The response of a global circulation model to a uniform geothermal heat flux of 50 mW m-2 through the sea floor is examined. If the geothermal heat input were transported upward purely by diffusion, the deep ocean would warm by 1.2°C. However, geothermal heating induces a substantial change in the deep circulation which is larger than previously assumed and subsequently the warming of the deep ocean is only a quarter of that suggested by the diffusive limit. The numerical ocean model responds most strongly in the Indo-Pacific with an increase in meridional overturning of 1.8 Sv, enhancing the existing overturning by approximately 25%

Adcroft, Alistair

Marotzke, Jochem

Scott, Jeffery R.

English

Southampton (e-Prints Soton)

GEOPHYSICALRESEARCHLETTERS,VOL.28,NO.9,PAGES1735-1738, May 1, 2001Impact of Geothermal Heating on the GlobalOcean CirculationAlistair Adcroft and Jeery R. ScottMassachusetts Institute of Technology, Cambridge, MassachusettsJochem MarotzkeSouthampton Oceanography CentreAbstract. The response of a global circulation model toa uniform geothermal heat ﬂux of 50 mW m−2 throughthe sea ﬂoor is examined. If the geothermal heat inputwere transported upward purely by diusion, the deep oceanwould warm by 1.2C. However, geothermal heating inducesa substantial change in the deep circulation which is largerthan previously assumed and subsequently the warming ofthe deep ocean is only a quarter of that suggested by thediusive limit. The numerical ocean model responds moststrongly in the Indo-Pacic with an increase in meridionaloverturning of 1.8 Sv, enhancing the existing overturning byapproximately 25%.1. IntroductionThe general circulation of the ocean in climate models isgenerally considered to be forced by wind stress, heat andfreshwater ﬂuxes at the sea surface. However, there are othersources of energy that can directly or indirectly lead to mo-tion that are normally ignored or argued to be insignicant.For example, tidal forcing is normally neglected in climatemodels and yet tidal energy might be an important sourcefor diapycnal mixing [Munk and Wuncsh, 1998].Another neglected energy source is the geothermal heatﬂux through the sea ﬂoor. This trickle of heat, which is dueto the slow cooling of the solid earth, is estimated to havea typical value of 50 mW m−2 (1 mW = 10−3 Watts) onabyssal plains and up to 200 mW m−2 on mid-ocean ridges[Sclater et al., 1980; Kadko and Baross, 1995; Stein et al.,1995; Murton et al., 1999]. Even these peak values are smallcompared to typical values of air-sea heat ﬂuxes, which areof order 100 W m−2. The consequences of the geothermalheat ﬂux have been considered in process and regional scalestudies and found to modify local water properties and cir-culation [Joyce and Speer, 1987; Hautala and Riser, 1989;Speer, 1989; Helfrich and Speer 1995; Thompson and John-son, 1996]. It is less clear whether geothermal heating inﬂu-ences the large-scale ocean circulation. Joyce et al. [1986]used a scaling argument to suggest that the weak back-ground geothermal heating is insucient to aect the vortic-ity balance of the abyssal circulation. In a companion study,Scott et al. [2000] examine the response of an idealized sec-tor ocean model to geothermal heating. Here we examinePaper number 1999GL000000.0094-8276/01/1999GL000000$05.00the response of a global circulation model to a simple rep-resentation of the geothermal heat ﬂux. In both studies wend substantial modication of the meridional overturningcirculation.2. Numerical model and experimentsWe compare two realizations of a numerical global cir-culation model which are congured and forced in identicalways except that the perturbation experiment has a steadyand uniform heat ﬂux of 50 mW m−2 applied at the oceanbottom, independent of depth. By examining the dierencebetween these two runs (perturbation - control) we establishhow the geothermal heat ﬂux can modify the ocean circula-tion.The model [Marshall et al., 1997] has 15 levels, with anupper layer thickness of 50 m, and a uniform horizontal res-olution of 2.8 in both latitude and longitude (128  64grid points). The topography is realistic at this resolu-tion except for the exclusion of the Arctic. In all otherrespects the model is standard (congured as in Danaba-soglu and McWilliams [1995]) including the use of the Gentand McWilliams parameterization [Gent and McWilliams,1990] with a thickness diusion coecient of 1000 m2s−1and forced by observed monthly averaged ﬂuxes and restor-ing to monthly averaged Levitus data. We use asynchronoustime steps of 1 day and 1 hour for the thermodynamic andthe momentum equations, respectively. The model is inte-grated for 3000 years of thermodynamic time, at which pointthe deep ocean has equilibrated with the surface boundaryconditions.The resulting ocean state is similar to that obtained inother models at this resolution, with some typical decien-cies; the Antarctic Circumpolar Current (ACC) transport isweak, 95 Sv (1 Sv = 106 m3 s−1), and deep water propertiesare too cool (1{2C). However, since we are examining