Pacific sea surface temperature field reconstruction from coral D 18 O data using reduced space objective analysis

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1 PALEOCEANOGRAPHY, VOL. 17, NO. 1, 17, 129/2PA59, 22 Pcific se surfce temperture field reconstruction from corl D 18 O dt using reduced spce ojective nlysis Michel N. Evns, 1 Alexey Kpln, nd Mrk A. Cne 2 Lmont-Doherty Erth Oservtory, Plisdes, New York, USA Received 5 Septemer 2; revised 25 July 21; ccepted 2 Septemer 21; pulished 27 Ferury 22. [1] A systemtic methodology for the reconstruction of climte fields from sprse oservtionl networks of proxy dt, employing the technique of reduced spce ojective nlysis, is pplied to the reconstruction of gridded Pcific Ocen Bsin se surfce temperture (SST) from corl stle isotope (d 18 O) dt for the period In this pproch we seek to reconstruct only the leding modes of lrge-scle vriility which re oth oserved in the modern climte nd resolved in the proxy dt. We find tht the corl dt verifily resolve two sptil ptterns of SST vriility. The first nd dominnt pttern is tht of the El Niño Southern Oscilltion (ENSO). A second pttern reflects uniform chnges over most of the Pcific Bsin. Clirtion nd verifiction tests for show tht root-men-squre vrince is smll ( C RMS) nd reconstruction errors re lrge (.6 C RMS), limiting interprettion to the tropicl region. Periods of enhnced ENSO ctivity similr to those oserved in the pst two decdes re evident in the reconstruction for the erly nineteenth century. The chnging frequency of ENSO wrm phse events ppers to coincide with wrming of the Pcific men stte inferred from similr reconstruction efforts using tree ring indictors over the lst two centuries. INDEX TERMS: 3344 Meteorology nd Atmospheric Dynmics: Pleoclimtology; 4215 Ocenogrphy: Generl: Climte nd internnul vriility (339); 4522 Ocenogrphy: Physicl: El Niño; KEYWORDS: ojective nlysis, ENSO, se surfce temperture, proxy, corl 1. Introduction 1.1. Motivtion [2] The motivtion for this work is the desire to understnd the nturl vriility of the climte system on timescles from yers to centuries nd plce the somewht unusul lst few decdes within longer-term context. However, our direct (or instrumentlly recorded) oservtions of most climte vriles only extend ck for out 1 15 yers. In their sence we cll upon high-resolution pleoproxy dt, tht is, oservtions of prmeters which vry in wy we understnd with the climte vrile of interest, to study preinstrumentl nd preindustril climtes. [3] Now, suppose we think of the climte system s consisting of only hndful of ptterns superimposed upon one nother, which together descrie some sizle frction of the lrge-scle, low-frequency vriility [Wllce, 1996, 1996]. Some exmples of such ptterns in tody s climte re the influences of the El Niño Southern Oscilltion (ENSO) phenomenon [Philnder, 199], ner-glol temperture trends [Hnsen nd Leedeff, 1988; Cne et l., 1997], Pcific nd Atlntic Bsin centered decdl phenomen such s the Pcific Decdl Oscilltion [Zhng et l., 1997] nd the North Atlntic Oscilltion [Hurrell, 1995], nd internnully vrying Antrctic circumpolr wves [White nd Peterson, 1996]. If the ptterns re indeed few, importnt, nd lrge scle, then we might e le to monitor them using only hndful of well-locted direct oservtions [Bretherton nd McWillims, 198; Bennett, 199; Jones et l., 1 Now t Lortory of Tree-Ring Reserch, University of Arizon, Tucson, Arizon, USA. 2 Also t Deprtment of Erth nd Environmentl Sciences, Columi University, New York, New York, USA. Copyright 22 y the Americn Geophysicl Union /2/2PA59$ ; Evns et l., 1998]. We my even e le to reconstruct pst climtic vriility using n oservtionl ntenn composed of proxy dt. [4] Our methodologicl sis for climte field reconstructions follows from recently developed reduced spce ojective nlysis procedures for the nlysis of historicl climte fields [Cne et l., 1996; Kpln et l., 1997, 1998; Evns et l., 21; Kpln et l., 2]. In this pproch we seek to reconstruct only the leding modes of lrge-scle vriility (spce reduction) which re oth oserved in the modern climte nd resolved in the proxy dt, given uncertinties [Evns et l., 21]. [5] At ll stges we emphsize the following. 1. The proxy dt re permitted to provide climtic informtion in regions fr removed from the smpling site (glolity). 2. The proxy dt re employed within the context of their oservtionl errors, which re determined vi clirtion studies (ojectivity). 