Reconstructing Tropical Atlantic Hydrography Using Planktontic Foraminifera and an Ocean Model

PALEOCEANOGRAPHY, VOL. 5, NO. 3, PAGES 409-431, JUNE 1990

RECONSTRUCTING TROPICAL ATLANTIC HYDROGRAPHY USING PLANKTONTIC FORAMINIFERA AND AN OCEAN MODEL

A. C. Ravelo and R. G. Fairbanks

Lamont-DohertyGeological Observatory of ColumbiaUniversity, Palisades, New York

S. G. H. Philander

GeophysicalFluid DynamicsLaboratory, NOAA, PrincetonUniversity, Princeton, New Jersey

Abstract.In thetropical Atlantic, planktonic modelcan be usedto simulatehydrographic changes fomminferaspecies are vertically distributed with to comparewith faunalpredicted past hydrographic highestabundances occunfng in thephotic zone changes.Since the oceanmodel is wind driven,this (approximately0-100 m). The tropicalAtlantic approachprovides a way of evaluatingthe validity of thermoclinedips from eastto westand varies estimatesof pastwind stresschanges and the seasonallydue to changesin the southeastand contributionof thesechanges to the faunalchanges northeasttrade winds. In the east, the thermoclineis in the past. A doublewind stressrun indicatesthat in thephotic zone, and in thewest, the well-mixed thecentral and eastern equatorial and southeast surfacelayer extends below the photic zone most of regionsof the studyarea are mostsensitive to wind theyear. As expectedfrom speciesvertical stressincreases. Factor analysis of the foraminifera distributionsin planktontows, the species abundancesfrom thelast glacial maximum (LGM) assemblageson the seafloorare correlatedto the showsthat species associations change downcore hydrographicconditions of theoverlying surface anddemonstrates how themethods developed in this oceanlayer. A new techniqueto reconstructpast studycan be applied.Comparison of thedouble tropicalAtlantic (20øN to 20øS)photic zone windstress experiment and the LGM faunalchanges hydrographyand surface wind field usesfaunal indicatessome areas of significantagreement assemblagedata from deep-sea cores. Planktonic suggestingthat faunal changes may reflect foraminiferaabundances in coretops correlate with thermoclinesu'ucmre response to theLGM wind observationsof modernphotic zone hydrography field. Discrepanciesmay reflectthe fact thatuniform definedhere as seasonal temperature variation and changesin thenorth and south trade wind strengths mixedlayer depth. The hydrographyis did not occur at the LGM. mathematicallydescribed using empirical orthogonal function(EOF) analysisof annualtemperature range INTRODUCTION asa functionof depth.Factor analysis of 29 species of planktonicforaminifera from 118core tops Generalcirculation models (GCMs) predict producesthree factors. The factorscorrelate to increasednortheasterly trade wind intensityand mixedlayer depthand the two EOF modes.The decreasedsouthwesterly monsoonal flow in the oceanmodel of theAtlantic ocean produces similar tropicalAtlantic during the last glacial maximum mappatterns of the EOF modes. Thereforethe [Williamset al., 1974; Gates,1976; Manabeand Hahn, 1977; Manabeand Broc•01i• •1983; Kutzbach andGuetter, 1986; Rind, 1987]. L•ger changesin Copyright1990 surfacewinds, increasedsoutheasterly and by the AmericanGeophysical Union. monsoonalflow, are predicted for 9000years ago whenthe Earth's precessional cycle was at a Papernumber 90PA00484. maximum(northem hemisphere summer solstice and 0883- 8305/90/90PA-00484510.00 perihelionwere aligned)and low-latitude seasonal 410 Raveloet al.' ReconstructingTropical Atlantic Hydrography insolationwas enhanced[Kutzbach, 1981; changesin wind stressmay affectthe thermocline Kutzbach and Otto-Bleisner, 1982; Kutzbach and structure. The model uses modem seasonal wind Street-Perrott, 1985; Kutzbach and Guetter, 1986]. stressobservations as input [Hellermanand Paleoclimaticobservations, used to predictindirectly Rosenstein,1983] and predictsseasonal pastchanges in wind stress,generally agree with the hydrographicconditions that are in closeagreement GCMs. Studiesof lake levels [Street-Perrottand with observationaldata [Garzoli and Philander, Roberts, 1983; Street and Grove, 1979; Street- 1985; Richardson and Philander, 1987]. The Perrott and Harrison, 1984; Butzer et al., 1972], of model has been discussed and evaluated in detail pastdune distribution of continentalAfrica [Philander and Pacanowski, 1986a,b.; Richardson [Sarnthein,1978; Fairbridge,1964], andof deep- and Philander, 1987] and demonstratesthat wind- searecords of sapropelsfrom the Mediterranean inducedsurface currents play a primaryrole in the [Rossignol-Strick,1983; Rossignol-Stricket al., adjustmentprocesses of the upperocean. The upper 1982; Rossignol-Strick,1985] andMelosira and oceanis decoupledfrom the deepocean, and its phytolithsfrom the tropicalAtlantic [Pokrasand seasonalthermal structure is determinedby surface Mix, 1985] indicatepast precipitation pattems which winds [Philander and Pacanowski, 1980]. indirectlyinfer changesin monsoonalflow. Deep- Sincethe model predicts that the thermocline searecords of aeolian materials [Samthein et al., structureand surfaceocean circulation adjusts to 1981; Kolla et al., 1979; Parkin and Shackleton, wind stresschanges, on geologictime scales, 1973; Diester-Haass, 1976] and faunal studiesfrom instantaneously(a few months)[Philander and the tropicalAtlantic and Indian oceans [Prell, Pacanowski,1984], wind stresschanges in the past 1984a,b; Mix et al., 1986; Mcintyre et al., 1989] canbe monitoredby measuringparameters that havebeen used to indicatepast changes in wind indicatechanges in thethermocline structure. stressmagnitude and direction, although the delivery Becauseforaminifera grow throughoutthe photic of aeoliandust to the oceanis dependenton source zonewhich has steep thermal gradients in theeastern areaaridity as well. tropics(Fairbanks et al. [1982], Curryet al. [1983], Althoughthese paleoclimatic observations are andthis paper, Figure 1) speciesabundances can be generallyin agreementwith the GCM results,the usedto recordvertical hydrographic gradients which timing andmechanism of wind stressresponse to respondto wind changes.Since the tropicsis a insolationchanges associated with theprecessional regionwhere last glacial maximum (LGM) cycle (19-23 kyr periodicity)are still unresolved. continental evidence contradicts estimated sea surface GCMs predictthat surfacewinds in the tropics temperatures[Rind andPeteet, 1984], thereis instantaneouslyrespond to low-latitudeinsolation further reason to examine the ocean faunal data. changesindependent of the extratropicalboundary While oneof theprimary purposes of the CLIMAP conditions of the model, and therefore, in the ProjectMembers [ 1981] approachof usingfaunal geologicrecord, low-latitude wind stresschanges assemblagesto reconstructsea surface temperature shouldbe in phasewith low latitudeinsolation (SST) fieldswas to generateinput for a LGM changesdominated by precessionalcyclicity atmosphericmodel simulation,the purposeof this [Kutzbach and Guetter, 1986]. Observational studyis to generatea reconstructionof theupper studies,however, indicate that maxima in monsoonal watercolumn hydrography to comparewith ocean wind intensitylag behindsolstice-perihelion model simulations.Therefore we take advantageof alignmentby thousandsof years[Street and Grove, thefact thatspecies abundances in the tropics 1979; Rossignol-Stricket al., 1982; Rossignol- respondto hydrographicconditions other than SST. Strick, 1983; Street-Perrott and Roberts, 1983; The integrationof the modelprovides a way of Street-Perrottand Harrison,1984; Rossignol- predictingpast changes in themagnitude and Strick, 1985; Pokrasand Mix, 1985; Mcintyre et directionof tropicalwind and a way of assessingto al., 1989]. The tropicalwind systemresponse whatextent faunal changes in the tropicalAtlantic mechanismsto changesin seasonalinsolation and canbe explainedby wind stresschanges predicted the degreeof decouplingbetween the tropical and by GCMs andpaleowind observations. extratropicalforcing have been discussed in some In thispaper, planktonic foraminifera assemblages studies [Prell, 1984a,b; Prell and Kutzbach, 1987]. from coretops are correlatedto the hydrographyof Our approachis to takeadvantage of therapid the oceansurface layer. The seasonalhydrography dynamicresponse of the surfacelayer of the tropical is characterizedby usingempirical orthogonal Atlantic Ocean to surface winds, and to combine function(EOF) analysisof the annualtemperature results from an ocean model with observational data rangeversus depth in the oceansurface layer. The to developa techniqueto monitorwind stress EOF modeweightings of a modelsimulation of the changesthat is independentof continental modemtropical Atlantic ocean circulation is paleoclimate. comparedto the EOF modeweightings of modem We usea high-resolutionocean model [Philander observations[Levims, 1982]. Finally, an andPacanowski, 1984] for the tropicalAtlantic to experimentwhere winds are systematicallydoubled simulatecirculation patterns in orderto seehow showswhich areasare hydrographicallymost Ravel: et al.: ReconstructingTropical Atlantic Hydrography 411

Temperature(øC) Temperature(øC) Temperature(øC)

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I t ;-' 6o• / 60J • / G. sacculifer 80• ] G. ruber 80• • G.ruber 8o• J (w/o sac-like • [ (whitevariety) 10ot ( (pinkvariety) lOOl . • .... •oo] [ finalchamber) 0 2000 4000 6000 0 5000 10000 15000 0 10000 20000 30•0 Number per 1000 cubicmeters Numberper 1000 cubicmeters Number per 1000 cubic meters

Temperature(øC) Temperature(øC)

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Chlorophyll (gM) Chlorophyll(gM) o ("q • 'qD 00 o, c? . c• . ,cs . o, . ,• 0,

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•' 20 - g6øt 80t lOOl loo o lOOO 2000 3000 0 200 400 600 800 1000 Number per 1000 cubicmeters Numberper 1000 cubicmeters Fig. 1. Eastequatorial plankton tow abundancesfor nine species.Histograms of G. ruber varieties,G. sacculifer(without sac-likefinal chamber),G. inflata, and G. glutinata are shown with a temperatureprofile measuredduring their collection(2øS, 1 løW). Histogramsof N. dutertrei,G. menardii,G. tumida,and P. obliquiloculataare shownwith a chlorophyllprofile measuredat the location of the tows (4øN, 6øW). 412 Raveloet al.' ReconstructingTropical Atlantic Hydrography sensitive to wind stressincreases. Thus far, there 1986; Hisard et al., 1986; Katz et al., 1986; havebeen no oceanmodel simulations of pasttimes Garzoli and Clements,1986; Katz, 1987]). usingGCM-generated surface wind fields. This is However,the exactrelationship between wind our next objective. However,LGM faunaldata are stress,surface currents,vertical meridional interpretedin light of thecore top correlationwith transport,and the regional thermocline structure modernhydrography. cannot be understood from observations alone. Thus the ocean model that we use is useful in TROPICAL ATLANTIC CIRCULATION understandingthese relationships.

