Spatial and temporal oceanographic variability of the eastern

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1 PALEOCEANOGRAPHY, VOL. 12, NO. 3, PAGES , JUNE 1997 Spatial and temporal oceanographic variability of the eastern equatorial Pacific during the late Pleistocene: Evidence from Radiolaria microfossils Nicklas G. Pisias and Alan C. Mix College of Oceanic and Atmospheric Sciences, Oregon State University, Corvallis Abstract. Eight 150,000 year long records of sea surface temperatures combined with two additional recordspanning 400,000 years constrain the spatial and temporal patterns of oceanographic change in the eastern equatorial Pacific and possible mechanisms of variability in the region. Empirical orthogonal function analysishows two important modes of variability, one associated with the eastern boundary current and another associated with the North Equatorial Countercurrent. The two long time series located in the equatorial divergence and within the Peru Current have very different patterns of change. The spectrum for the time series from the Peru Current is dominated by orbital periods of 100, 4 I, and 23 kyr and is similar in variance distribution and phase to records from the Southern Ocean. In contrast, the equatorial divergence site has spectral concentrations the orbital frequencies and also concentration of variance at the nonorbital 31,000 year period. The phase and amplitude spectra of these two sites support the importance of changes in eastern boundary advection and also document a nonlinear response of the equatorial Pacific to orbital changes. Finally, these data provide a new evaluation of the temperature change in the eastern equatorial Pacific during the last glacial maximum. Cooling in the Peru Current region is predicted to be about 4øC, and cooling in the equatorial divergence is estimated to be 3 ø to 5øC. The estimated cooling of the region is of the order of 2øC greater than the cooling predicted by Climate: Long-Range Investigation, Mapping, and Prediction (CLIMAP). Introduction Objectives Global studies of oceanic response to climate change have shown that the equatorial current system was strongly affected by glacial climatic conditions [Climate: Long-Range Investigation, Mapping, and Prediction (CLIMAP), 1976, 1981], as indicated by cooler sea surface temperature estimates in equatorial upwelling areas, and possibly stronger flows of cold water into these regions from eastern boundary currents. The CLIMAP studies have been important to the understanding of these general circulation patterns, but they lack the temporal sampling needed to delineate the time sequences of changes necessary to establish regional linkages of climate variations. Post-CLIMAP studies in the Atlantic and Antarctic point to a large spatial variability of climate responses, both in timing and amplitude [e.g., Hays et al., 1976; Ruddiman and Mcintyre, 1984; Mix et al., 1986a, b]. In this spatial and temporal variability lies the key to determining the mechanisms of climate change. To determine the spatial patterns of changes in equatorial Pacific current flow and heat transport related to glaciation and deglaciations, we present a compilation of 10 marine sediment records from this complex oceanographic region. Each of these records span the last 150,000 years, and through Copyright 1997 by the American Gee;physical Union. Paper number 97PA /97/97PA the analysis of stable isotopes ( 5180), combined with water mass and sea surface temperature estimates based on Radiolaria microfossil assemblages, we can reconstruct the temporal and spatial response of this region to climate change over the last full glacial cycle. In addition, two of these time serie span at least the last 400,000 years and thus provide insights into the response of the eastern equatorial Pacific to orbital insolation changes induced by changes in the Earth's orbital parameters. In this paper we presenthe results of the study of radiolarian microfossils. In other papers we will presenthe results from foraminiferal and carbon isotopic studies. Oceanographic Setting The general distribution of surface currents is illustrated in Figure 1. This circulation pattern to a large extent reflects the tropical atmospheric circulation and the effects of the change in the sign of the Coriolis force across the equator. A fundamental feature of this circulation pattern is the asymmetry of surface currents north and south of the equator. This asymmetry reflects the general position of the Intertropical Convergence Zone (ITCZ), which marks the convergence between the northeast and southeas trade winds. In the present-day climate system the position of the ITCZ is essentially always north of the equator in the eastern equatorial Pacific. Divergence along the equatoresults in a depression of the topography and a pressure gradient, which is balanced by geostrophic flow to the west, both north and south of the equator. This flow is the South Equatorial _. 381

2 382 PISIAS AND MIX: LATE PLEISTOCENE VARIABILITY OF THE PACIFIC Current (SEC). Waters from both the Peru Current, which carries colder waters from high latitudes into the equatorial current system, and the eastward flowing subsurface Equatorial Undercurrent (EUC) provide source waters for the SEC. Seasonal variation in the equatorial current system reflects the seasonal movement of the ITCZ and the seasonal change in the strength of the trade wind systems. The strength of the SEC reflects changes in the strength of the southern hemisphere trade winds. Wyrtki [1965] describes three patterns of circulation which reflect the movement of the ITCZ. During August to December the ITCZ is in its most northerly position at about 10øN and the southeast trades are at their strongest. The SEC is at its strongest, and the North Equatorial Countercurrent (NECC) is fully developed. As the NECC flows eastward, it turns in a cyclonic cell around the feature known as the Costa Rica Dome and is a major contributor of water flowing into the North Equatorial Current NEC south of 20øN. The California Current turns away from the American coast at about 25øN and only contributes water to the (NEC) north of about 20øN [Wyrtki, 1965]. This circulation pattern seems to be the most stable pattern associated with a northerly position of the ITCZ [Wyrtki, 1965]. During February to April the ITCZ is at its most southerly position, the southeast trades are weakened, and the northeast trades are strongest. During this period the NECC does not develop because of the increased intensity of the northeast trades and southern position of the ITCZ. The SEC is much weakened because of the decrease in the southeast trades. The California Current is strengthened during this interval and penetrates to about 3øN and is the major contributor of waters in the NEC. Within the Panama Basin and the Gulf of Tehuantepec two large gyres form. A cyclonic gyre flows around the region of the Costa Rica Dome (at about 8øN, 