3. All sttisticl estimtes mde during nlysis re filtered to ring out the most roustly resolved fetures (spce reduction). The reltionship of the proxy dt to climte vriility is treted similrly. [6] Given explicit ssumptions, the nlysis produces fields nd error estimtes. The results my e susequently checked for consistency with prmeter choices nd procedurl ssumptions y comprison with direct nd proxy oservtions not used in the reconstruction procedure nd with results from enchmrk experiments. [7] Here we ttempt reconstruction of the Pcific Bsin se surfce temperture (SST) field using the corl stle isotope (d 18 O) dt set studied y [Evns et l., 2]. We egin y giving n overview of the methodology (section 2), which is developed in detil elsewhere [Evns et l., 21]. The SST field reconstruction uilds on previous work [Evns et l., 2] which sought to determine which, if ny, lrge-scle ptterns of SST vriility re resolved y the corl dt (section 3.3). We conclude y compring the results with SST reconstructions sed on tree ring-derived indictors from the Pcific costs of North nd South Americ 7-1

2 7-2 EVANS ET AL.: PACIFIC SST FIELD RECONSTRUCTION Proxy records Climte Field Reconstruction Definition of C, E, α Clirtion Trget field chrcteriztion Proxy dt chrcteriztion Definition of H, R Instrumentl dt Anlysis procedure Verifiction Reconstruction of fields, indices, theoreticl errors Verifiction Study of climte dynmics Figure 1. Methodology of ojective nlysis climte field reconstruction from proxy dt. [Evns et l., 21] nd comment on the ehvior of the ENSO system inferred from these results. 2. Methodology [8] Our ojective is to otin the liner lest squres fit to ville oservtions nd model of the lrge-scle modes of sptil field vrition. The procedure is schemticlly digrmmed in Figure 1 nd descried riefly elow. More detils re given y Evns et l. [21] Trget Climte Field [9] Missing oservtions, chnges in mesurement protocols nd qulity, nd other fctors introduce uncertinty in ll geophysicl dt sets. Hence we egin with nlysis of the historicl oservtions of the climte field we seek to reconstruct to emphsize the lrgest-scle fetures which re likely to hve the smllest reltive errors. This is conveniently performed using empiricl orthogonl function (EOF) nlysis, which decomposes the sptil covrince of the climte field T(x, t) into set of orthogonl vectors nd their corresponding vrinces: C ¼ TT T EE T ; ð1þ where oldfce vriles indicte vectors or mtrices, quntities within ngle rces re time-verged, nd in ll equtions herefter, superscript T denotes the mtrix or vector trnspose. Here E is mtrix whose columns re smll numer of the eigenvectors of C, nd is squre mtrix whose digonl elements re the eigenvlues corresponding to E. We then ssume reduced spce form for the reconstruction solution: Tx; ð tþ ¼ Ex ð ÞAðÞþresidul; t where T is the mtrix whose rows re time series of climtic oservtions, with ech row corresponding to loction, nd A(t) is the low-dimensionl vector of mplitudes with which the modes E contriute to the climte field T. The long-term men of T is zero. E, A, nd T re estimted from the modern oservtions nd provide the sis for clirtion of the proxy dt Clirtion [1] We next choose set of pleoproxy oservtions suitle for the reconstruction of the trget climte field nd clirte the proxy dt over some contemporneous time intervl. Note tht y clirting the proxy dt in terms of the lrge-scle ptterns identified in the modern climte oservtions, we permit the proxies to descrie nonlocl s well s locl climte vriility. For instnce, proxy climte dt set including oservtions from the estern equtoril Pcific will likely permit clirtion of (t ð2þ lest) the lrge-scle ENSO climte pttern. We write Dt ðþ¼haðþ: t The mesurement opertor H represents liner reltionship etween the climte field nd proxy oservtions. If H is known, (3) sets up Guss-Mrkov oservtionl scheme for estimtion of A(t) from the mtrix of proxy oservtions D [Gndin, 1965; Mrdi et l., 1979; Ro, 1973]. Note tht there re importnt ssumptions nd cvets in this pproch; see sections nd 2.3.2for discussion. We use the proxy dt nd modern climte oservtions to estimte H over clirtion intervl, using the singulr vlue decomposition (SVD) [Bretherton et l., 1992; Fritts et l., 1971; Cook et l., 1994] to regress the proxies on only the leding ptterns in the modern oservtions. The error vrince in the model (eqution (3)) is estimted from the covrince mtrix of the proxy-climte clirtion discrepncy: D E R ¼ ðd HAÞðD HAÞ T : ð4þ 2.3. Anlysis [11] We now seek the field T(x, t) which is the est fit to the clirted proxy oservtions nd to the set of leding ptterns we oserve in the modern climte, given uncertinty. We construct cost function: SðAÞ ¼ðHA DÞ T R 1 ðha DÞþA T 1 A where S evluted t ech time t is