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Late Quaternary Paleoceanography of the Tropical Atlantic, 1&Colon

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Late Quaternary Paleoceanography of the Tropical Atlantic, 1&Colon PALEOCEANOGRAPHY,VOL. 1, NO. 1, PAGES 43-66, MARCH 1986 LATE QUATERNARY PALEOCEANOGRAPHY OF THE TROPICAL ATLANTIC, 1: SPATIAL VARIABILITY OF ANNUAL MEAN SEA-SURFACE TEMPERATURES, 0-20,000 YEARS B.P. Alan C. Mix College of Oceanography, Oregon State University, Corvallis, OR 97331 William F. Ruddiman Lamont-Doherty Geological Observatory, Columbia University, Palisades, NY 10964 Andrew Mcintyre Department of Earth and Environmental Sciences,Queens College, City University, New York, NY Abstract. At least two modesof glacial-interglacial INTRODUCTION climate changehave existedwithin the tropical Atlantic Ocean during the last 20,000 years. The first mode (defined The tropical oceansact as a solar collector and heat by cold glacial and warm interglacial conditions) occurred source for the earth's climate. Heat collected here is symmetricallynorth and south of the equator and advectedaway from the tropics in oceanic currents. Some dominatedthe easternboundary currentsand tropical of this heat is transferred to the atmospherein the middle upwelling areas. This pattern suggeststhat mode 1 is latitudes [Von der Haar and Oort, 1973] and is thus driven by a glacial modification of surface winds in both transported to the polar regions, where net radiative heat hemispheres.The secondmode of oceanicclimate change, loss occurs. defined by temperature extremescentered on the This paper examinesthermal variability of the tropical deglaciation, was hemispherically asymmetrical, with the Atlantic Ocean during the transition from the last northern tropical Atlantic relatively cold and the southern glaciation to the modern interglaciation between 20,000 tropical Atlantic relatively warm during deglaciation. A years B.P. and the present. Our approach is to study likely causefor this pattern of variation is a reduction of spatial variability of tropical Atlantic paleotemperatures the presentlynorthward cross-equatorialheat flux during estimatedfrom fossil foraminiferal assemblages.By deglaciation. No single mechanism accounts for all the determiningthe spatial effects of climate changes,we data. Potential contributorsto oceanicclimate changesare attempt to trace patterns of variability to the mechanisms linkage to high-latitude climates, modification of mon- governingtheir occurrence.Topics to be addressedinclude soonalwinds by ice sheetand/or insolation changes, (1) linkage betweennorthern and southernhemisphere atmosphericCO2 and greenhouseeffects, indirect effects oceanicclimate, (2) variations in Atlantic interhemispheric of glacial meltwater, and variations in thermohaline heat transport, and (3) relationshipsbetween low-latitude overturn of the oceans. and high-latitude climate changeson a glacial-interglacial time scale. •Previouslyat Lamont-Doherty GeologicalObservatory, PHYSICAL OCEANOGRAPHY and Department of Geology, Columbia University, Palisades, New York. The modern seasurface temperatures of the tropical 2Alsoat Lamont-Doherty Geological Observatory, Atlantic Ocean are controlled by the winds. On an annual Palisades, New York. average, warmest temperatures (> 27øC) occur mostly in the westernequatorial Atlantic (Figure la), especiallyin Copyright 1986 the northern hemisphere[Levitus, 1982; Reynolds, 1982]. by the American GeophysicalUnion. This reflectsradiative heating of surfacewaters advected westward in the North and South Equatorial currents, the Paper number 5P0836. northern hemisphereposition of the Intertropical Conver- 0883-8305 / 86/005 P-0836510.00 genceZone (!TCZ), and northward cross-equatorialheat 4/4 Mix et al.' Tropical Atlantic Paleoceanography a 70W 50 30 10 10E --' 30 N 10 10S 70W 50 30 10 10E b 70W 50 30 10 10E 30N 30N ß 25 ii'"'.•NNUAL MEAN ß::;..':'." SEA-SURFACE "'.TEMPERATURE ß . ß . ß ,•.•ø .' o . ß ß ß . 10 10 ßß ß ß ß ß ß ß ., ß . ß ß . 10S 10S ":CLIMAPATLAS ß . .. ß ß . 70W 50 30 10 10E Fig. 1. Modern mean annual seasurface temperature. Dotted linesmark estimatedmean annual positionsof the thermalequator. (a) Atlas values(plotted from data tablesof Levitus [1982]). (b) Averageof Augustand February temperatures,from the CLIMAP atlas values(see text) usedto simulatethe annual mean in the transferfunction calibration. (c) Foraminiferaltransfer function estimates of meanannual temperature; the coretop calibrationdata set (N. G. Kipp, personalcommunication, 1982). transport in the ocean [Hastenrath and Lamb, 1978; radiative heating of the large continental landmass in the Hastenrath, 1980]. northern hemisphere. Cool temperatures south of the Sea surface temperatures are relatively cool off equator in the Gulf of Guinea reflect a combination of northwest Africa ( < 21 øC) and in the easternAtlantic advection