Ocen ling 39 (11) 17 18 Contents lists ville t ScienceDirect Ocen ling journl homepge: www.elsevier.com/locte/ocemod Testing n eddy-permitting model of the Southern Ocen cron cycle ginst oservtions M. Woloszyn, M. Mzloff, T. Ito, Deprtment of Atmospheric Science, Colordo Stte University, 1371 Cmpus Delivery, Fort Collins, CO 853-1371, USA Scripps Institution of Ocenogrphy, University of Cliforni Sn Diego, 95 Gilmn Drive, L Joll, CA 993-3, USA rticle info strct Article history: Aville online 3 Decemer 1 Keywords: Southern Ocen Cron cycle Eddy-permitting modeling Air-se CO flux Mesoscle processes ply fundmentl roles in the dynmics nd iogeochemistry of the Southern Ocen. Previous modeling studies with corse resolution models show vrying estimtes of cron uptke there due to the differences in the representtion of physicl circultion nd mesoscle processes. We develop new eddy-permitting cron cycle model of the Southern Ocen with improved representtion of eddies, jets, nd frontl structures. The model employs physicl circultion fields from Southern Ocen Stte Estimte: model solution tht is constrined y suite of remotely sensed nd in situ mesurements to ensure relistic hydrogrphic structures nd trnsport fields. A simple iogeochemistry model is coupled to the physicl model to simulte the cycling of lklinity, cron, nd nutrients. In this pper we compre simulted iogeochemicl trcers with oservtions, nd dignose mechnisms controlling the regionl cron cycle. -dt comprisons in Drke Pssge nd CLIVAR repet sections provide tests for the model performnce in cpturing oserved iogeochemicl properties. Temporl vriility of ir-se CO flux is primrily controlled y surfce pco on sesonl timescle, modulted y rpid fluctutions in surfce wind speed on shorter timescle. Anlysis of simulted cron trnsport indictes tht the zonl men circultion trnsports cron southwrd, prtly compensted y the northwrd sttionry nd trnsient eddy fluxes. Vriility of zonl men circultion, which is strongly correlted with Ekmn trnsport, domintes the temporl vriility of meridionl cron trnsport cross the Polr Front. Ó 1 Elsevier Ltd. All rights reserved. 1. Introduction Recent oservtionl nd modeling studies revel crucil roles of mesoscle processes in the trnsport of uoyncy nd iogeochemicl trcers in the Antrctic Circumpolr Current (herefter, ACC) (Rintoul et l., 1; Ito et l., 4; Olers et l., 4; Mrinov et l., 6). The ACC, consisting of multiple fronts, jets nd eddies, connects ll mjor sins, nd cts s trnsport rrier seprting reltively cold nd fresh Antrctic wters from wrm nd slty sutropicl wter msses. The cross-frontl trnsport is key prmeter controlling the glol strtifiction (Gnndesikn, 1999), ir-se uoyncy fluxes (Speer et l., ; Mrshll nd Rdko, 6), nd the uptke nd inventory of cron dioxide in the Southern Ocen (Mrinov et l., 6; Ito et l., 1). The meridionl uoyncy nd iogeochemicl trcer trnsport cn e mesured using residul men strem function, which includes the effects of oth eddy nd men flow dynmics (Mrshll, 1997; Speer et l., ; Krsten et l., ; Mrshll nd Rdko, 3; Olers nd Viseck, 5). Corresponding uthor. Tel.: +1 97 491 86; fx: +1 97 491 8693. E-mil ddress: ito@tmos.colostte.edu (T. Ito). Sensitivity of the ACC to chnging extrtropicl southern hemisphere climte hs een the focus of intense reserch. The tmosphere overlying the Southern Ocen is undergoing multidecdl climte chnge chrcterized y the positive trend in the Southern Annulr Mode, (Thompson et l., ; Mrshll, 3) which my e in prt driven y ozone depletion nd glol wrming (Thompson nd Solomon, ; Miller et l., 6). Stronger nd polewrd-shifted winds my enhnce Ekmn divergence nd the upwelling of deep wters in the polr Southern Ocen. However, the response of the residul men overturning circultion is uncertin due to incomplete understnding of the response of eddy dynmics to incresed wind stress (Hllerg nd Gnndesikn, 6; Meredith nd Hogg, 6; Boning et l., 8; Riccrdo et l., 1). The meridionl overturning of the Southern Ocen hs profound influence on the glol cron cycle. The deep wters upwelling in the Southern Ocen re highly enriched in dissolved inorgnic cron (herefter, DIC). Thus, enhnced upwelling in the region will likely relese cron dioxide into the tmosphere (Le Quéré et l., 7; Lovenduski et l., 8). On the other hnd, intensifiction of the overturning circultion my lso enhnce the uptke nd sequestrtion of cron dioxide in the northwrd flnk 1463-53/$ - see front mtter Ó 1 Elsevier Ltd. All rights reserved. doi:1.116/j.ocemod.1.1.4
M. Woloszyn et l. / Ocen ling 39 (11) 17 18 171 of the ACC where thermocline wters re formed (Lovenduski nd Ito, 9). The residul men overturning circultion plys crucil role in controlling the simulted ir-se fluxes of cron nd trnsient trcers (Ito et l., 4). Numericl ocen models hve een mjor tool to investigte the iogeochemistry nd cron cycle of the Southern Ocen. Significnt model disgreements hve een found, however, in the simulted uptke nd inventory of nthropogenic cron dioxide (Orr et l., 1; Doney et l., 4). Mesoscle eddies, with length scle of tens of kilometers, re not resolved in corse resolution ocen models nd must e prmeterized. Explicitly resolving eddies in ocen iogeochemistry simultions cn significntly lter the ehvior of iologicl processes in mny different regimes including costl ses nd oligotrophic gyres (McGillicuddy et l., 1998; Gruer et l., 11). The representtion of mesoscle eddies in ocen iogeochemistry models is mjor source of uncertinty in the current nd future estimte of ocenic cron uptke nd iogeochemicl trcers. Recent developments in numericl ocen models nd computtionl infrstructures re eginning to llow the explicit representtion of ocen eddies nd mesoscle fetures in ocen ecosystem nd iogeochemistry models. In this pper we descrie nd evlute recently developed eddy-permitting cron cycle model covering the entire circumpolr ocens south of 5 S t lterl resolution of sixth of degree (Ito et l., 1). This model is one of the first ttempts to simulte the cron cycle of the Southern Ocen t