Hofmann, M.

Morales Maqueda, M. A.

English

NERC Open Research Archive

GEOPHYSICAL RESEARCH LETTERS, VOL. ???, XXXX, DOI:10.1029/,Geothermal Heat Flux and its Influence on the Oceanic AbyssalCirculation and Radiocarbon DistributionM. HofmannResearch Domain I, Potsdam Institute for Climate Impact Research, Potsdam, GermanyM. A. Morales MaquedaProudman Oceanographic Laboratory (Natural Environment Research Council), Liverpool, United KingdomGeothermal heating of abyssal waters is rarely regardedas a significant driver of the large-scale oceanic circulation.Numerical experiments with the Ocean General CirculationModel POTSMOM-1.0 suggest, however, that the impact ofgeothermal heat flux on deep ocean circulation is not negligi-ble. Geothermal heating contributes to an overall warmingof bottom waters by about 0.4 ◦C, decreasing the stabilityof the water column and enhancing the formation rates ofNorth Atlantic Deep Water and Antarctic Bottom Water by1.5 Sv (10% ) and 3 Sv (33% ), respectively. Increased influxof Antarctic Bottom Water leads to a radiocarbon enrich-ment of Pacific Ocean waters, increasing ∆14C values in thedeep North Pacific from -269◦/◦◦ when geothermal heatingis ignored in the model, to -242◦/◦◦ when geothermal heat-ing is included. A stronger and deeper Atlantic meridionaloverturning cell causes warming of the North Atlantic deepwestern boundary current by up to 1.5 ◦C.1. IntroductionGeothermal heating through the seafloor adds only a verysmall contribution to the oceanic energy budget. The aver-age geothermal heat flux currently entering the oceans isestimated to be between 93 to 101 mW m−2 (1 mW = 10−3W) (Pollack et al. [1993]). In comparison, sea surface turbu-lent and radiative heat fluxes can be hundreds to thousandsof times larger (Huang [1999]). Geothermal heating is also aseemingly lesser player from the point of view of the oceanicmechanical energy budget. Of the 2 TW (1 TW = 1012W) of mechanical energy injected into the ocean by windsand tides, approximately 0.4 TW are directly available forturbulent mixing, that increases the available potential en-ergy of the ocean and partly fuels the Meridional Overturn-ing Circulation, or MOC (Huang [1999]; Kuhlbrodt et al.[2007]). In comparison, Huang [1999, 2002] found that thepotential energy geothermal heating can generate is almost10 times smaller: 0.05 TW. In spite of the small energiesassociated with geothermal heating, geothermal heat fluxesmight have a measurable effect on the large-scale circulationand tracer distribution (Adkins et al. [2005]; Huang and Jin[2002];Mullarney et al. [2006]; Roussenov et al. [2004]). Thereason for this is that, unlike surface heat fluxes, geothermalfluxes, however small, are unidirectional, always contribut-ing towards increasing the buoyancy of the deep ocean. Inaddition, geothermal heat sources are situated at the bot-tom, while buoyancy losses occur at the surface, a config-uration which, according to Sandstro¨m’s Theorem (Huang[1999]), supports a closed MOC.Copyright 2008 by the American Geophysical Union.0094-8276/08/$5.00Numerical studies with ocean circulation models of vari-ous complexities have found that oceanic geothermal heatingcan create bottom waters several tenths of a degree warmerthan would exist in its absence, and help maintain a vigor-ous abyssal circulation (Mullarney et al. [2006]; Huang andJin [2002]; Adcroft et al. [2001]). It has also been arguedby Roussenov et al. [2004] that, since oceanic concentra-tions of natural radiocarbon, notably in the Pacific Basin,are strongly controlled by the export rate of Antarctic Bot-tom Water (AABW), geothermal heating should affect theglobal distribution of ∆14C.Adcroft et al. [2001] investigates the impact of geother-mal heating on the simulated MOC of a global ocean gen-eral circulation model (OGCM) with realistic topography.They found a 25 % increase in the amount of AABW thatenters the Indo-Pacific Ocean upon application of a uniformgeothermal heat flux of 50 mW m−2. In their study, thedeep North Pacific region experiences the most pronouncedwarming, with a temperature rise of up to 0.4 ◦C . The studyof Adcroft et al. [2001] used a spatially uniform geothermalheating source. However, geothermal heat fluxes vary con-siderably in space. The global compilation of geothermalheat fluxes of Pollack et