´ophysique G., Lemaitre,Universite´ Catholique de Louvain, Louvain-la-Neuve, Belgium.A. Timmermann, Leibniz Institut fu¨r Meereswissenschaften, Kiel,Germany. (atimmermann@ifm.uni-kiel.de)L04303 TIMMERMANN AND GOOSSE : WIND STRESS FORCING OF MERIDIONAL OVERTURNING L043034 of 4

Is the wind-stress forcing essential for the meridional overturning circulation?

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Is the wind-stress forcing essential for the meridional overturning circulation?

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