n eddy-permitting resolution. A gol of this study is to evlute the relism of the simulted cron cycle y directly compring the model to oservtions. A second gol is to ttriute the cuses of model-dt misfit y evluting the individul physicl nd iogeochemicl trcers nd their influence on simulted cron dioxide. Section descries the model rchitecture nd initil nd oundry conditions used for the simultion. In Section 3, we discuss the results of the model-dt comprison, nlysis of simulted cron trnsport, nd its sensitivity to tmospheric vriility. We discuss the implictions of the results nd conclude in Section 4.. Methods.1. descriptions.1.1. An eddy-permitting Southern Ocen cron cycle model Estimtes of three-dimensionl ocen circultion fields constrined y in situ nd remotely sensed oservtions hve ecome ville through the recent progress in ocen dt ssimiltion. We employ the circultion fields determined from the Southern Ocen Stte Estimte (SOSE) in which the time-dependent, threedimensionl circultion fields of the Southern Ocen re estimted t the eddy-permitting scle using 4DVAR method (Mzloff et l., 1). The model is sed on the Msschusetts Institute of Technology ocen generl circultion model (Mrshll et l., 1997,), covering the regionl domin etween 78 S nd 4 S with lterl resolution of one-sixth of degree nd 4 verticl levels. The model includes the KPP mixed lyer scheme (Lrge et l., 1994), ulk formul for ir-se momentum nd uoyncy fluxes, nd se ice model (Losch et l., 1). The model reproduces oserved hydrogrphic structures nd wter mss distriution s discussed in Section 3. Sesonl growth nd retret of se ice cover is in very close greement with oservtions. There re some regionl discrepncies in the se ice concentrtion t high ltitudes, including slight overestimtion in the South Indin nd Pcific regions, slight underestimtion in the South Atlntic, nd polyny formtion in the Weddell Se. The ocen cron cycle nd ir-se fluxes of CO re simulted in the offline mode using five-dy verged stte estimte fields nd simple iogeochemistry model (Njjr et l., 199) sed on the OCMIP scheme, in which iologicl cron uptke is prmeterized using the liner relxtion of ner surfce (top 75 m) phosphte to the monthly men climtology of the World Ocen Atls 5 (Grci et l., 6) when the model concentrtion is greter thn tht of climtology. The model trnsports five trcers including dissolved inorgnic cron (herefter, DIC), lklinity, phosphte, dissolved orgnic phosphte, nd oxygen. Constnt stoichiometric rtios re used for the formtion of orgnic mteril with phosphte, cron nd oxygen t 1:117: 17, nd constnt rin rtio of.7 is used for clcium cronte formtion. One third of cron uptke is llocted to the sinking prticulte pool which hs verticl dissolution profile given y simple power function. The model simultions re initilized in 1995 using initil conditions sed on oservtionlly-derived DIC, lklinity nd nutrient concentrtions, interpolted from the GLODAP dtset (Key et l., 4), nd then spun up for 1 yers to the end of 4. Initil model drifts diminished fter few yers of the spin-up integrtion. In this study we focus on the intr-nnul vriility over the -yer period from Jnury 5 to Decemer 6. These clcultions require mssive computtionl resources nd re mde possile thnks to Sn Diego Supercomputer Center s DtStr supercomputer nd NASA s Columi nd Pleides supercomputer..1.. Corse resolution model In order to evlute the impct of model resolution, the eddypermitting model is compred to nother simultion with corser resolution for which the Estimting the Climte nd Circultion of the Ocen (ECCO) product (version 3 itertion 73, provided y Dr. P. Heimch of MIT) is used insted of SOSE with the sme OCMIP scheme for the iogeochemicl component of the model. The model domin covers the glol ocen with lterl resolution of one degree in ltitude nd longitude nd with 3 verticl lyers. The model uses third-order upwind dvection scheme, nd the verticl trcer diffusivity is set to 1. 1 5 m s 1. Since the model does not explicitly resolve mesoscle eddies, su-grid scle processes re prmeterized using n isopycnl thickness diffusion scheme (Gent nd McWillims, 199). We use long-isopycnl thickness nd trcer diffusivity of 1. 1 3 m s 1 with the tpering scheme following (Dnsoglu nd McWillims, 1995). Initil guess of the tmospheric wind field is sed on the NCEP-1 renlysis product (Klny et l., 1996) for oth eddy-permitting nd corse resolution models. The model is first spun up using the climtologicl circultion fields nd pre-industril tmospheric cron dioxide concentrtion of 78 ppmv over yers to rech the sttisticl stedy stte. Then the model is further spun up using the historic tmospheric cron dioxide concentrtion from 1765 through 1991 where the model fields t the end of 1991 re used s the initil condition for the 17-yer long simultion from Jnury 199 through Decemer 8. The model fields re rchived in monthly verges for the susequent nlysis... Oservtionl dt The model performnce in simulting the regionl cron cycle is evluted y compring the model output to concurrent oservtions throughout the Southern Ocen. The model-dt comprison is performed in terms of surfce prtil pressure of cron dioxide (herefter, pco ) s well s the physicl nd iogeochemicl trcers (T, S, DIC, nd lklinity) in Drke Pssge nd in Climte Vriility nd Predictility (CLIVAR) hydrogrphic sections from the Atlntic nd Pcific sins...1. Drke Pssge oservtions The R/V Lurence M. Gould tkes underwy pco mesurements from Plmer Sttion to the South Americn ports cross