thedierence between perturbation and control experiments weassume that our results are not dependent on these aspects.The global meridional overturning and peak northward heattransports are 22 Sv and 1.5 PW (1 PW = 1015 Watts), re-spectively.3. ResultsBefore presenting our model results, we consider a purelydiusive response in a hypothetical ﬂat bottom ocean; thisresponse represents an extreme limit where advection playsno role. If we add 50 mW m−2 at the bottom and diuse theheat vertically with a diusivity of 5  10−5 m2 s−1,t h e nover a 5 km water column we obtain a temperature anomaly12 ADCROFT ET AL.: IMPACT OF GEOTHERMAL HEATING ON OCEAN CIRCULATION0 0.1 0.2 0.3 0.4 0.5−5000−4000−3000−2000−10000Temperature difference (oK)Depth (m)Global         Atlantic       Indo−Pacific   ACC            Diffusive LimitFigure 1. Horizontal mean temperature dierence betweengeothermal and control runs. Shown are proles for the globalaverage, Atlantic basin and Indo-Pacic basins and the SouthernOcean (ACC). Also plotted, for comparison, is the theoreticalprole in a purely diusive limit.that is linear with depth, approximately zero at the surfaceand +1.2C at the bottom. In the context of deep watermass properties, this represents a substantial perturbation.Fig. 1 shows the global horizontal mean temperature per-turbation (solid black) obtained with the numerical model.The prole is close to linear, except for a slight minimumin the upper 300 m, and reaches a value of almost 0.3Ca t5000 m depth. This is only a quarter of the prediction fromthe diusive limit (dotted curve, shown in part). Hence,the more signicant portion of the heat transfer is accom-plished through advection. There are regional variations inthe perturbation temperature which we have measured bybreaking the horizontal average into three non-overlappingregions: the Atlantic basin, north of the Cape of Good Hope,the Indo-Pacic basins, north of Cape Horn, and the South-ern Ocean (the remaining model). The Indo-Pacic prole(red) has the largest perturbation, almost 0.4Ca tt h eb o t -tom, but is still far from the diusive limit. The Atlantic(blue) has the most vertical structure, a narrow 1000 mband of high gradient accounting for most of the bottomtemperature anomaly. The Southern Ocean (green) has theweakest perturbation with the most linear prole. These re-−80 −60 −40 −20 0 20 40 60 8000.20.40.6LatitudeZonal average Qsurf    (W m−2)Figure 2. Zonally averaged surface ﬂux anomaly. The globalaverage is 50 mW m−2, necessary to balance the geothermal heat-ing.50 100 150 200 250 300 350−80−60−40−20020406080LongitudeLatitudeFigure 3. Dierence map of potential temperature, ,a tadepth of 3010 m. Contour interval is 0.02C. Peak value is NorthPacic in 0.4C.gional dierences indicate that the circulation within thesebasins/regions responds dierently, much as the regions playvery dierent roles in the ocean circulation in general.All the proles in Fig. 1 appear to approach zero at thesurface. In our model the global mean SST anomaly is deter-mined by the air{sea exchange coecient (32 W m−2 K−1),which balanced with the anomalous heat ﬂux leads to a1.5  10−3 C warming. Fig. 2 shows the zonally averagedanomalous surface ﬂux associated with the SST anomaly.Despite the uniformity of the applied geothermal heating,the majority of anomalous surface heat loss occurs in theSouthern Ocean.A horizontal map of temperature anomaly at a depthof 3010 m is shown in Fig. 3. The Pacic basin is clearlywarmed the most with a peak of 0.5Ca n dt h ee n t i r eo c e a nat this depth is generally warmer ( 0.1 C). There is, how-ever, a region in the tropical Atlantic where the geothermalrun is slightly cooler than the control. Later we will see thatthis is due to increased upwelling of cold water from depth.Fig. 4 shows the anomalous meridional overturning cir-culation (MOC) for a) the globe, b) the Atlantic and c) theIndo-Pacic. The global anomalous MOC is dominated bythe Indo-Pacic which shows a cross-equatorial transport of1.8 Sv (1 Sv entering the Pacic across 35S), with south-ward ﬂow above 3500 m and northward ﬂow below. An over-turning circulation acting in the presence of a large back-ground temperature stratication transports heat meridion-ally. Here, deep anomalous ﬂow transports cold water north-ward and anomalous mid-depth ﬂow transports warmer wa-ter southward thus achieving an anomalous southward heattransport necessary to match the surface heat ﬂux (Fig. 2).The perturbation experiment must also transport heatvertically to connect the geothermal source to the surface.Fig. 5 shows vertical proles of horizontally averaged verti-cal heat ﬂux for a) the Indo-Pacic and Atlantic and b) theSouthern Ocean. The contribution of the large scale circu-lation acting on the anomalous temperature dierence (redcurve) is upward in the Northern hemisphere; the upwellingIndo-Pacic water is anomalously warmed more than waterat the same depth in the Atlantic. However, the contribu-tion due to anomalous advection of a large-scale tempera-ture distribution is downward (green curve) in