Tropical Pacific Ocean thermocline

Transcription

1 PALEOCEANOGRAPHY, VOL. 12, NO. 3, PAGES , JUNE 1997 Tropical Pacific Ocean thermocline reconstructions for the last glacial maximum depth D. J. Andreasen University of California, Earth Sciences Board, Santa Cruz A. C. Ravelo University of California, Institute of Marine Sciences, Santa Cruz Abstract. We evaluate the relationship between ten surface ocean (0-300 m) hydrographic parameters and the spatial distribution of factor-analyzed core top planktonic foraminiferal abundances in the tropical Pacific Ocean (24øN-24øS) for core tops <3800 m. The spatial distribution of the first three faunal factor loadings (88% of the variance) are most highly correlated to subsurface variability (mixed layer depth, thermocline depth) and resistant species percent (RSP). However, RSP is not related o dissolution but is related to thermocline depth. Factor I (mixed layer species G. #lutinata, G. tuber) and factor III (G. tuber) can be distinguished from each other by low abundances of G. #lutinata in factor III. Both assemblagespatially comprise the deep mixed layer region of the western tropical and equatorial Pacific Ocean, but are associated with distinct water mass properties. A combination of Factor I and III loadings shows a higher correlation to thermocline depth (R ). Factor II loadings (dominated by hermocline dwelling species N. dutertrei) are most significantly correlated with the thermocline depth (R 2 = 0.73). Most factorshow only marginally significan correlation o sea surface temperatures (SSTs), indicating that SST is not the primary forcing factor on the planktonic foraminiferal species distributions in the tropical Pacific. A new transfer function was calculated to predict tropical Pacific thermocline depth from plank onic foraminiferabundances using the Imbrie-Kipp Method (IKM)(standard deviation of residuals 4-22 rn (la)). An additional +5-m error is attributed o low species counts in the core top database. The modern analog technique (MAT) was also used to predict thermocline depth (standard deviation of residuals q- 21 m). While last glacial maximum (LGM) thermocline depth changes by IKM and MAT were generally within error, estimated changes were geographically uniform, suggesting an oceanographic response to climate forcing. We estimate that the thermocline depth of the LGM was shallower than present by -20 rn sou h of 8øS, possibly due to a shift in he South Pacific anticyclone to the northeast. Both the IKM and MAT estimate a steeper east-westhermocline slope along the equator, suggesting hat zonal wind stress (Walker circulation) was intensifie during he LGM. Collectively, the thermoclin estimates for he LGM suggest an equatorward compression of the climate zones in both hemispheres. Introduction Recent debate has questioned whether the vestern tropics has a thermostat [Ramanathan and Collins, 1991; '1992, 1993; Fu et al., 1992] that maintains sea Copyright 1997 by the American Geophysical Union. Paper number 97PA õ'328305/97/97pA:00822 $i2.00 : " '... " surface temperature (SST) within a narrow temperature range (27ø-31øC). The stability of tropical SST has global climatic ramifications because tropical SST is linked to moisture content, lapse rates, atmospheric albedo, and cloud cover, which are integral to the dynamics of oceanic and atmospheric circulation. Climate model sensitivity studies indicate that tropical SSTs have a large effect upon model-generated atmospheric circulation patterns at all latitudes [Intergovernmental 395

2 396 ANDREASEN AND RAVELO: TROPICAL PACIFIC THERMOCLINE DEPTHS Panel on Climate Change (IPCC), 1990]. In the tropical Pacific, strongly coupled ocean-atmospheric oscillations (El Nifio-Southern Oscillation) reflect marked reorganization of both the SST field and subsurface thermocline coincident with a weakening or reversal of prevailing easterly trade winds [Philander, 1990]. Thus tropical thermocline structure is determined to a large extent by the direction and strength of seasonal winds. Reconstruction of the spatial structure of the past thermocline in the tropics provides insight into past surface circulation and can serve as a valuable means of verifying general circulation model (GCM) predictions of the tropical wind field response to changes in high- and low-latitude climate forcing. In addition, thermocline depth reconstructions offer the opportunity to investigate changes in tropical climate dynamics and causal mechanisms of SST changes. Transfer-function-based SST estimates derived from the spatial distribution of core top planktonic foraminifera [CLIMAP Project Members, 1976, 1981] have shaped paleoceanographers' view of the last glacial maximum (LGM) climate. In the tropics these estimates have become controversial [e.g., Rind and Peteet, 1985; Guilderson et al., 1994; Schrag et al., 1996], leading to extensive efforts to verify their accuracy. For the tropical Pacific, foraminiferal transfer function validation efforts have focused on reliability [Prell, 1985; Le, 1992], methodological errors at the calibration extremes [Le and Shackleton, 1994], and problems with dissolution [Prell, 1985; Thunell et al., 1994]. Molfino et al. [1982] did an exhaustive statistical analysis of the Imbrie-Kipp method (IKM [Imbrie and Kipp, 1971]) used by the CLIMAP group, which used three faunal groups (radiolaria, coccolithophores, and foraminifera). They concluded that SSTs were not the primary ecological control on the distributions of microfossil species in the tropical surface sediments. Numerous tropical ocean studies suggest foraminiferal species abundances are controlled by upper water column temperature and nutrient gradients. Groundtruthing of foraminiferal species seasonality and vertical abundances using multiple opening-closing net and environmental sensing system (MOCNESS) plankton tows [Fairbanks and Weibe, 1980; Fairbanks et al., 1982; Bg et al., 1985; Thunell and Reynolds, 1984], in combination with determination of calcification depth from oxygen isotopic measurements [Fairbanks et al., 1980; Fairbanks et al., 1982; Curry et al., 1983; Ravelo et al., 1992], indicates that specieshow specific depth habitat preferences within the photic zone. Consequently, steep vertical hydrographic gradients within the photic zone influence microfossil species distributions in tropical sediments [Luz, 1973; CLIMAP, 1976, 1981; Molfino et al., 1982; Mcintyre et al., 1989; Molfino and Mcintyre, 1990, Ravelo et al., 1990; Chen, 1994]. Thus, in the tropical Atlantic, higher core top abundances of nutricline/thermocline dwelling foraminiferal species are found in regions where the thermocline shoals into the photic zone ( m)[mcintyre et al., 1989; Ravelo et al., 1990]. Because faunal abundances can provide subsurface hydrographic information, the upper water colunto structure, and not simply SST, can be reconstructed. The upper surface (thermocline) structure in the tropical Pacific Ocean has an equilibrium response to seasonal changes in the zonal pressure gradients and wind stress [McPhaden and Taft, 1988], so changes in wind stress are instantaneously (on geologic timescales) linked to changes in the upper ocean hydrography. Therefore it should be possible to reconstruct the upper water column thermocline structure using foraminifer abundances. The purpose of this study is to examine and quantify correlations between modern hydrographic conditions and the distribution of planktonic foraminiferal species abundances in the tropical Pacific Ocean deep sea surface sediments. We examine only shallow sites (<3800 m) in order to minimize the influence of calcite dissolution. Our results show that the core top spatial distri- bution of planktonic foraminiferal species in the tropics is most strongly influenced by the depth of the thermocline relative to the photic zone and that SST influence is weak and often not statistically significant. We develop a transfer function to predict thermocline depth in the tropical Pacific Ocean and apply it to predict the spatial distribution of the depth of the thermocline during the LGM. Tropical Pacific Ocean Circulation The four components of modern tropical Pacific upper ocean circulation (0-300 m) are the surface westward flowing North Equatorial Current (NEC), located above 10øN; the eastward flowing North Equatorial Counter Current (NECC), centered at about 6øN; the westward flowing South Equatorial Current (SEC), between 8øS and 3øN; and the eastward flowing subsurface Equatorial Undercurrent (EUC); at 0 ø [Pond and Pickard, 1983; Hastenrath, 1985](Figure 1). These currents represent a dynamic, seasonally responsive coupling between the atmosphere and ocean [Philander, 1990]. The location, direction, magnitude, and seasonality of surface currents in the tropical Pacific Ocean is determined by the balance of wind stress and oceanic geostrophic forces. The magnitude of wind stress and heat fluxes dictates the extent of vertical mixing and thus modulates the depth of the mixed layer and sea- sonal thermocline [Hastenrath, 1985]. The current system is asymmetrically arranged about the equator due to interhemispheric differences in land-sea areal distributions that lead to a stronger southern hemisphere wind field component and thus wind field convergence north of the equator. As such, seasonal changes in the strength and direction of the wind stress modulate latitudinal migration of the Intertropical Convergence Zone (ITCZ) 7øN and determine seasonal changes in the