unitless sclr quntity. Here A is the vector of temporl mplitudes we seek to reconstruct from the proxy dt. The first term of (5) represents the misfit of the reconstruction to the clirted oservtions nd is weighted y the clirtion error. The second term represents the misfit to the Mmodern climtology nd is weighted y the covrince of the modern climte signl. At ech time t the nlysis is punished (S! lrge) for either putting too much stock in oservtions with lrge error or in ptterns which explin little of the vrince in the modern climte. If the proxy oservtions nd sttisticl climte model errors re unised nd if the prior covrinces C, R, nd re good estimtes of the true covrinces, then minimiztion of (5) produces the optiml lest squres reduced spce estimte of A [Kpln et l., 1997]: A ¼ PH T R 1 D: The error covrince in the estimte is P ¼ H T R 1 H þ 1 1; ð7þ ð3þ ð5þ ð6þ

3 Tle 1. Corl d 18 O Records Employed in This Work EVANS ET AL.: PACIFIC SST FIELD RECONSTRUCTION 7-3 Site Nme Loction Time Period Genus Reference Aq AQ N, 35 E :A Porites Heiss [1994] Aq AQ N, 35 E :A Porites Heiss [1994] Ceu CEB 1 N, 124 E :A Porites Ptzöld [1984] Secs Islnd SEC 8. N, 82. W :M Porites Linsley et l. [1994] Kiritimti KIR 2 N, 157 W :M Porites Evns et l. [1999] Trw Atoll TAR 1 N, 172 E :M Porites Cole et l. [1993] Urvin By URV S, 91.2 W ,62-81:A Pvon Dunr et l. [1994] Punt Pitt PUN.7 S, 89 W :S Pvon Shen et l. [1992] Mhe, Seychelles MAH 4.6 S, 55.8 E :M Porites Chrles et l. [1997] Mdng, PNG MAD 5.2 S, E :S Porites Tudhope et l. [1995] Espiritu Snto ESP 15 S, 167 E :A Pltygyr Quinn et l. [1993] New Cledoni CAL 2.7 S, E :S Porites Quinn et l. [1998] Arhm Reef ABR 22 S, 153 E :B Porites Druffel nd Griffin [1993] Nominl temporl resolution of d 18 O dt: M, monthly; S, sesonl; A, nnul; B, iennil. Sites re ordered in decresing ltitude (north to south). which is weighted verge of the error in the clirted oservtions nd the error in our model of the modern climte. When the oservtions re reltively ccurte (R is smll) or sufficiently numerous (the ptterns in H re well resolved), the nlysis error (7) is low, nd the nlysis puts vrince into the solution T. When the oservtions re poor (R is lrge) or sent (H is ill resolved), the nlysis error ecomes lrge, nd the resolved vrince pproches climtology or the trivil solution T =. Similrly, if the reltionship etween proxy dt nd direct oservtions hs little skill, the clirtion error covrince R will e lrge, the numer of clirted ptterns contined in H will e smll, the nlysis error P will e lrge, nd the nlysis estimte A will e smll. In ll cses the pleoclimte estimte A is constructed in such wy to e consistent with the prior estimtes of uncertinty nd with the chnging vilility of proxy dt over time Assumptions. [12] At this stge we summrize the ssumptions of the methodology. 1. The nlysis of modern climte vriility produces unised estimtes of E, A,, nd T. All errors re rndom. 2. E, which is sed on modern oservtions, descries the ptterns of climtic vriility recorded in the pleoproxy dt. Liner comintions of the columns of E my e used to descrie the fields oserved vi the proxy dt. 3. Errors in the modern oservtions re smll reltive to those in the proxy dt. 4. The reltion etween climtic ptterns nd the proxy dt is liner out the men vlue; in other words, proxy oservtions re unised estimtes of the trgeted climte vrile to e reconstructed or else re linerly relted to prmeter which covries with the trget climte field [Evns et l., 2]. 5. The estimte of this clirtion reltionship H nd the covrince of the model residul R, which re developed over specified clirtion intervl, re unised, correct, nd constnt. 6. There is no ge model error in the nnully verged proxy dt Cvets. [13] Shortcomings of the pproch re descried elow. 1. Spce reduction (EOF filtering) emphsizes common fetures expressed cross trget climte field nd the proxy dt set, which we expect to e climtic in origin. It discounts oth climtic nd proxy vriility, which explins little common vrince, nd is most likely to e dominted y oservtionl errors. However, since the historicl oservtions nd the proxy dt re not sptilly nd temporlly complete, estimtes of EOFs nd principl components my e ffected y selection is [Lwley, 1956; Schneider, 21]. 9S 6S 3S 3N 6N 9N AQ18,19 MAH ABR CEB TARKIR MAD ESP CAL SEC.7 PUN.6 URV 3E 6E 9E 12E 15E 18W 15W 12W 9W 6W 3W Figure 2. Smpling sites for the 13 corl dt sets exmined in this study. Contours give the RMS SST nomly from the Kpln et l. [1998] nlysis (contour intervl is C). Site nmes re revited s in Tle 1. The reconstruction domin will e the region 11 E 7 W.