Seasonalchanges in surfacewinds of thetropical METHODS Atlantic,associated primarily with meridional migrationsof theIntertropical Convergence Zone In orderto useabundances of multiplespecies of (ITCZ), drive the tropicalsurface currents. In March planktonicforaminifera as indicators of thermal andApril the ITCZ is closeto the equator,winds at gradientsand seasonality, it is importantto havea the ITCZ are relaxed and surface currents are weak goodunderstanding of foraminiferadepth habitat and and westward, and the strengthof the northeast seasonality.Much of the basicgroundwork for such tradesand the North EquatorialCurrent (NEC) are at an understandinghas been done. Severalauthors a maximum. The thermoclinegenerally slopes from documentthe verticaldistribution and seasonalityof the southeast where it is shallow to the northwest foraminiferaspecies and zooplankton using whereit is belowthe photic zone (the top layer of the abundances[Jones, 1967; Fairbanksand Wiebe, oceanthrough which light penetrates). In May the 1980; Ormer et al., 1980; Tolderlund and Be, 1971; southeasttrades intensify and the ITCZ moves Be, 1982; Thunelland Reynolds, 1984]. Isotopic measurements of foraminifera from tows and northward. At 3øN the westwardflowing South EquatorialCurrent (SEC) intensifiesand between sedimenttraps indicate calcification depth and 3øNand 10øN,the Noah EquatorialCurrent seasonality[Fairbanks et al., 1980; Deuseret al., becomesthe eastwardflowing Noah Equatorial 1981; Williams et al., 1981; Fairbanks et al., 1982; Countercurrent(NECC). Garzoli and Katz [1983] Curry et al., 1983; Deuserand Ross,1989]. suggestthat the reversal of thetrade winds at this Typically,certain species are found in greatest time is responsiblefor thereversal of theNECC. abundancein the mixedlayer, and someare foundin Shipdrift measurementsshow that the reversal the subsurfacethermocline where primary occursin winterand spring in the westernAtlantic productivityis at a maximum.Figure 1 shows [Richardson,1984]. Overall, the NECC flow is speciesabundances from plankton tows in the warm andmost intense in Augustand October when easterntropical Atlantic, confuming vertical it depressesthe thermocline just north of theequator distributionpatterns first measuredin the eastern all theway acrossthe Atlantic Ocean. The tropicalPacific [Fairbanks et al., 1982; Curryet al., magnitudeand direction of thetropical Atlantic 1983]. These studies,which demonstratethat in the oceaniccurrents as well asthe upwellingof cold tropicsdepth-stratified planktonic foraminifera watersand the advectionof extratropicalwaters can speciescan be usedto monitorsurface layer all haveprofound effects on thethermal structure of hydrography,are the basisfor the methodused in the watercolumn (the depthof the mixedlayer and this studyand provide insight for the interpretation the gradientof the thermocline)[Merle, 1983]. of the resultsof the coretop factoranalysis. In orderto correlatehydrography to the coretop Regionalupwelling brings cool waters and foraminiferafaunal assemblages,the seasonal nutrientsinto thephotic zone making the eastern hydrographywas reduced to threehydrographic tropicsa regionwith highsurface productivity and "modes"(Figure 2), andthe relativeabundances of steepthermal gradients. Upwelling is notlimited to 29 speciesof planktonicforaminifera from 118 core the coastalregions; upwelling causes the regional topsfrom between20øN and20øS were reduced to "doming"of the thermoclineas well. Generally,the threefactors using factor analysis. 104 coretops are thermoclinedips downward from eastto west,with from Prell's[ 1985] compilationof the CLIMAP depressionsin downwellingareas and doming in calibrationdata set, and the remaining 14 coretop upwellingareas. In theeast the thermoclineis abundancesare from a studyby Mix [ 1985]. Scores shallowand steep,and in the westthe mixedlayer is for threefactors were calculatedfrom the coretop deepand the thermoclineis belowthe photic zone. abundancedata. The coretop factorloadings were Variationsin the strengthof the tradewinds control thencorrelated to hydrographicparameters discussed the observedspatial and temporalvariations in below. upwellingand downwelling intensity and therefore in the depthof the thermoclineand mixed layer. There havebeen a numberof projectsdesigned to EOF Analysisof Hydrography obtain direct observations of surface flow and wind patternsfrom the tropicalAtlantic in thepast few The hydrographyof the tropicalAtlantic (20øN to years(Sequal/Focal program [Weisberg, 1984; 20øS)was characterized using monthly temperature Garzoli and Katz, 1983; Richardson and Walsh, observations[Levitus, 1982]. The first Raveloet al.: ReconstructingTropical Atlantic Hydro•aphy 413

Deep Mixed THREE -25 Layer HYDROGRAPHIC Hydrographic Mode ENDMEMBERS -50 (Deep Mixed Layer and Two EOF Modes) a

I . • -7o'"i5 .... io' 30 Temperature(øC)

[ Seasonality _5o-25 • -25-5o[Hydrøgraphice Mod j

C. -7510 15 20 25 30 -7 Temperature(øC) SurfaceSeasonality Mode (StandardizedScores of EOF Mode 1) 0 ..... 0 \ HighSubsurfacel X,, Seasonality. -25 '•-25 x• Hydrographic'

-50 •2 -50

-7 15 20 25 30 -7 -1 0 1 2 Temperature(øC) ShallowThermocline Mode (StandardizedScores of EOF Mode 2) Fig. 2. The threehydrographic end-members. Monthly temperatureprofiles are shownfor (a) the deepmixed layer end-member,(b) the high surfaceseasonality end-member, and (d) the high subsurfaceseasonality end-member. The modescores from the EOF analysisof temperaturerange as a functionof depthare plottedfor (c) EOF mode 1 and (e) EOF mode 2. hydrographicparameter used was annualaverage et al., 1980, 1982], and therefore their fluxes to the mixed layer thickness.Mixed layer depthfor each seafloorare probablynot affectedby deeper monthwas calculated by findingthe minimumof the temperaturevariations. Thereforewe focusonly on secondderivative of temperatureas a functionof the top part of the surfaceocean layer (upper75 m), depthbelow sealevel. Monthly mixed layer depths wherethe largestsubsurface temperature variability were then averaged.The annualaverage mixed layer is in the eastin regionswhere seasonalupwelling depthgenerally increases westward in the tropical bringscool watersinto the photic zone. Most of the Atlantic (Figure3). The annualaverage ocean westerntropical Atlantic, despite large seasonal surface flow from east to west causes warmer thermoclinevertical movements, has a deepmixed surfacewater to "pile up" in the westand cooler layer throughoutthe yearand little seasonalityin the subsurfacewater to upwellin the east. photic zone (Figure2a). The largestseasonal variations in the thermocline In orderto characterizethe seasonalvariability in depthoccur primarily in the westernand central thephotic zone we reducethe monthly temperature tropicalAtlantic along and just noah of the equator. databy doingan empiricalorthogonal function However,isotopic measurements on foraminifera analysis(EOF) on annualtemperature range as a collectedin planktontows showthat the dominant functionof depth. For locationsspaced 2 øin latitude speciesof planktonicforaminifera in the tropics and2 ø in longitude,annual temperature range at 5-m grow in the photiczone (top 75 m or so) [Fairbanks intervals from 0 to 75 m was calculated from cubic 414 Raveloet al.: ReconstructingTropical Atlantic Hydrography

-80 ø -70 ø -60 ø -50 ø -40 ø -30 ø -20 ø -10 ø 0 ø 10ø 20 ø 20 ø

ß.:.:.:.:.:.:.:.:.:.:.:.:.:.:.:.:.:.:.:.:.',•...:.:.:.:.:.:.:.:.:.:.:.:.:.' ..:.:.:.:.:.:.'..:.:.:.:.:.:.:.:, :.:.'... • 10 ø -10 o

0 o

_10 ø

_20 ø -80 ø -70 ø -60 ø -50 ø -40 ø -30 ø -20 ø -10 ø 0 ø 10ø 21)ø Fig. 3. Observedmixed layer depth(m). splineinterpolated, smoothed monthly average eastemtropical Atlantic in the northand the south observationscompiled by Levitus[1982]. EOF (Figure4). The negativeamplitude factors represent analysison thesetemperature range profiles shows areaswith low surfaceseasonality and high that 95% of the variancein temperaturerange versus seasonalitybetween 60 and75 m. Theseareas do depthfor thetropical Atlantic can be explainedby not correlatewith any of the faunalfactors probably two modes. Amplitudefactors of the modesare becausevariability at thesedepths has little influence calculatedin the following way: on foraminiferaecology. Although the negative amplitudefactors are notmapped, they are briefly AFij = VEij * sqrt(VAj)*oi discussedin the following section. The positiveamplitude factors of the secondEOF whereAFi' is the amplitudefactor at the ith location moderepresent subsurface seasonality centered at for thejth mode,VE •s the e•genvector,VA is the about50 m (Figure 2d and 2e). This modehas eigenvalue,and • is the standarddeviation of the positiveamplitude factors in theeastern tropics temperaturerange as a functionof depth. particularlybetween 10øN and 3øS(Figure 5). Mode The first modeexplains 70% of the variance. two is referred to as the "shallow thermocline" Positiveamplitude factors of modeone represent hydrographicmode since it representsan end- highsurface seasonality (down to approximately30 memberwith high subsurfaceseasonality due to m) andlow seasonalityat approximately60-75 m largeannual variations in thedepth of the shallow (Figures2b and2c). For areaswith positive thermoclinecentered at approximately50 m. amplitudefactors, mode one is calledthe "surface Negativeamplitude factors of EOF modetwo seasonality"hydrographic mode. The mappattern of representareas where there is relativelylow the "surfaceseasonality" mode shows high- seasonalityat 50 m anddo not consistentlycorrelate amplitudefactors concentrated at themargins of the with any of the faunal factors.