86øW), and an anticyclonic gyre in the Panama Basin is centered at about 5øN and 88øW. During May to July the ITCZ returns to its northerly position. During this interval, however, the California Current is strong as the NECC begins to develop and strengthen. Unlike the period from August to December the NECC turns northward and contributes waters to the Costa Rica Coastal Current, which flows along the Central American coast. This pattern, according to Wyrtki [1965], is less stable than the first pattern discussed. The asymmetry found in the distribution of surface currents in the equatorial Pacific is also seen in the response of these currents to large-scale climate events. Observations of sea level changes across the equator during El Nifio events clearly demonstrate that while along the equator El Nifio is characterized by a weakening of the surface circulation, off the equator in the NECC and NEC it is associated with a marked increase in circulation [Wyrtki, 1974]. Empirical orthogonal function (EOF) analysis of sea level records from the central and eastern equatorial Pacific further confirms the asymmetry of the equatorial circulation [Baumgartner and Christensen Figure 1. General circulation of the equatorial Pacific showing major surface and subsurface currents. NEC, North Equatorial Current; NECC, North Equatorial Countercurrent; SEC, South Equatorial Current; PC, Peru Current; EUC, Equatorial Undercurrent; CHC, Chile Current; CAC, California Current. Surface divergence is found at the equator and at the boundary between the NECC and NEC. Surface convergence is 1985]. Baurngartner and Christensen [1985] suggest that the associated with the boundary between the NECC and SEC. equatorial circulation can be viewed as two closed cells, the circulation made up of the South Pacific gyre, including the SEC and Peru Currents, and the cell that includes the NEC and NECC systems. The change in the strength of divergence along the equator and between the NEC and NECC is seen in the out-of-phase response of organic carbon production and flux to the deep sea as observed in sediment trap experiments. During 1983 El Nifio organic carbon flux decreased along the equator but showed a maximum at the boundary between the NEC and ECC [Pisias et al., 1986; Dyrnond and Collier, The Equatorial Undercurrent plays a critical role in defining the characteristics of the water associated with "equatorial upwelling." Through an analysis of A]4C combined with ocean circulation models, Toggweiler et al. [1991] help to clarify the complex nature of circulation processes of the EUC. Toggweiler et al. [1991] show that a significant component of water entering the EUC has its origin as the subantarctic Mode Water. Toggweiler et al. [ 1991 ] also note that water forming the core of the Equatorial Undercurrent in the central Pacific does not reach South America. These waters become entrained into the surface layer west of the Galapagos between 110 ø and 95øW. Bryden and Brady [1985] suggest that the cold water tongue at the equator results from the entrainment of 20ø-21øC waters from the EUC. Toggweiler et al. [1991] argue that the vater upwelled along the Peru coast comes from the lower layers of the EUC. These waters have TS properties of 11ø-14øC and This water is cooler, saltier, and contains higher nutrients than water upwelled along the equator in the equatorial divergence [Toggweiler et al., 1991]. This new picture of the nature of the EUC combined with the conceptual view of the circulation of the equatorial Pacific leaves us with a complex interaction of a number of processes that would effect the characteristics of surface waters at our study sites. Nutrient fluxes to sites within the equatorial band, for example, will reflect the intensity of equatorial upwelling. The strength of advection of waters upwelled

3 PISIAS AND MIX: LATE PLEISTOCENE VARIABILITY OF THE PACIFIC 383 along the Peru margin (processes controlled mostly by local wind stress) have the physical and chemical characteristics that reflect oceanographic processes occurring nearly a quarter of the Earth's circumference away, at much higher latitudes. Finally, the chemical characteristics of this water reflect the chemical composition at its source in the subantarctic region as well as the mixing and chemical alteration that occurs as it flows from the southwest Pacific to the equator and finally eastward to the Peru region. Variability of the Tropical Oceans During The Late Pleistocene: Previous Studies There are four regions of high productivity in the eastern tropical Pacific Ocean, each responding to different aspects of equatorial wind fields and surface currents. For simplicity, we discuss these regions separately, but in reality the areas overlap and blend. Along the eastern margin of the tropical Pacific, primarily near the coasts of Peru and Ecuador and along the Mexican coast in the Gulf of Tehuantepec, meridional winds cause coastal upwelling. The nutrient-rich, upwelled waters are transported westward in the NEC and SEC. Changes in meridional wind strength thus affect productivity in these regions. The Costa Rica Dome is a region of intense seasonal upwelling related to the intensification of the cyclonic wind stress curl in conjunction with the seasonal migration of the ITCZ and the eastward flow of the NEC [Hoftnann et al., 1981]. If climate were to change, the intensity and location of the Costa Rica Dome upwelling would depend upon the intensity of the winds and location of the ITCZ. The third area of high productivity is found along the equatorial divergence. The southeast trade winds blowing across the equator create divergence and consequently cause upwelling at the equator. On the basis of heat budget arguments, Wyrtki [1981] suggests that the upwelled water must come from above the thermocline; that is, from less than 50 m depth in the eastern Pacific. If this is correct, the major flux of nutrients into the region is advected westward from the coast of Peru. Upwelling at the equator would increase in intensity if the southeast trade winds were to increase. However, net productivity may instead be controlled by advection of nutrients from the southeast. The last important high-production area is associated with the return flow of waters in the EUC. This subsurface current becomes enriched in nutrients as it flows eastward across the Pacific underneath the equatorial divergence, and portions of it are upwelled in the region near the Galapagos Islands [Leettnaa, 1982]. In 1981, approximately half of the total transport of the EUC disappeared between 110 ø and 94øW, and the remainder disappeared by 85øW. However, the simple heat budget calculations of IVyrtki [1981] suggesthat much of this water cannot have been upwelled, as solar heating is insufficient to raise the temperature of the undercurrent to observed surface water temperatures. yrtki [1981] speculated that a large fraction of the water in the undercurrent must recirculate