4 7-4 EVANS ET AL.: PACIFIC SST FIELD RECONSTRUCTION 2. In prctice, the smll numer of ville proxy dt sets limits the numer of ptterns which my e clirted. Hence it is likely tht pst climte vriility, s represented y the proxy oservtions, is not spnned y the clirted ptterns. 3. We ssume tht ll errors in historicl oservtions nd in the proxy dt re rndom. This is most certinly not the cse. For instnce, chnges in oservtionl methods nd in the distriution of oservtions over time in historicl oservtions re typicl in mny types of climtic vriles [e.g., see Prker et l., 1994; Hnsen nd Leedeff, 1988]. Proxy dt often hve nlogous error sources s well s uncertinty in the interprettion of the proxy mesurement nd in ge model development. See elow for discussion of the prolem of ge model error in the proxy dt. 4. Proxy oservtions re not lwys unised or liner estimtes of climte vriles. 5. The numer of ptterns clirted (rnk of H) involves somewht sujective choice y the investigtor. In principle, this cn give rise to overclirtion. The influence of potentil overclirtion prolems my e exmined using verifiction tests such s those descried in section 2.4. See section 4 for n rgument tht given the fetures of the methodology presented here, this introduced sujectivity hs miniml influence on the results. 6. We ssume tht there re no errors in the ssignment of time to proxy records, ut this cnnot e true since time is never mesured directly, only inferred, in such records. Rndom or systemtic ge model errors in individul corl dt sets must introduce errors in the reconstructions. In the pproch descried here they do so implicitly y degrding the clirtion estimte with rndom error or is, incresing the clirtion uncertinty, nd offsetting timing of reconstructed events, resulting in degrded reconstructions Verifiction [14] The finl stge of the procedure is testing of the nlysis results for consistency with prior estimtes, ssumptions, nd independent oservtions. We my test the results y comprison of the proxy-reconstructed climte estimtes with (1) historicl climte oservtions not used in clirtion of the proxy dt, (2) proxy oservtions not employed in the reconstruction, (3) reconstructed climte sed on synthetic proxy dt (these synthetic dt my e historicl climte oservtions chosen to mimic the expected qulities of the rel proxy dt or red noise time series with the utocovrince sttistics of the rel proxy dt), nd (4) other proxy-sed reconstructions of pst climtes produced from independent dt sets. Once the strengths nd weknesses of the reconstruction re clerly known, we my proceed to the study of the reconstructed climte vriility. 3. Pcific SST Field Reconstructions from Corl D 18 O Dt 3.1. Trget Climte Field [15] As n exmple of the methodology descried in section 2, we reconstruct the Pcific Bsin se surfce temperture field, using 13 d 18 O time series derived from reef corls collected from 12 tropicl Pcific, Indin Ocen, nd Red Se sites (Tle 1). These sme dt were systemticlly compred with nlyzed instrumentl SSTs y Evns et l. [2]. We choose Pcific SSTs s trget climte field for its importnce to glol tmospheric circultion nd climte nomlies [Glntz et l., 199]. [16] Our description of historicl SST vriility is sed on the nlysis of [Kpln et l., 1998] of the MOHSST5 [Prker et 6S 3S 3N 6N 6 S 3 S 3 N 6 N E 15E 18W 15W 12W 9W - 12E 15E 18 W 15 W 12 W 9 W l., 1994] product of the U.K. Meteorologicl Office. MOHSST5 is 5 5 monthly compiltion of historicl ( ) shipsed oservtions of SST, qulity-controlled for ovious outliers nd corrected for estimted oservtionl ises. The nlysis of this oservtionl dt set employs the newly developed technique of the reduced spce optiml smoother to ojectively extrct lrge-scle vritions of SST on ner-glol grid (4 S 6 N) from incomplete sptil nd temporl oservtions. The smllscle fetures of SST vriility, totling C in verge mplitude nd not constrined y modern high qulity oservtions, re filtered out in this pproch. Kpln et l. [1997, 1998]discuss the pproch in detil, providing error estimtes, numerous verifiction exercises, nd tests of the roustness, strengths, nd limittions of the nlyzed fields. For our purposes we will seek reconstruction of nnul (April to Mrch) men SST, s this definition is nturl climte yer for tropicl phenomen expected to e resolved in the corl dt [Ropelewski nd Hlpert, 1987]. Herefter we will refer to these nnully verged nomlies s KSST. While we elieve this product is good description of historicl internnul nd longer-term, lrge-scle SST vriility, no existing historicl nlysis of glol SST fields is free of prolems owing to severe dt Figure 3. () Reconstructed sptil pttern 1, illustrted s the correltion of the SST field [Kpln et l., 1998] with the time series of the two clirted ptterns over the clirtion period, Contour intervl is. () Sme s in Figure 3, except for reconstructed sptil pttern 2; contour intervl is.

5 EVANS ET AL.: PACIFIC SST FIELD RECONSTRUCTION 7-5 mplitude mplitude c 15 PUN,KIR MAD numer time series 1 5 URV ABR CAL SEC AQ18 MAH CEB ESP AQ19 TAR Figure 4. () Time series of reconstructed pttern 1. () Time series of reconstructed pttern 2. (c) Numer of corl time series ville. Arevitions for corl nmes s in Tle 1; position of lel shows pproximte strt time of individul corl time series. deficiencies [Hurrell nd Trenerth, 1999]. Specific prolems nd prospects of the nlysis techniques used here re discussed y Kpln et l. [22] Proxy Dt [17] Mny high-resolution pleoclimtic studies hve exploited corl d 18 O mesurements to construct decdes- to centuries-long records of tropicl ner-surfce conditions. The oxygen isotopic composition of corlline rgonite is n pproximtely liner function of the SST in which the corl secretes its rgonite, the locl net freshwter flux, nd disequilirium offset from se wter d 18 O. In previous pper [Evns et l., 2] we rgued tht such dt could e used together to study lrge-scle ptterns of SST vriility, through resolution of the covrint locl SST or locl se wter d 18 O signl recorded y the corls. Here we employ the sme 13 corl dt set studied in our previous work. These records re listed in Tle 1, nd their loctions re shown in Figure 2.