_80ø _70ø _60ø _50ø _40ø _30ø _20ø _10ø 0 ø 10ø 20 ø 20 ø'I • 'I'X--•• ='/ I I •I/ I •::::.:.:.:.:-:.:.:[.:-:.:t-:-:-:.:.:.••.."?:ii!i!::!::iliii::iiiii::!::!i!:::•i.•I I I ' '20øI

10 o 10 o

0 o 0 o

_10o -10 o

_20ø -20 ø -80 ø -70 ø -60 ø -50 ø -40 ø -30 ø -20 ø -10 ø 0 ø 10ø 20 ø Fig. 4. Distributionof the surfaceseasonality hydrographic mode (EOF mode1). Raveloet al.' ReconstructingTropical Atlantic Hydrography 415

-80* -70* -60* -50* -40* -30* -20* - 10' O* 10' 20* 20* 10' • 10o o oø [EOF Hydrographic Mode2 / -10' I (ShallowThermocline Mode) -10'

-20* • Observations• [Levitus,1982]• . • I • -200 -80' -70 ø -60 ø -50 ø -40 ø -30 ø -20 ø -100 00 100 20 ø Fig. 5. Distributionof the shallowthermocline hydrographic mode (EOF mode2).

Factor Analysisof Foraminifera The fourthhighest eigenvalue indicates that a fourth factorwould increasethe totalvariance explained by The planktonicforaminifera relative abundance only 3%. Speciesscores for the threefactors are datawere reducedusing Q- modefactor analysis listed in Table 1. with varimax rotation. 89% of the variance of the Speciesscores for factor 1 are highestfor abundancedata can be explainedby threefactors. Globigerinoidesruber (whitevariety),

TABLE 1. Varimax FactorScores (Core tops)

Species Mixed Layer Thermocline Seasonal Factor Factor Succession F•½•or. Orbulina uni versa 0.011 0.060 0.038 Globigerinoides conglobatus 0.016 0.033 -0.025 G. tuber (pink) 0.139 -0.028 0.256 G. tuber (white) 0.906 -0.056 -0.036 G. tenellus 0.019 -0.016 0.038 G. sacculifer (without saclike final chamber) 0.278 0.232 0.094 G. sacculifer (with saclike final chamber) 0.129 0.216 0.031 Sphaeroidinella dehiscens -0.008 0.066 -0.019 Globigerinella aequilateralis 0.092 0.040 0.015 Globigerina calida 0.022 0.001 0.033 G. bulloides -0.021 -0.034 0.297 G. falconensis 0.021 -0.020 0.105 G. digitata -0.002 0.035 0.011 G. rubescens 0.035 -0.021 0.021 G. quinqueloba -0.001 -0.001 0.009 Neogloboquadrina pachyderma (left-coiling) -0.002 0.003 0.010 N. pachyderma (right-coiling) -0.015 -0.005 0.179 N. dutertrei -0.052 0.612 0.136 Pulleniatina obliquiloculata -0.000 0.352 -0.110 Globorotalia inflata -0.075 -0.005 0.759 G. truncatulinoides (left-coiling) 0.002 -0.001 0.010 G. truncatulinoides (right-coiling) 0.018 -0.001 0.024 G. crassaf ormis -0.002 0.075 0.039 "P-D intergrade" -0.019 -0.007 0.324 G. hirsuta 0.000 0.001 -0.000 G. scitula 0.007 -0.003 0.030 G. menardii 0.047 0.359 0.139 G . t urni da -0.045 0.498 -0.198 Globigerinita glutin• 0.200 -0.021 0,•%• 416 Raveloet al.: ReconstructingTropical Ariantic Hydrography

Globigerinoidessacculifer (without a sac-likefinal Wiebe, 1980; Fairbankset al., 1980; Ortner et al., chamber),and Globigerinitaglutinata (Table 1). 1980; Fairbankset al., 1982]. Sincethe species Thesespecies are found in greatestabundance in the with thehighest factor scores for factor2 arefound mixedlayer throughout the tropicalAtlantic (Figure in greatestabundance in thethermocline, factor two 1). Factorone is, therefore,called the "mixedlayer" is referred to as the "thermocline" factor. The factor. The mixedlayer factor explains 73.3% of the thermoclinefactor has highest loadings in theeastern varianceof the abundancedata. Relativelyhigh tropicalAtlantic between the equatorand 10øN weightings(>0.75) of the mixedlayer factor (Figure7) wherehydrography is bestcharacterized dominatethe tropical Atlantic where the mixed layer by EOF mode2 (Figure 5). is greaterthan about 40 rn (Figures3 and6). Factor3, whichexplains 5% of the abundancedata Speciesscores for factor2 (whichexplains 11% of is dominatedby G. ruber (pink variety),Globigerina the variance)are highestfor Neogloboquadrina bulloides,Globorotalia infiata, and P/D intergrade. dutertrei,Pulleniatina obliquiloculata, Globorotalia Outsidethe tropics, G. infiata,the specieswith the menardii, and Globorotalia turnida. Plankton tow highestscore, is thoughtto grow in greatest studiesin thetropical Atlantic (Figure 1) indicatethat abundance in surface waters between 13 ø and 19øC thesespecies are concentrated in greatestabundance [Tolderlandand Be, 1971] and closeto the surfacein in narrowdepth ranges in the subsurfacethermocline the cold season[Deuser et al., 1981; Deuserand wherethe chlorophyll concentration and primary Ross,1989]. The pink varietyof G. ruberis productivityare at a maximum[Fairbanks and thoughtto proliferatein the warm season[Tolderland

_80ø _70ø _60 ø _50ø _40ø _30ø _20ø _10ø 0 ø 10 ø 20 ø

_80ø _70ø _60 ø _50ø _40ø _30ø _20ø _10ø 0 ø 10 ø 20 ø Fig. 6. Mixed layerfactor loadings distribution with core top locations. The mixedlayer factor is an indicationof mixed layer depth.

_80 ø .70 ø _60 ø _50 ø _40 ø _30 ø _20 ø _10 ø 0 ø 10 ø 20 ø 20 ø[•.---,•øI ß -V Tt% ß I ..I ß ßI z[ I [,./ ' r • I I ' 20ø

_10 o _10 o

-20ø • • • I • . • . • ß _20ø _80ø _70ø _60 ø _50ø _40ø _30ø _20ø _10ø 0 ø 10 ø 20 ø Fig. 7. The•ocline factorloadings distribution with core top locations. The the•ocline factoris an indicationof the••line depth.•en thermoclinefactor loadings •e high,the the•ocline is in thephotic zone all ye• andthere is highsubsurface mmperat•e season•ity in thephotic zone. Raveloet al.: Reconsu'uctingTropical Atlantic Hydrography 417 and Be, 1971; Williams et al., 1981; Deuser and temperatureestimates in the tropicsmust be Ross,1989]. Sincefactbr 3 is dominatedby a approachedwith cautionis illustratedby the fitctthat combinationof cold seasonspecies and G. ruber, a the seasonalsuccession faunal factor also has a good warm seasonspecies, it is called the "seasonal linearcorrelation with temperatureat 50 m (r2 = succession" factor. The seasonal succession factor 0.570). (Figure8) hashighest weightings in the northernand There is goodfirst ordercorrelation between the southernmargins of the easterntropical Atlantic. mixedlayer faunal factor and mixed layer depth, The abundanceof G. infiata alonemay not always between the thermocline faunal factor and the be an indicationof the seasurface temperature during shallowthermocline hydrographic mode, and the cold season,since in someareas of the tropics between the seasonal succession faunal factor and wheresurface temperatures are relatively warm, they the surfaceseasonality hydrographic mode. The may grow in the subsurface.In 21 MOCNESS tows correlationscan be seenin the mappatterns of the from the EastEquatorial Atlantic, G. infiatais faunalfactors and the hydrographicmodes. There recoveredonly from the subsurfacethermocline. An are a few locations where the correlation between a exampleis shownin Figure 1. factorweighting and a hydrographicvariable is not good,but theselocations can be identifiedby the CORRELATION AND CALIBRATION weightingsof the otherfactors. CorrelationBetween Mixed Layer Factor Unlike otherregions at higherlatitudes, the faunal and Mixed Layer Depth factorsin thetropical Atlantic are not strongly correlatedto seasurface temperature. Neither the Thereis an exponentialrelationship between mixed thermoclinefactor nor the mixed layer factor are layerdepth and the mixed layer faunal factor (Figure correlatedwith annualaverage, seasonal minimum, 10a). High weightingsof themixed layer factor are or seasonalmaximum sea surface temperature in the foundin areaswith a deepmixed layer, and low tropics(Figure 9a and 9b). The seasonalsuccession weightingsare foundin areaswith a thin mixed faunalfactor does, however, have a fairly good layer. Of thecore tops with factorweightings correlation with seasonal minimum sea surface greaterthan 0.8, 95% are locatedin areaswhere the temperature(linear regression r2 = 0.616)but not mixedlayer is 35 m or deeper.For thosecore tops seasonalmaximum sea surface temperature (Figure with factorweightings less than 0.8, 90% are located 9c). Thisresult is encouraging,and it is possible in areaswhere the mixed layer is lessthan 35 m. thatsea surface temperature estimates as well asthe The few areasthat have mixed layer depths more surfacelayer hydrography can be derivedfrom thanabout 40 m andmixed layer factor weights less faunalabundances in the past. Becausethe than0.8 havehigh seasonalityin thedeep part of the circulationand vertical thermal gradients in the analyzedwater layer between60 and75 m. These tropicalAtlantic are quite different relative to the areascan be identifiedin the sedimentby alsohaving higherlatitudes, a new equationfor seasurface low weightingsof the seasonalsuccession factor temperatureestimates should be derivedfor the (indicatinglow surfaceseasonality), and moderate tropicalAtlantic with considerationof theunique weightingsof thethermocline factor (indicating hydrographyof the tropics. The fact thatsea surface thermoclineseasonality below 50 m).