3 ANDREASEN AND RAVELO- TROPICAL PACIFIC THERMOCLINE DEPTHS E W 20N 20N S 20S 120E W Figure 1. Contour plot of annual average thermocline (18øC isotherm) depth (in meters) in the tropical Pacific. Thermocline depth is derived from the world ocean atlas of Levztus [1982]. The squares mark the geographic position of 189 core tops used in this study. The arrows represent the main oceani currents in the tropical Pacific: the South Equatorial Current (SEC), the North Equatorial Current (NEC), the North Equatorial Countercurrent (NECC), and the Equatorial Undercurrent ((EUC) denoted by the dashed arrow). Shaded areas reflect the latitudinal breadth of the easterly currents. strength and position of tropical surface currents and Methods subsurface hydrography. Thermocline depth structure (Figure 1) is determined primarily by the prevailing easterly trade winds that force the oceanic upper layer to pile up at the western boundary of the basin, maintaining a deep thermocline in the west ( m deep). In the east, trade-;vind-driven westward export of ;vater causes subsurface shoaling of the nutrient-rich thermocline into the euphotic zone, creating conditions for high eastern equatorial Pacific (EEP) productivity (Figure 1). Positive feedbacks stemming from the large temperature gradient along the equator enhance the zonal circulation and east-westhermocline gradient [Bjerknes, The tropical Pacific Ocean core top and LGM foraminiferal species abundances used in this study, located between 24øN and 24øS, are a subset of the CLIMAP calibration data set compiled by Prell [1985]. A total of 189 core tops were selected, all from <3800 m water depth in order to minimize the influence of differential dissolution. The cutoff depth (3800 m) was chosen to include primarily cores above the foraminifera] lysocline, which ranges between 3500 and 4200 m with shallower regions farther east [Berger et al., 1982; Valencia, 1977; Parker and Berger, 1971; Farrell and Prell, 1989; Berger, Most of the core tops (142; 75%) have The ;vestward extending lobe of off-equatorial,, 100 (96-108) individual specimens counted, with the divergence north of the ITCZ is caused by the Coriolis force acting on the opposing flow between the NECC and NEC. This divergence outlines a lobe of shallow remaining cores (47) having counts of >250 tests. To reconstruct the thermocline depth at the LGM, t ventyseven tropical Pacific LGM CLIMAP sites (with a maxithermocline which extends west to 145øW and is cen- mum depth of 4600 m) were used. This maximum depth tered at,, 9øN(Figure 1). The NECC, sandwiched between the equatorial and off-equatorial divergence, is a geostrophic eastward flow of warm surface vater. The NECC converges with the adjacent SEC to the south, was chosen because the lysocline was m deeper during the LGM compared to today [Farrell and Prell, 1989]. We performed Q-mode factor analysis with VARIcreating a distinct and seasonally variable depression MAX rotation using the Calgary Brown factor analyof the thermocline, most prominent at,, 5øN, between 160 ø and 120øW (Figure 1). West of 110øW, the thermocline depth has a seasonal equilibrium response to the wind stress in the eastern central equatorial Pacific [McPhaden and Taft, 1988]. The southeast trades extend to their northward maximum at 3øN in July, causing surface Ekman divergence along the equator exsis (CABFAC) program [Klovan and ImbrUe, 1971] on foraminifer count data (27 species) from the 189 core top sites in order to reduce the basin-wide variance to a few characteristic end-member species assemblages (factors) limbtie and Kipp, 1971]. For this study, all factors that explained variances greater than the noise level in the data set (i.e., greater than 1% variance) tending westward nearly to 180øW. Ekman drift from were retained. A transfer function that calculates mean coastal winds drives upwelling along South America, shoaling the thermocline to <50 m. annual thermocline (18øC isotherm) depth from factor loadings (weightings) was developed using the technique