6 7-6 EVANS ET AL.: PACIFIC SST FIELD RECONSTRUCTION 6S 3S 3N 6N S 3S 3N 6N 12E 15E 18W 15W 12W 9W 12 E 15E 18 5W 12W W 1 9W c d 6S 3S 3N 6N 6S 3S 3N 6N E 15E 18 W 15 W 12 W 9 W 12E 15E 18 W 15 W 12 W 9 W Figure 5. Clirtion ( ) sttistics for the corl-sed SST reconstruction (CoSST). () Correltion. Shding indictes significnce t the 95% confidence level, ssuming 44 effective degrees of freedom. () Rootmen-squre difference etween CoSST nd historicl SST [Kpln et l., 1998] ( C). (c) Men mplitude of CoSST ( C). (d) Estimted error in CoSST ( C). Reported resolution of the records vry from monthly (TAR, KIR, MAH, nd SEC) to sesonl (MAD, CAL, nd PUN) to nnul (AQ18, AQ19, CEB, URV, nd ESP) to iennil (ABR). We clculte nomlies reltive to the sesonl cycle (if ny is resolved) in the dt nd verge ech record to nnul (April Mrch men) resolution Clirtion [18] We clirte the corl d 18 O dt using KSST. To provide for testing of the results, the potentil full clirtion period ( ) ws split into two hlves of equl length ( nd ). The corl d 18 O-sed reconstruction ws mde using the clirtion intervl The complementry period ( ) ws reserved for verifiction (section 2.4). Herefter we refer to the corl-sed SST reconstruction s CoSST. Results of the clirtion were consistent with Evns et l. [2], who rgued tht two ptterns could e verifily resolved from the thirteen corl time series Anlysis [19] Reconstruction ws performed for the intervl , over which t lest one corl time series ws ville. The sptil ptterns which were reconstructed re shown in Figure 3 s the correltion of the SST field with the time series of the two clirted ptterns over the clirtion period, The time series of the reconstructed ptterns together with the numer of corl time series ville for nlysis re shown in Figure 4. Consistent with the results of Evns et l. [2], the two ptterns look like the ocenogrphic signture of the ENSO phenomenon (Figure 3) nd sin-wide positive trend, in which portions of the estern equtoril nd North Pcific show the opposite sense (negtive) trend (Figure 3). [2] To check our initil ssumptions (section 2.3.1), we show four field sttistics compring CoSST to KSST (Figures 5 9). Correltion etween CoSST nd KSST shows where the clirted ptterns hve skill, regrdless of signl mplitude (Figures 5 9). The root-men-squre difference etween CoSST nd KSST gives the men ctul error in the reconstructed fields (Figures 5 9). Figures 5c 9c shows the RMS vrince of CoSST; this mp my e compred to Figure 2 to ssess the extent to which the nlysis resolves vrince in the trget climte field. Figures 5d 9d gives the estimted error in CoSST, clculted from the error covrince mtrix P (eqution (7)) s dig (EPE T ). This mp my e compred to the RMS difference descried ove (Figures 5 9) to determine consistency of the reconstruction s estimted error estimte. These sttistics my e

7 EVANS ET AL.: PACIFIC SST FIELD RECONSTRUCTION 7-7 6S 3S 3N 6N S 3S 3N 6N.6 c 12E 15E 18W 15W 12W 9W d 12 E 15E 18 W 15W 12W 9W 6S 3S 3 N 6 N 6S 3S 3 N 6 N E 15E 18 W 15 W 12 W 9 W 12E 15E 18 W 15 W 12 W 9 W Figure 6. Verifiction ( ) sttistics for the corl-sed SST reconstruction (CoSST). ( d) Sme s Figures 5 5d. compred etween clirtion nd verifiction periods to determine whether there is rtificilly clirted skill, nd to explore the extent to which prior ssumptions re violted. [21] Clirtion ( ) sttistics for the CoSST re shown in Figure 5. The correltion mp (Figure 5) shows tht the reconstruction clirtes vrince in the centrl nd estern equtoril Pcific s well s in the north nd south sutropicl gyres. RMS error etween CoSST nd KSST is lso lrgest in the centrl nd estern equtoril Pcific. This is ecuse of the significnt error in the clirtion. Since the errors re lrge, the reconstructed vrince is smll, nd there re lrge errors even where skill is high. This is illustrted y Figure 5c, which shows tht the mplitude of reconstructed SST vriility in the centrl nd estern equtoril Pcific is out the mgnitude of the error in this region. Figure 5d shows the estimted error in the reconstruction; since these results re computed for the clirtion period, it mtches y design the ctul error in the reconstruction over this period, with smll differences due to the chnges in dt vilility over this period Verifiction Comprison with withheld historicl oservtions. [22] The sttistics show tht fetures of SST vriility in the centrl nd estern equtoril Pcific re verifily resolved y the reconstruction (Figure 6), with ctul men error (Figure 6) similr to the estimted error (Figure 6d), leit with smll signl vrince (Figure 6c). Skill is significnt not only t the corl loctions ut in rod regions of the estern equtoril Pcific s well nd, to lesser extent, in the sutropicl gyres. An experiment with switched clirtion nd verifiction periods gives similr results (not shown). Use of one or three ptterns (insted of two) for reconstruction shows slightly poorer verifiction skill. Becuse the nlysis is punished for putting too much weight into ptterns which do not contriute much to modern climte vriility, overclirtion prolems re minimized. These results suggest tht our ssumptions (section 2.3.1) on the stility of the clirtion, numer of ptterns clirted, nd estimtion of prior covrinces, lthough violted in prctice, re not ftl to the results, given the cvets discussed. [23] These results lso illustrte the glolity of the methodology employed here (section 2): corls remote from these locles nd even those from outside of the Pcific Bsin (e.g., MAH, ABR, AQ19, CEB, MAD, ESP, nd CAL) mke significnt contriution to the reconstruction skill, s judged y the correltion etween individul corl records nd time series of the clirted ptterns [Evns et l., 2, Tle 3]. However, with the reduction in dt vilility over time (Figure 4c) the vrince resolved decreses, nd the error in the nlysis increses. On the sis of these results,