back to the west as a subsurface flow, however, Toggweiller et al. [1991] suggesthat waters from the deeper part of the undercurrent actually upwell in the Peru Current region. Manabe and Hahn [1977] and Kutzbach and Guetter [1986] have modeled the ice age climate based on the 18,000 years B.P. August sea surface temperatures, ice cover, and land areas estimates made by CLIMAP [1976, 1981]. Their modeling suggests that for August, at least, the glacial wind pattern is marked by stronger southeastrade winds and an ITCZ that, at least in the eastern Pacific, does not move significantly from its present position. If this scenario is correct for the glacial ocean, one would expect that during a glacial climate there would be higher productivity along the eastern margins and possibly more advection of nutrient-rich waters westward. Upwelling would be more intense in the Costa Rica Dome and the position of the upwelling would be in approximately the same location as in the present. Stronger southeast trade winds would force more intense upwelling at the equator, with the magnitude of increase in productivity dependent upon nutrient advection from the eastern margins. The enhanced westward advection and divergent upwelling would also probably decrease the influence of the Equatorial Undercurrent in the highproductivity region around the Galapagos. How does this scenario compare with available paleoceanographic evidence from the area? One of the more detailed studies of the regional response of the eastern equatorial Pacific to late Pleistocene climate change is presented by Rotnine [1982]. This study was an extension of the 18,000 years B.P. reconstruction of Molina-Cruz [1977a]. Rotnine [1982] developed six time slice reconstructions of the eastern equatorial Pacific during the last 127,000 years. The time slices were selected to examine the pattern of circulation during extremes of global climate. The levels included the major isotopic events of the last 127,000 years: isotope stages 2, 3, 4, and 5e. This work provides important insights into the nature of oceanographi changes but leaves many questions unanswered. Rotnine [1982] examined changes in the distribution of radiolarian assemblages in 11 cores from the equatorial Pacific. Chronostratigraphy was based on both isotopic records (only six cores had isotope records available, of which only three were analyzed at high resolution), while the other five cores were correlated based on carbonate variations. As noted by Snoechx and Rea [1994], there are distinct carbonate variations patterns in the equatorial Pacific and the synchroniety of all carbonate events is not yet tested. The time slice approach of Rotnine [1982] combined with a detailed time series approach presented in this study provides a framework for unraveling the response of the equatorial Pacific to late Pleistocene climate change. Time series studies of the eastern equatorial Pacific core V [Molina-Cruz, 1977a, b; Rotnine and Moore, 1981] provide a detailed temporal study of the eastern equatorial Pacific. Data from V19-29 include microfossil assemblages changes, as well as changes in the abundance of the continentally derived mineral quartz. Molina-Cruz [1977a, b] used this record to infer changes in eolian deposition assuming that the only source for quartz at this site was from eolian input. However, based on the proximal location of the site to the South American continent and the Quayaquial River, Reaet al. [1985] question this assumption and suggest that the quartz record is more influenced by fluvial process

4 3 84 PISIAS AND MIX: LATE PLEISTOCENE VARIABILITY OF THE PACIFIC than by eolian deposition. Thus, while the interpretation of faunal changes by Romine [1982] are in part based on the interpretation of the quartz record from core V19-29, the description of faunal changes are not dependent on the assumption of Molina-Cruz [1977a, b]. The studies of Molina-Cruz [1977a, b], Rornine [1982] and Rornine and Moore [1981] generally show a pattern of increased circulation and inferred wind stress during glacial times. However, inferred changes in circulation from the assumed eolian quartz record of core V19-29 do not directly follow a glacial-interglacial pattern [Motina-Cruz, 1977a, b]. The time slice studies also show large changes in annual mean ITCZ position, from 10ø-15øN during interglacial stage 5 to 0ø-5øN during peak glaciations (stages 4 and 2). These large variations in ITCZ position are not consistent with glacial atmospheric circulation inferred from the climate models [Manabe and Hahn, 1977; Kutzbach and Guetter, 1986], which showed essentially no changes in the position of the ITCZ in the eastern tropical Pacific. This apparent contradiction between paleoclimate records and model results presents an important problem if we are to be able to use model results to predict potential changes in global climate. Clearly, the eastern equatorial Pacific has a very complex geographic pattern in its response to climate change on seasonal to interannual-to-glacial-interglacial timescales. The objective of this study is to better quantify and describe the regional variability of the eastern equatorial Pacific during the last 150,000 years. Strategy The strategy used in this paper differs from previous studies of this region in a number of important ways. To document regional variability, we chose not to use the time plane approach used by CLIMAP and others but rather a geographic time series strategy called empirical orthogonal function (EOF) analysis. This strategy has been used to look at large-scale variability in the atmosphere [Kutzbach, 1967], in surface ocean conditions, and to document the regional response of the surface ocean to deglaciation [Mix et at., 1986a, b]. Hagelberg et al. [1995] used EOF analysis to examine oceanographic variability in the eastern equatorial Pacific for most of the late Neogene. High-resolution stratigraphy based on oxygen isotope records allows marine sequences to be correlated with very high precision (< 2000 years; [Pisias et at., 1984; Prell et at., 1986]. In this study all marine sections have high-quality stable isotope records spanning the last 150,000 years. In addition, development of high-resolution chronostratigraphies for the marine isotope record [e.g., Irnbrie et al., 1989] allows us to place paleoceanographic records into a precise temporal framework. However, the analytical approaches are more dependent on the precision of the stratigraphic framework (our ability to define time lines between all records) than on our ability to accurately date each time line. The primary objective of this study is to document the variability of the eastern tropical Pacific during the past few hundred thousand years and from this information examine the important processes associated with these oceanographic changes. Time series analysis of two paleoceanographic time series which span the last 400,000 years are used to place the