al. [1993] shows that maximumvalues of up to 325 mW m−2 occur along the geologicallyyoung crests of the ocean ridges (Fig. 1(a)) and that, as theoceanic crust ages away from the ridges, geothermal heatfluxes decline to a background value of about 50 mW m−2.According to this data set, the global mean of the geother-mal heat flux through the seabed is 101 mW m−2, which isabout twice the value used by Adcroft et al. [2001].In this article, we revisit the problem of assessing theimpact that geothermal heating has on the MOC and theoceanic distribution of ∆14C. We use a global ocean climatemodel and compare results from experiments with and with-out geothermal heat fluxes.These results of our study mainly confirm the findingsmade by Adcroft et al. [2001], reach, however, beyond them.Besides the circulation and temperature changes we obtain,which tend to be larger than theirs, we find an intensificationof the Atlantic MOC by 10 % accompanied by a deepeningof the NADW cell by up to 500 meters and a warming of thedeep North Atlantic western boundary current by up to 1.5◦C . The most striking feature in our simulations is a cool-ing of the deep Weddell Sea by up to 0.3◦C caused by thegeothermally enhanced convection in the Southern Ocean.2. Model and experimental designThe model is an ocean-sea ice general circulation modelcoupled to an anomaly model of the atmospheric energy-moisture balance (Hofmann and Maqueda [2006]). Theocean model is the Modular Ocean Model, version 3.0, fea-turing a number of modifications outlined in Hofmann andMaqueda [2006]. Henceforth, this model will be referred toas the Potsdam version of MOM, version 1.0 (POTSMOM-1.0). The horizontal model grid has a uniform resolution of3◦×2◦ . In the vertical, the grid consists of 29 levels with1X - 2 HOFMANN AND MORALES MAQUEDA: GEOTHERMAL FLUXES AND OCEAN CIRCULATIONFigure 1. (a) Geothermal heat fluxes of Pollack et al.[1993] (in mW m−2) and (b) Temperature difference at3500 m between experiments GEOVAR and CNTRL av-eraged over the last 200 years of integration (in ◦C).thickness increasing from 10 m at top to 405 m at the bot-tom. The model includes an empirical parameterization ofbottom-enhanced vertical mixing (Hasumi and Suginohara[1999]), with a low background diffusivity of 0.1 cm2 s−1.Isopycnic Redi stirring is set to 1.25 × 107 cm2 s−1, whileGent-McWilliams thickness diffusivities vary between 0.275and 0.55 107 cm2 s−1. The ocean model tracers are poten-tial temperature, salinity and radiocarbon (Toggweiler et al.[1989]).Three 5000-year experiments were run with this model.First, the control run (CNTRL) ignores geothermal heating.In the second run (GEOVAR), the non-uniform geothermalheat distribution of Pollack et al. [1993] (GEOVAR), shownin Fig. 1(a), was used. To test the effect of a homogeneousforcing a third experiment (GEOCONST) was conductedwith a spatially uniform geothermal heat flux of 101.0 mWm−2, the global average of the geothermal flux in GEOVAR.Results from the last 200 years of integration are presentedhere.3. ResultsIn accord with the results of Adcroft et al. [2001] and theidealized experiments of Scott et al. [2001], our simulationsshow that, on global average, geothermal heating leads toa warming of waters below the main thermocline by a fewtenths of degree Celsius (0.26 ◦C for GEOVAR and 0.22 ◦Cfor GEOCONST). Parts of the cold newly formed AABWleave the deep Antarctic Circumpolar Current (ACC), movenorthward and enter the Atlantic and Indo-Pacific Basinsat about 40◦ S. As this bottom water masses flow furthernorth, they gradually gain heat from the geothermal sourcesat the sea floor.At a depth of 3500 m, the warming in the Pacific Oceanand in the eastern Atlantic Ocean simulated by GEOVARamounts to 0.3 to 0.4 degrees (see Fig. 1(b)). Far away fromthe strong geothermal sources of the mid-ocean ridges, thewarming patterns calculated in experiment GEOCONSTare very similar to those of GEOVAR. This is explainedby the fact that geothermal warming is an integrative pro-cess: whether it occurs progressively, as a result of a uniformheat flux (GEOCONST), or discontinuously, as waters movethroughout hot and cold spots (GEOVAR) is immaterial, aslong as the total heat absorbed is the same in both cases.Results from GEOVAR and GEOCONST qualitativelydepart from those discussed in Adcroft et al. [2001] in