8 7-8 EVANS ET AL.: PACIFIC SST FIELD RECONSTRUCTION 6S 3S 3 N 6N S 3 S 3 N 6 N c 12E 15E 18W 15W 12W 9W d 12 E 15 E 18 W 15 W 12 W 9 W 6 S 3 S 3 N 6 N 6 S 3 S 3 N 6 N E 15 E 18 W 15 W 12 W 9 W 12 E 15 E 18 W 15 W 12 W 9 W Figure 7. Verifiction ( ) sttistics for the enchmrk SST reconstruction (see text). ( d) Sme s Figures 5 5d. we elieve tht the reconstruction is interpretle ck to 18 nd is most skillful in the NINO3.4 index region (17 12 W, 5 N 5 S). [24] Results lso suggest tht ge model errors in individul corl records my not hve much influence on the reconstruction results. Suspected prolems with the ge model of the Urvin By record of Dunr et l. [1994] re not lrge component of the reconstruction error: with the Urvin By record removed from the experiment, verifiction correltion with historicl SST (s in Figure 6) chnges y less thn tenth of correltion unit in the most skillfully resolved regions (results not shown). The methodology employed here ((4) (7)) ensures tht the influence of ny individul record is weighted y its oservtionl uncertinty nd tht no one proxy record inordintely domintes the solution. More comprehensive experiments such s this my led to improved oservtionl error estimtes for individul corls. They lso highlight the importnce of record repliction to reduce overll proxy nd reconstruction errors [Evns et l., 1998]. [25] The reconstruction of the NINO3.4 index presented here is n improvement on tht otined vi liner regression of the NINO3.4 SST index on the sme 13 corl time series (Tle 2). Clirtion correltions retrieved y the two pproches re out equl; however, correltion with the NINO3.4 index of Kpln et l. [1998] shows tht our reconstructed NINO3.4 index is more roust in the verifiction period. Verifiction period error rises only smll mount from the clirtion period error, while the multiple liner regression error rises sustntilly. These results illustrte tht limiting the reconstruction to wht my e verified (spce reduction) nd incorporting clirtion error into the reconstruction methodology (ojectivity) improves the qulity of the outcome Comprisons with reconstructions sed on synthetic dt. [26] We perform two dditionl experiments to ssess the qulity of the results. The first is reconstruction of the SST field using KSST time series from the corl loctions. This enchmrk experiment is intended to illustrte reconstruction from the est possile dt. A second experiment ( noise ) employs red noise time series with utocovrince sttistics like those of the corl dt nd is intended to illustrte the influence of dt contining no climtic informtion side from persistence. [27] The verifiction ( ) results for these experiments re shown in Figures 7 nd 8, respectively. Compred to CoSST, the enchmrk reconstruction (Figure 7) hs higher skill, smller estimted error, nd greter resolved vrince. This is not surprising given the multivrite dependencies of d 18 O nd the higher uncertinties of individul d 18 O time series. However, we lso note

9 EVANS ET AL.: PACIFIC SST FIELD RECONSTRUCTION 7-9 6S 3S 3N 6N S 3S 3N 6N c 12E 15E 18W 15W 12W 9W d 12 E 15E 18 W 15W 12W 9W 6 S 3 S 3 N 6N 6 S 3 S 3 N 6N E 15E 18 W 15 W 12 W 9 W 12E 15E 18 W 15 W 12 W 9 W Figure d. Verifiction ( ) sttistics for the noise SST reconstruction (see text). ( d) Sme s Figures (Figure 8) tht the skill, resolved vrince, nd error in CoSST re etter thn tht expected if the proxy dt contined no climtic informtion whtsoever. These results suggest tht dditionl proxy time series from importnt regions like the centrl nd estern equtoril Pcific will improve CoSST y resolving the reconstructed ptterns with less error [Evns et l., 1998] Comprison with other ENSO reconstructions. [28] A further nd desirle verifiction is tht our reconstruction grees with similr ttempts using different methodologies nd proxy dt sets. Two exmples re the wintertime Southern Oscilltion Index (SOI, difference etween stndrdized se level pressure nomlies t Thiti nd Drwin) reconstructed from tree ring dt y Sthle et l. [1998] nd the nnul NINO3 SST index (SST nomly verged over 15 9 W, 5 N 5 S) reconstructed from vriety of proxy dt y Mnn et l.