equatorial Pacific into a global framework defined by Imbrie et al. [1992, 1993]. Irnbrie et al. [1992, 1993] show that different parts of the climate system display very different time domain patterns of variation over the past few hundred thousand years. However, differences in the appearance of their time series and the relative amounts of variance in the different frequency components associated with orbital forcing varies systematically. The documentation of the geographic distribution of the phase of the climate response to solar insolation represents one of the most significant contributions of the Irnbrie et al. [1992, 1993] studies. What might we expect the phase of these responses to be? If the response to local solar radiation is direct and immediate, then we would expect the signal associated with orbital tilt (41,000 year period) to be a standing wave: The phase of solar radiation change at the tilt frequency is constant as a function of latitude, and the amplitude of this forcing increases with increasing latitude. At the frequencies of precession (23,000 and 19,000 years) the phase insolation progresses with latitude and season. Thus we might expect a very different geographic distribution of phases at the different orbital frequencies. Irnbrie et at. [1992] show that the geographic distribution of phases for climate time series with respect to orbital forcing fall into three groupings: an early response in high-latitude southern hemisphere (indicated by a phase angle following insolation forcing but leading ice sheet response), an ice sheet associated response, and a late response in the North Atlantic (indicated by a phase angle following ice sheets). This pattern is observed at both the primary solar insolation frequencies, the 23,000 year precession cycle and the 41,000 year tilt cycle, a pattern not expected if the ocean responds directly to insolation. To account for observation of the same phase pattern being observed in both frequencies, Irnbrie et al. [1992] hypothesized a simple four end-member model describing the response of the ocean to solar radiation changes. In this model, Irnbrie et at. [1993] propose that the very high northern latitudes (the Arctic-Nordic Seas) is the critical region which responds very quickly to insolation changes, the very early response region. A very early response in the northern hemisphere is proposed to explain the close tie between ice volume changes and the solar radiation forcing of the northern hemisphere summer. Through changes in deep ocean convection patterns in the far North Atlantic this response is quickly propagated to the Southern Ocean. This very early response in the North Atlantic/Arctic region is not evident in long time series but is inferred from limited shorter paleoceanographic records [Irnbrie et al., 1993]. The early response of the Southern Ocean is, however, well documented [Irnbrie et al. 1993, and references therein]. While this model is useful in discussing deep ocean linkages, how might this model explain all the observed phase relationships? Specifically, we might ask these following two questions: (1) Are the same set of processes responsible for propagating the response at all frequencies? (2) While the model deals with issues of deep ocean circulation, what are the processes responsible for the surface ocean response at specific geographic settings? An additional question that needs to be addressed in future work

5 PISIAS AND MIX: LATE PLEISTOCENE VARIABILITY OF THE PACIFIC Y ß RC VNTR1-8 ß RC ß Y69-106P ß Y69-71P RC ß ß V19-29 Y ß Y Figure 2. Locations of cores used to reconstruct sea surface conditions in the eastern equatorial Pacific. is, Are the processes that work at the frequencies associated with Milankovitch also responsible for the large geographic response to even higher-frequency components of the climate system? Building on previous studies of oceanographic variability in the eastern equatorial Pacific on timescales from interannual to glacial-interglacial timescales, we hypothesize at least three processes that might play a role in controlling oceanographi change in this region: (1) changes in local wind forcing, (2) changes in the advection of eastern boundary currents related to hemispheric scale wind forcing, and (3) changes in source waters associated with eastern boundary and equatorial upwelling. Methods In this study we have utilized 10 marine sediment cores (Figure 2 and Table 1). All cores span the last full glacial cycle (150,000 years), while two cores provide longer time series, recording 400,000 years of variability in the Peru Current (Y ) and over 700,000 years at the equatorial divergence (RC13-110). For all cores, stable isotopes 5180 and 513C have been analyzed. These data provide the stratigraphic and chronostratigraphic framework for further detailed analysis. The 10 cores listed in Table 1 were sampled to provide a temporal resolution of at least 3,000 years. The sampling strategy depended on the initial stratigraphy to span at least the last full glacial cycle. Splits of each sample were used to collect radiolarian and foraminiferal species census and stable isotopic data. The downcore radiolarian data collected as part of this study used Radiolaria microscope slides prepared following the technique of RoeloJ3 and Pisias [1986]. Estimates of mean annual sea surface temperatures (SST) and seasonal temperature ranges are based on census data of radiolarian microfossils. The transfer functions to estimate mean annual SSTs and annual temperature range are presented by Pisias et al. [this issue]. These transfer functions were developed using the technique of Imbrie and Kipp [1971]. Surface sediment samples from the entire Pacific basin were used in the transfer functions. Unlike previous radiolarianbased transfer function the census percentage data were first transformed with a logarithmic transformation. This transformation is useful to make a percentage data set more normally distributed and basically assumes that the species abundance data for Radiolaria responds exponentially to environmental change. In addition, extensions to the Q-mode factor analysis [Klovan and Miesch, 1976] used in the Imbrie and Kipp [1971] technique were employed to provide more objective criteria for selecting which Radiolaria species should or should not be retained in the factor analysis model. The standard error of estimates for both the mean annual SST temperature range equations is 1.6øC. Table 1. Cores Used in This Study Core Name Latitude, a -=S Longitude, W Water Depth m Species Used for Isotopic Analysis Y ø 17' 85 ø 15' Y69-106P 2059 ' ' Y69-71P 0006 ' ' 2164 Neogloboquadrina dutertrei 2870 N. dutertrei 2740 N. dutertrei V ø15' Y o27' 83o34 ' 77o34 ' 3159 benthie 2734 N. dutertrei RC ø12 ' 101o26 ' 3120 N. dutertrei RC o06 ' 95.65' 3231 benthie VNTR 1-8 0o02 ' 110o29 ' 3791 N. dutertrei RCI lø23' 104o30 ' 3621 N. dutertrei Y o38' 106o57 ' 3175 N. dutertrei anegative latitudes are south.