_80ø _70ø _60ø _50ø _40ø _30ø _20ø _10ø 0 ø 10 ø 20 ø , .,.. ., . , , -...... , , , 20ø

ß lO00 o ...... o'9Q• -_0lO0 o • FaunalFactor 3 ] _• ! (SeasonalSuccession Factor)l -10 ø-• com•oPsJ . '.• ...... -10 ø -20 ø • • • , _20ø _80ø _70ø _60ø _50ø _40ø _30ø _20ø _10ø 0 ø 10 ø 2( ø Fig. 8. Seasonalsuccession factor loadings distribution with coretop locations. The seasonal successionfactor is • indicationof surfacetemperature season•ity. 418 Raveloet al.: ReconstructingTropical Atlantic Hydrography

30 ooeIoø eeß e . ßß ß• ß t ø:.-'., ,lb 28 i 0 '- ß ' - : o o . 26 . •o•o o o o o oO ø o • o o o OoC•o oo oß O O O O O O

O O

14 o rninSST=22.95+2.34* Factor 1 r•2=0.118 12 - ß max SST = 27.61 + 0.35 * Factor 1 r•2 = 0.008

10 ß i ß I ß I ß i ß i ß i ß i ß i ß i ß i ß ! ß ao ).2 -0.1 -0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 Mixed Layer Factor Loadings

30 ßß 28 ß ß ß 0 ß % 0 26 0 0 • o oßo o oß o o •" 22 ße o o o o o o o 0 o o • 20 o o o 0

•' 18 o o

14 o min SST = 24.40 + 0.99 * Factor 2 r•2 = 0.014 12 ß max SST = 27.37 + 1.66 * Factor 2 r•2 = 0.122

10 b. -0.2 -0.1 -0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 Thermocline Factor Loadings 30

28 ß ß ß 26 o •o• ø

o o o o 0% • o 8

o o

o min SST = 26.21 - 8.72 * Factor 3 r•2 = 0.616 12 ß max SST = 28.43 - 3.18 * Factor 3 r•2 = 0.260

10 ...... , ...... C. -0.2 -0.1 -0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 Seasonal SuccessionFactor Loadings Fig. 9. Factorloadings plotted against annual minimum and maximum sea surface temperature for (a) the mixed layer factor,(b) the thermoclinefactor, and (c) the seasonalsuccession factor. There is goodcorrelation only betweenthe seasonalsuccession factor and minimumsea surface temperature. Raveloet al.: ReconstructingTropical Atlantic Hydrography 419

ß ß ß ß ß ß ß

sJol:)e,lapnl!Idm• opoIA[:).[qdgJ•oJp,(H ,(l.lll•tlOSl•oS o:)l•JJnS (m) qldo O Jo,(e"I pox!IAI

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ß g ß i ß i ß ß - i ß ß i ß i ß o '

sJol:•ej opnl!ldmg (m) t13d•P 3•"I P•x!BI apoIAI :)!qdgJl•oJp•H OU!l:)OmJoqJ•a•OllgqS 420 Raveloet al.- Reconstructing Tropical Atlantic Hych'ography

Correlation Between the Seasonal Succession weil, it is more useful to think of the thennocline Factor and EOF Mode 1 factoras filling in whenthe surfacetemperature seasonalityis not largeand when the mixed layer The seasonalsuccession faunal factor (positive depthis small (lessthan about40 m). When the EOF mode 1) correlateslinearly with the surface thermocline factor dominates or when it has seasonalityhydrographic mode (r 2 = 0.699)(Figure moderateweightings with the seasonalsuccession 10b). Typically,in areaswith high amplitudefactors factor,the overlying thermocline is in thephotic zone of the surfaceseasonality hydrographic mode andthe mixedlayer is shallow(Figure 10d). (Figure2b) the thermoclineis in thephotic zone for Negativeamplitude factors of EOF mode2 are at leastpart of the year. Combinedwith the foundin regionsthat have either positive amplitude thermoclinefactor weightings, the seasonal factorsof EOF mode1 or havedeep mixed layer successionfactor can alsobe usedto predictmixed depths.Therefore the hydrographyin theseareas art layer depth(Figure 10d). representedby the seasonalsuccession or the mixed The negativeamplitude factors of EOF mode1 layer factor. representa very differenttype of hydrographywith maximum seasonalityat 75 m. All locationswith Testingthe Correlations overlyinghydrography represented by negativeEOF mode 1 has seasonal succession faunal factor In order to test the correlations described above, weightingsbelow 0.2. It is not surprisingthat the theyare recalculated using 1/3 of thecore top data seasonalsuccession factor, which records surface set,and the calibrationto the hydrographic temperaturevariation, does not covarywith parametersis testedon the remaining2/3 of the data seasonalityat depthrepresented by the negative set. For the calibrationbetween mixed layer factor amplitudefactors. andmixed layer depth, the residuals (predicted There is alsoa fairly goodcorrelation between the mixedlayer depth minus observed mixed layer seasonal succession faunal factor and seasonal depth)have a standarddeviation of 10.7 m. The minimumsea surface temperature (Figure 9c). residualsproduced by usingthe thermoclineplus the However, the fact that G. ruber hassignificant score seasonalsuccession factors to calculatemixed layer weightingsin the seasonalsuccession faunal factor depthhas a standarddeviation of 10.9 m. Residuals indicatesthat this factor is moreclosely related to from theprediction of surfaceseasonality seasonalitythan to minimumsea surface temperature hydrographicmode have a standarddeviation of in the modemtropical Atlantic. In addition,it is 0.28, andresiduals from the predictionof the importantto considerin paleo-reconstmctionsthat shallowthermocline hydrographic mode have a G. infiata growsbelow the surfacein someregions standard deviation of 0.39. The standard deviation of the tropicalAtlantic (Figure 1). of the residualsgive an indicationof theminimum changein hydrographyneeded to significantlyalter the faunal abundances..These calculations of the Correlation Between the Thermocline Factor residuals of standard deviation are maximum values and EOF Mode 2 since in all casesthe calibration with 1/3 of the data setexplains less of thevariance of thehydrographic The thermoclinefaunal factor is a goodindication parametersthan the calibrationusing the entire core of thermoclinedepth and subsurface seasonality. top data set. When thelargest seasonal variations in temperature are centered at 50 m below the surface in the water column(positive EOF mode2), thenthe thermocline OCEAN MODEL factorweakly correlates with the shallowthermocline hydrographicmode (Figure 10b). This correlationis The oceanmodel from theGeophysical Fluid not strongfor severalreasons. As discussedabove, DynamicsLab [Philanderand Pacanowski, 1986a,b] moderate values of the thermocline faunal factor can covers an area from 28øS to 50øN, has 27 levels in alsooccur where there is high subsurfaceseasonality thevertical down to 4149 m with 10 levelsin thetop due to movementsof the thermoclinein the deeper 100 m of thewater column. The longitudinal part of the water column(60-75 m). In this case,the resolutionis 1ø, or 100 km nearthe equator.The thermocline faunal factor will not correlate well with north-southgrid spacingincreases from about33 km EOF mode2 which indicatesseasonality shallower near the equatorto about200 km at 20øN and 20øS. than 75 m. In some cases, the location of the The modeloutput has been compared to observations thermoclinethrough most of the year,and the depth in paststudies [Garzoli andPhilander, 1985; of maximum subsurfaceseasonality is very shallow Richardsonand Philander,1987]. Below, thephotic (approximately30 m). In theselocations the zonetemperature output from the standardmodel run thermocline faunal factor will not correlate with EOF is comparedto observationscompiled by Levitus mode2 which indicatesseasonality centered at 50 m, [ 1982]in termsof the threehydrographic parameters not 30 m. discussedearlier (mixed layer depth, the high Since the thermocline factor and the shallow seasonalityEOF mode,and the shallowthermocline thermoclinehydrographic mode do not correlate EOF mode). Changesin the mappatterns of these Raveloet al.: ReconstructingTropical Atlantic Hydrography 421 parametersdue' to doublingthe wind stressshow agreementwith the observations(Figure 3). In the which areasare mostlikely to showfaunal westthe modelsimulates mixed layer depths which assemblagechanges due to increasingwind strength. are approximately10-20 m too shallow. This is a For the standard wind stress run used in this resultof problemswith the parameterizationof study,the initial conditionsare zerocurrents and mixing processesdiscussed by Richardsonand climatologicaltemperature and salinity fields of Philander[1987]. Despitethese discrepancies the January[Levitus, 1982]. The modelwas forced by modelsimulation provides a fairly accurate the observedwind field compiledby Hellermanand predictionof deepmixed layer hydrography Modeled Rosenstein[1983]. A 40-min time stepwas used, minusobserved residuals of mixedlayer depthhave with seasonalequilibrium reached in 2 years. The a standard deviation of 10.4 m. standardmodel run outputdescribed below is from The mappattern of the surfaceseasonality the third year. hydrographicmode for the standardmodel run (Figure 12) is similar to that of the observations Standard Wind Stress Run (Figure 4) with the northeasternand southeastern marginsof the studyarea having the highestvalues. Mixed layer depthand amplitude factors of the In the northeastregion of high amplitudefactors EOF modes were calculated from the standard model (15øN, 20øW), the model underestimatesthe surface temperatureoutput. Projectionof theEOF modeson seasonaltemperature range with the seasonal the modeloutput explains 80% of the model minimum temperatureabout 2øC warmer thanthe temperaturevariance. The modelpredicts mixed observations.In a regionin the southeast(centered layerdepths of 30 m andless in theeastern part of at about 10øS, 10øW), the model overestimatesthe the tropicalAtlantic (Figure 11) thatis in close surfaceseasonality hydrographic mode because the

-80 ø -70 ø -60 ø -50 ø -40 ø -30 ø -20 ø -10 ø 0 ø 10 ø 20 ø 20 ø 20 ø

10 o 10 o oo 3o,• oo -100 -100

_20ø _20ø -80 ø -70 ø -60 ø -50 ø -40 ø -30 ø -20 ø -10 ø 0 ø 10 ø 20 ø Fig. 11. Mixed layerdepth (m) predictedby the standardmodel run.