[1998]. Rigorous intercomprison efforts re hindered y differences in the reconstructed vrile, reconstructed seson or yer definition, selected clirtion intervl, nd the overlp etween proxy dt sets employed. Nevertheless, given tht the methods pplied in these studies re ll forms of multiple liner regression nd use nnul or higher-resolution proxy oservtions nd tht the reconstructed quntities (NINO3.4, NINO3, nd SOI) re strongly covrint in the modern oservtions, it is useful exercise to compre the results. [29] Twentieth century correltions etween ll ENSO pleoestimtes nd instrumentl indices re significnt t or ove the 95% level (Tle 3, top). A stricter test is intercomprison of reconstructions over the nineteenth century (Tle 3, ottom). Notly, two of the three proxy-proxy correltions remin significnt, ut this is not surprising ecuse of common proxy dt input. For exmple, 7 of the 14 time series employed y Sthle et l. [1998] re used y Mnn et l. [1998], nd 4 of 5 1 corl d 18 O records employed in the present study (Figure 4) were lso used in the study of Mnn et l. [1998]. Only the Sthle et l. [1998] study nd the present work re sed on independent sets of proxy oservtions; correltion etween these proxy ENSO reconstructions wekens from.61 in the twentieth century to.9 in the nineteenth century. While it should e noted tht the greement etween these two pleoreconstructions improves through the nineteenth century, s numer of corls ville for nlysis increses, this is discourgingly consistent with the results of Schmutz et l. [2] regrding proxy North Atlntic Oscilltion reconstructions. Disgreement is proly due to the smll numer of proxy oservtions ville for the most importnt ENSOinfluenced regions nd proxy dt ge model error. Additionl sites nd more repliction of dt series will e required for etter oservtionl error estimtes nd improved reconstructions. Further improvements my lso stem from systemtic proxy-

10 7-1 EVANS ET AL.: PACIFIC SST FIELD RECONSTRUCTION 6S 3S 3N 6N S 3S 3N 6N.6.7 c 12E 15E 18W 15W 12W 9W d 12 E 15E 18 W 15W 12W 9W 6S 3S 3 N 6 N 6S 3S 3 N 6 N E 15E 18 W 15 W 12 W 9 W 12E 15E 18 W 15 W 12 W 9 W Figure 9. Verifiction ( ) sttistics for the tree ring indictor-sed SST reconstruction (TrSST). Correltions significnt t the 95% confidence level ssuming 4 degrees of freedom re shded. ( d) Sme s Figures 5 5d. instrumentl nd proxy-proxy intercomprison efforts s well s detiled study of the processes controlling proxy responses to climte vrition. 4. Discussion 4.1. Reconstruction Fetures [3] The CoSST reconstruction presented here is sed on t most 13 corls, which together resolve t most two ptterns of SST vriility. The ENSO pttern of SST vriility is resolved much Tle 2. Skill of NINO3.4 Estimtes, Sttistic NINO3.4 (This study) NINO3.4 (MLR) Clirtion correltion Clirtion error 5 C 5 C Verifiction correltion.62 4 Verifiction error 1 C 9 C Correltions nd error estimtes formed vi comprison with Kpln et l. [1998] NINO3.4. Clirtion period: ; verifiction period: MLR, multiple liner regression. more roustly thn the second pttern; this is to e expected given tht tropicl smpling sites from the Pcific nd Indin Ocens re expected to resolve ENSO [Evns et l., 1998]. It is lso consistent with the results of mny of the corl reserchers referenced here who find tht their individul dt series index ENSO quite relily on internnul timescles. Correltion of the first pttern reconstructed here with the NINO3.4 SST index (SST nomly verge for the region 17 W 12 E, 5 N 5 S) from Kpln et l. [1998] is.71 for the clirtion ( ) period nd.62 for the verifiction ( ), significnt t the 99% level ssuming 45 degrees of freedom [Trenerth, 1984]. The trend pttern reconstructed hs very smll mplitude (Figures 3 nd 4), much smller thn tht found in mny individul corl records. This is consistent with our finding [Evns et l., 2] tht individul corl d 18 O time series often hve decdl nd seculr vriility ut tht much of this vrince does not linerly reflect SST vriility or SST covrint vriility in net freshwter flux. By nlyzing the corl dt s set we cn isolte the climticlly induced frction Comprison With Results Bsed on Tree Ring Dt [31] Results from CoSST my e compred to those otined y Evns et l. [21], which ttempted reconstruction of the sme trget climte field using 15 tree ring indictors from Pcific

11 EVANS ET AL.: PACIFIC SST FIELD RECONSTRUCTION 7-11 Tle 3. Comprison of ENSO Pleoreconstructions Correltion r M-NINO3 E-NINO3.4 K-NINO3 K-SOI S-SOI M-NINO E-NINO K-SOI S-SOI M-NINO E-NINO K-SOI.76 Comprisons with K-NINO3 in period re for , nd comprisons with K-SOI re for Sources: M-NINO3, Mnn et l. [1998]; E-NINO3.4, this study; K-NINO3, Kpln et l. [1998]; S-SOI, Sthle et l. [1998]; K-SOI, Können et l.[1998]. Correltions re significnt t or ove the 95% confidence level (considering effects of seril utocorreltion in the time series [Trenerth, 1984]). influenced regions of North nd South Americ. The tree ring indictors were selected y Villl et l. [21] ecuse they sense locl precipittion nd temperture nomlies which re in turn ssocited with Pcific Bsin SST vriility. In this work we found tht the tree ring indictors could e used to reconstruct one pttern of SST vriility. On the sis of dt vilility nd error estimtes we elieve this reconstruction, here termed TrSST, is interpretle for the intervl (Figure 9). This pttern is sptilly ENSO-like [Villl et l., 21; Evns et l., 21] ut is not s equtorilly trpped s ENSO is. Also, unlike the ENSO pttern, its dominnt frequency of vriility is rodly decdl to interdecdl. [32] Cvets tht should e considered here re tht oth proxy dt sets re smll, the numer of resolved ptterns is smll, nd the errors in the reconstructions re quite lrge. Nevertheless, it ppers tht these two independent sources of