4 ANDREASEN AND RAVELO: TROPICAL PACIFIC THERMOCLINE DEPTHS of Imbrie and Kipp [1971]. Our use of their approach thermocline depth estimates due solely to low species relates coefficients from a least squares "best fit" of core top factor loadings (independent variables), and their squares and cross products, to the thermocline depth (dependent variable) via curvilinear multivariate regression. For the transfer function, annual average thermocline depth is defined as the depth of the 18øC counts. An alternative approach to estimating paleoecological parameters is via the modern analog technique (MAT) [Hutson, 1980]. Following previou studies and our own investigation, we used the squared chord distance measure to calculate the dissimilarity coefficient [Prentice, isotherm depth [Houghton, 1991] and was determined 1980; Overpeck et al., 1985; Prell, 1985; Andreasen, by a cubic spline fit of the ocean atlas data compiled 1995]. The threshold critical value of the dissimilarity by Levitus [1982]. However, five core sites adjacento Peru lie beneath a region of seasonally intense upwelling coefficient is used to distinguish analog fi'om no-analog samples. A critical value of 0.4 was chosen in this study where SSTs below 18øC occur through part of the year. based on two criteria: The first is historical use with the For these sites, we calculated thermocline depth as the depth of the minimum of the first derivative of their annual average temperature-depth profiles. The correlation between these two methods for defining thermotropical Pacific CLIMAP data set [Prell, 1985; Chen, 1994; Thunell et al., 1994], and second, samples at the LGM with low communalities (<0.8) based on the IKM corresponded to no-analog samples identified by MAT cline depth is high (R2=0.82), and agreement between using a critical value of 0.4. Estimates of the thermothese two thermocline depth proxies is best for depths cline depth were made from a veighted chord distance between 50 and 150 m. The 18øC isotherm is chosen similarity measure [Prell, 1985] using the three closest for the transfer function because it is generally more analogs drawn from the same 189 core tops used in the highly correlated to the foraminiferal assemblages than is the minimum of the first derivative thermocline depth IKM calibration. determination. Because the core top factor analysis was derived from shallow cores (<3800 m), the application of the transfer function downcore is constrained Results Factor Analysis of Core Top Foraminifera to regions which have preservation conditions similar to those found above the modern foraminiferal lysocline in the tropical Pacific. We quantified the error in thermocline depth estimates due to low core top specimen counts (approximately 100 specimens, as opposed to 330 specimens needed for statistical rigor). We "randomly" modified the species abundances for the 189 core tops and recalculated thermocline depths based on these modifications through 1000 iterations. We determined the range within which we should modify species abundance counts by calculating the range of standard error within 95% confidence limits for each species abundance (Xi) in each of 189 core tops, as follows Xi ( Xi(1- Xi) < Xim < Xi N +1.96(Xi(1-Xi) N (1) where Xim is the modified species abundance and N is the number of core specimens counted [Patterson and Fishbein, 1989]. We calculated the probability based on the normal distribution that this change Xi,, could represent a true species abundance and applied the modification only if it passed a numerical test which we weighted to make small modifications more successful than large ones. After renormalizing the revised abundances we calculated thermocline depth from a revised factor analysis/multivariate regression thermocline depth equation. We repeated this entire process 1000 times to determine an error distribution in our Factor analysis reduced the 189 core top relative abundance data to seven factors, describing 96% of the variance (Table 1). Factor I is donfinated by two mixed layer dwelling species, Globigerinita glutinata and Globigerinoides tuber, and describes 66% of the total variance in the faunal data. Central equatorial Pacific core top site TET-39 (4.95øN, 165øW) has the highest loadings on factor I (Figure 2a) and therefore represents one end-member of the faunal distribution. High loadings (>0.75) are found in two western Pacific regions, one in the central equatorial Pacific and a second region centered at about 14øS and 115øW (Figure 3a). This factor, labeled "mixed layer I" for descriptive purposes, has highest loadings of greater than 0.75 in regions where the mixed layer depth is greater than 70 m. The distribution of high loadings (>0.75) on factor II (16% of the variance), which is dominated by Neogloboquadrina dutertrei (Table 1), is centered about the eastern equatorial Pacific divergence (Figure 3b). There is remarkable correspondence between the region where factor II has high weightings (Figure 3b) and the region where the thermocline depth is less than 80 m (Figure 1). At the location of the core top with highest loadings on factor II, the thermocline is within the photic zone (Figures 2b and 3b). This factor is called "thermocline factor II". Factor III explains 6% of the variance and highest loadings in the off-equator western tropical Pacific Ocean where nutrients are depleted in the mixed layer (Figure 3c). Mixed layer depths in regions dominated by this factor are typically deeper than 70 m (Figure 2c), similar to regions dominated by mixed layer factor I (Figure 2a). Mixed layer III scores indicate a

5 ANDREASEN AND RAVELO: TROPICAL PACIFIC THERMOCLINE DEPTHS 399 Table 1. Q-mode, VARIMAX Rotated Factor Scores for Seven Factors From 189 Tropical Pacific Ocean (24øN-24øS) Core Tops (_<3800 m) Varimax Factor Scores Species I II III IV V VI VII Orbulina universa Globigerinoides conglobatus G. tuber G. tenellus G. sacculifer (total) $phaeroidinella dehiscens b Globogerinella aequilateralis G. calida G. hulloides G. falconensis G. digitata G. rubescens Neogloboquadrina pachyderraa 1 b N. pachyderraa r b N. dutertrei b Globoquadrina conglomerata G. hexagona Pulleniatina obliquiloculata b Globorotalia infiata b G. truncatulinoides 1 b G. truncatulinoides r b G. crassaforrais b G. hirsuta G. scitula G.menardii b G. tufaida b Globigerinita glutinata % variance explained Cumulative % variance Species with factor score (absolute values) >0.20. bused to calculate the resistant species percent (RSP). RSP is the sum of percent abundances of these species in the core tops [Berger, 1968; Parker and Berger, 1971]. negative correlation between species G. ruber and G. glutinata and is distinguished from mixed layer factor I by the absence of G. glutinata in factor III. Factors IV-VII together account for 8.5% of the variance (Table 1). The largest weightings on factors IV, VI, and VII are in limited geographic regions with unique hydrography associated with continental boundary dynamics or land-sea interactions [Andreasen, 1995]. Factor V loadings are high at two open ocean sites with high scores for species Globigerinoidesacculifer. These factors are retained to derive the IKM transfer function, as suggested by Le [1992]. Correlations to Modern Hydrographic Parameters Faunal factors do not show a strong linear correlation to minimum, maximum, or annual average SSTs in the tropical Pacific Ocean. However, except for one in- stance, the weak correlations are statistically significant (c = 0.05; Table 2, and Figures 4a and 4b). Factors have higher correlations to subsurface hydrographic features: mixed layer depth and thermocline depth. Mixed layer factor I has good first-order correlation with the mixed layer depth (R ). Of the core tops with high loadings (>0.8) for mixed layer factor I, 89% are located in areas where the mixed layer is deeper than 80 m. Factors II and III are best correlated to thermocline depth, as defined by the 18øC isotherm. Linear correlation between thermocline factor II and thermocline (18øC isotherm) depth is high (R2=0.731), and 91% of the sites with loadings >0.8 had a thermocline depth shallower than 70 m (Figure 4d) Significant correlations to salinity (at 0 and 50 m) occur with mixed layer factor I and with thermocline factor II. Factors IV-VII.(8.5% of variance) show only weak correlations (R z <0.1) with all hydrographic parameters [Andreasen,

6 400 ANDREASEN AND RAVELO' TROPICAL PACIFIC THERMOCLINE DEPTHS (a) Temperature (øc) (b) Temperature (øc) (c) Temperature (øc) I I I Mixed Layer Factor I Hydographlc Endmember (4.5"N, 160.5"W) Thermocl,ne,13 Hydograph,c Endmember (2"S, SS"W) II Mixed Layer Factor III Hydographic Endmember (18.5"N, 173.5"W) Lev tus, 1982 Levltus, 1982 Levitus, o g 10. RIS.34 2o TET-39 5øN- 160øW 20 3øS-85øW ' o 100 o lo 20 MPC øN- 173øW t ' Figure 2. (top) Surface ocean hydrography (12-month and annual average temperature profiles) for the upper 200 m of the water column and (bottom) the planktonic foraminiferal relative abundances for end-member core sites for factors I-III. Figure 2a is mixed layer factor I endmember core TET-39 and associated hydrography, Figure 2b is thermocline II end-member RIS- 34, and Figure 2c is mixed layer factor III end-member core site MPC-31. i_ i i i i i i i i i i i i i /,,,,,,,,,,,,, i 24 : / 0!15 xx.,e: j -,,/ Factor I - _ d [a]. 0., ' 4 ' ' t ' ' ' ?..' ' Factor II '= 16. '...., 0.0 ' -8,,.., :, 0.,'.' '-8.s :" ' : x.' ': " ' i'"' \\\/ f' ow [bl i, i, I, i, i, i, i, i, i, i,!, i, i, i. i, i, i. i Factor III 16. ' ' " 8-0 g. 0.7 : ' ' _ 8 0',v.( '.."' :i:. 'al0. p -----, /,,,...:..o. o 'x_ " _ -16 ' ':'"" ', ' ''. ' ' "I' ' N, ' ' Figure 3. Contour maps of factor loadings derived from Q-mode factor analysis for (a) mixed layer factor I (G. glutinata), (b) thermocline factor II (N. dutertrei), and (c) mixed Layer factor III (G. tuber) for 189 core top locations (squares). [cl