the Northernhemisphere; this is due to the anomalous upwelling in thePacic acting on the climatological temperature dierencebetween Pacic and Atlantic. The net vertical heat trans-port is near zero in the upper layers (above 2000m) anddownward in the deep layers.ADCROFT ET AL.: IMPACT OF GEOTHERMAL HEATING ON OCEAN CIRCULATION 3The Southern Ocean is the primary region of upwardanomalous heat transport (Fig. 5b) with an average netﬂux of around 200 mW m−2, or four times larger than thegeothermal heat ﬂux. At depth (below 3000m), the anoma-lous circulation (green curve) accounts for most of the ver-tical heat transport. Despite the vigorous wind-driven over-turning in the upper layers (above 2000m), the majority ofvertical heat transport is due to parameterized ﬂuxes; thestrongly sloping isopycnals associated with the ACC allowthe geostrophic eddy parameterization to eciently trans-port anomalous heat vertically.The overall picture, then, is one of net anomalous heatgain in the northern hemisphere due to little heat lossthrough the sea surface. A vigorous anomalous meridionaloverturning carries heat southward to the Southern Oceanwhere parameterized geostrophic eddies eciently transportanomalous heat upward to the surface.4. DiscussionWe have shown that the addition of a uniform geother-mal heat ﬂux of 50 mW m−2 can lead to deep basin-scaletemperature anomalies as large as 0.5C in the Indo-Pacicand on average about 0.3C. To explain this response weconsider the direct eect of the background circulation onabyssal water that has been anomalously warmed. Theproperties of the water in the deep western boundary cur-rent (DWBC) in the far North Atlantic are dominantly set−80 −60 −40 −20 0 20 40 60 80−5000−4000−3000−2000−10000Latitude (oN)Depth (m)0000000000.40.40.40.40.40.80.80.80.8 1.21.21.21.2 1.61.6 2 20000 0000 a)−80 −60 −40 −20 0 20 40 60 80−5000−4000−3000−2000−10000Latitude (oN)Depth (m)−0.2−0.2−0.20.20.20.40.4 0.80000 000000 0 b)−80 −60 −40 −20 0 20 40 60 80−5000−4000−3000−2000−10000Latitude (oN)Depth (m)0.20.20.20.20.40.40.40.4 0.40.80.80.8 1.21.21.61.60 0000 c)Figure 4. Dierence in meridional overturning stream functionfor a) the globe, b) the Atlantic and c) the Indo-Pacic. Contourinterval is 0.2 Sv. Positive numbers indicate clockwise circulationanomaly.−100 −50 0 50−5000−4500−4000−3500−3000−2500−2000−1500−1000−5000Indo−Pacific and Atlanticw¢T_NetSGSwT¢_Vertical heat flux  (mW m−2)Depth (m)a)−100 0 100 200 300−5000−4500−4000−3500−3000−2500−2000−1500−1000−5000Southern OceanwT¢_SGSw¢T_NetVertical heat flux  (mW m−2)Depth (m)b)Figure 5. Horizontally averaged vertical ﬂux anomaly for a) theIndo-Pacic and Atlantic and b) the Southern Ocean. In blackis the net anomalous ﬂux, in red is the component due to largescale advection of the temperature anomaly, wT0,i ng r e e ni st h ecomponent due to anomalous advection, w0T, and in blue is thecomponent due to sub-grid scale parameterizations (SGS).by surface boundary conditions. Water temperature rises asit passes near the bottom but the strong circulation leadsto a relatively short residence time, essentially \ﬂushing"the geothermally heated water out of the Atlantic at thedepth of the deep western boundary current. In the Pacic,the deep inﬂow carries abyssal water that has already beenanomalously warmed and hence has a warmer starting pointbefore its more sluggish transit northward. This leads to anenhanced warming of the abyssal Pacic with respect to theother basins and also explains the vertical structure in theAtlantic (Fig. 1).We also showed a strong meridional circulation anomalyin the Indo-Pacic basin essentially in the same sense as theexisting circulation; that is, deep inﬂow and shallow out-ﬂow. This 1.8 Sv of transport constitutes a 25% increasein the Pacic overturning. It is not immediately obviouswhy the Pacic should respond so strongly, though we mightspeculate that it is connected to the sense of the thermoha-line circulation; high latitude sinking in the North Atlanticand Antarctic and broad upwelling in the Pacic. The rela-tively sluggish northward ﬂow in the deep Pacic allows thegeothermal heat ﬂux more time to act on water parcels. TheAtlantic overturning has a much weaker and less coherentresponse. The North Atlantic Deep Water overturning cellis not inﬂuenced much by the geothermal heating partly dueto the \ﬂushing" mentioned above but also since it is gener-ally disconnected from the bottom. The Antarctic BottomWater cell is in contact with the bottom in all the basinsand is therefore most inﬂuenced by the geothermal heating.In the Southern Ocean, we noted a transition in mech-anism for vertical heat transport. In contrast to the otheroceans, the Southern Ocean is characterized by relativelylarge meridional gradients of temperature by virtue of wind-driven zonal channel dynamics. The combination of themeridional temperature gradients and the anomalous MOCresults in particularly ecient vertical heat transport. Since4 ADCROFT ET AL.: IMPACT OF GEOTHERMAL HEATING ON OCEAN CIRCULATIONthis anomalous circulation does not