6 386 PISIAS AND MIX: LATE PLEISTOCENE VARIABILITY OF THE PACIFIC EOF (1) 0N EOF (2) 20N _ T=0 _-_' :-.- -I- T = 50kyr T = 100kyr Figure 3. Schematic illustrating how the empirical orthogonal function (EOF) partitions a data matrix. Each variable in the data matrix is a time series location, and each sample is a vector whose elements are the observed sea surface temperature (SST) at each locations. This data matrix is equal to a weighted sum of orthogonal functions (in this illustration two are shown) which have a geographic representations. The weights are two time series representing the temporal importance of each mode of variation. Stratigraphic and chronologic control is provided by stable isotopic analysis completed for all cores. In Table 1 we list the species used for these isotopic analyses. Analyses were completed at the Oregon State University College of Oceanography on a Finnigan/MAT 251 automated mass spectrometer equipped with an Autoprep Systems carbonate preparation device. The analytic precision of j180 measurements on this instrument is better than 0.08%0. Isotopic records were placed into a chronologic framework using the timescale of Imbrie et al. [1984]. To examine the spatial-temporal response of the eastern equatorial Pacific during the last glacial cycle, we use empirical orthogonal function (EOF) analysis. EOF analysis is an extension of the multivariate technique of principal components analysis (PCA) [Morrison, 1967]. PCA is used to simplify a multivariate data set. In PCA the data matrix is composed of a number of samples (observations; rows of the matrix) with several variables (columns of the matrix). However, in the case of EOF analysis the data matrix is composed of a set of time series from different geographic locations. In this case the number of variables (columns) is determined by the number of geographic locations, while the number of samples (rows) is the number of time steps in the time series. In Figure 3 we show a schematic diagram of a data matrix generated from sea surface temperatures from the eastern equatorial Pacific. Each time series must be sampled with the same temporal resolution and contain the same number of observations. For the data series used in this study, linear interpolation was used to generate time series at each site with a sampling resolution of 3000 years. Each time series contains 49 data points. From these data we could make 49 maps of sea surface temperature distribution, but such an exercise would be very difficult to illustrate and does not help to find the underlying mechanisms for climate variability in the eastern equatorial Pacific during the time interval spanned by these time series. Thus the goal of empirical orthogonal function analysis is to simplify the data and to extract the smallest set of independent modes of variability that can adequately describe the data set. In EOF analysis the modes of variation are extracted from the correlation matrix generated from the 10 time series. The eigenvectors of the correlation matrix describe the spatial mode of variability in the time series. EOF analysis produces eigenvalues and eigenvectors that preserve all of the information contained in the correlation matrix, and thus each eigenvector can be used to describe a specific fraction of the data variance. This fraction is given by the eigenvalues associated with each eigenvector. We have found that the EOFs determined using the correlation and the covariance matrix do not differ in the regional response. As illustrated in Figure 3, the data matrix can be described as the product of the EOF modes (the eigenvectors whose elements are associated with a geographic location and thus can be represented by a contour map) times a time series reflecting the temporal history of the importance of each mode. Thus, for any given time, the spatial pattern of change is equal to a weighted sum of each EOF mode. The scaled eigenvectors derived in this study are shown in Table 2. This table represents the scaled factor loading matrix. The length of each eigenvector (sum of squares of the columns) is equal to the associated eigenvalue times the sum of the variances of all time series, while the sum of squares by rows equals the variance explained at each site. Thus the Table 2. Scaled Factor Loadings and Percentage of Variance Explained for Each Time Series by the First Two EOFs Y Y69-106P Y69-71P V19-29 Y RC RC VNTR1-8 RC Y Site Variance, % EOF 1 EOF

7 PISIAS AND MIX: LATE PLEISTOCENE VARIABILITY OF THE PACIFIC 387 scaled factor loadings have units of degrees. By dividing the row sum of squares by the time series variance we can calculate the percentage of each time series varianc explained by the modes of variation considered in the analysis (Table 2). Time series analysis is used to examine the response of the eastern equatorial Pacific to Milankovitch forcing. Crossspectral techniques are used to compare the sea surface temperature records with the records of global ice volume (the 5180 records from each core). The spectral techniques used are based on the work of Jenkins and Watts [1968] and are the same ones used in the works of Irnbrie et al. [1992, 1993]. Results and Discussion Last Glacial Maximum Our new data from the castera equatorial Pacific allow us to r½½valuat½ the surface ocean conditions in this region during the last glacial maximum-(lgm). These data improve on CLIMAP in that the chronology for all cores is based on oxygen isotope stratigraphy rather than a mixture of isotope and lithologic stratigraphi½s. Indeed, in cores that were also studied by CLIMAP, significant differences in the LGM stratigraphic 1½v½1 based on our new isotope-based stratigraphy versus the older CLIMAP age picks were found [Pisias et al., this issue]. Reconstructions based on CLIMAP [1976, 1981] show that part of this region warmed slightly during February and cooled during August. Maximum temperature changes indicated in the region are of the order of 2øC [Moore et al., 1980]. In Figure 4 we show the mean annual sea surface temperatures for "modern conditions," for the LGM, and an anomaly map of the modern minus LGM. For modern conditions we use the estimates for core top SST from the 10 cores. Thus the anomalies shown in Figure 4 are independent of the local regional bias in the sea surface temperature transfer function [Pisias et al., this issue]. These data suggest a significant cooling in mean temperatures in this region. All sites show a cooling except for Y in the northeast corner of the study area. The warming estimated in core Y is less than the standard error for the transfer function and may not be significant. Cooling in cores at or near the equator range from 2.7 ø to 4.8øC with cooling in the Peru Current region of about 3.5øC (Figure 4c). In all cases the changes in mean annual temperature are greater than the change in the seasonal range estimated from radiolarian assemblages (Figure 4d). Thus we would predict that during the LGM no sites in the Peru Current or equatorial divergence region of the eastern equatorial Pacific were warmer than A - "Modern" SST 184!iii :'-"::. -:.. -' iicg-!';;'-'. -"- ß t *' '¾ '"'"'"'"' ':':' ': i , t r...,..., 2. 0 ß -10.:s:: : 8?s s :g::: : : :t<::::sa:: :::: : : :::::: ============================ LGM T ' :'-' Seasonality Modern - LGM I I I Figure 4. (a) "Modern" mean sea surface temperatures core sites; (b) mean sea surface temperatures during the last glacial maximum (LGM) (18,000 years B.P.); (c) modem minus LGM temperatures; and (d) change in season range estimated from radiolarian transfer functions (Modem minus LGM).