7 . ANDREASEN AND RAVELO- TROPICAL PACIFIC THERMOCLINE DEPTHS o. ' oo.' o ' '..edge. 't,.. o øo... coo -o w _,. : Ooo ø o... o ;. o? ø % oo o oo o o o o oo o oo )o o - o o o 08 ø0 o o o o o, 28 L,.-'ww""-.o echo... o o _ i ec o o... -_.-;.. e'-i o o o o ' ' 'o o o ooo... Ooo o- - o-ooo.. o [o o o ol 24 Li, o o -o o... SST (max) = O.9x r = 0.05 SST (mln) = x r2= 0.04 * i!, i i i i I, i, i i,,,,... i,, o Mixed Layer Factors I+III (Loadings) R I G o o o o o'... SST (m.x)= x/= O.O4 "SST (rain) = 2S.8-1.6x r2= O.O3 I,, I I,,, I,,, I,,, I O.G 0..0 he ocline ac of II ( oadinks) 3OO I I I I I I I I I I I I I I I.,, Thermocllne Depth = x x' il i,. r': 0.8 ' ' ' ø o ' ' /C. 4.' ' ' ' i ' ', i,,, i,,, i ' ed --Thermocline Depth = x r2= 0.73 'o o ø 'eilb,.,,bl - e :, _ Mixed Layer Factors I+III (Loadings) t i I,,, I,,, I,,, I,,, Thermocline Factor II (Loadings) d Figure 4. (a) Simple linear regression R 2 correlations of maximum and minimum sea surface temperature (SST) plotted against mixed layer factors I plus III. Here the loadings of mixed layer factor I are added to the absolute values of negative mixed layer factor III loadings. (b) Maximum and minimum SST plotted against thermocline factor II loadings. There is a poor correlation for the factors with SST. In Figure 4a, SST varies from -20 ø to 30øC for mixed layer factor I and III loadings > 1.0 that are representative of the deep mixed layer regions of the tropical Pacific. (c) The second-order relationship of mixed layer factor (I and III) loadings and thermocline (18øC isotherm) depth. Six samples (open squares) were omitted from the least squares fit; these sites all contained high loadings for factors IV-VII. (d) Linear fit of thermocline factor II and thermocline depth. 1995]. The correlation of thermocline factor II with sea surface salinity appears to be indirect, arising from the geographical correspondence of the Panama Basin lowsalinity lens with the easterlies-driven upwelling and shoaling of the thermocline in the eastern Pacific. Factor analysis defined two deep mixed layer assemblages (factors I and II!) and distinguished slight differences between the responses of G. glutinata and G. ruber to variations in water mass properties and equatorial dynamics [e.g., Wyrtki, 1966; Andreasen, 1995; Watkins et al., 1996]. Both factors have the highest correlations to subsurface features (thermocline depth, mixed layer depth, temperature at 50 m). A secondorder polynomial relationship is found between mixed layer factor I plus the absolute value of factor III loadings and thermocline depth (R 2 = 0.81, N=189; Figure 4d). Combined mixed layer factor I and III loadings (Figure 5) reveal a map pattern with high weightings in areas where the mixed layer is deep and low weightings where the mixed layer is shallow. We investigated whether the spatial distribution of factor loadings reflects an artifact of dissolution in the tropical Pacific. Factor loadings are not correlated with core depth in the range of water depths of the core tops used in factor analysis (Table 2). As an alternative, we iffvestigated the resistant species percent ((RSP), defined in Table 1) which has been used as a dissolution indicator for foraminifera [Cullen and Prell, 1984]. This is done with caution because high RSP could reflect high species abundances due to optimal water column conditions (e.g., shallow thermocline regions) unrelated to dissolution. This effect is precisely what we found. RSP is highly correlated with factor loadings (R in one case, Figure 6 and Table 2). However, like the factor loadings, RSP is much more highly correlated to thermocline depth (R 2 = 0.67) than core

8 402 ANDREASEN AND RAVELO- TROPICAL PACIFIC THERMOCLINE DEPTHS 120E ,,_ / ' Factor I+III 70W ,;,: E z '. 0.s0/ W Figure 5. Contour map of combined mixed layer factor I and (absolute value of) factor III loadings for the core top locations. These combined factors encompass the deep mixed layer regions of the western tropical Pacific Ocean. depth (R 2 = 0.02; Figure 6). While it would be ideal to be able to make a comparison between our factor loadings and more appropriate dissolution data such as percent fragments (not available for this data set), our comparison to core water depth indicates that the factor loadings and the distribution of resistant species are strongly based upon surface ocean processes and not dissolution. Calculation of Thermocline Depth Transfer Function Strong correlations between faunal factor loadings and thermocline depth in the tropical Pacific indicates that the development of a transfer function to estimate thermocline depth is appropriate. The IKM relates factor loadings and their squares and cross products to Table 2. Simple Linear Regression Correlation Coefficients for Factors I-III With 10 Hydrographic Parameters N= 189 Hydrographic Parameters R 2 Correlation Coefficients Factor I Factor II Factor III Factor I+III Annual temperature range Annual average temperature Minnimum sea surface temperature Maximum sea surface temperature Temperature at 50 m Mixed layer depth c Thermocline (minimum of first derivative) d Thermocline depth (18øC isotherm) e Temperature at Thermocline Depth Avgerage surface salinity Avgerage salinity at 50 m Core depth Resistant species percent (RSP) % variance explained Cumulative % variance b b b b b b b b *** The two mixed layer factors (I and III) are combined by adding the absolute values of the loadings for mixed layer factor III to mixed layer factor I. 4Not significant (c =0.05). bthe mixed layer depth was defined as the depth of a 2% deviation of temperature from a slope of temperature with depth. The slope is calculated from the surface temperature and a temperature at depth less than determined by an empirical first estimate of the mixed layer depth. cthe two most highly correlated parameters for each factor. d Thermocline depth defined as the minimum of the first derivative from a temperature versus depth profile. ethermocline depth as defined by the depth of the 18øC isotherm. This includes five cores defined by footnote d adjacent to Peru (see the methods section).