reach the surface someother mechanism must achieve the vertical transport ofanomalous heat in the upper water column. The wind-driven overturning in the ACC is associated with some ofthe strongest large-scale upwelling but in the absence ofmeridional gradients of anomalous temperature this circula-tion transports relatively little heat vertically. Instead, thegeostrophic eddy parameterization [Gent and McWilliams,1990] is able to eciently ﬂux anomalous heat upwards alongthe strongly tilted isopycnals.The response that we describe is much larger than ex-pected from naive comparisons of surface and geothermalheat ﬂuxes, because heating a ﬂuid from below is much moreecient in creating circulation than heating and cooling atthe same level [Huang, 1999]. The changes in deep circula-tion have little impact on total meridional heat transport,but they might well inﬂuence the transports of carbon andother properties that have concentration maxima at depth.Changes in deep carbon and nutrient transports might leadto signicant changes in outgassing rates and potentially inatmospheric CO2 concentrations. Currently, model errorsin deep temperature with respect to observations are proba-bly larger than the changes reported here; nevertheless, theresponse in deep temperature is large compared to spatialand temporal variability. Moreover, the modications in cir-culation change the temperature response qualitatively. Wehave used a uniform geothermal heat ﬂux of 50 mW m−2corresponding to the average ﬂux away from ridges. Inho-mogeneities in geothermal heat ﬂux (for example, due to fastsea-ﬂoor spreading in the East Pacic) and enhanced ﬂuxesalong ridges could presumably lead to even stronger localresponses.Acknowledgments. This work was supported by theFord, General Motors, and Daimler-ChryslerCorporations throughthe MIT Climate Modeling Initiative (Adcroft), the MIT JointProgram on the Science and Policy of Global Change (Scott),the U.S. Department of Energy's Oce of Biological and En-vironmental Research through Grant No. DE-FG02-93ER61677(Scott) and NSF grant 9810800 (Marotzke). We would like tothank Bill Young, Carl Wunsch, John Marshall, Xin Huang andBram Murton for many helpful discussions and comments on thework.ReferencesMunk, W., and Wunsch, C., Abyssal recipes II: energetics of tidaland wind mixing, DSR, 45(12), 1977{2010, 1998.Sclater, J. G., Jaupart, C., and Galson, D., The heat ﬂow throughoceanic and continental crust and the heat loss of the earth,Rev. of Geophysics and Space Physics, 18(1), 269{311, 1980.Kadko, B. A., and Baross, J., The magnitude and global implica-tions of hydrothermal ﬂux, in Seaﬂoor Hydrothermal Systems:Physical, Chemical, Biological and Geological Interactions, 91,446{466, 1995.Stein, C. A., Stein, S., and Pelayo, A. M., Heat ﬂow and hy-drothermal circulation, in Seaﬂoor Hydrothermal Systems:Physical, Chemical, Biological and Geological Interactions, 91,425{445, 1995.Murton, B. J., Redbourn, L. J., German, C. R., and Baker, E. T,Sources and ﬂuxes of hydrothermal heat, chemicals and biologywithin a segment of the mid-atlantic ridge, Earth Planet. Sci.Lett., 171, 301{317, 1999.Joyce, T. M., and Speer, K. G, Modeling the large-scale inﬂuenceof geothermal source on abyssal ﬂow, JGR, 92(C3), 2843{2850,1987.Hautala, S. L., and Riser, S. C., A simple model of abyssal cir-culation, including eects of wind, buoyancy and topograhpy,JPO, 19(6), 596{611, 1989.Speer, K. G., The Stommel and Arons model and geothermalheating in the south pacic, Earth and Planet. Sci. Lett., 95,359{366, 1989.Helfrich, K. R., and Speer, K. G., Oceanic hydrothermal circula-tion: Mesoscale and basin-scale ﬂow, in Seaﬂoor HydrothermalSystems: Physical, Chemical, Biological and Geological Inter-actions, 91, 347{356, 1995.Thompson, L., and Johnson, G., Abyssal currents generated bydiusion and geothermal heating over rises, DSR, 43(2), 193{211, 1996.Joyce, T. M., Warren, B. A., and Talley, L. D., The geothermalheating of the abyssal subartic pacic ocean, DSR, 33(8),1003{1015, 1986.Scott, J., Marotzke, J., and Adcroft, A., Geothermal heating andits inﬂuence on the meridional overturning circulation, sub-mitted to JGR.Marshall, J., Adcroft, A., Hill, C., Perelman, L., and Heisey, C.,A nite-volume, incompressible navier stokes model for studiesof the ocean on parallel computers, JGR, 102(C3), 5753{5766,1997.Danabasoglu, G., and McWilliams, J. C., Sensitivity of theglobal ocean circulation to parameterizations of mesoscaletracer transports, JC, 8(12), 2967{2987, 1995.Gent, P., and McWilliams, J. C., Isopycnal mixing in oceancirculation models, JPO, 20(1) 150{155, 1990.Huang, R., Mixing and energetics of the thermohaline circulation,JPO, 29(4) 727{746, 1999.A. Adcroft and J. Scott, Massachusetts Institute of Technol-ogy, 77 Massachusetts Avenue, Cambridge MA 02139. (email: ad-croft@mit.edu; jscott@mit.edu)J. Marotzke, Southampton Oceanography Centre, Universityof Southampton, Southampton, UK. (e-mail:Jochem.Marotzke@soc.soton.ac.uk)(Received August 7, 2000; revised January 24, 2001;accepted February 6, 2001.)