17 M. Woloszyn et l. / Ocen ling 39 (11) 17 18 the Drke Pssge severl times ech yer. Underwy pco mesurements re ville on sesonl sis during 5 nd 6, nd re provided y the LDEO cron dioxide group (Tkhshi et l., 9). Here, the model-dt comprison ws conducted sesonlly (we refer to the ustrl sesons) s well s nnully. We focus our nlysis in the open ocen from 56 Sto63 S nd exclude the highly vrile cruise trcks ner the costs long Antrctic nd South Americ. output nd oservtions were compred in the open ocen within these ltitudes nd the longitude rnge of 58 W 65 W. All cruises hve consistent ship-sed mesurements from 56 S to 63 S, ut the longitude t which smpling occurs vries. Two sections re considered for ech trnsect, one for ech of the northound nd southound cruise legs. To clculte men ises, the longitudinl pth of ech cruise ws tken into ccount: the model pco fields were mpped to the cruise trcks using nerest neighor interpoltion. The specific ltitudinl nd longitudinl positions of the trcks re importnt since surfce pco vries on such smll scles in the Drke Pssge.... Atlntic nd Pcific Ocen oservtions In order to compre model nd oservtionl dt in the lrge scle, two ocen trnsects from the CLIVAR repet hydrogrphy progrm re used: A16S nd P16S. The Atlntic cruise, A16S, occurred Jnury 11 Ferury 5, 5 nd covered ltitudes 4.5 S 6 S nd longitudes 5 W 36 W. The Pcific cruise, P16S, ws conducted Jnury 9 Ferury 19, 5 nd covered ltitudes 4 S 71 S while keeping long longitude 15 W. -dt comprison is performed etween 5 S nd 6 S for oth the A16S nd P16S cruises. Oservtions of temperture, slinity, lklinity, nd DIC re compred to model fields from the surfce to 1 m. Oservtionl dt is interpolted onto the depth levels of the model, nd then the corresponding model outputs with pproprite ltitude, longitude nd time re chosen for ech hydrogrphic sttion. Surfce pco is clculted from equilirium cronte chemistry equtions using the mesurements of temperture, slinity, DIC nd lklinity..3. Dignosing the model-dt misfit In the Drke Pssge, the simulted surfce pco fields from the SOSE OCMIP model re compred ginst oservtions. As method of evlution, model men is is clculted y seson, y yer, nd over the two-yer period. The verged difference etween the model nd oservtion is used to clculte the men is for ech cruise leg. M ¼ P pco model pco os N where N represents the totl numer of dt points for specific cruise trck. If the men is is positive, the model over-predicts pco compred to the oservtions nd vice vers. Next, the sesonl men is is clculted for ech seson in 5 nd 6 s the verges of ises of two cruise legs (southwrd nd northwrd) from ech seson, weighted y the numer of smples for ech cruise leg..4. Anlysis of surfce pco vriility nd ttriution of model-dt misfits ð1þ The cuses of the vriility nd model-dt misfit of pco cn e determined y evluting the four vriles tht control surfce pco : temperture (T), slinity (S), lklinity (ALK), nd DIC. One of the dvntges of the CLIVAR repet hydrogrphy cruises is tht ccurte mesurements of these prmeters hve een tken. Fctors controlling the model-dt misfit of pco re ssessed y tking liner pproximtion to the equilirium cronte chemistry (Lovenduski et l., 7). A Tylor expnsion is pplied to the equilirium pco t locl T, S, DIC nd ALK: dpco ¼ pco pco ref ¼ @pco @T dt þ @pco @pco ds þ S @S @Alk dalk S þ S Alk @pco S @Alk þ DIC @pco ds S @DIC @pco @DIC ddic S þ where S is the reference men slinity. Surfce pco nd its sensitivities to T, S, ALK, nd DIC re numericlly clculted sed on stndrd cronte chemistry solver using n efficient lgorithm (Follows et l., 6). Sensitivities of @pco pco, ; @pco ; @pco ; nd @pco @T @S @Alk @DIC, re defined for ech prmeter using reference vlues. The reference vlues for the Atlntic re: T =13 C, S = 35 psu, DIC =.15 mol m 3, ALK =.39 mol m 3. The sme reference T, S nd DIC re used for the Pcific dt, ut more representtive ALK for the Pcific sector of.37 mol m 3 is sustituted. dt nd ds re found y sutrcting the reference vlues from the either the oservtions or model output, nd ddic S nd dalk S re slinity-normlized vlues, which re found y the equtions: dalk S ¼ Alk os=mod Alk ref S os=mod S ref ; ddic S ¼ DIC os=mod S os=mod DIC ref S ref Slinity normliztion is used in order to seprte the effect of the freshwter udget, which dilutes mesurements of DIC nd ALK. Normlizing y the surfce slinity isoltes the effect of iology nd ocen trnsport from model slinity ises. The dpco eqution ove is clculted twice, once using the model output nd once using the oservtionl dt. The oserved vlues re then sutrcted from the model results in order to ttriute the model-dt misfit. The resulting difference gives the contriution of T, S, DIC, ALK, nd freshwter to the totl misfit tht is seen etween the simulted nd oserved surfce pco. 3. Results The primry ojective of this section is to evlute the simulted fields nd compre them ginst oservtionl dt. First the model fields re dignosed for the concurrent loction nd time where the oservtionl dt is ville. Our nlysis focuses on ville oservtions during the simultion period for the Drke Pssge from 56 S to63 S nd the Atlntic nd Pcific sins south of 3 S. In oth regions, output nd oservtionl surfce pco re compred nd nlyzed. To investigte the verticl structure, the wter mss properties re compred with hydrogrphic dt in the Atlntic nd Pcific sectors from the surfce to 1 m depth. 3.1. -dt comprison in the Drke Pssge Surfce ocen pco dt is ville in the Drke Pssge, nrrow 7 km chnnel locted etween the tips of South Americ nd the Antrctic Peninsul. This dtset hs reltively high temporl resolution enling model comprison on sesonl timescle. Dt ws otined from cruises covering ll sesons in 5 nd 6 nd etween the ltitudes of 56 S nd 63 S. Due to the constriction of the Drke Pssge, this region experiences significnt mesoscle vriility. Therefore, oservtionl dt from the Drke Pssge provides the most stringent test of the model s the representtion of frontl nd mesoscle vriility strongly impcts on the surfce ocen pco there. Hovmöllr digrms re creted for the simulted nd oserved surfce pco output etween 56 S nd 61 S for the two-yer model period (Fig. 1 nd ). Sptil nd temporl vriility of pco in the ðþ ð3þ