of cool eastern boundary current waters and south of the equator (<25øC) (Figure la). Off northwest Ekman divergencesouth of the equator responding to Africa this reflects southward advection of cool water in meridional winds [Cane, 1979; Philander, 1979] and of the Canaries Current and local "coastal" upwelling, which thermocline adjustmentsresponding to remote (western chills surfacewaters as much as 600 km offshore [Wooster Atlantic) wind forcing [Moore et al., 1978; McCreary et et al., 1976]. These effects are driven by northeasterly al., 1984]. trade winds that blow parallel to the coast [Newell et al., The asymmetrical pattern of cool sea surface tempera- 1972; Picaut et al., 1985]. tures south of the equator results in high radiative heat In the eastern equatorial Atlantic the winds are from the gain and low latent and sensibleheat loss in the South south [Newell et al., 1972; Picaut et al., 1985], driven by Equatorial Current [Hastenrath, 1980]. The mean annual Mix et al.' TropicalAtlantic Paleoceanography /45 C 70w 50 30 10 10E 30N 30N ':i'"•NNUALMEAN .:::.::'."'.SEA-SURF ACE •!.:.:.'.".T.EMPE RATURE ß ß ß . ß 10 10 ß CORE-TOO ß ß '. "" ß 10S 10S .2 . ESTIMATES.'.'... 70W 50 30 10 10E Fig. 1. (continued) position of the ITCZ in the northern hemisphereis tropical Atlantic sedimentsby B• et al. [1976], Gardner associated with a net flow of warm near-surface waters and Hays [1976], Prell et al. [1976] and Thiede [1977]. from the southern to the northern hemisphere in the Their resultsindicated that areas along the equator and off Guiana Current along South America. Return flow of cool northwest Africa were significantly cooler during the last water occursas North Atlantic Deep Water (NADW) flows glacial maximum than at present. They inferred that southward [Bryan, 1982; Stommel, 1980; Worthington, tropical Atlantic temperatures responded to equatorial 1976]. Through this cross-equatorialflow and thermohaline divergenceand upwelling of cool water driven by trade overturn, the Atlantic Ocean transports heat northward wind intensity. In their view, faster winds at the glacial acrossthe equator [Oort and Von der Haar, 1976; maximum reflected increasedhemispheric thermal gradi- Hastenrath, 1977]. Possible variations in these cross- ents causedby glaciation of the high-latitude continents. equatorial fluxes may have large consequencesfor climates This view of the oceansas a systemresponding elsewherethat are maintained by this redistribution of heat passivelyto external forcing by ice volume contrasts with between the hemispheres. Ruddiman and McIntyre's [1981] thermodynamic view of the Atlantic Ocean's amplifying role in the mechanics of PALEOCEANOGRAPHY glaciation. Ruddiman and Mcintyre hypothesized that a warm North Atlantic during ice growth provides the latent The Atlantic Ocean has played an extensiverole in the heat necessaryto fuel rapid glaciation, while a cold North development of paleoceanography. Early workers used the Atlantic, partially covered with sea ice, denies latent heat presenceor absenceof the tropical planktonic foraminifera (and moisture) during ice decay. G!oborotalia menardii to infer warm or cold climates in Still unansweredin Ruddiman and McIntyre's work is the tropical Atlantic [Schott, 1935; Ericson and Wollin, the question of what regulatesthe oceanic heat source that 1968]. Ruddiman[1971], however, showed that abundance acts as a glacial amplifier. One possibility they cite is that fluctuations of single speciessuch as G. menardii are not the ice sheets chill the North Atlantic with their melt optimal indicesof temperature. Emiliani [1955] used productsduring deglaciation. Chilling and fresheningof oxygen isotope analysesof planktonic foraminifera as a the high-latitude North Atlantic would reduce moisture paleothermometer. Shackleton [1967], however, demon- flux to the continentsby enhancingwater column strated that much of the oxygen isotope signal reflected stratification and seaice formation, thus reducing changingice volume on land rather than temperature. evaporation. This scenarioimplies that although the More rigorousapproaches to estimatingpaleotempera- oceansplay a role in glacial feedback, they are ultimately tures began with the work of Imbrie and Kipp [1971]. forced by glacial melt products and therefore are not Their "transfer function" approach related orthogonal independent of the ice sheets. representationsof modern foraminiferal faunas to modern Alternatively, the changing heat content of the North sea surface temperatures through regressionequations. By Atlantic may reflect variations in the oceanic heat flux applying theseequations to the downcore record of from the tropics. In this scenario the oceanscould be truly foraminiferal faunas, quantitative estimatesof paleo- active in the sensethat tropical climate changesnot forced temperatures were made.
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