_80 ø _70ø _60ø _50ø _40ø _30ø _20ø _10 ø 0 ø 10 ø 20 ø 20 ø 20 ø

10 o 10 o

0 o 0 o -10'• StandardModelRun'/ •iiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiii}iiiiiiiiiiiiii ....."'""'• 1-10' -200 'i , , ; , , -20 ø -80 ø -70 ø -60 ø -50 ø -40 ø -30 ø -20 ø -10 ø 0 ø 10 ø 20 ø Fig. 12. Distributionof surfaceseasonflity hydrographic mode (EOF mode1) predictedby the standard model run. 422 Raveloet al.: ReconstructingTropical Ariantic Hych'ography differencebetween the temperaturerange at the simulationof the last glacialmaximum or of the surfaceand at 75 m is greaterthan what is observed. deglaciafionand does not give us insightinto The residuals, modeled minus observed values of the changesin seasonalityof the winds. surfaceseasonality hydrographic mode, have a The differencebetween mixed layer depth standard deviation of 0.43. predictedby the doublewind stressrun and the In the model,the highestamplitude factors of the standardwind stressrun is contouredin Figure 15. shallowthermocline hydrographic mode occurs The prominentincrease in mixed layer depthall .i. alongand just northof theequator in the eastern alongthe equator, particularly close to the continents regionof the tropicalAtlantic (Figure 13). This map in both the eastand the west, is causedby increased patternis generallyin agreementwith the flow in thelate northernhemisphere summer of the observations(Figure 5) with the exceptionof the NECC. With doubledwind magnitudein the very high amplitudefactors in the Gulf of Guineain summer,the SEC is stronger.The returnflow of the modelsimulation. This discrepancycan be warm waterin the late summerand early fall when explainedby an underestimationof theobserved the southeasterlytrades begin to relax is enhanced, subsurfacetemperature range at about50 m due to andthe thermoclineis depressedall theway across averaging. Observedinstantaneous temperature the equatorialregion from westto east. The profilesfrom thisregion [Mele andKatz, 1985] from increasedstrength of the SEC causesthe shoalingof Septemberand March (Figure14) indicatethat the the thermoclineand the decrease in mixedlayer depth rate of temperaturechange with depthat any onetime in the southeast.North of about8øN doubling the is higherthan the monthlyaverage because the wind stresscauses the mixed layer depthto decrease thermoclineis not locatedexactly at the samedepth by about10 m. In thisregion the standardmodel duringthe samemonth from one year to the next. run simulatesmixed layer depthsthat are less than Thereforecomputing monthly averages effectively observations.Therefore it is possiblethat in addition decreasesthe steepnessof the thermoclineand the to problemswith theparameterization of mixing subsurfacetemperature range. In this case,the processes,the standardmodel wind field [Hellerman modelprovides a more accurateestimate of andRosenstein, 1983] is strongerthan reality subsurfacetemperature gradients than does the [Richardsonand Philander, 1987]. The changesin averagedobservations. Any otherdiscrepancies themixed layer depth due to doublingthe wind input betweenthe model (Figure 13) and the observations wouldbe largeenough to dramaticallyaffect faunal (Figure5) are comparableto the standarddeviation abundances,particularly in theeastern half of the of the residualsproduced by the faunalpredicted tropicalAtlantic where the mixedlayer faunal factor amplitudefactors. canbe usedto predictmodern mixed layer depth to within about 10 m. Doubled wind stress has little affect on the Doubled Wind Stress Run distributionof amplitudefactors of the surface seasonalityhydrographic mode with theexception of The doublewind stressrun was forcedby winds the regionclose to the equatorand the African coast with the samedirection and doubled magnitude between15øW and 10øE(Figure 16). This region relativeto the standardrun. The purposeof the showsextremely large changes relative to the range doublewind stressrun was to identify the regions of amplitudefactors calculated for the standardwind most sensitiveto increasesin wind stresschanges. stressrun. The decreasein amplitudefactors results The doublewind stressexperiment is not a realistic from a deepeningof the thermoclineparticularly in

_80 ø _70 ø _60 ø _50 ø _40ø -30' _20 ø _10 ø 0 ø 10 ø 20 ø , 20 ø • •.:,• =, , , , I I I I I 20 ø

10 ø -10 ø

0 o o

x • EOFHydrographic Mode 2 -10' -\ [(ShallowThermocline Mode)/ _10 ø

-20 ø • StandaldModelRun • , -20 ø _80 ø _70 ø _60 ø _50 ø _40 ø _30• _20ø _1o o 0 o 10 o 20 ø Fig. 13. Distributionof shallowthermocline hydrographic mode (EOF mode2) predictedby the standard model run. Raveloet al.: ReconstructingTropical Atlantic Hydrography 4 2 3

the northernhemisphere summer and early fall and -10 the consequentincrease in seasonalityat depth(60- 75 m) relativeto the seasonalityat the surface. ' (2'N,2'W)September I / The regionof largestchange, due to doublingthe . wind stressinput, for the surfaceseasonality mode is ' (2'N,2'W) alsothe region of largestchange for the shallow -40 thermoclinemode (Figure 17). The amplitude factors for the shallow thermocline mode decrease -50 whenthe windsare strengthenedbecause annual -60 thermoclinedepth variability is centeredbelow 50 m. As canbe seenin the differencemap for mixedlayer -70 depth(Figure 15), the annualmixed layer is asmuch -80 as 20 m deeperin thisregion. Amplitudefactors of the shallowthermocline hydrographic mode -90 increaseswith doubledwind stressin the region between0 øand 10øSin the eastcentral part of the -100 lO 12 14 16 18 20 22 24 26 28 30 32 tropicalAtlantic because the thermoclinein on averageshallower than it is in the standardmodel run Temperature (øC) (centeredcloser to around50 m). Mixed layerdepth changes(Figure 15) areconsistent with a shoalingof the thermocline relative to the standard model run. -10 SEQUALXBT profiles t The changesin the shallowthermocline XBT:210 (2'N, 3'W) September XBT: 117(3'N, 4'W) March hydrographicmode due to doublingthe wind stress are muchgreater than what the faunalabundances d would respondto and thereforeconfirm the value of -40 usingthe faunalfactor weightings to monitor changesin the hydrographicmodes. -50 / The regionsof smallestchange due to doublingthe wind stress,north of 5øN and south of 5øS in the -60 westernhalf of the studyarea, are the bestplaces to -70 - ,, obtaindowncore records of nutrient-depletedstable surfacewater. Downcoresignals from otherregions -80 are influencedby regionalwind stresschanges and shouldnot be usedfor "global"records of nutrient- -90 free surfacewater. This resultcould have profound -100 I i i i i i effectson studiesthat stackrecords from the tropics •0 12 14 16 18 20 22 24 26 28 30 32 [Matthews and Poore, 1980; Mix and Ruddiman, Temperature (øC) 1985; Curry and Crowley, 1987; Prenticeand ß i ß i ß i ß i ß i ß i ß i ß i ß i ß i , Matthews,1988]. It is possiblethat a singlecore from a regionthat is well chosenfor thepurposes of StandardModel Run I ' a given studywould provide a more accuratesignal thanone produced by stackingrecords from different I ' ' (2'N'2'W)(2'$, SeptemberI I regions in the tropics. ii -40 APPLICATION TO THE LAST GLACIAL MAXIMUM -50

-60 The doublewind stressrun comparedto the standard wind stress run indicates that one of the -70 most sensitive areas to wind stress increases is in the

-80 easternto centralequatorial region. This regionof large changeis the bestregion for downcore -9o monitoringof the wind magnitudeincreases. C. Althoughwe havenot yet donea modelrun forced ß i , i , -100 10 12 14 16 18 20 22 24 26 28 30 32 by realisticwinds for pasttimes, the doublewind stressexperiment can be usedto makea first guessat Temperature (øC) whetherfaunal changes can be explainedby wind Fig. 14. Temperatureprofiles of Septemberand forcedchanges in hydrography. March from (a) Levitus [1982], (b) Sequalproject We chose the LGM because it is for this time slice [Mele and Katz, 1985], and (c) the standardmodel thatthe most complete spatial sediment sampling is run. available. First, we mustquestion the validity of 424 Raveloet al.: ReconstructingTropical Atlantic Hydrography

_80ø _70ø _60ø _50ø _40ø _30ø _20ø _10ø 0 ø 10 ø 2t) ø 20ø •20 ø

i i}iii!}iiiiiii!}iii!½iii?f......

_80ø _70ø _60ø _50ø _40 ø _30 ø _20 ø _10 ø 0 ø 10 ø 21)ø Fig. 15. Difference in mixed layer depth(m) betweenthe doublewind stressrun and the standard modelrun. Postivenumbers indicate deepening and negative numbers indicate shoaling of mixed layer with double wind stress.

-80 ø -70 ø -60 ø -50 ø -40 ø -30 ø -20 ø -10 ø 0 ø 10 ø 20 ø 20ø • ••,x•, ' 20 ø

10ø ':•:'"'::•0.0 .:.' 10 ø

0ø ....:.:.:.:.:.:, ,,:',iiii!ii:'•'i'"•:::::::::::::::::::':'.'.'.:.:-,....•.•.'.'.'.'.'•T• ...... ß - 0 ø x f EOFMode 1 - -Xx [(SurfaceSeasonality Mode) /

-20 ø • ndardWnd• , , , _20 ø _80ø _70ø _60ø _50ø _40ø _30ø _20 ø _10ø 0 ø l0 ø 20 ø Fig. 16. Differencein the surfaceseasonality hydrographic mode (EOF mode 1) betweenthe double wind stress run and the standard model run.