9 ANDREASEN AND RAVELO: TROPICAL PACIFIC THERMOCLINE DEPTHS 40 loo i I loo r.a 40 uu 20 RSP = '(FA1+111) r2= _.' : ' 0 - o Mixed Layer Factors I+III [Loadings] rj 40 u0 20 O//" 0 -- $. "'oeo _o _.o /. - - 'I- ; L,,,/o ø t, _o q)øl I J:tsp = *J:Atj r = I Thermocline Factor II [Loadings] b 3001 I I J ] [ 0 I, J, J J 250 ], TCD = *RSP ] * r2: ] 500 : Core Depth= r2= *RSP- / - - I *. ** / * =oo ø ø 50 :. 3 0 I ",'..'."... ; "' ' "1 o 4 o I I I I I I Resist t Sp ies Percent [%] C Resistant Species Percent [%] d loadings, and (b) factor II loadings. The relationship of RSP and (c) thermocline depth, and (d) core water depth are also shown. Resistant species are highly correlated with thermocline depth but weakly related to core depth. This indicates that surface water processes, and not dissolution, determine the spatial distribution of resistant foraminiferal species in the core tops used in this study. thermocline depth (N=189 core tops), to yield a transfer function that has a multiple correlation coefficient of 0.930, and thus describes 86% of the thermocline depth variance (Table 3). The residuals (estimated minus observed thermocline depth) have a standard deviation of 4-22 rn (la) and are a measure of the random error in the estimation (Figure 7). An additional error of 4-5 rn is caused by the low number of specimens in counts from the core top samples (refer to Methods section for a description of how this error was calculated). Core top estimation of thermocline depth by the MAT for core tops (N=186) with analog cores having dissimilarity coefficients less than the critical value of 0.4 yields a standard deviation of residuals error of 4-21 m. (Figure 7b). Residuals (estimated minus observed thermocline depth) represent the deviation of the dependent vari- able (thermocline depth), as calculated by the transfer functions, from a description of the total systematic variance in the modern calibration data set. Significant linear correlation between the residuals or its slope (at c =0.05) and independent ecological variables (e.g., SST, SSS) indicates biasing of thermocline depth estimates by these variables (Table 4, and Figure 8). The relationships between the residuals of the thermocline depth estimates and mixed layer depth, RSP, and annual SST range are not significantly different from zero slope or zero correlation via either MAT or IKM. However, both methods show statistically significant biasing by latitude and SSS via both tests (of slope and correlation). In addition, the IKM thermocline depth estimates are weakly biased by maximum SST, salinity at 50 m, and temperature at 50 m, yet these parameters are not significantly correlated with

10 404 ANDREASEN AND RAVELO: TROPICAL PACIFIC THERMOCLINE DEPTHS Table 3. Curvilinear Regression Coefficients for the Tropical Pacific Ocean (24øN-24øS) Shallow core (<3800 m) Thermocline Depth Trans- fer Function 189 Observations 36 Parameters Parameter Coefficient Parameter Coefficient intercept Factor I-Factor V Factor I Factor I-Factor VI Factor II Factor I-Factor VII Factor III Factor II-Factor III Factor IV Factor II-Factor IV Factor V Factor II-Factor V Factor VI Factor II-Factor VI Factor VII Factor II-Factor VII Factor Factor III-Factor IV Factor II Factor III-Factor V Factor Factor III-Factor VI Factor IV Factor III-Factor VI Factor V Factor IV-Factor V Factor VI Factor IV-Factor VI Factor VII Facto IV-Factor VII Factor I-Factor II Factor V-Factor VI Factor I-Factor III Factor V-Factor VII Factor I-Factor IV Factor VI-Factor VII Multiple R Multiple R Adjusted for degrees of freedom. the MAT residuals. Spatially, IKM and MAT residuals show a coherent latitudinal bias (R2=0.185 for IKM, and r2=0.017 for MAT), with underestimates predominating south and overestimates predominantly north of the equator in the western tropical Pacific (Figure 9). Underestimates (Figure 9) occur where the thermocline (>250 m) is below the photic zone (Figure 1), nutricline/thermocline productivity is weak, and the faunal assemblage is unresponsive to differences in the thermocline depth. In sum, biasing effects on IKM and MAT thermocline depth estimates, by independent hydrographic, bathymetric, and geographic variables, are IKM Standard Error + 27m 250 MAT Standard Error + 21m a 50 b Observed Thermocline Depth [m] Observed Thermocline Depth [m] Figure 7. Estimated versus observed core top thermocline depth calibrations for the the transfer function (left) and the modern analog technique (MAT) (right). Thermocline depth data set is the modern core top (189) sites (Figure 1). One hundred eighty-six sites were used for the MAT because three sites had no analogs with dissimilarities less than the critical value of 0.4.

11 _ ANDREASEN AND RAVELO: TROPICAL PACIFIC THERMOCLINE DEPTHS 405 Table 4. Curvilinear Regression Coefficients for the ropical Pacific Ocean (24øN- 24øS) Shallow core (<3800 m) Thermocline Depth ransfer Function Geographic, Bathymetric, and Hydrographic Parameters Imbrie-Kipp Modern Analog Slope R Slope Latitude Longitude Core top depth Annual surface temperature range Average sea surface temperature Minimum sea surface temperature Maximum sea surface temperature Temperature at 50 m Mixed layer depth c Thermocline depth (minimum of first derivative) a Thermocline depth (18øC isotherm) e Temperature at TCD Sea surface salinity Salinity at 50 m Resistant species percent (RSP) , , , , , , , , , , , , , , , , , b b b b , i' b b b b , , , , , Statistically significant correlations or slope of the thermocline depth residuals indicates biasing of the IKM and/or MAT thermocline depth estimates by these parameters. Residuals are the estimated minus observed thermocline depths by IKM and MAT methods. Correlation coefficient bnot significant (a=0.05). cthe mixed layer depth is defined as the depth of a 2% deviation of temperature from a slope of temperature with depth. The slope is calculated from the surface temperature and a temperature at depth less than determined by an empirical first estimate of the mixed layer depth. d Thermocline depth defined as the minimum of the first derivative from a temperature depth profile. ethermocline depth as defined by the depth of the 18øC isotherm. This includes five cores defined by footnote d adjacent to Peru (see the methods section). significant but small (within error) and affect the IKM more so than the MAT. Last Glacial Maximum Thermocline Depth Ve applied our thermocline depth estimation techniques (using IKM and MAT) to LGM samples from 27 available sites (Table 5). Of the 27 sites investi- gated, 21 were found suitable to apply the thermocline depth transfer functions (Table 5). Communalities (h2), a measure of the amount of information in the downcore fauna represented by the modern faunal factors, were used to evaluate which LGM cores were suitable for analysis. Six LGM sites have communalities less than 0.8, the arbitrary cutoff we have chosen to repre IKM r= 0.33 a ** f r'l._.... *,*,"' C.* I... I... I... I... I Observed Thermocline Depth [m] MAT r= b o_ " s e_ eqo -. '...,, J,,,,,,,,...,,, f,,, Observed Thermocline Depth [m] Figure 8. Residuals (estimates minus observed thermocline depth) for core top sites using the methods of Imbrie and Kipp, [1971](left) and the modern analog technique (right).