tworemarkable ways. Firstly, the deep Southern Ocean, withthe exception of the Amundsen and Bellingshausen Seas,experiences a cooling of up to −0.3 ◦C. Secondly, the deepwestern boundary current in the North Atlantic warms bybetween 0.9 ◦C and 1.5 ◦C. Salinity changes of about −0.1Figure 2. (a) Observed natural ∆14C (in ◦/◦◦ ) at150◦ W from the Global Ocean Data Analysis Project,GLODAP (Key et al. [2004]). (b) Color: as in (a) butsimulated in CNTRL. Contours: 200-year mean of Indo-Pacific meridional overturning transport north of 34 S (inSv) in CNTRL. The contour interval is 2 Sv. (c) As in(b) but simulated in GEOVAR.HOFMANN AND MORALES MAQUEDA: GEOTHERMAL FLUXES AND OCEAN CIRCULATION X - 3psu in the Southern Ocean and +0.1 psu in the North At-lantic accompany these temperature changes (not shown).The seemingly paradoxical cooling of the deep SouthernOcean in GEOVAR and GEOCONST is caused by an inten-sification of open ocean deep convection in these simulations.In all three model experiments, the occurrence of open oceandeep convection in the Southern Ocean is a prevailing fea-ture. The main area of convection, with winter mixed layersoften stretching down to the seafloor, sits astride the easternlimb of the Weddell Sea Gyre, and is centered around theGreenwich Meridian at 65◦S. Smaller convection areas occurover the Cosmonaut Sea and the western Ross Sea, off CapeAdare. The convective activity in the Weddell Sea exhibitsa striking inter-decadal variability. Experiment CNTRL re-peatedly reveals the occurrence of open deep convection cov-ering a maximum area of about 6×105 km2 and lasting forabout 20 years followed by a convection free period for about30 years. In case of GEOCONST and GEOVAR the area ofthis intermittent deep convection almost triples, while thetime period of the convective activity increases by a factorof two (not shown). We do not yet fully understand thefactors that control the frequency and intensity of the in-termittent convection events, but, clearly, the more intensedeep convection in GEOVAR and GEOCONST gives rise tohigher rates of formation and export of AABW (Fig. 2 b andc). Convection episodes are associated with strong sea icemelt and the formation of embayments in the sea-ice coverthat, in the Weddell Sea, occasionally close into a polynya,a feature also captured in the modeling study of Goosseand Fichefet [2001], although their polynya reappeared ev-ery winter. While the formation of recurrent polynyas inthe Weddell and Cosmonaut Seas is well documented (e.g.,Figure 3. (a) Color fill of 200-year mean of zonally av-eraged σ2 (in kg m−3) and contours of 200-year mean ofmeridional overturning transport (in Sv) in the AtlanticOcean north of 34 S for CNTRL. (b) As in (a), but dif-ference between GEOVAR and CNTRL.Moore et al. [2002], Geddes and Moore [2007]), the decadalcyclicity of the simulated polynyas and associated convec-tion episodes is probably unrealistic, however, can not beentirely excluded because of the paucity of observationaldata.The impact of geothermal heating on AABW circulationis most prominent in the Indo-Pacific Ocean. In CNTRL,the northbound flow of AABW at 30◦S is 10 Sv, while inGEOVAR and GEOCONST this flow increases by about 40% thus leading to a more vigorous ventilation of the deepNorth Pacific compared to CNTRL (Fig. 2 b and c). Thisresult is in qualitative agreement with Adcroft et al. [2001],who, with a mean geothermal heat flux equal to roughly halfours, reported an increase of 25% in deep AABW flow intothe Indo-Pacific. A larger export of AABW is expected todecrease the age of North Pacific waters, and this is borneout by the fact that the deep ∆14C minimum of -269 ◦/◦◦in CNTRL, increases in GEOVAR to -242 ◦/◦◦(Fig. 2 band c), a value that compares well with recent observationaldata Key et al. [2004] (Fig. 2 a).The warming of the deep western boundary currentshown in Fig. 1(b) is the manifestation of a deepening ofthe North Atlantic Deep Water (NADW) overturning cellby 300 to 500 m, which also causes an increase in salinityand in ∆14C (by up to 14 ◦/◦◦ not shown). This deepen-ing results from the increased contrast between the densityof newly formed NADW and that of the AABW reachingthe North Atlantic, which is as much as 0.1 kg m−3 lighterin GEOVAR than in CNTRL (Fig. 3b). Ultimately, up-welling of this lighter water in the