M. Woloszyn et l. / Ocen ling 39 (11) 17 18 173 ltitude 56S 57S 58S 59S 6S 61S 5 5.5 6 6.5 7 time ltitude 56S 57S 58S 59S 6S Oservtion 61S 5 5.5 6 6.5 7 time surfce ocen pco [ppmv] 35 345 365 385 45 Fig. 1. Hovmöllr digrm of the surfce ocen pco in Drke Pssge. Hovmöllr digrm of simulted surfce pco in units of ppmv in Drke Pssge from 56 S to61 S from Jnury 1, 5 to Decemer 31 sed on () the simultion nd () oservtions. Drke Pssge is stronger in the simultion with temporl stndrd devition of 6.1 ppmv reltive to the oserved stndrd devition of 17.6 ppmv. However, overll ptterns of the surfce ocen show similrities etween the model nd oservtion. There re reltively high surfce pco during lte winter nd lte summer, nd reltively low surfce pco occurs during erly summer nd fll. These ptterns re more extreme in the model. One mjor difference etween the model nd oservtion occurs t high ltitudes during erly qurter of 6 where the model shows high pco vlues wheres the oservtion shows low vlues. The sptilly verged is of surfce pco is clculted in order to determine the overll model is y yer nd y seson. Positive is denotes tht the model is overpredicting pco reltive to the oservtions, nd negtive vlues denote underprediction (Fig. ). Even though the sptil vriility is significntly overestimted in Men Bis Summer Fll Winter Spring TOTAL Fig.. Men is of the model in Drke Pssge. Men differences etween modeled nd oserved surfce pco in units of ppmv y seson, yer, nd two-yer totl in the Drke Pssge. Positive vlues represent the model overestimting the oservtionl vlues. the model, when verged over the two-yer period in the Drke Pssge, the model-dt misfit ecomes remrkly smll, with sptilly verged is of only +.65 ppmv. The sesonl model ises re somewht smller thn the oserved temporl vriility. Sesonl is shows greter mgnitudes in the fll when the is is round +1. ppmv. The model est reproduced the oservtionl vlues of surfce pco in the spring sesons with men is of only 4.5 ppmv. The modeled surfce ocen pco reproduces sptilly verged oservtions resonly well. Evluting model-dt misfit for individul cruises, the spring nd summer sesons show lrger discrepncies, specificlly ner the Polr Front etween 58 S nd 61 S. -dt misfit in this region my e due to the incomplete representtion of the iologicl processes in the model. However the underwy pco dt from the Drke Pssge is not ccompnied y the mesurements of ALK nd DIC, which mkes it difficult to ttriute the misfit to specific physicl or iogeochemicl trcer. Biologicl cron uptke is miniml during winter sesons, nd the cuse of error is likely relted to the representtion of physicl processes including se ice concentrtion. errors in the se ice concentrtion cn significntly impct the regionl circultion s well s ir-se exchnge of gses. Errors in se ice concentrtion field cn leve n imprint on the slinity of the surfce wters. The ship-sed oservtion shows higher slinity t high ltitudes thn the model, nd stellite-derived se ice cover t this time nd loction is lso greter thn the model, indicting the potentil importnce of se ice in the model error in this region. Between the two simulted yers (5 nd 6), similr sesonl fetures exist in the model pertining to iologicl nd se ice processes. Overll iologicl ctivity during the summer sesons nd processes ssocited with se ice re likely to hve contriuted to the model-dt misfit of surfce pco. More explicit representtion of iologicl processes my e le to resolve some of the deficiencies of the simplified iologicl scheme in this model.
174 M. Woloszyn et l. / Ocen ling 39 (11) 17 18 Drke Pssge pco nomly [ppm] Drke Pssge mixed lyer depth [m] Os Totl DIC ALK T S MAM JJA SON DJF MAM JJA SON 5 6 MAM JJA SON DJF MAM JJA 5 6 SON Fig. 3. Mechnisms ehind the sesonl pco vriility in Drke Pssge. () Time series of sptilly verged surfce pco is plotted from the simultion (lck solid) nd the oservtion (lck dots). Vriility of surfce pco in Drke Pssge is ttriuted to vriility in DIC (lue dsh), lklinity (mgent solid), temperture (red dsh) nd slinity (green solid) using linerized cronte chemistry. () Time series of sptilly verged mixed lyer depth is plotted. For oth pnels, ech seson is mrked with grey shding. (For interprettion of the references to colour in this figure legend, the reder is referred to the we version of this rticle.) We nlyze the model fields to determine mechnisms driving the vriility of surfce pco in this region nd its impct on the ir-se CO fluxes. The vriility depends on multiple processes including iologicl cron sources nd sinks, horizontl trnsport, verticl entrinment, surfce het fluxes, nd ir-se gs trnsfer. There re two nnul peks in the surfce pco during lte winter nd summer in the simultion. The vrition of surfce pco cn e explined y the four prmeters (DIC, ALK, T nd S) s discussed in Section.4. Fig. 3 shows the vrition of surfce pco in the Drke Pssge nd the contriutions from the four prmeters. Oservtionl vlues re lso plotted showing the resonle greement except for the erly 6 where model nd oservtion re showing opposite nomly. In the model, the effects of temperture nd DIC vriility primrily control the vriility of the pco, nd they re mutully compensting with one nother. The summertime high pco is due to the increse in the se surfce temperture, nd the wintertime high pco is due to the high DIC which my e driven y the entrinment of susurfce wters. The mixed lyer depth sesonlly vries from 5 m to 5 m in this region, nd the timing of deepest mixed lyer mtches the highest concentrtion of DIC in the winter sesons (Fig. 3). Significnt northwrd grdient of pco exists oth in the model nd oservtions, which is primrily cused y the temperture grdient cross the Polr Front, prtilly compensted y the opposing DIC grdient. During lte spring nd erly summer sesons, there is strong decrese of surfce pco ssocited with the decrese in DIC. This lte spring/ erly summer decrese in DIC is driven y the enhnced iologicl cron uptke. While the low vlues of surfce DIC remins throughout the summer, the surfce heting strongly increses the pco during the middle to lte summer sesons. The vriility of surfce pco in comintion with the surfce wind speed controls the vriility of the ir-se CO fluxes. Fig. 4 shows the normlized indices of the ir to se CO flux, negtive of surfce ocen pco nd the wind speed t 1 m ove the se level. The normlized indices re defined s the deprtures from the temporl men divided y the stndrd devition. The temporl vriility of the ir-se CO flux is primrily controlled y the surfce ocen pco (correltion coefficient of.8). Reltively rpid, weekly vriility of CO flux is influenced y the tmospheric wind vriility (correltion coefficient of.3). These correltions re sttisticlly significnt with the confidence intervl of 95%. 