12 406 ANDREASEN AND RAVELO: TROPICAL PACIFIC THERMOCLINE DEPTHS 16. mbrle- lpp W 24-[ -- ' 0' ' ' ' ' /. ode qlog - [a] ' " ' ' 'g" ' ' ' ' I --16 Figure 9. Contour map of the residuals (estimated-observed thermocline depth) as determined by (a) the Imbrie-Kipp method, and (b) the modern analog technique. Positive values indicate regions where the thermocline is overestimated. [b] Table 5. Last Glacial Maximum Thermocline Depth Estimates Using the Imbrie and Kipp Method and the Modern Analog Technique Core Observed Imbrie-Kipp Modern Analog Latitude Longitude Depth, Communalities Thermocline Thermocline Depth Thermocline Depth m Depth, m Est.' Est.-Obs. b Est.' Est.-Obs. b V RC RC RCll RCll RCll RCll RC RC13-1? RC RC c..f V V V f V V V Vl!} V ' V V f..f V V V V V V 'LGM estimated thermocline depth b Estimated minus observed thermocline depth. No modern analog with dissimilarity coefficients <0.4.

13 ANDREASEN AND RAVELO: TROPICAL PACIFIC THERMOCLINE DEPTHS 407 sent sites with no analog to the modern fauna. Four of the six rejected sites are located in the EEP (h , 0.36, 0.36, and 0.71), one at 2øN, 140øW (RCll-210, h 2 = 0.72) and one at 17øS, 113øW (V19-53, h ; Table 5). The same four sites from the EEP were also rejected by the MAT, having dissimilarity coefficients in excess of the critical value of 0.4 and indicative of a LGM fauna with no modern analog. However, MAT vas able to find a subset of modern analogs for the LGM samples from sites RCll-210 and V Core V19-53 is near the East Pacific Rise, and though most of its foraminiferal tests are well preserved [Luz, 1973], this site receives a significant portion of its noncarbonate material from local volcanism [Bender et al., 1971], which could potentially alter the assemblage in some unexplained way. RCll-210 is a deep site (4420 m) and may be influenced by dissolution. Our estimates of the thermocline depth anomalies (estimated LGM thermocline depth minus modern thermocline depth) using the IKM (Figure 10a) indicate a general shoaling in the south and equatorial tropical Pacific Ocean (east of 165øW) and a general deepening of the thermocline in the region encompassing the modern warm pool of the western tropical Pacific. Clearly, a detailed geographical pattern of thermocline depth changes would require more sites. The MAT (Figure 10b) indicates that the thermocline was shallower during the LGM in the southern tropics east of 165øW. The MAT also indicates that the ther- mocline in the warm pool of the western equatorial and tropical Pacific was possibly deeper than today. Close to the equator in the central and eastern Pacific the thermocline depth is estimated to have been shallower at a few sites (by 17 and 38 m) and deeper at others (by 26, 29, and 32 m). Several sites exhibit substantial discord between IKM and MAT estimates (-84, 6, -10, 1, and-51 m; Figure 10a). It is difficult to determine the reasons for the discrepancies, although these sites have high species diversity at the LGM and may reflect a sensitivity in the transfer function to species that are rare in the core top data set. One site (6 m, Figure 10a) where there is disagreement deep (4305 m) and may be biased by dissolution. While most LGM thermocline depth estimates by the IKM and MAT are within their respective estimate errors, the estimates are geographically coherent and there is general agreement between methods, suggesting that the hydrographic changes are relatively small, but real. Discussion Hydrographic Control on Foraminiferal Assemblages in the Tropical Pacific Ocean Mixed layer factors I and III have marginal to insignificant correlations to SST. This supports previous findings that suggest that the unique hydrography of the tropics makes prediction of tropical SSTs by changes in faunal assemblages problematic [Luz, 1973; Molfino [a] 16 -s4 ' " Imbrie-Kipp '5 ' 6-27 '.n , , [b] , 9 2' ' :}2-17.n ' q -24 o ? "- - '., ' Fig e 10. Anomaly m p of the l st gl ci m ximum (20 k minus present) thermocline depth s determined by the ( ) Imbrie-Kipp method nd (b) the modern nalog technique. Positive wlues indicate deeper thermoc]jne depth 20 km

14 408 ANDREASEN AND RAVELO: TROPICAL PACIFIC THERMOCLINE DEPTHS et al., 1982; Ravelo et al., 1990; Chen, 1994]. Fau- Interpretations of LGM Thermocline Depth nal stability in the tropics does not necessarily indicate Estimates SST stability in the western equatorial warm pools. The results of this study suggest that while changes Small differences in foraminifer assemblages between in the spatial distribution of faunal assemblages found core top and LGM samples indicate modest adjustments in the geologic record do not reflect SST, they do re- to the subsurface hydrographic structure in the tropical flect movement of the thermocline depth relative to the Pacific during the LGM. Although predicted changes in photic zone. Thermocline depth movements therefore thermocline depth are close to the error of the predictive reflect wind stress changes that may help constrain po- methods, they have geographically coherent structure, tential mechanisms of LGM tropical dynamics and SST which suggests that the changes may be small but real. change. Shoaling of the thermocline in the east, at the modern Ravelo et al. [1990] found that in the tropical At- position of the NECC, and a deepening in the vest could lantic, interpretations of the faunal factors and their be interpreted as either equatorward movement of the correlations with photic-zone hydrographic parameters ITCZ and/or a strengthening of the northeast trades. are readily understood when upper layer ocean ecology In the southeastern region of our study area, annual is considered. Pelagic regions where the mixed layer ex- average LGM thermocline depth vas shallower, which tends below the photic zone have underlying sediments suggests northward displacement of the South Pacific rich in species with high scores in mixed layer factors I high-pressure zone (Figure 11). We know from the efand III ( G. tuber, G. glutinata, and G. sacculifer). High fect of the seasonal migration of the high-pressure zone scores for G. tuber and G. sacculifer are consistent with that this displacement would have caused reduced wind their adaptation to low-nutrient surface waters due to stress curl, Ekman pumping, and thermocline depth in their symbiotic algae. However, the high abundances the southeast region of our study area during the LGM. of asymbiotic species G. glutinata in these oligotrophic In addition, it would have caused an increase in the waters are less easily explained, because G. glutinata magnitude of the zonal component of the vind stress shows a preference for specific water masses within the over the east central equatorial Pacific and of the meridnutrient-depleted deep mixed layer regions of the trop- ional component of the vind stress in the EEP near the ical Pacific. In regions where upwelling of the thermo- coast of Peru. Both the increase in zonal equatorial cline/nutricline into the photic zone enhances primary winds and the inferred cooling of surface waters due productivity, subsurface-dwelling foraminiferal species stronger meridional vinds and enhanced Peruvian upflourish [Fairbanks et al., 1982; Mcintyre et al., 1989; welling would perpetuate stronger Walker circulation. Ravelo et al., 1990], and these species are well repre- Below, we discuss these climatic conditions as they resented in the seafloor fossil assemblages. As expected, late to this and previous interpretations of LGM equatoeastern tropical Pacific cores are almost all dominated rial and southern hemisphere circulation in the tropical by thermocline-d velling species N. dutertrei. These ob- Pacific. servations are supported by high anticorrelation of the Our interpretation that the average position of the thermocline depth to loadings of factors I-III (Table 2, subtropical high-pressure system was closer to the equaand Figure 4d). tor and the Peruvian coast in the LGM relative to to- 140W W 140W W / / /. :?.,:..:- / / 10 lo ( 20S 10 I lo,,, \......========== :...'..! d. :O': :?7 :: ' : 1 0 :'C:: i a "g ::::; : : : " :: S I I I I Reduced-- : :... :, 10. -W:d?:dl 2os 140W W 140W W [al [bl Figure 11. Schematic illustration of the changes in circulation inferred from this study in the eastern topical Pacific: (a) present climate and (b) the interpretation of the last glacial maximum climate. Arrow sizes reflect relative changes in the wind field. The position of the subtropical high-pressure cell (H) and the intertropical convergence zone (ITCZ) are labeled.