-800-70 ø -60 ø -50 ø -40 ø -30 ø -20 ø -100 00 10020 ø

10*• "•:'':':"...... '"":"'"""'•.•.._...... :•' [o -100

•..... EOFMode - -10ø I(Sha-\ 11owermTh o lcine Mode) I / "•'.u,• / \ - 10ø

-20 ø • •Double-Standard • WindStress• Run)• • • • k• • • (• -20 ø _80 ø _70 ø _60 ø _50 ø _40 ø _30 ø _20 ø _100 0 o 10 o 20 ø Fig. 17. Differencein the shallowthermocline hydrographic mode (EOF mode2) betweenthe double wind stress run and the standard model run. Raveloet al.: ReconstructingTropical Atlantic Hydrography 4 2 5

assumingthat foraminifera species associations are Regardlessof the areafrom which the factorscores constantthrough time. Only about35% of the are derived,the LGM faunalassemblages in the varianceof the the LGM speciesabundances can be tropicalAtlantic are not well representedby coretop explainedby thecore top factors calculated in this faunal scores.Factor analysis of speciesabundances study. However,61% of thevariance in the tropical from the LGM sectionof 77 coresin the tropical AtlanticLGM level canbe explainedby therevised Atlanticdoes not producefactors with the same CLIMAP AtlanticOcean FA-20 equationsused by scoresas the coretop factors(Table 2). In the LGM Mix [1985]. This may indicatethat factors calculated scores,G.infiata is an importantspecies in boththe from a largerregion than 20øN to 20øSmay proveto thermocline and seasonalsuccession factors, several be morerobust. Nevertheless,it is importantto importantspecies in modemsediments are not considerthe unique hydrography of thetropics and found, and G. bulloidesbecomes an important thefact that the samespecies that proliferate in the speciesin the seasonalsuccession factor. Thus surfacewater in the subtropicsmay proliferatein the factoranalysis can be usedon the abundancedata subsurfacein the tropics.For the purposeof setsfrom pasttime slicesto determineif species detailedstudies of hydrographicchanges in the associationschange through time. tropics,the faunal assemblages relationships to Contouredmaps of the threefaunal factors hydrographicparameters should be examined producedby factoranalysis of theLGM abundance regionallyrather than oceanwide. Although the data setare shown(Figures 18, 19 and20). There is regionof this studyfrom 20øNto 20øScovers most a mixed layer factor(explaining 67% of the variance) of thetropics, we expectthat the variance explained whoseloadings suggest reduced mixed layer depth in theLGM level by thecore top factors may in the centralpart of the tropicalAtlantic (Figure 18). increaseif the studyarea is increasedin sizeby 5* In the secondfactor of the LGM analysis,like in the 10ø farther north to makethe region more symmetric coretop thermoclinefactor, species which live in the aboutthe ITCZ ratherthan the equator. thermocline,N. dutertrei, G. infiata, and P/D

TABLE 2. V arimax Factor Scores(LGM) Species Mixed Layer The rmoc line Seasonal Factor Factor Succession

Orbulina uni versa 0.013 0.015 0.054 Globigerinoides conglobatus 0.031 0.019 -0.020 G. tuber (pink) 0.070 0.019 0.242 G. ruber (white) 0.928 0.006 0.097 G. tenellus 0.036 -0.019 0.025 G. sacculifer (without saclike final chamber) 0.169 0.212 -0.021 G. sacculifer (with saclike final chamber) 0.079 0.209 -0.062 Sphaeroidinella dehiscens 0.000 0.000 0.000 Globigerinella aequilateralis 0.101 -0.010 0.021 Globigerina calida 0.009 0.017 0.028 G. bulloides -0.073 -0.150 0.700 G. falconensis 0.027 -0.011 0.114 G. digitata 0.004 0.010 -0.001 G. rubescens 0.042 0.005 0.015 G. quinquel oba -0.002 -0.002 0.012 Neogl oboquadrina pachyderma (left-coiling) -0.002 0.0O4 0.015 N. pachyderma (right-coiling) -0.050 0.047 0.245 N. dutertrei -0.037 0.845 -0.124 Pulleniatina obliquiloculata 0.010 0.004 0.001 Globorotalia inflata -0.069 0.188 0.447 G. truncatulinoides (left-coiling) 0.006 -0.002 0.008 G. truncatulinoides (right-coiling) 0.064 0.103 -0.006 G. crassaformis -0.001 0.106 0.031 "P-D intergrade" -0.116 0.335 0.357 G. hirsuta 0.001 -0.001 0.000 G. scitula 0.010 0.004 0.030 G. menardii -0.004 0.006 0.036 G . t urni da -O.002 0.006 0.009 Globigerinita glutina½• O ,250 0,043 O,il$ 426 Raveloet al.: ReconstructingTropical Atlantic Hydrography

_80ø _70ø _60 ø _50ø _40ø _30ø _20ø _10 ø 0 ø 10 ø 20 ø , I I I I 20 ø .

ß.:.. ::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::ß ß -10 o ..::i:!:!:i:i:i:i:i:i:!:i:i:i:i:i:i:i:;:!:i:i:i:i:i:i:!:i:i:i:i:i:i:i:i:!:i:!:i:i.•aa

0 o

_10 ø -10ø ..!:i:i:i:i:i:i:i:i:i:i:i:!:i:i:i:::?.x?...::•i•i•i•?i•i•i•i•!•i(..a.,.• ß '.

-20 ø • ,I _20 ø _80ø _70ø _60ø _50ø _40ø _30ø _20o _10 ø 0 o 10 o 20 ø Fig. 18. LGMmixedlayerfacmr Badingsdis•ibufionwith corelocations.

-80 ø -70 ø -60 ø -50 ø -40 ø -30 ø -20 ø -10 ø 0 ø 10 ø 20 ø 20 ø 20ø1I ß"-" •.•.', , ß , , .1\_ ,- .) ,. , t-" . 10øE •• ' ' . -10 o

0 o

_10 ø

_20 ø _80 • _70 • _60 • _50 • _40 • _30 • _20 • _10 • 0 • 10 • 20 ø Fig. 19. LGM thermoclinefactor loadings distribution with corelocations.