3.. -dt comprison long CLIVAR repet A16S nd P16S While the Drke Pssge oservtions llow evlution of the model t the mesoscle, dignosing the cuses of the misfit surfce pco is difficult ecuse of the lck of ALK nd DIC oservtions. Here we further evlute the model using CLIVAR repet hydrogrphy dt from line A16S (5 W 4 W) nd P16S (15 W). These cruises include the mesurements of four vriles tht control surfce pco ; T, S, ALK nd DIC. Thus it is possile to exmine the misfit of ech vrile, nd ttriute the cuses of model-dt misfit of surfce pco to the four vriles. Simulted model fields re compred ginst concurrent shipsed mesurements for the top 1 m. Fig. 5 demonstrtes tht the model is cpturing the oserved pttern of temperture nd slinity in the Pcific Ocen; the sptilly verged model-dt misfit is only.15 C nd.194 psu. The gretest differences etween model nd oservtions re generlly in the surfce mixed lyer nd t the Polr Front (53 S), s the modeled front is slightly south of the oserved loction. In the regions of lrgest disgreement, the model-dt misfit is never greter thn out 3 C for temperture nd.8 psu for slinity. In ddition to reproducing the oserved temperture nd slinity, the model reproduces the mgnitude nd sptil ptterns of oserved lklinity in the Pcific sector (Fig. 6). The lrgest disgreement in lklinity is south of the Polr Front etween the surfce nd m depth, ecuse the model produces verticl lklinity grdient tht is too steep. Below out 3 m, the model produces slightly higher vlues of lklinity equtorwrd of 55 S nd slightly lower vlues south of 55 S. The men nd stndrd devition of the model-dt misfit for lklinity is 3. ±. leq kg 1, which demonstrtes the overll ility of the model to cpture oservtionl lklinity. The model lso is le to cpture the overll mgnitude nd ptterns of DIC from the surfce down to 1 m depth in the Pcific Ocen (Fig. 6). The region where the model shows the most disgreement is elow m t the Polr Front with lower thn oserved DIC vlues, the lrgest difference eing out 6 lmolc kg 1 ner the frontl region. Throughout the upper m of the domin, the model overestimtes DIC up to 6 lmolc kg 1. However, the men nd stndrd devition of the model-dt misfit for DIC re only 3.8 ± 1.7 lmolc kg 1. Dt from the Atlntic sector shows tht the model reproduces the oserved temperture, slinity, lklinity, nd DIC. Fig. 7 demonstrtes tht the model cptures the overll distriution of
M. Woloszyn et l. / Ocen ling 39 (11) 17 18 175 +4 Stndrdized indices MAM JJA SON DJF MAM JJA SON 5 6 Fig. 4. Vriility of ir-se CO flux in Drke Pssge. Time series of sptilly verged ir-se CO flux, negtive of surfce ocen pco nd 1 m wind speed re plotted. Plotted vlues re stndrdized through dividing nomlies y the temporl stndrd devition. Ech seson is mrked with grey shding. CLIVAR Line P16S Oservtions Temperture [degree C] Difference in Temperture [degree C] -1 1 Oservtions Slinity [psu] Difference in Slinity [psu] Fig. 5. Comprison of temperture nd slinity (P16S). Comprison etween simulted (left column) nd oserved (center column) distriution of () temperture [ C] nd () slinity [psu] in the top 1 m from the CLIVAR repet hydrogrphy P16S line (Pcific Ocen). The right column shows the difference etween the model nd oservtions. The oservtionl dt is interpolted onto regulr verticl grid consistent with the model nd then the model dt is horizontlly interpolted onto the loction of hydrogrphic sttions evluted t the time when the cruise took plce during erly 5.
176 M. Woloszyn et l. / Ocen ling 39 (11) 17 18 Oservtions Oservtions Fig. 6. Comprison of lklinity nd DIC (P16S). Comprison etween simulted (left column) nd oserved (center column) distriution of () lklinity in units of leq kg 1 nd () DIC in units of lmolc kg 1 in the top 1 m from the CLIVAR repet hydrogrphy P16S line (Pcific Ocen). The right column shows the difference etween the model nd oservtions. The oservtionl dt is interpolted onto regulr verticl grid consistent with the model nd then the model dt is horizontlly interpolted onto the loction of hydrogrphic sttions evluted t the time when the cruise took plce during erly 5. temperture nd slinity south of 5 S. The model-dt misfit for oth vriles is firly smll; the temperture hs men modeldt difference of.19 C nd the slinity hs men difference of.111 psu. Fig. 8 demonstrtes tht the model is le to cpture the lrge-scle distriution of lklinity nd DIC. The men nd stndrd devition of the model-dt misfit re.83 ±.7 leq kg 1 for lklinity nd 1.7 ±.7 lmolc kg 1 for DIC. Overll, the simulted lklinity nd DIC vlues in the Atlntic sector re closer to oservtions thn the Pcific sector. The stndrd devition of the misfit is greter in the Atlntic sector, however, which my e ttriuted to stronger vriility in this prticulr section. Fig. 9 nd summrizes the model-dt comprison for the hydrogrphic distriution of temperture, slinity, ALK nd DIC in the form of Tylor digrm (Tylor, 1) in which the model performnce is displyed in single digrm in terms of the model-dt correltion, their root-men-squre difference, nd the rtio of their vrinces. In the polr plot, the rdius (r) is the mesure of normlized stndrd devition nd the cosine of the ngle (h) is the correltion coefficient etween the model nd oservtions. All dt points re close to the x-xis (correltion coefficients >.85) in Fig. 9 nd indicting tht the simulted ptterns of temperture, slinity, lklinity, nd DIC compre very well with the oservtions. The distnce etween oservtionl point (r = 1, h = ) nd the modeled point mesures the centered root-men-squre error. In the Pcific section (Fig. 9), the model reproduces the pttern nd mplitude of