8 388 PISIAS AND MIX: LATE PLEISTOCENE VARIABILITY OF THE PACIFIC 26, 22 '7 20 >- 18 t- z > 22 L--:, > '- 23,- 22 o 22, 20 ß " 18 > 16 o & >' Age x 1000 Years o ß - 18 >' Age x 1000 Years Figure 5. A 150,000 year time series of sea surface temperatures reconstructed from radiolarian microfossils in the core shown in Figure 2. Each time series has been interpolated to a 3 kyr sampling interval. modem values. These data suggesthat the region cooled by about 2øC more than CLIMAP predicted. There are two possible explanations for the differences between our study and that of CLIMAP: (1) In this study, improved stratigraphy gives us confidence that the 18,000 levels are accurately picked [see Pisias et at., this issue], and (2) these estimates are based on one fossil group rather than an integration of a number of different data sets. Since our estimates agree with revised foraminiferal SST estimates made on these same cores [Pisias et at., this issue; Mix et at., 1997], we suggesthat these new data are providing a more realistic estimate of conditions during the last glacial maximum. Regional Response to Climate Change During the Past 150,000 Years To examine the regional response, we presenthe analysis of the ten, 150,000 year long, time series of sea surface temperature. While this data set provides much higherregional resolution, the short time series will be complemented by the long time series to evaluate the frequency domain response of the region. In Figure 5 we show the time series for the 10 cores illustrated in Figure 2. The data shown in Figure 5 are the interpolated time series with "sampling" at 3 kyr intervals. The cores are organized in approximate transect order. Figure 5 shows that the pattern of variability during the last glacial cycle is complex over this region. Except for core Y within the Peru Current, cores farther away from the equator show relatively small temperature changes (Y71-3-2, RC10-62, Y ). Cores under the influence of the Peru Current and the SEC have relatively similar patterns of variations but also show some differences (Y , V19-29, RC13-115, VNTR1-8, and RC13-110). Equatorial cores located in the easternmost part of the area show some cooling associated with the last glacial maximum but generally show relatively small changes during the rest of the glacial cycle (Y69-106P and Y69-71P). EOF analysis is used to extract the spatial and temporal modes of variability from these data. In Figure 6 we show the first and second EOF for these time series. The EOF were extracted using the correlation matrix, and we plot the scaled factor loadings. In Figure 6 we also show the temporal records for each EOF. The first EOF accounts for 42% (Figure 6a) of the data variance, and the second EOF accounts for 15% (Figure 6b). The spatial pattern defined by the first eigenvector of our EOF analysis is very suggestive of an eastern boundary current influence. It is most important in the time series of cores V19-29 and Y At the equatorial sites it is most important at RC and somewhat less important to the west at VNTR1-8 (Figure 6a). At the northeastern most site, Y71-3-2, the mode of variability represented by EOF 1 has a reversed sign. The temporal variability of this mode is essentially one of the glacial/interglacial cycle (Figure 6c). Thus, unlike the equatorial Atlantic where a precession related mode of variation appears to dominate [Mcintyre et at., 1989], the eastern equatorial Pacific is more influenced by the global long period pattern of climate change. The second EOF is most important at sites away from the equator and the influence of the SEC (Figure 6b). It is most important at the northern most sites, RC10-62, Y69-106P, and Y Interestingly, it is also relatively important in the record from core V The regional pattern of this EOF suggests that it is associated with the North Equatorial Counter Current (NECC). The temporal variability of this

9 PISIAS AND MIX: LATE PLEISTOCENE VARIABILITY OF THE PACIFIC Age x 1000 Years ß" -40 -'+ø. J46 -ø ' 4 '-"'""'"' '"'"'"'"'"' :':' ::" ' i:i ' :' ' I I I /, 0-9*...?...:!F.:...:.`.... :...!i!:i:ii? i[...:.:.:...::...!! Age x 1000 Years Figure 6. (a) The first EOF extracted from the 10 time series shown in Figure 7. EOF accounts for 42% of data variance; (b) second EOF accounting for 15% of data (units of factor loadings in Figures 6a and 6b are øc x 100); (c) temporal variability associated with the first EOF. Dashed line is the isotopic record from core RC13-110; (d) temporal variability for second EOF. Dashed line is the isotopic record from core RC mode shows shorter-scale variation superimposed on a longterm trend from 0 to about 90,000 years ago with a distinct minimum at 115,000 years ago. The mode again is important during the isotope stage 6 (Figure 6d). For this EOF pattern a positive valued factor loading (generally found north of the equator) is suggestive of a long-term warming, while a negative factor loading (predominately found south of the equator) would be indicative of a long-term cooling associated with this mode of variability. Regional Response on Orbital Timescales Two cores in our data set have time series of estimated sea surface temperatures that span much of the late Pleistocene (Figure 7). Core RC is located at 0ø6'N and 95ø39'W and provides an 800,000 year time series in the equatorial divergence zone. Core Y is located at 16ø16'S and 77ø20'W providing a temperature time series for the eastern boundary of the South Pacific. Chronologies for these cores are provided by detailed isotopic records based on benthie foraminifera. The isotope record for RCI3-110 can be easily correlated to the standard late Pleistocene isotope record [Irnbrie et at., 1984] and thus provides a high-resolution chronology for this site [Pisias et at., 1990]. The upper part of core Y , spanning the last glacial cycle, was studied by Motina-Cruz [1977b]. Resampling of the core 5as allowed complete isotopic analysis based on benthie foraminifera (Uvigerina sp. and, when available, Cibicides sp.). Correlation of this isotope record