15 ANDREASEN AND RAVELO: TROPICAL PACIFIC THERMOCLINE DEPTHS 409 day is consistent with other studies. From a theoret- Collins, 1991]. Cooler EEP SSTs agree with radiolarical standpoint, Flohn [1978] calculated the latitudi- ian faunal evidence that shows an LGM enhancement nal extent of the low-latitude Hadley cell and demon- of upwelling adjacento South America [Romine, 1982]. strated, with high correlation to modern observations, that an increase in the equator to pole temperature No-Analog Fauna in the Eastern Equatorial gradient ( 7Teq-pole) compresses the poleward extent Pacific (EEP) of the Hadley cell. A probable increase in the LGM West Antarctic ice volume [Stuiver et al., 1981] and Foraminifer and radiolarian LGM faunal assemblages in the EEP east of 110øW, unlike other regions of the winter sea ice expansion to nearly twi( e the areal ex- tropical Pacific, are distinctly unlike (communalities tent of the Antarctic ice sheet [Cooke and Hays, 1982] 0.8) modern tropical faunal assemblages [CLIMAP, suggest significant Antarctic LGM cooling and larger 1976, 1981; Moore et al., 1981; Moore et al., 1980; 7Teq-pole. Thus the LGM subtropical high-pressure Romine, 1982; Romine and Moore, 1981]. In this region, systems were probably compressed in latitudinal ex- the modern thermocline factor II (N. dutertrei) assemtent and seasonally intensified [Nicholson and Flohn, blage slightly resembles the EEP foraminifer assemblage 1980]. Estimates of thermocline depth presented here of 21,000 years ago, which has significant abundances agree with previous interpretations of faunal records, of modern temperate and sub-polar species Globogerradiolarian assemblages, and SST transfer functions, inella bulloides, Neogloboquadrina pachyderma (r), and which suggesthe southern hemisphere subtropical cli- Globorotalia infiata, in addition to N. dutertrei. In mate zones were compressed equatorward at the LGM the modern ocean, SSTs in regions with N. dutertrei- [CLIMAP, 1976; Moore et al., 1980; Moore et el., 1981; dominated assemblages rarely cool below 21øC (Fig- Romine, 1982]. In addition, the core top G. glutinata- ure 2b), except during La Nifia episodes. Significant dominated assemblage in the southeastropical Pacific is dominated by G. tuber during the LGM; this indiabundances of temperate and subpolar latitude fauna in the LGM sediment may suggesthat SSTs cooler than cates a less saline water mass and suggests a shift in 21øC existed, at least seasonally, in the EEP during the the position of the South Pacific anticyclone [Wyrtki, LGM. A modern Kuroshio foraminifer assemblage is the 1966]. On the other hand, GCM simulations of the LGM that incorporate expanded circum-antarctic sea closest analog to the EEP LGM fauna (identified by the MAT as having a dissimilarity coefficient less than the ice and ice sheets find that the latitudinal position of critical value; Figure 12a). The Kuroshio site has a the low-latitude Hadley cell (convection about 10øN and hydrographic profile that suggests that for part of the subsiding at 30øS in July) remained constant [Gates, annual cycle, SSTs were considerably cooler in the EEP 1976; Manabe and Hahn, 1977; Kutzbach and Guetter, during the LGM (Figure 12b). 1986; Rind and Peteet, 1985; Rind, 1987] and that the Processes that may be responsible for cooling SSTs subtropical high pressure was positioned partially over below 21øC in the EEP may be understood by look- South America at the LGM [Gates, 1976; Kutzbach ing at two separate physical processes, on seasonal and and Guetter, 1986]. However, these GCM results are interannual timescales, that are responsible for modheavily dependent upon the small changes estimated ern EEP SST variability [KSberle and Philander, 1994]. by CLIMAP [1976, 1981] SSTs and a coarse resolution Interannual (El Nifio) control of EEP SSTs is caused in determination of Hadley cell poleward extent [Rind, by the dynamic response of the surface ocean to large- 1987]. Despite some model-data discord, paleoclimatic scale changes in the wind field. Historically, large-scale observation and theory uniformly suggesthe South Pacific high-pressure zone was closer to the equator and to the coast at the LGM. Today, the westward dipping thermocline is a result of the transport toward Indonesia and buildup of water by zonal circulation changes (analogous to a strong La Nifia mode) do not cool EEP SSTs belo v -19ø-20øC [Philander, 1990] but do shoal the thermocline in the east central Pacific up to 180 m [McPhaden and Hayes, 1990]. Zonal wind field changes of greater magnitude strong westerly wind stress (Figure 1). On interannual than are seen in a modern La Nifia are incompatible timescales, the steepness of the transequatorial thermocline dip is associated with the strength of the trades in the EEP. With stronger trades, the east-west temperature gradient intensifies because of stronger windwith the modest shoaling (about 20 m) of the LGM thermocline shoaling estimated in the east central equatorial Pacific. Thus we conclude that a change in the wind field comparable in magnitude to interannual cirdriven divergence and coastal upwelling in the EEP culation changes in the tropical Pacific can play a sup- (Figure 11). These conditions are associated with a porting, but not primary role, in cooling SSTs in accord steepening of the east-westhermocline slope (e.g., La with the LGM EEP fauna. Nifias) [Philander, 1990]. At the LGM we observe a steeper than modern east-west thermocline slope, which is consistent with stronger zonal winds, a greater eastwest temperature gradient, enhanced Walker circulation, and cooler EEP SSTs, since it is unlikely that LGM SSTs were higher in the west [Ramanathan and Since basin-wide circulation changes analogous to E1 Nifio/La Nifia variations appear insufficient to generate the cooling consistent with EEP LGM fauna, local processes that control seasonal SSTs in the EEP may offer insight into the hydrographic conditions consistent with the cooler-dwelling LGM fauna. In the modern