temperture nd DIC vriility very well (with the correltion coefficients >.95 nd lmost perfect vrince) ut it tends to overestimte the mplitude of slinity nd lklinity vriility with slightly weker correltion coefficients (>.9). As result the root-men-squre error is slightly greter for slinity nd ALK reltive to temperture nd DIC. In this section, the men is of DIC is significnt, contriuting to the model-dt misfit of surfce pco. In the Atlntic section (Fig. 9), the simultion cptures the stndrd devition of temperture, slinity, ALK nd DIC vriility within ±1% of the oserved vlues. The sptil ptterns of temperture nd slinity re well reproduced (with correltion coefficients >.95) ut less so for the ALK nd DIC (with correltion coefficients >.85). In this section, the model-dt misfit of lklinity is gretest in the sutropicl region, while the model-dt misfit of DIC is gretest t higher ltitudes. The direct model-dt comprison shows tht the model is le to cpture overll distriution of temperture, slinity, lklinity nd DIC resonly well. There re still significnt model-dt misfits in the comprison of surfce pco which is very sensitive to smll perturtions in lklinity nd DIC. Here we quntittively dignose wht hs cused the misfit using the mesurements of four vriles tht determine the surfce pco. First, the totl model-dt misfit of surfce pco is clculted in the Pcific nd Atlntic sectors. Then, the model-dt misfit of surfce pco is dignosed using the model-dt misfit of temperture, slinity, lklinity, nd DIC s outlined in Section.4. It is cler in the Pcific sector tht the model-dt misfit of surfce pco is minly driven y the difference etween modeled nd
M. Woloszyn et l. / Ocen ling 39 (11) 17 18 177 Oservtions Temperture [degree C] Difference in Temperture [degree C] Oservtions Fig. 7. Comprison of temperture nd slinity (A16S). Sme s Fig. 5 ut for the CLIVAR repet hydrogrphy A16S line (Atlntic Ocen). oserved DIC (Fig. 1). Specificlly in the region from 35 S to 53 S, which corresponds to the re of the gretest model-dt difference in surfce DIC. Alklinity nd DIC seem to e countercting in tht when one cuses positive is to the surfce pco (i.e. model is over-predicting oservtions), the other is doing the opposite. Alklinity is somewht offsetting the effect of DIC nd reducing the totl model-dt misfit of surfce pco. The difference etween modeled nd oserved temperture hs miniml contriution to the totl model-dt misfit, which is expected since the comprison of simulted nd oserved surfce temperture showed the model reproduced oservtionl vlues very well. The slinity nd freshwter (trcer dilution) effects re lmost negligile throughout the whole ltitudinl domin, suggesting little contriution to the totl model-dt misfit of surfce pco. Fctors controlling the model-dt misfit in the Atlntic sector (Fig. 1) re not s cler s the Pcific sector. DIC is controlling the totl misfit etween 5 S nd 55 S where there is the mximum DIC nd pco model-oservtion difference. The totl misfit in the sutropics, specificlly from 4 S to 45 S, is dominted y the difference in model nd oservtionl lklinity. In the other regions not mentioned, the totl model-dt misfit is from contriution of DIC, lklinity, nd temperture. Similr to the Pcific sector, there is little contriution to the totl misfit from the slinity nd freshwter effects. 3.3. Comprison with the corse resolution model A comprison to lower-resolution model is performed in order to evlute the impct of model resolution on the simulted iogeochemicl properties. The comprison ws crried out using the physicl circultion from n ocen stte estimte with lterl resolution of one degree provided y the ECCO GODAE project (see Section.1.). In contrst to the eddy-permitting model, the corse resolution model run does not explicitly resolve of mesoscle processes. Fig. 11 compres simulted nnul men ir-se CO flux etween the Tkhshi climtology, SOSE OCMIP high resolution model, nd the ECCO low-resolution model run. Overll, the modeled ir-se CO flux ppers resonly similr to the oservtionl fluxes, cpturing the mjor regions nd mgnitudes of outgssing (1 3 molc m yer 1 ) long the ltitudinl nd from 5 S to 6 S. There is some discrepncy etween exctly where the strongest regions of outgssing occur, ut the generl pttern nd flux vlues re well cptured in oth models. The eddy-permitting model cptures the loctions nd mgnitudes of cron dioxide uptke (depicted y shdes of lue in Fig. 11), which occurs minly north of 5 S. The model shows stronger nd more equtorwrd shifted region of outgssing thn oserved. Similr to the eddy-permitting model, the corse resolution model (Fig. 11c) cptures the pttern nd mgnitude of ir-se cron flux resonly well. The corse resolution model shows very similr sptil pttern of ir-se CO flux s the high resolution model. Both runs hve the most outgssing in the ltitudes of the Antrctic Circumpolr Current. However, the mgnitude of outgssing is much stronger in the eddy-permitting model. A significnt difference etween the models occurs in the region of the Weddell Se djcent to Antrctic where the eddy-permitting model shows strong outgssing. The climtology does not depict outgssing in the Weddell Se. However, the oservtionl dt from this
178 M. Woloszyn et l. / Ocen ling 39 (11) 17 18 Fig. 8. Comprison of lklinity nd DIC (A16S). Sme s Fig. 6 ut for the CLIVAR repet hydrogrphy A16S line (Atlntic Ocen). () Repet P16S () Repet A16S 1.8.1..3.4.5 C o r r e l t 1.8.1..3.4.5 C o r r e l t Normlized stndrd devition 1.6 1.4 1. 1.8.6 1.6 1..8.6 ALK i o n.7 C o e f i c i e n t.8.9.95 Normlized stndrd devition 1.6 1.4 1. 1.8.6 1.6 1..8 DIC.6 i o n.7 C o e f.8 f i c i e n t.9.95.4..4 DIC T Os S.99.4. ALK.4 S T Os.99 Fig. 9. Tylor digrm of the model-dt comprison. Tylor digrms re constructed to quntify the model performnce in reproducing the four trcers (T, S, DIC nd ALK) in the mixed lyer in the CLIVAR repet hydrogrphy () P16S nd () A16S line. Multiple vriles re plotted on single digrm using normlized stndrd devition. Refer to text for figure interprettion.