_80 ø _70 ø _60 ø _50 ø _40 ø _30 ø _20 ø _10 ø 0 ø 10 ø 20 ø 20 ø 20 ø

10 ø

0 o

_10 ø

_80 ø _70 ø _60 ø _50 ø _40 ø _30 ø _20 ø _10 ø 0 ø 10 ø 20 ø Fig. 20. LGM seasonalsuccession factor loadings distribution with core locations. intergrade,and species which live in the mixedlayer, photiczone and subsurfaceseasonality (Figure 19) G. sacculifer(with and without a sac-likefinal in an area from 10øN to 10øS in the central and chamber),have high scores.Loadings of the LGM easternregions and generallyin the west. The third thermoclinefactor (explaining 14% of thevariance) factor(explaining 7% of the variance)is similarto indicatethat therewas a permanentthermocline in the the core top seasonalsuccession factor but alsohas Raveloet al.: ReconstructingTropical Atlantic Hydrography 427 high weightingsof G. bulloides,G. pachyderma Most specieswith high scoresin the thermocline (fight-coiling)and P/D intergrade.This factor factorare known to proliferateat the level of the indicatesreduced surface seasonality in the south thermoclinewhere primary productivity and easterncomer of the studyarea near the continentbut microzooplanktonare at a maximum[Fairbanks and slightlyincreased surface seasonality in thecentral Wiebe, 1980; Fairbanks et al., 1980; Ortner et al., regionbetween 5 ø and 20 ø S (Figure20). 1980; Fairbanks et al., 1982]. The thermocline factorhas high weightingsin regionswhere the DISCUSSION ecologicalhabitat that supportsthe mixed layer speciesis reducedby nutrient-richwaters upwelling Correlationof Faunal Factorsto Hydrography into the photic zone. G. sacculifer,a mixed layer species,has fairly high scoresin the thermocline There are somevery importantimplications from factorbecause a permanentthin mixed layer is still the resultsof this studythat shouldbe considered maintained. The thermocline factor does not whentrying to predictpast oceanic changes from correlate well with the "shallow thermocline" foraminiferaassemblages. Since the faunal hydrographicmode because the hydrographicmode assemblagesof thepresent ocean are not well is definedas havingthe thermoclinecentered at 50 correlatedwith seasurface temperature in the tropical m. However, the fact that in combination with the Atlantic, the usefulnessof usingchanges in faunal seasonal succession factor it does correlate with assemblagesto estimatesea surface temperature in mixed layer depthmeans that high weightingsof the tropicsshould be reconsidered.For example, eitherfactor or moderateweightings of both should high mixed layer factorweightings (>0.9) andlow be usedas an indicationof a thin mixed layer. seasonalsuccession and thermoclinefactors (<0.1) Finally, two thingsthat might improve our ability canbe foundin areasthat havedeep mixed layers to predictphotic zone hydrography from the faunal with an averagesea surface temperature anywhere factorsare a betterunderstanding of how much between22øC and 27øC(Figure 9). Sincesea differential dissolution there is and the use of surfacetemperature estimates using transfer absolute flux rates. Thus far we have chosen not to functionsrequire unreasonable lapse rates to explain focus on the effect of dissolution because there continental snow line observations[Rind and Peteet, appearsto be no correlationbetween core top depth 1984], there is reasonto believe that the estimatesare and the factors,but it is still possiblethat that too warm. The inabilityto resolvepast sea surface dissolution effects create some of the scatter in the temperaturechanges using foraminifera regressionfits. Absoluteflux ratesof planktonic assemblages,particularly for the areaof deepmixed foraminiferaspecies to the seafloorwould probably layer (westerntropics), means that other methods providemore information that could be tied to photic suchas oxygenisotopic analyses may be the best zonehydrography. While relativeabundances way of estimatingpast sea surface temperatures in indicatethe dominanceof a certainecological habitat, this region. absolutefluxes might indicatethe combination of The faunalfactor correlations with the parameters habitatsmore clearly, thus providing a more detailed of photic zone hydrographyare straightforward reconstructionof hydrography. when foraminiferalecological habitats are considered.Species with high scoresof the mixed Comparisonof Model Resultsand layer factorhave symbioticalgae and are adaptedto Faunal Factorsfor theLGM relativelynutrient depleted upper levels of the photic zone [Hemlebenet al., 1989]. In regionswith a The double wind stress run is not intended to be a well-developedmixed layer that extendsbelow the simulationof the LGM sinceGCMs predictsmall photiczone, mixed layer speciesdominate over increases,between about 10% [Kutzbach and speciesof the othertwo factorswhich grow in Guetter, 1986] and 40% [Rind, 1987], only in the associationwith the highernutrient, cooler upwelling northeast trades for the LGM. The double wind waters. stressrun is only usedto locateareas that are The seasonal succession factor correlates with the sensitiveto wind stressincreases. The comparison surfaceseasonality hydrographic mode because the of double to standard wind stress model runs importantspecies of this factorproliferate in different predictsthat some of the areasof largestchange for seasons.Regions with high surfaceseasonality are mixedlayer depth are the sameas those areas which in the eastat theperipheries of the studyarea where havedecreased mixed layer faunal factor weightings. thereis a cold deepmixed layer in the seasonof high A comparisonof the coretop mixedlayer factor wind stress(winter) anda well-developedshallow weightings(Figure 6) with theLGM (Figure18) thermoclineand thin warmermixed layer in the indicatesthat the modemeastern boundary of summer. Besideshaving high weightingsin the relativelydeep mixed layer (indicatedby the0.75 seasonalsuccession faunal factor, these areas have contour)was shiftedwestward in the LGM. The moderateweightings in the thermoclinefactor modelshows that theregions that showthe major becausethere are timesof the yearwhen cool faunalchanges in the LGM relativeto today(0-10øW nutrientrich waters are supplied to thephotic zone. in the south and 30-40øW in the north) are sensitive 4 2 8 Raveloet al.: ReconstructingTropical Atlantic Hydrography to wind stresschanges. However, the modelresults rangeat the bottomof the analyzedwater column (75 alsosuggest that in termsof mixedlayer depth,the m). The shallowthermocline hydrographic EOF equatorialregion responds in theopposite sense modehas high subsurface temperature range due to relative to 5-20øN or 5-20øS. This is not seen in the seasonalvariations in thedepth of the thermocline. LGM mixedlayer factorweightings. The 2. Factoranalysis reduces 29 speciesof planktonic thermocline and the seasonal succession factors of foraminiferafrom 118 coretops in the tropical the LGM showthe mostchange relative to the Atlanticto threefactors which explain 89% of the modembetween 20øW and30øW along the equator variance of the abundance data. Because of the and between 0 ø and 10øE in the southern half of the knowndepth stratification of tropicalspecies in the studyarea. Althoughthe differencesin the double tropicalAtlantic, the threefactors are called the and standard wind stress model runs indicate that mixedlayer factor,the thermoclinefactor, and the these areas are somewhat sensitive to wind stress seasonal succession factor. changes(Figures 16 and 17), they are not the most 3. Modem hydrographycan be predictedby the sensitive. foraminiferafactor weightings using simple To summarize,there is moderatelygood agreement correlations.Mixed layerdepth correlates with the betweenregions where the modelis sensitiveto mixed layer faunalfactor and with the thermocline wind stress increases and observations of LGM plus seasonalsuccession factors. Seasonal faunalchanges in the westernequatorial region, and successionfaunal factor weightings are linearly northand southof the equatorin the east. However, correlatedwith the surfaceseasonality hydrographic thereis significantdifference between the model and mode. Moderateto high thermoclinefaunal factor observationseast of centeralong the equator. weightingsindicate seasonal temperature variations Discrepanciesbetween the regions predicted to be in the subsurface due to seasonal vertical movements themost sensitive to wind stresschange and the of the thermoclinewithin the photiczone. These regionsthat showthe largestchanges in the LGM correlationsraise the possibility that more variance in relative to the modem will not be resolved until a faunalabundances downcore may be explainedby realistic LGM wind field is used to drive the ocean thermoclineand seasonality changes than by sea model. There are severalways that these surfacetemperatures. discrepanciescan be explained.(1) It is possible 4. The oceanmodel of thetropical Atlantic is that the double-standard wind stress model runs do comparedto modemhydrography in termsof the not provideinformation about where the threehydrographic parameters. Map patternsof hydrographyis most sensitiveto wind stress mixedlayer depthand amplitudefactors of the two decreases,seasonal changes in the strengthof the EOF modespredicted by themodel are very similar trades,or changesin the strengthof the monsoons. to thoseobserved. Some discrepancies can be (2) The LGM faunalchanges could be forcedby attributedto theeffect of averagingdata in orderto extratropicalocean circulation changes rather than producethe modemobservation temperature fields. wind stresschanges [Mix et al., 1986; Mcintyre et Mostdiscrepancies are not significantrelative to the al., 1989]. By changingthe boundaryconditions of standarddeviation of theresiduals produced by the the oceanmodel we may be able to simulatethe faunalfactor predictions of the hydrographicmodes. effectof extratropicalchanges on theregion in future The resultsdemonstrate that the model is appropriate modelruns. (3) The LGM faunalchanges could be for paleoceanographicapplications. due to dissolutionof lessresistant species which 5. The standardwind stressrun outputis would effectivelyincrease the thermocline subtractedfrom outputof a modelmn in which the weightingsand decrease the mixed layer factor wind field usedas inputis doubledin magnitude. weightings.N. dutertrei,a relativelyresistant The changesdue to doublingthe wind stressindicate species[Berger, 1971; Be, 1977], hasthe highest areas that are most sensitive, and should show scorein the thermoclinefactor which has high substantialfaunal changes that can be attributedto weightingsin areaswhere coresare locatedoff of the pastincreases in the surfacewind field. Areas which Mid-AtlanticRidge. showsmall changes are the bestplaces for downcore records of stable surface waters. CONCLUSIONS 6. Factoranalysis of LGM faunalabundances producesthree factors that represent the three 1. The photiczone seasonal hydrography of the hydrographicparameters. In general,changes in the tropicalAtlantic can be characterizedby theaverage LGM factorweightings relative to thecore top arein annualmixed layer depth and two hydrographic some of the same areas which are sensitive to wind modes.EOF analysison annualtemperature range stressincreases. The areaof largestdiscrepancy is variationas a functionof depthindicates that the eastof centeralong the equator. Discrepancies annualtemperature range profiles at anyone location couldreflect the fact thatatmospheric GCM can be explainedby two modes. The surface experimentsindicate that uniform changes in north seasonalityhydrographic EOF modehas high andsouth trade wind strengthsdid not occurat the surfacetemperature range, and low temperature LGM, or could be a result of dissolution effects. Raveloet al.' ReconstructingTropical Atlantic Hydrography 429

Model runswith realisticwind fieldsfor pasttimes variations in Northwest Africa deduced from east may resultin moreagreement between model and Atlantic sedimentcores, Quat. Res., 6, 299-314, data in the future. 1976. Fairbanks, R. G., and P. H. Wiebe, Foraminifera Acknowledgments.We would like to thankSilvia andchlorophyll maximum; vertical distribution, Garzoli for assistance and advice on the EOF seasonalsuccession and paleoceanographic analysis,Ted Baker,Ron Pacanowskiand Bill significance,Science, 209, 1524-1526, 1980. Hurlin for their computerexpertise, Barbara Fairbanks, R. G., P. H. Wiebe, and A. W. H. Be, Molfino, Bill Ruddiman, and Peter Wiebe for useful Verticaldistribution and isotopic composition of discussions,and Tom Crowleyand an anonymous living planktonicforaminifera in the westernNorth reviewerfor their comments.Thanks also go to Atlantic, Science, 207, 61-63, 1980. Warren Prell for makingthe CLIMAP datasets Fairbanks, R. G., M. Sverdlove, R. Free, P. H. available. This researchwas made possible by a Wiebe, and A. W. H. Be, Vertical distributionand NationalScience Foundation Minority Graduate isotopicfractionation of living planktonic Fellowship(to A.C.R.), and NSF grantsOCE86- foraminifera from the PanamaBasin, Nature, 09743 and OCE84-02055 (to R.G.F.). This is 298(5877), 841-844, 1982. LDGO contribution 4615. Fairbridge,R. W., Africanice-age aridity, in Problemsin Paleoclimatology,edited by E.M. REFERENCES Nairn, pp. 356-363, JohnWiley, New York, 1964. Be, A. W. H., An ecological,zoogeographic and Garzoli, S. L., and M. E. Clements, Indirect Wind taxonomicreview of recentplanktonic Observationsin the SouthwesternAtlantic, J. foraminifera,in OceanicMicropaleontology, vol. Geophys.Res.,91(C9), 10551-10556, 1986. 1, editedby A. T. S. Ramsay,pp. 1-100, Garzoli, S. L., and E. J. Katz, The forced annual AcademicPress, San Diego, Calif., 1977. reversalof the AtlanticNoah Equatorial Be, A. W. H., Biology of planktonicforaminifera, Countercurrent,J. Phys.Oceanogr.,13(11), 2082- in Foraminifera-Notesfor a Short Course,Stud.in 2090, 1983. Geol., vol. 6,editedby T. W. Broadhead,pp. 51- Garzoli, S. L., and S. G. H. Philander, Validation 89, Universityof Tennesseeat Knoxville,1982. of anequatorial Atlantic simulation model using Berger,W. H., Sedimentationof planktonic invertedecho sounderdata, J. Geophys.Res., foraminifera, Mar. Geol., 11,325-358, 1971. 90(C5), 9199-9201, 1985. Butzer, K. W., G.L. Isaac, J.L. Richardson, and C. Gates,W. L., The numericalsimulation of ice-age Washbourn-Kamau,Radiocarbon dating of East climatewith a generalcirculation model, J. Atmos. African lake levels, Science, 175, 1069-1076, Sci., 33, 1,844-1,873, 1976. 1972. Hellerman,S., andM. Rosenstein,Normal monthly CLIMAP ProjectMembers, Seasonal reconstruction windstress over the world ocean with error of the Earth'ssurface at thelast glacial maximum, estimates,J. Phys. Oceanogr.,13, 1093-1104, Geol. Soc. of Am. Map and Chart Ser., MC-36, 1983. 1981. Hemleben,C., M. Spindler,and O. R. Anderson, Curry,W. B., andT. J. Crowley,The •3C of ModernPlanktonic Foraminifera, 363 pp., equatorialAtlantic surface waters: implications for Springer-Verlag,New York, 1989. ice agepCO2 levels, Paleoceanography, 2(5), Hisard, P., C. Henin, R. Houghton,B. Piton, and 489-517, 1987. P. Rual, Oceanicconditions in the tropicalAtlantic Curry, W. B., R. C. Thunell, and S. Honjo, during 1983 and 1984, Nature, 322(6076), 243- Seasonalchanges in theisotopic composition of 245, 1986. planktonicforaminifera collected in PanamaBasin Jones,J. I., Significanceof distributionof sedimenttraps, Earth Planet. Sci. Lett., 64, 33-43, planktonicforaminifera in theEquatorial Atlantic 1983. Undercurrent,Micropaleontology, 13(4), 489- Deuser,W. G., and E. H. Ross,Seasonally 501, 1967. abundantplanktonic foraminifera of the Sargasso Katz, E. J., Seasonalresponse of the seasurface to Sea: Succession,deep-water fluxes, isotopic the wind in the equatorialAtlantic, J. Geophys. compositions,and paleoceanographic implications, Res., 92(C2), 1885-1893, 1987. J. Foraminiferal Res., 19(4), 268-293, 1989. Katz, E. J., P. Hisard, J. M. Verstraete, and S. L. Deuser, W. G., E. H. Ross, C. Hemleben, and M. Garzoli,Annual change of seasurface slope along Spindler,Seasonal changes in species the equatorof the AtlanticOcean in 1983 and composition,numbers, mass, size and isotopic 1984, Nature, 322(6076), 245-247, 1986. compositionof planktonicforaminifera settling into Kolla, V., P. E. Biscaye,and A. F. Hanley, the deepSargasso Sea, Palaeogeogr., Distributionof quartzin lateQuatemary Atlantic Palaeoclimatol., Palaeoecol., 33, 103-127, 1981. sedimentsin relationto climate,Quat. Res., 11, Diester-Haass,L., Late Quaternaryclimatic 261-277, 1979. 430 Raveloet al.: ReconstructingTropical Atlantic Hydrography