16 _ 410 ANDREASEN AND RAVELO: TROPICAL PACIFIC THERMOCLINE DEPTHS G.Rub T G.Sac T G.Aequi G.Bullo G.Pac G.Duter R P.Obliq G.Infla G.Menar G.Tumid G.Gluti Other I ;[] V21-33 RC (LGM (Modern Core) Analog) 20 ' '"':':' " :'".'-' "."i i:i:i... '' 60 :.:.:.... m (a)... I... I... i I... I % relative abundance o 8O Temperature (øc) 15 2o 25 3o ',/ ' (b) ' ' ac.f [ II] I / / J/ // /LGMMødernAnaIog! [[ I/////// Hy_dography,... Vl//////,,.',,,5øW, ), / Figure 12. (a) Bar graph of subtropical modern analog site (RC12-139, 33øN-139øW) foraminiferal abundances of abundant species compared to the last glacial maximum site (V21-33, 2.2øS-91øW) species abundances, and (b) modern surface hydrography for waters above site RC (and LGM) EEP, between the coast and 110øW the pro- ing mechanisms need to be addressed. Some potenfile of the maximum temperature gradient below 50 m tial mechanisms include (1) an enhanced or longer La remains nearly constanthroughouthe year [Kb'berle Nifia mode, (2) enhancement of the Walker circulation and Philander, 1994, Figure 2c]. Seasonal SSTs re- leading to increased upwelling, (3) changes in radiative flect variability in the temperature gradient and ver- responses due to a combination of decreased greenhouse tical mixing above 50 m. K6berle and Philander [1994] gases (e.g., CO2 and CH4), increased aerosols and/or inconcluded that seasonal variations in EEP SST result creased albedo (e.g., increased stratus clouds), etc., (4) from two local processes: (1) seasonal upwelling by the changes in the lapse rate, or (5) changes in midlatitude wind-driven divergence of surface currents (equatorial forcing of the thermocline and tropical conditions. The divergence) and (2) seasonal changes in heat flux that results presented here can provide constraints on the modulate mixing processes and influence the extent to first two of these mechanisms and suggest that neither which SST is controlled by upwelling induced by mean mechanism can explain cooling LGM tropical Pacific annual winds. For example, lowering the heat flux re- SSTs by 4ø-6øC. duces the temperature stratification and allows deeper This study suggests that the degree of thermocline mixing by winds, bringing up colder waters from within adjustments to changes in tropical climate dynamics the permanent thermocline. Thus cooling the sea sur- was fairly small but was significant in some regions. face in the EEP requires a seasonal reduction in the Comparison of our results, moderate equatorial shoalheat flux to allow for deeper mixing, and intensification ing of the thermocline between 110øW and 130øW, with of mean winds to drive this deeper mixing. Because of modern observations of La Nifia/E1Nifio cycles I-Philanthe relatively small thermocline depth changes in the der, 1990] indicates that increases zonal circulation rest of the basin, zonal winds (Walker circulation) dur- and equatorial upwelling in the LGM had to be less ing the LGM were insufficiento cause profound eastern than the interannual amplitude. Furthermore, using equatorial cooling. However, meridional winds (intensi- the modern E1 Nifio/La Nifia variability as an analog, fied Hadley circulation) may have been strong enough expected SST changes in the LGM would have been to drive deeper mixing in the LGM (Figure 11). The minimal. reduction of the heat flux suggests that there was an in- It is difficult to explain a lowering of LGM SSTs crease in stratus cloud cover or aerosols above the EEP (>3øC) in the western tropical Pacific with enhanced during the LGM. Walker circulation as inferred by our thermocline recon- Implications for LGM Tropical SST Cooling structions. Organized convection, the engine of Walker circulation fueled by latent heat release, requires as a Ongoing debate regarding the degree of cooling in the necessary condition SSTs >27.5øC [Gad#il et al., 1984; tropics during the LGM [Webster and $treten, 1978; Graham and Barnett, 1987]. An additional requirement Rind and Peteet, 1985; CLIMAP, 1976, 1981; Brassel for large-scale atmospheric convection is the advection et al., 1986; Broecker, 1986; $tute et al., 1995; Guilder- and convergence of moist air into convective regions son et al., 1994] will require additional research in order [Cornejo-Garrido and Stone, 1977; RamaRe and Hori, to reach consensus. If substantial cooling in the trop- 1981]. Since saturation vapor pressure is a nonlinear ics (by 4ø-6øC) occurred, hypotheses regarding cool- function of air temperature, warm SSTs (>27øC)"up-

17 ANDREASEN AND RAVELO: TROPICAL PACIFIC THERMOCLINE DEPTHS wind" of organized convective cells seem necessary for convection, and that requires that a sizable warm pool exist in the western tropics at the LGM. As such, our reconstructions require that mechanisms to explain cooling western tropical Pacific SSTs by more than 3øC in the LGM must also provide a mechanism for enhancement of the east-west dip of the thermocline due to enhanced equatorial zonal winds (i.e., Walker circulation) while removing its convective engine. Future data collection and modeling experiments should be aimed at evaluating SST response to changes in radiative forcing, lapse rates, and midlatitude forcing. Future interpretations of LGM data and models can be constrained by the thermocline reconstructions presented here. Summary We have shown that subsurface ocean dynamics, specifically the depth of the thermocline relative to the photic zone, serve as the primary hydrographic control governing the spatial distribution of planktonic foraminifer species in tropical deep sea sediments in the tropical Pacific. Using factor analysis on the abundances of 27 species of planktonic foraminifera from 189 core sites in the tropical Pacific, we reduced the variance in the modern assemblages to three pelagic factors (I-III) that describe 88% of the variance. On the basis of known depth habitats of the dominant species in the assemblages, we labeled these factors mixed layer factor I, thermocline factor II, and mixed layer factor III. We retained four additional factors for use in the IKM transfer function. We have shown that the assemblages of foraminifera are poorly correlated to SSTs. Thus we suggest that the use of faunal foraminiferal abundances to estimate SSTs in the tropics is likely to give erroneous SST estimates. However, abundances can be used to estimate the wind-forced subsurface features of the upper 300 m of the tropical ocean. Thus we have a tool that can potentially constrain the mechanisms of possible SST changes during the LGM. We developed a new trans- /. fer function to estimate the thermocline depth designed for the tropical Pacific sites with preservational conditions similar to those above the modern foraminiferal lysocline of the tropical Pacific Ocean. Standard deviation of the residuals for our IKM thermocline depth transfer function is 4-22 m. An additional error associated with low faunal species counts reflect a 4-5-m error in the transfer function. Standard deviation of residuals for our MAT thermocline depth estimates is 4-21 m. Significant but weak correlations of residuals (estimated thermocline depth minus observed thermocline depth) with the dependent variable, thermocline depth, indicate that while thermocline depth is the primary correlative variable, there are other secondary water mass/equatorial dynamical influences on foraminifer faunal distributions. Our estimates of the thermocline depth in the tropical Pacific during the LGM using two transfer function techniques, IKM and MAT, indicate that subsurface hydrography of the tropical Pacific Ocean during the LGM was only slightly different than the present tropical Pacific subsurface structure. Our evidence indicates a slight shoaling of the thermocline in the southern hemisphere tropics (south of 8øS), explainable by a reduction of he wind stress curl of the southeast trades over this region. On the basis of evidence presented here and in previous studies, a shift in the subtropical high-pressure zone toward Peru and the equator at the LGM is consistent with the data. We show a moderate increase in the east-west thermocline slope along the equatorial Pacific, which suggests that zonal winds were stronger and are consistent with cooler EEP SSTs. The largest hydrographic difference in the tropical Pacific is reflected by the presence of modern temperate and subpolar species in the far eastern equatorial Pacific during the LGM, possibly due to an increase in meridional winds associated with intensified Hadley circulation combined with a moderate increase in the zonal winds (Walker circulation). Finally, a moderate enhancement of Walker circulation observed in this study indicates that significant SST changes cannot be caused by cooling from divergent upwelling along the equator. Acknowledgments. 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