with the record of Irnbrie et al. [1984] is straightforward and provides a high-resolution chronology (Figure 7). Radiolarian-based mean SST estimates for both cores are also shown in Figure 7. At both sites the most extreme changes in sea surface temperatures are associated with the last and penultimate glacial-to-interglacial transitions (between isotope stages I and 2 and between stages 5 and 6; Figure 7). In general, both records show generally cooler temperatures throughout the records as compared to the recent. Neither records show clear long period glacial-tointerglacial changes, but they do show variance at intermediate frequencies. Visually, the variations in Y appear more regular than seen in RC In the longer record of core RC the early Brunhes seems to have less variability than the later part of the record. Finally, visual inspection suggests that there is a slight lead in the SST records at both sites relative to the oxygen isotope records, though the different character of the isotope records versus the SST records makes this assessment difficult. To better quantify the differences between these time series, spectral calculations of the SST time series from cores RC and Y are shown in Figure 8. Also shown are the

10 390 PISIAS AND MIX: LATE PLEISTOCENE VARIABILITY OF THE PACIFIC o '16 A : , 3.6 B c ø,., o ' D Age x 1000 Years Figure 7. Time series of 5180 mean SST for RC and Y (a) Mean SST for Y ; (b) 5180 for Y ; (c) mean SST for RC13-110; (d) 5]80 RC spectral density functions for the SST time series and the coherence between the SST and oxygen isotope records from each core. In core Y we see spectral peaks at the frequencies associated with orbital forcing, the 100,000 year eccentricity peak and the 41,000 and 23,000 year peaks of the tilt and precession (Figure 8a). At these frequencies the SST record is significantly coherent with the isotope record from core Y In the SST record, nearly equal amounts of variance are seen in the precession and tilt frequency bands. The pattern of relative contribution of variance from each of the Milankovitch frequency bands, a dominance of the 100 kyr period and nearly equal contributions from the tilt and precession bands, is similar to the SST records for both the higher northern latitudes as well as in the Southern Ocean [Imbrie et at., 1992]. The variance spectrum for RCI3-110 is shown in Figure 8b. Like the Y SST record, the dominant frequency of variation is the 100,000 year period with significant variance also concentrated at the 41,000 year tilt period. Both of these frequencies are significantly coherent with the isotope record from this same core. A significant difference is seen in the spectra for the RC SST record at frequencies higher than the 41,000 year period of tilt. The RC SST time series does not have concentration of variance at the precession frequencies and are not coherent with the ]80 time series at this frequency. Rather, there is a spectral peak in the SST time series centered at a 31,000 year period (Figure 8b). Core RCll-210 from the central equatorial Pacific has significant variance at a 31,000 year period in both the SST record and record of eolian grain size [Pisias and Rea, 1988]. This is interpreted to reflect changes in local wind intensity that drives changes in sea surface temperatures in the equatorial band. A similar frequency component is observed in records from the tropical Indian Ocean [Clemens et at., 1991] but is not seen in the tropical Atlantic [Imbrie et at., 1989]. In the variance spectra of SST at Y there is also some evidence for a 31,000 year period. The peak is not well isolated because of the presence of the strong precessional peak in Y This might be evidence of a mode of variability being transferred from the equatorial region to the eastern boundary current. To place these sea surface temperature records into the framework developed by Imbrie et al. [1992], we plot the phase vectors superimposed on those of Imbrie et al. [1992] in Figure 9. We plot two phase vectors for the SST record from core Y that coincide with the significant coherence in both frequencies bands of the solar insolation time series (tilt and precession) and one vector for the significant coherence between the SST data from core RC and the isotope record at the 41,000 year tilt period. In Figure 9, vectors R 2 representhe early responselements of the climate system studied by Imbrie et al. [1992]. These elements include Southern Ocean SST, sea ice, and change in atmospheric CO2. Vectors R 3 representhe phase of global ice

11 1'01 A PISIAS AND MIX: LATE PLEISTOCENE VARIABILITY OF THE PACIFIC 391 :BW: volume relative to the 65øN summer solar insolation record. The vectors R4 represent late responding North Atlantic SST. Our result show that the 41,000 year phase vector for Y71-6- ß :"-.' : ' o Y plots exactly in the early response field associated with If I / / ß EBC the response seen in the Southern Ocean; the phase vector for I t t / % *%oo. SST the precession band plots between the vector of the early Southern Ocean response and the response of global ice volume. The response vector for RC plots in the early 0.9 ' response field between the Y vector and the vector representing the response of global ice volume.... The position of the response vectors for estimated sea surface temperatures at core sites Y and RC suggesthat the eastern equatorial Pacific is linked to climate response of the Southern Ocean. As discussed above, we BW * *% CI might consider two possibilities: (1) transmission through the eastern boundary current of the South Pacific and/or (2) *i RC J Eq. Upwelling Mode Water from the subantarctic. Distinguishing between these two possibilities is difficult given the limited number o% o SST of long, high-quality time series. Important constraints are provided by detailed analysis of the spectral calculations of 8.";':: V :.': these mean annual SST records. While the SST response vector at the 41,000 year periods in core Y is earlier than RC13-110, given the errors associated with these estimates, we cannot