Kutzbach,J. E., Monsoonclimate of theearly maximum in the northwesternAtlantic Ocean,J. Holocene:Climate experiment using the Earth's Mar. Res., 38, 507-531, 1980. orbitalparameters for 9000 yearsago, Science, Parkin, D. W., and N.J. Shackleton, Trade wind 214, 59-61, 1981. andtemperature correlations down a deep-seacore Kutzbach, J. E., and P. J. Guetter, The influence of off the Sahara Coast, Nature, 245, 455-457, 1973. changingorbital parameters and surface boundary Philander, S. G. H., and R. C. Pacanowski, The conditionson climatesimulations for thepast generationof equatorialcurrents, J. Geophys. 18,000 years,J. Atmos.Sci., 43, 1726-1759, Res., 85(C2), 1123-1136, 1980. 1986. Philander, S. G. H., and R. C. Pacanowski, Kutzbach, J. E., and B. L. Otto-Bliesner, The Simulationof the seasonalcycle in the tropical sensitivityof the African-Asianmonsoonal climate Atlantic Ocean, Geophys.Res. Lett., 11,802-804, to orbitalparameter changes for 9000 YearsB. P. 1984. in a low-resolutiongeneral circulation model, J. Philander, S. G. H., and R. C. Pacanowski, The Atmos. Sci., 39, 1177-1188, 1982. massand heat budget in a modelof the tropical Kutzbach, J. E., and F. A. Street-Perrott, Atlantic Ocean,J. Geophys.Res., 91(C12), Milankovitchforcing of fluctuationsin thelevel of 14,212-14,220, 1986a. tropicallakes from 18 to 0 kyr B. P., Nature, Philander, S. G. H., and R. C. Pacanowski, A 317(6033), 130-134, 1985. modelof the seasonalcycle in thetropical Atlantic Levitus,S., Climatologicalatlas of theworld ocean, Ocean, J. Geophys.Res., 91(C12), 14,192- NOAA Prof. Pap. 13, 173 pp.,U.S. Government 14,206, 1986b. PrintingOffice, Washington,D.C., 1982. Pokras, E.M., and A.C. Mix, Late Pleistocene Manabe, S., and A.J. Broccoli, Influence of the climaticchanges in tropicalAfrica recordedin CLIMAP ice sheeton the climateof a general Atlantic deep-seasediments, Quat. Res.,24, 137- circulationmodel: Implications for the 149, 1985. Milankovitchtheory, in Milankovitchand Climate, Prell, W. L., Monsoon climate of the Arabian Sea editedby A. Bergeret al., pp. 789-799, D. Reidel, duringthe Late Quaternary:A responseto Hingham, Mass., 1983. changingsolar radiation in Milankovitchand Manabe, S., and D. G. Hahn, Simulation of the Climate,Part 1, editedby A. L. Berger,pp. 349- Tropical Climateof an Ice Age, J. Geophys.Res. 366, D. Reidel, Hingham,Mass., 1984a. 82(27), 3889-3911, 1977. Prell, W. L., Variationof monsoonalupwelling: A Matthews,R. K., andR. Z. Poore,Tertiary •5•80 responseto changingsolar radiation, in Climate recordand glacio-eustaticsea-level fluctuations, Processesand Climate Sensitivity,Geophys. Geology, 8, 501-504, 1980. Monograph29, editedby J. E. Hansenand T. Mcintyre, A., W.F. Ruddiman,K. Karlin, and A. Takahashi,pp. 48-57, AGU, Washington,D.C., C. Mix, Surfacewater responseof the equatorial 1984b. ArianticOcean to OrbitalForcing, Prell, W. L., and J. E. Kutzbach, Monsoon Paleoceanography,4(1), 19-55, 1989. variabilityover the past 150,000 years, J. Mele, P.A., and E.J. Katz, SequalXBT cruise Geophys.Res., 92(D7), 8411-8425, 1987. reportLDGO-85-2, reportof XBT datafrom four Prentice, M. L., and R. K. Matthews, Cenozoic ice- Sequalcruises (1983-1984), Lamont-Doherty volumehistory: Development of a composite Geol. Obs., Palisades, N.Y., 1985. oxygenisotope record, Geology, 16, 963-966, Merle, J., Seasonalvariability of subsurfacethermal 1988. structurein the Tropical ArianticOcean, in Richardson,P. L., Drifting buoytrajectories in the Hydrodynamicsof theEquatorial Ocean, editedby AtlanticNorth Equatorial Countercurrent during J. C. J. Nihoul, pp. 31-49, Elsevier,New York, 1983, Geophys.Res. Lett., ••(8), 745-748, 1983. 1984. Mix, A. C., Late Quaternarypaleoceanography of Richardson, P. L., and S. G. H. Philander, The the Atlantic Ocean: Foraminiferal faunal and seasonal variations of surface currents in the stable-isotopeevidence, Ph.D. dissertation, tropicalAtlantic Ocean: A comparisonof shipdrift Columbia Univ., New York, 1985. datawith resultsfrom a generalcirculation model, Mix, A. C., and W. F. Ruddiman, Structure and J. Geophys.Res., 92 (C1), 715-724, 1987. timingof thelast deglaciation: Oxygen isotope Richardson,P. L., andD. Walsh,Mapping evidence, Quat. Sci. Rev., 4, 59-108, 1985. climatologicalseasonal variations of surface Mix, A. C., W. F. Ruddiman,and A. Mcintyre, currentsin the tropicalAtlantic using ship drifts, J. Late Quaternarypaleoceanography of the tropical Geophys. Res., 91(C9), 10,537-10,550, 1986. Atlantic,2, The seasonalcycle of seasurface Rind,D., Componentsof theice agecirculation, J. temperatures,0-20,000 years B.P., Geophys. Res., 92(D4), 4,241-4,281, 1987. Paleoceanography,1 (3), 339-353, 1986. Rind, D., and D. Peteet, LGM terrestrial evidence Ortner, P. B., P. H. Wiebe, and J. L. Cox, andCLIAMP SSTs:Are theyconsistent?, Quat. Relationshipsbetween oceanic epizooplankton Res., 19, 1-22, 1984. distributionsand the seasonal deep chlorophyll Rossignol-Strick,M., African monsoons,an Raveloet al.: ReconstructingTropical Atlantic Hydrography 431

immediateclimate response to orbitalinsolation, Thunell, R. C., and L. A. Reynolds,Sedimentation Nature, 303(5921), 46-49, 1983. of planktonicforaminifera: Seasonal changes in Rossignol-Strick,M., MediterraneanQuaternary speciesflux in the PanamaBasin, sapropels,and immediate response of the African Micropaleontology,30(3), 243-262, 1984. monsoonto variationof insolation,Palaeogeogr., Tolderlund, D. S., and A. W. H. Be, Seasonal Palaeoclimatol., Palaeoecol., 49, 237-263, 1985. distributionof planktonicforaminifera in the Rossignol-Strick,M., W. Nesteroff,P. Olive, and westernNorth Atlantic, Micropaleontology,17(3), C. Vergnaud-Grazzini,After the deluge: 297-329, 1971. Mediterraneanstagnation and sapropelformation, Weisberg,R. H. (Ed.), Sequal/focal:First year Nature, 295(5845), 105-110, 1982. resultson the circulationin the equatorialAtlantic, Samthein,M., Sanddeserts during glacial maximum Geophys.Res. Lett., 11,713-802, 1984. and climaticoptimum, Nature, 272, 43-46, 1978. Williams, D. F., A. W. H. Be, and R. G. Sarnthein,M., G. Tetzlaff, B. Koopmann, K. Fairbanks,Seasonal stable isotopic variations in Wolter, and U. Pflaumann,Glacial and interglacial living planktonicforaminifera from Bermuda windregimes over the eastern subtropical Atlantic planktontows, Palaeogeogr., Palaeoclimatol., and North-West Africa, Nature, 293(5829), 193- Palaeoecol., 33, 71-102, 1981. 196, 1981. Williams, J., R. G. Barry, and W. M. Washington, Street, F. A., and A. T. Grove, Global mapsof Simulationof theatmospheric circulation using the lake-level fluctuationssince 30,000 B P, Quat. NCAR globalcirculation model with ice age Res., 12, 83-118, 1979. boundaryconditions, J. Appl. Meteorol.,13(3), Street-Perrott,F. A., and S. P. Harrison,Temporal 305-317, 1974. variationsin lake levels since30,000 yr BP: an index of the globalhydrological cycle, in Climate R. G. Fairbanks and A. C. Ravelo, Lamont- Processesand Sensitivity,Geophys. Monogr. DohertyGeological Observatory, Palisades, N Y Ser., 29, edited by J. E. Hansenand T. 10964. Takahashi,pp. 18-129, AGU, Washington,D. S. G. H. Philander,Geophysical Fluid D•(namics C., 1984. Lab, NOAA, P.O. Box 308, Princeton, N I Street-Perrott, F. A., and N. Roberts, Fluctuations 08542. in closed-basinlakes as an indicatorof past atmosphericcirculation, in Variationsin theGlobal WaterBudgets, edited by A. Street-Perrotet al., (ReceivedSeptember 1, 1989; pp. 331-345, D. Reidel, Hingham,Mass., 1983. acceptedFebruary 20, 1990.)