M. Woloszyn et l. / Ocen ling 39 (11) 17 18 179 Delt (in ppm) Delt (in ppm) Delt pco Fig. 1. Attriution of model-dt misfit in surfce pco. Totl model-dt misfit (thick lck) of pco in units of ppmv in the surfce mixed lyer from () CLIVAR line P16S nd () CLIVAR line A16S, tht re decomposed into temperture (solid red), slinity (solid green), normlized lklinity (solid lue), normlized DIC (dshed lck), nd freshwter (dshed pink) contriutions. (For interprettion of the references to colour in this figure legend, the reder is referred to the we version of this rticle.) Tkhshi Oservtions SOSE-OCMIP YR men: YR5-7 ECCO Low-Resolution YR men: YR5-7 3 o W o 3 o E c 6 o W 6 o E 9 o W 9 o E 7 o S 1 o W 6 o S 5 o S 1 o E 15 o W 4 o S 18 o W 15 o E Fig. 11. -model comprison of ir-se CO flux. Air-se cron flux [molc m yer 1 ] from () Tkhshi et l. 9 climtology (nnul men referenced to YR), () the eddy-permitting model ( yer men from 5 to 7) nd (c) the corse resolution model simultion ( yer men from 5 to 7). Positive (negtive) vlues represent flux out of (into) the ocen. region is limited during the winter seson nd cre must e tken, since the climtologicl vlues re likely ised towrd summer conditions. This sesonl is cn miss the entrinment of susurfce wters tht my e strongest during winter months when mixing nd convection is most ctive. Another issue my exist in the ove nlysis in terms of the timing. The climtologicl fluxes
18 M. Woloszyn et l. / Ocen ling 39 (11) 17 18 re referenced to the yer, wheres the models re evluted for the two-yer period from 5 to 6. In some regions, simulted pttern of pco is significntly different etween the two yers, which is nother complicting issue in compring with the climtology. Atmospheric pco hs incresed over the period etween nd 6, which might hve ffected the reltive strength of nturl nd nthropogenic cron fluxes. 3.4. Impct of tmospheric winds on the cross-frontl cron trnsport The men flow nd eddy components of the lterl cron flux is dignosed from the model output. First, we pply trditionl decomposition of the zonl men nd eddy components, where n eddy is defined s devition from the zonl men tken t fixed ltitude. Eddy trnsport cn e further decomposed into trnsient nd sttionry components. The zonl men component is representtive of the trnsport y the temporl nd zonl men circultion, reflecting the Eulerin men component of the meridionl overturning circultion. The sttionry eddy flux represents the mendering of the time-men ACC circultion while the trnsient eddy flux reflects the temporlly fluctuting component of the circultion. The model trnsport is dignosed from the five-dy time verged output where DIC (herefter, C) nd meridionl velocity (v) re decomposed into zonlly nd temporlly verged men ðc xt ; m xt Þ, sttionry eddy (C,v ) nd trnsient eddy components (C,v ). C ¼ C xt ðy; zþþc ðx; y; zþþc ðx; y; z; tþ Tking the product of C nd v, nd then tking the zonl nd temporl verges yields the totl northwrd flux in three components. mc xt ðy; zþ ¼m xt C xt þ m C x þ m C xt The first term on the right hnd side is the zonl nd temporl men component of the northwrd CO trnsport. The second term is the sttionry eddy component nd the lst term is the trnsient eddy component. Fig. 1 shows the lterl cron trnsport in the three components, where the net southwrd trnsport is driven y the zonl ð4þ ð5þ men component, prtilly compensted y the northwrd eddy components. The ckground, men concentrtion of DIC is incresing southwrd, nd so the net trnsport is up-grdient nd tends to increse the DIC concentrtion in the high ltitude Southern Ocen. This southwrd trnsport is prtly lnced y the outgssing of CO in the Southern Ocen to the tmosphere s shown in Fig. 11. Recent increse in the tmospheric CO hs slowed this outgssing, leding to the ccumultion of nthropogenic DIC in the Southern Ocen (Ito et l., 1). Fig. 1 lso indictes tht the sttionry eddy component trnsports more cron dioxide thn the trnsient eddy component, which is not discussed in previous works. The northwrd eddy fluxes re down-grdient, nd the two eddy components together compenste the effect of the zonl men circultion y pproximtely 5% in the ACC. Recognizing the importnce of the eddy trnsport, we dignose the zonl men nd eddy trnsport cross the sptilly mendering men circultion of the ACC. The trditionl zonl men tends to verge different wter msses t constnt ltitudes. Thus the stremline verging llows us to focus on the trnsport cross the wter mss oundries s well s eliminting the sttionry eddy component. Here, we define the men pthwy of the ACC using the time-men rotropic strem function (w = 5 Sv) which is very close to the position of the Polr Front s defined y the hydrogrphic structure. Then velocities nd trcer concentrtions cn e seprted into two components: (i) long-stremline (zonl) men nd (ii) eddies (i.e. deprture from the men). The stremline-verged quntities re clculted for ech five-dy verge fields, nd the time series of the cross-frontl northwrd cron trnsport is plotted in Fig. 13. The net trnsport is southwrd (negtive) nd is pproximtely out 1. PgC yer 1, consistent with the results from the trditionl zonl men dignostics. The temporl vriility of the cross-frontl cron trnsport is primrily driven y the wind-driven vriility of ocen currents. Anomlous cross-frontl mss flux is lmost entirely driven y the Ekmn trnsport vriility (with correltion coefficient of.98, confidence intervl of 99%), nd this explins 8% of the vrince for the cross-frontl cron flux. The de-correltion timescle of the cross-frontl Ekmn trnsport is out 1 dys, nd on this timescle the vriility of the net CO flux is primrily driven y the Ekmn trnsport. northwrd cron trnsport [Pg C/yer] 8S 7S 6S 5S 4S 3S Northwrd cron trnsport [Pg C/yer] 4 6 8 1 1 14 Net Zonl Men Eddy (trnsient) (sttionry) Time Net Zonl (stremline) men Eddy Fig. 1. Zonlly integrted northwrd cron trnsport. Northwrd cron trnsport is integrted long constnt ltitude circles. Net trnsport (lck solid) is seprted into zonl men (lue solid), sttionry eddy (red dsh) nd trnsient eddy (red dot) components. (For interprettion of the references to colour in this figure legend, the reder is referred to the we version of this rticle.) Fig. 13. Time series of cross-frontl cron trnsport. Northwrd cron trnsport is integrted long the position of the Polr Front. The net trnsport (lck solid) is seprted into the zonl, long-stremline men (lue dsh) nd eddy component (red dsh dot). (For interprettion of the references to colour in this figure legend, the reder is referred to the we version of this rticle.)