A  incompressible navier stokes model for studies of the ocean on parallel computers,

A simple model of abyssal circulation, including eects of wind, buoyancy and topograhpy,

Abyssal currents generated by diusion and geothermal heating over rises,

Abyssal recipes II: energetics of tidal and wind mixing,

Heat ﬂow and hydrothermal circulation,

Isopycnal mixing in ocean circulation models,

Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge MA 02139. (email: adcroft@mit.edu; jscott@mit.edu)

Mixing and energetics of the thermohaline circulation,

Modeling the large-scale inﬂuence of geothermal source on abyssal ﬂow,

Oceanic hydrothermal circulation: Mesoscale and basin-scale ﬂow,

Sensitivity of the global ocean circulation to parameterizations of mesoscale tracer transports,

Sources and ﬂuxes of hydrothermal heat, chemicals and biology within a segment of the mid-atlantic ridge,

The geothermal heating of the abyssal subartic paci ocean,

The heat ﬂow through oceanic and continental crust and the heat loss of the earth,

The magnitude and global implications of hydrothermal ﬂux,

The Stommel and Arons model and geothermal heating in the south paci

Impact of geothermal heating on the global ocean circulation

The response of a global circulation model to a uniform geothermal heat flux of 50 mW m(-2) through the sea floor is examined. If the geothermal heat input were transported upward purely by diffusion, the deep ocean would warm by 1.2 degreesC. However, geothermal heating induces a substantial change in the deep circulation which is larger than previously assumed and subsequently the warming of the deep ocean is only a quarter of that suggested by the diffusive limit. The numerical ocean model responds most strongly in the Indo-Pacific with an increase in meridional overturning of 1.8 Sv, enhancing the existing overturning by approximately; 25%

Adcroft, A.

Scott, J.

Marotzke, J. 

MPG.PuRe

http://hdl.handle.net/11858/00-001M-0000-0014-189A-5

Impact of geothermal heating on the global ocean circulation

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