proxy dt resolve SST vriility which is roughly seprle in the frequency domin (Figure 1). It my e possile to reconstruct ll three ptterns with skill from the comined dt sets if the oservtionl error for the different proxies (eqution (4)) cn e ccurtely determined. However, we lso note tht oth proxy reconstructions contin much more vrince t low frequencies thn do historicl oservtions. Furthermore, the two proxy reconstructions suggest differing levels of interdecdl vriility. Multiproxy reconstructions, in which the type of proxy dt employed chnges over time, my reflect spurious frequency evolution due to skill prtitioning in the frequency domin. Reconstruction experiments in which susets of proxy dt re sequentilly removed from the nlysis my indicte whether this prolem is significnt Low-Frequency Behvior of ENSO in the Preindustril Er [33] Since CoSST hs verifile skill only in certin prts of the reconstruction domin (Figure 6), we form n re-verge index for time series nlysis from region where skill is high. As suggested y Figure 6 we exmine chrcteristics of the NINO3.4 SST index formed from CoSST. The uncertinty in CoSST is such tht it precludes sophisticted quntittive nlyses. Nevertheless, the results suggest tht ENSO ws more frequent fter 198, lower in the epoch, nd gin more frequent 1 1 KSST spectrum CoSST spectrum TrSST spectrum power spectrl density frequency (cyc/yr) Figure 1. Power spectr of KSST, CoSST, nd TrSST over the common intervl

12 7-12 EVANS ET AL.: PACIFIC SST FIELD RECONSTRUCTION numer wrm events/31 yers NINO3.4 SST nomly Figure 11. () Numer of ENSO wrm phse events (NINO3.4 SST nomly > C reltive to the men over running 31 yer window), s reconstructed in CoSST. () TrSST NINO3.4 SST nomly ( C), filtered with 31 yer Hnning window. On the sis of error nlysis nd numer of proxy time series ville, TrSST is interpretle ck to 155; CoSST is interpretle ck to out 187. round the eginning of the 19s. This is consistent with previous work y Trenerth nd She [1987] nd confirmed lter y corl pleoclimte studies [Cole et l., 1993; Urn et l., 2]. CoSST NINO3.4 further suggests tht the period ws lso period of reltively vigorous ENSO ctivity, while the period ws reltively quiescent (Figure 11), the very strong event of 1877 excepted. Comprison with Pcific decdl SST vriility inferred independently from TrSST suggests tht ENSO wrm phse frequency is ssocited with periods of wrm Pcific men stte, consistent with the hypothesis put forwrd y Gershunov nd Brnett [1998] nd Urn et l. [2]. As these oservtions extend t lest into the preindustril period, ttriution of the unusully ENSO-rich pst few decdes my lie in prt with nturl vriility. 5. Conclusions [34] We hve pplied methodology to verifily reconstruct pst climte vriility from proxy dt to the prolem of SST field reconstruction using the ville network of corl d 18 O dt. Two ptterns re reconstructed: ENSO nd trend in which regions of the North Pcific nd estern equtoril Pcific respond in the opposite sense. The reconstruction hs lrge errors ut is proly qulittively interpretle ck into the erly nineteenth century. A similr ttempt to reconstruct Pcific SST using n independent set of tree ring indictors suggests tht these different types of proxy dt my provide reconstruction skill roughly seprle in the frequency domin. Comprison of the results of these two reconstructions of Pcific Bsin SST suggests tht the frequency of ENSO is tied to the men stte of the Pcific, tht sid frequency fluctutes on interdecdl time scles, nd tht this ssocition extends into the preindustril er. If these oservtions re correct, they will e orne out nd strengthened y the incorportion of dditionl pleoproxy dt into the nlysis. [35] Acknowledgments. This work ws supported y NOAA/ESH grnt NA86GP437 nd NOAA/CCDD grnt NA16GP1616. We thnk Mrk Ekin, the WDC-A for Pleoclimtology, nd the cited scientists for the use of their dt. LDEO contriution References Bennett, A. F., Inverse methods in ssessing ship-of-opportunity networks nd estimting circultion nd winds from tropicl expendle thythermogrph dt, J. Geophys. Res., 95, 16,111 16,148, 199. Bretherton, C. S., C. Smith, nd J. M. Wllce, An intercomprison of methods for finding coupled ptterns in climte dt, J. Clim., 5, , Bretherton, F. P., nd J. C. McWillims, Estimtions from irregulr rrys, Rev. Geophys., 18, , 198. Cne, M. A., A. Kpln, R. N. Miller, B. Tng, E. C. Hckert, nd A. J. Buslcchi, Mpping tropicl Pcific se level: Dt ssimiltion vi reduced stte spce Klmn filter, J. Geophys. Res., 11, 22,599 22,617, Cne, M. A., A. C. Clement, A. Kpln, Y. Kushnir, R. Murtugudde, D. Pozdnykov, R. Seger, nd S. E. Zeik, 2th century se surfce temperture trends, Science, 275, , Chrles, C. D., D. E. Hunter, nd R. G. Firnks, Interction etween the ENSO nd the Asin monsoon in corl record of tropicl climte, Science, 277, , Cole, J. E., R. G. Firnks, nd G. T. Shen, Recent vriility in the Southern Oscilltion: Isotopic results from Trw Atoll corl, Science, 26, , Cook, E. R., K. R. Briff, nd P. D. Jones, Sptil regression methods in dendroclimtology A review nd comprison of 2 techniques, Int. J. Clim., 14, , Druffel, E. R. M., nd S. Griffin, Lrge vritions of surfce ocen rdiocron: Evidence

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