M. Woloszyn et l. / Ocen ling 39 (11) 17 18 181 Velocity c Cron Trnsport 5 5 1 1 V s 1 VC s 15 15 Depth 3 5 3 35 5 3 35 4 4 45 4 45 5 5-1 +1 + Northwrd Cron Trnsport [Pg C/yr] 1.5 +.5 +1 Correltion coefficient 1.5 +.5 +1 Correltion coefficient Fig. 14. Verticl structure of cross-frontl cron trnsport. () Verticl distriution of cross-frontl cron trnsport is plotted for ech 5 m ins. The size of error rs represents the temporl stndrd devition for ech depth in. () Correltion coefficient etween cross-frontl velocity nd wind-driven Ekmn trnsport. (c) Correltion coefficient etween cross-frontl cron flux nd the Ekmn trnsport. For () nd (c), shded regions indictes the 95% confidence intervl of the null hypothesis (no correltion) sed on the t-test. Fig. 14 shows the verticl distriution of northwrd cron flux in units of PgC yer 1. The wter column is divided into 1 lyers with 5 m thickness ech, nd the cross-frontl cron trnsport is integrted for ech lyers. The size of errorr indictes the mgnitude of temporl stndrd devition. The southwrd cron trnsport occurs in the deep ocen elow the depth of 1 m, which is prtilly compensted y the northwrd trnsport in the surfce lyer. This reflects the pttern of meridionl overturning circultion, which rings up deep wters to the surfce Southern Ocen nd suducts intermedite nd mode wters to the thermocline. On the timescles of weeks to severl months, the vriility of the meridionl overturning circultion is primrily controlled y the tmospheric winds. Fig. 14 nd c shows the verticl distriution of correltion coefficients etween wind-driven Ekmn trnsport nd the mss nd cron flux cross the Polr Front. Agin we divide the wter column into 1 lyers with 5 m thickness ech, nd the cross-frontl cron trnsport is integrted for ech lyers. For most lyers, the vriility of cross-frontl mss nd cron trnsport is sttisticlly significnt. Ner the surfce, trnsport vriility is strongly correlted with the wind-driven Ekmn flow. In the deep ocen elow 1 m, the trnsport vriility is negtively correlted with the Ekmn flow suggesting the importnt role of the verticl momentum trnsfer nd the southwrd return flow. When there re nomlously strong westerly winds, Ekmn flow trnsports cron northwrd in the surfce ocen, nd the geostrophic return flow trnsports cron southwrd in the deep ocen. The net effect is dominted y the southwrd, deep trnsport due to the higher DIC concentrtion there. While eddies ply significnt roles in the meridionl cron trnsport in the zonl men cron udget, our nlysis finds reltively smll contriution of eddies to the vriility of cron trnsport cross the Polr Front on the intr-nnul timescles. The dominnt trnsport mechnism is the wind-driven, Ekmn flow. Similr to other high resolution model studies, this study finds tht the eddy trnsport is lwys in n opposing direction to the zonl men circultion trnsport in the Southern Ocen. 4. Summry nd conclusions We developed n eddy-permitting ocen cron cycle model of the Southern Ocen. The cron cycle is simulted in the offline mode in which the physicl circultion is prescried y the eddy-permitting ocen stte estimte product constrined y suite of in situ nd remotely sensed oservtions yielding relistic, time-vrying three-dimensionl circultion fields. High resolution models such s this one re vlule tool to investigte the trnsport nd fluxes of cron nd other iogeochemicl trcers. The nlysis presented in this pper tests the relism of the simulted cron cycle over the intr-nnul timescles. The direct modeldt comprison revels tht the model reproduces the oserved distriution of temperture, slinity, lklinity nd dissolved inorgnic cron resonly well in the Pcific nd Atlntic sins, yet there re still some differences prticulrly close to frontl regions nd in sesonlly ice covered regions. The model-dt comprison is performed to evlute the model s ility to simulte the oserved surfce ocen pco, ir-se cron fluxes, nd relted iogeochemicl trcers. First the surfce ocen pco is compred to the underwy pco mesurements from the Drke Pssge. The results highlight tht even in region with significnt mesoscle vriility, the model only hs smll men is over the two-yer simultion of out.65 ppmv. There re, however, significnt discrepncies etween the model nd oservtions in the temporl nd sptil vriility t the scle of fronts nd eddies. The sin-scle model-dt comprison in the Atlntic nd Pcific Ocens shows tht the model cn well reproduce the vriles tht control surfce pco : temperture, slinity, lklinity, nd DIC. The comprisons from the surfce to 1 m depth revel remrkly smll model-dt misfit in oth ocen sins for ll vriles. The nlysis llows dignosing the fctors controlling the model-dt misfit of surfce ocen pco. In the Pcific sector differences in simulted nd oserved DIC re the mjor cuse of the model-dt misfit in surfce pco. In the Atlntic sector discrepncies etween the modeled nd oserved lklinity drives
18 M. Woloszyn et l. / Ocen ling 39 (11) 17 18 the surfce pco misfit in the Suntrctic region (4 S 45 S), while discrepncies in DIC control the misfit t higher ltitudes. Finlly, corse resolution model employing the identicl iogeochemicl scheme hs een compred to the eddy-permitting model. The corse resolution model is driven y the ocen stte estimte from the ECCO GODAE project. The high- nd low-resolution model runs exhiit similr sptil ptterns of ir-se CO flux, ut differences exist in the strength of fluxes t high ltitudes. In this study, simple iogeochemistry model hs een used in which surfce nutrients re simply restored towrd monthly climtologicl vlues. This is perhps the most significnt source of error in our model s the prmeterized iologicl sources nd sinks of cron cnnot cpture the vriility ssocited with foodwe nd ecosystem processes tht re present in the rel ocen. Inclusion of more sophisticted iologicl model is in progress to improve the simultion of the regionl cron cycle. Acknowledgement Woloszyn nd Ito re supported y the Ntionl Aeronutics nd Spce Administrtion (NASA) grnt NNX8AL7G nd the Ntionl Ocenic nd Atmospheric Administrtion (NOAA) grnt NA8OAR43893. Mzloff is supported y NSF grnt OCE 96118. NASA Advnced Supercomputing division nd Sn Diego Supercomputer Center provided computing nd dt storge resources. Additionl computtionl resources re lso provided y NSF through TerGrid resources under grnt numer MCA6N7. The Southern Ocen Stte Estimte is supported y the NASA nd the Ntionl Ocenogrphic Prtnership Progrm (NOPP) contrcts to Msschusetts Institute of Technology. Two nonymous reviewers provided very helpful comments on the mnuscript. References Boning, C.W., Dispert, A., Viseck, M., Rintoul, S.R., Schwrzkopf, F.U., 8. The response of the Antrctic Circumpolr Current to recent climte chnge. Nt. Geosci. 1 (1), 864 869. Dnsoglu, G., McWillims, J.C., 1995. Sensitivity of the glol ocen circultion to prmeteriztions of mesoscle trcer trnsports. J. 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