Southern Ocean creates astronger meridional density contrast in GEOVAR comparedto CNTRL, and a concomitant increase of 1.5 Sv (10 % )in the strengths of the Atlantic MOC maximum and 30◦Soutflow.4. ConclusionsNumerical experiments with a coarse resolution ocean-sea ice model indicate that geothermal heat fluxes are anon negligable forcing of the ocean circulation, substan-tially strengthening the AABW and NADW overturningcells. Geothermal heating stimulates convective activity inthe Southern Ocean and, consequently, increases the rate offormation and export of AABW by 33 % . While the re-current open ocean convection that produces most of theAABW in the model has rarely been observed, there is in-direct evidence that sporadic thermobaric convection couldindeed substantially contribute to the formation of AABW(McPhee [2003]). This enhanced production of AABW leadsto a better representation of the distribution of natural∆14C. In a recent study, Matsumoto and Key [2004] pointedout that, in ocean models with vertical diffusivities as lowas observed, such as ours, the deep ocean will become lessventilated, and natural ∆14C values will thus be underesti-mated. Here we have shown that the inclusion of geothermalheat fluxes can partly correct this pathological behavior. Wehave also reason to believe that the simulation of CFCs (e.g.Broecker et al. [1999]), and maybe other tracers, will as wellbe improved thanks to the enhanced ocean ventilation af-forded by the inclusion of geothermal heat fluxes. Geother-mal heat fluxes also intensify by around 10 % both the max-imum of the Atlantic MOC and the outflow of NADW intothe Southern Ocean, while the penetration depth of NADWin the North Atlantic is increased from 2500 m to a morerealistic 3000 m.Acknowledgments. M.H. was funded by the Garry Comerfoundation. We are grateful to E. Bauer and R. Calov for con-structive comments on the manuscript.X - 4 HOFMANN AND MORALES MAQUEDA: GEOTHERMAL FLUXES AND OCEAN CIRCULATIONReferencesAdcroft, A., J. R. Scott, and J. Marotzke (2001), Impact ofgeothermal heating on the global ocean circulation, Geo-phys. Res. Lett., 28, 1735 – 1738.Broecker, W., S. Sutherland, and T. Peng (1999), A possible 20thcentury slowdown of Southern Ocean deep water formation,Science, 286, 1132 – 1135.Adkins, J., A. P. Ingersoll, and C. Pasquero (2005), Rapid cli-mate change and conditional instability of the glacial deepocean from the thermobaric effect and geothermal heating,Quat. Sci. Rev., 24, 581 – 594.Geddes, J. A., and G. W. K. Moore (2007), A climatology ofsea ice embayments in the Cosmonaut Sea, antarctica, J. Geo-phys. Res., 34, L02505, doi:10.1029/2006GL027910.Goosse, H., and T. Fichefet (2001), Open-ocean convection andpolynia formation in a large-scale ice-ocean model, Tellus, Ser.A, 53, 94 – 111.Griffies, S. M. 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Hughes (2006),The effect of geothermal heating on the ocean overturn-ing circulation, Geophys. Res. Lett., 33, L02607, doi:10.1029/2005GL024956.Pollack, H. N., S. J. Hurter, and J. R. Johnson (1993), Heatflow from the earth’s interior: Analysis of the global data set,Rev. Geophys., 31, 267 – 280.Roussenov, V., R. G. Williams, M. J. Follows, and R. M. Key(2004), Role of bottom water transport and diapycnic mix-ing in determining the radiocarbon distribution in the pacific,J. Geophys. Res., 109, C06015, doi:10.1029/2003JC002188.Scott, J. R., J. Marotzke, and A. Adcroft (2001), Geothermalheating and its influence on the meridional overturning circu-lation, J. Geophys. Res., 106, 31,141 – 31,154.Toggweiler, J., K. Dixon, and K. Bryan (1989), Simulation of ra-diocarbon in a coarse-resolution world ocean model 1. steadystate prebomb distributions, Rev. Geophys., 18, 8217 – 8242.M. Hofmann, Research Domain I, Potsdam Institute for Cli-mate Impact Research, Telegrafenberg A26, 14473 Potsdam, Ger-many. (hofmann@pik-potsdam.de)M. A. Morales Maqueda, Proudman Oceanographic Labora-tory, 6 Brownlow Street, Liverpool L3 5DA, United Kingdom.(mamm@pol.ac.uk)

Geothermal Heat Flux and its Influence on the Oceanic Abyssal

Circulation and Radiocarbon Distribution

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Geothermal Heat Flux and its Influence on the Oceanic Abyssal Circulation and Radiocarbon Distribution

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