say the timing of the response of these 0.8 two time series at the 41,000 year period is significantly different. The estimated signal amplitudes found in the tilt frequency band is also the same for both time series, of the Frequency (kry-1) order of IøC (0.7øC for both records). The significant difference between these two time series is found in the Figure 8. Variance spectra and coherehey for the 8180 and precession frequency band. From this evidence we conclude mean SST time series in cores (a) ¾ and (b) RC13- that the linkage for core Y is associated with the 110. Three curves are shown in each frame. The spectra for the eastern boundary currents of the South Pacific. If the linkage oxygen isotope (solid curve) and sea surface temperatures between the Southern Ocean and the eastern equatorial Pacific (crosses) are plotted on a log scale (scale not shown). The coherence (solid curve with crosses) between these two time is through Antarctic Mode Water, we would expect frequencies found in the subantarctic to also be found in the series is plotted on a hyperbolic arctangent axis so that the 80% confidence interval for significant coherence can be equatorial zone as well as in the eastern boundary current of simple represented by C]. Horizontaline represents the 80% the South Pacific. An eastern boundary linkage is supported confidence level testing the null hypothesis that the by the observation that the amplitude of the precession band coherence at any given frequency is nonzero. Bandwidth of spectral calculations is represented by horizontal bar (BW). relative to the 41,000 tilt in core Y is similar to that observed in the Southern Ocean [see Imbrie et al., 1992, (23K') R R 3 (Ice) E Y O R 3 (Ice) _. v R4 O(41K) Figure 9. Phase vectorshowing the relationship of the sea surface temperature records from core Y and RC relative to Vectors for Y represent the significant coherence at both 23 and 41 kyr periods, while only the coherence at the 41 kyr period for RC is shown. Vectors are plotted with respect to the response vectors calculated by Imbriet al., [1992, Figure 14]. The vectors representing the orbital forcing at the 23 and 41 kyr periods (precession and tilt) are plotted at 12 o'clock on these phase wheels. Vector R 2 represents Antarctic SST, sea ice, Afi13C, and atmospheric CO2 response; R 3 represents the /5]80 response; and R 4 is the North Atlantic SST response. Shaded areas around the Y and RC vectors represent +1 o.

12 392 PISIAS AND MIX: LATE PLEISTOCENE VARIABILITY OF THE PACIFIC Figure 8]. Thus we speculate that this linkage is sufficiento transmit the 41,000 and 23,000 year response of the Southern Ocean to the mid-south American margin, yet not sufficiento transmit a precession response to the equator. This does not rule out a role of Mode Water in the EUC on the divergence zone of the equatorial Pacific. The presence of a clearly identified 31,000 period along the equatorial and other tropical sites suggests that this apparent nonlinear response [see Pisias and Rea, 1988] is of tropical origin. The possibility that this frequency component is also present in the eastern boundary current data suggests that it might be transmitted from the equatorial to other regions of the Pacific. Whatever process provides the linkage between the high southern latitudes and the eastern equatorial Pacific, it must account for the earlier response along the central South American margin than at the equator, the presence of a response at precessional frequencies along the eastern margin, and the lack of precession response at the equator. Conclusions 1. Radiolarian mean annual SST estimates from the LGM suggests that the CLIMAP temperatures estimates for the eastern equatorial Pacific underestimate surface ocean cooling by about 2øC. 2. The dominant modes of spatial/temporal variability defined from 10 time series spanning the last 150,000 years can be associated with changes in the Peru/South Equatorial Current systems and in the North Equatorial Countercurrent. The character of this variability suggests warming in the NECC during glacial times. 3. Time series analysis of a 400,000 year long SST time series from the Peru Current shows that the response of this region to Milankovitch scale climate variability is similar to high-latitude Southern Ocean locations. The response in the Peru Current is similar in terms of the distribution of variance within each of the Milankovitch bands, significant coherence at both the tilt and precessional frequency bands, as well as with respecto the phase of the response relative to changing global ice volume. 4. Time series analysis of an equatorial divergence time series shows that the frequency/variance distribution is different from the Peru Current region in that variations similar to precessional forcing are not evident. In addition, the equatorial divergence SST record contains evidence of a 31,000 year frequency component which may result from nonlinear response of the equatorial circulation system to orbital forcing. The phase of the coherent 41,000 period frequency band is similar to the response observed in the Southern Ocean response and the Peru Current region. 5. Taken together, these results suggesthat changes in the intensity of the eastern boundary current of the South Pacific play a bigger role in controlling climate variability on these timescales than do changes induced by the transport of subantarctic Mode Waters into the region via the Equatorial Undercurrent. Acknowledgments. This research was supported by NSF grant OCE We wish to thank Mysti Weber for completing the radiolarian counts and June Wilson and Ann Morey for isotopic sample preparation. We thank Mitch Lyle for many stimulating discussions abouthe paleoceanography of the equatorial Pacific. The manuscript benefited by careful reviews from T.C. Moore Jr., Tim Herbert, and Bill Chaisson. References Baumgartner, T.R., and N. Christensen Jr., Coupling of the Gulf of California to large-scale interannual climatic variability, d. 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