Climate and Environment of the Subtropical and Tropical Americas

(NH) in the mid-Holocene: comparison of observations with climate

model simulations

Authors:

Anthony Ruter, Jennifer Arzt, Steven Vavrus, Reid A. Bryson, John E. Kutzbach*

Center for Climatic Research

Gaylord Nelson Institute for Environmental Studies

University of Wisconsin – Madison

1225 W. Dayton Street

Madison, WI 53706

USA

Corresponding author. Telephone: 608-262-2839; Fax: 608-263-4190

Corresponding e-mail address: jek@facstaff.wisc.edu (J.E. Kutzbach)

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Abstract

Published paleoecological records from 36 sites in the subtropical and tropical

Americas, spanning the region from the equator to 30N, are used to summarize the

history of Holocene environmental and climatic changes. Although there is considerable

regional variability, and although site coverage is still sparse in many places, it is clear

that significant parts of the USA Southwest, Northern and Central Mexico, the Yucatan,

and Central America were wetter than present in the early to mid Holocene and then

exhibited a drying trend toward the late Holocene, with many sites undergoing especially

significant drying between 6000 and 5000 years ago. In contrast, the USA southeast was

drier than present in the early to mid Holocene and became wetter in the late Holocene.

These observations are compared to simulations of climate for 6000 years ago, and for

the present, made with four different climate models. The models showed fair agreement

both with each other and with the proxy record in many locations, particularly in the

subtropics, although significant differences were also noted, especially in the tropics. The

accuracy of the models is simulating correctly the sign of observed changes (wetter or

drier) was, when the simulated change was large, from 45% to 69%, with a pooled

accuracy among four models of 61%. This result is a strong indication that, although

currently useful for studies of mechanisms, models still have considerable room for

improvement in their accuracy of simulation. Our result underlines the value of using

more than one model for data/model comparison as a means of assessing the robustness

of simulated climate patterns. The observed general drying trend from early through

middle Holocene is captured in part by two climate models that simulated all or portions

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of the Holocene climate sequence. This drying trend coincides with the timing of some

events in the environmental history of the region.

1: Introduction

This paper focuses on the climate and environmental history of the region of the

Americas extending from the equator to about 30N, and spanning the past 11000 years,

but with primary emphasis on comparing the period around 6000 years ago to the present.

This region of the tropical Americas lies within the broader area of the Americas that has

been studied by the IGBP PAGES project under the topic of Pole-Equator-Pole (PEP)

transects, a project that resulted in a monograph on the climate and vegetation histories of

the Americas (Markgraf, 2001). Our paper draws upon these and other sources to

summarize Holocene environmental records, presents results from climate simulations for

6000 years ago derived from several different climate models, compares the simulations

with the paleoenvironmental data, and then summarizes climate, vegetation and other

environmental records for the Holocene for Central America, a subregion within our

study area. This study has been a focus of the interdisciplinary seminar of the Climate,

People and Environmental Program at the University of Wisconsin-Madison.

Changes in earth’s orbital parameters during the Holocene have caused changes in

the seasonality of incoming solar radiation. In the northern hemisphere, insolation was

increased in summer and decreased in winter in the early- to mid-Holocene. These

changes of insolation caused changes in the seasonality of climate, producing generally

warmer summers and colder winters in the northern hemisphere in the early- to mid-

Holocene (compared to present), as inferred from paleoclimatic observations and as

simulated by climate models. The COHMAP group (COHMAP,1988; Wright et al,

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1993) produced charts for many regions of the world showing the observed changes in

pollen records and in lake levels records for various times in the Holocene: 9000, 6000,

3000 years BP (in this introduction, we refer to calendar years Before Present (BP);in

later sections we will use either radiocarbon-estimated ages or calendar ages as inferred

from radiocarbon analyses). The COHMAP group then used a climate model to help

explain how these changes in vegetation or lake status resulted, in part, from changes in

the seasonal cycle of insolation. A major focus of this early work was to document

changes in the monsoon climates of northern Africa and southern Asia, where both

simulations and observations indicated stronger summer monsoons and increased

precipitation in the early to mid Holocene, compared to the present. Because these

continents are large, the response of northern hemisphere monsoons to changed

seasonality of insolation was also large.

By comparison to the studies in northern Africa and southern Asia, relatively less

attention has been given to the role of orbital forcing in causing changes in climate in the

subtropical and tropical Americas. However, this situation is now changing. The

aforementioned monograph (Markgraf, 2001) contains extensive new analyses and

summaries of Holocene environmental records from the Americas, including both

northern South America and Central America (Bradbury et al,2001; Fritz et al, 2001).

Metcalfe et al( 2000) summarize records of climatic change from Mexico. For the multi-

millenial orbital time scale, some studies have noted a general confirmation of the role of

changed seasonality of insolation in forcing the broad envelope of Holocene climate

changes. For example, Cross et al (2000, 2001) and Baker et al (2001) find, in the early

to mid-Holocene, that the southern tropics (in particular, the region of Lake Titicaca)

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were relatively dry while regions of northern South America and the circum-Caribbean

were relatively wet; Leyden et al (1994) note an orbital-forced component to climatic

changes in Yucatan. An out-of-phase relationship between summer precipitation in the

tropics of the two hemispheres is to be expected because of the corresponding out-of-

phase relationship of insolation: early to mid-Holocene summertime insolation was

increased in the northern hemisphere and decreased in the southern hemisphere. This

relative increase in insolation in the northern hemisphere was accompanied by a

northward shift of the ITCZ; after the mid-Holocene, the ITCZ shifted southward (Mayle

et al, 2000;Maslin and Burns,2000; Haug et al, 2001). However, when one examines

regional records in greater spatial and temporal detail, the picture is less clear. While the

early or mid- Holocene is wetter than present (and wetter than the late Glacial) at sites in

Venezuela ,Colombia, Panama, Guatemala and parts of Mexico and Yucatan (Bradbury

et al 2001; Fritz et al, 2001;Metcalfe et al 2000) the timing of this wet phase is far from

synchronous, there is considerable regional variability, and rather large changes on

millennial or shorter time scales can, in some regions, mask or obscure the broad

envelope of orbitally-forced changes.

There are several possible explanations for why the climate’s response to orbital

forcing may be muted or more spatially and temporally variable in the northern

subtropical and tropical Americas than in northern Africa or southern Asia: the relatively

small land mass of the northern tropical Americas reduces the magnitude of the

insolation-forced heating of the land surface, the complex topographic features of the

region cause spatially-variable climatic responses to the changed insolation , and the late-

glacial climatic conditions over northern North America and the subtropical and tropical

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oceans to the south may have played a more significant role in the tropical Americas than

in Africa or Asia. Specifically, the remnant ice sheet over northern North America, or

meltwater outflow to the Gulf of Mexico, may have influenced late-glacial and early

Holocene climates of the northern tropical Americas (see review of climate model

sensitivity studies by Peteet 2001). Because climate models of relatively coarse spatial

resolution cannot simulate the relationships between local climates and local topography,

the need for studies using models of higher spatial resolution has been stated (Metcalfe

et al, 2000).

Until recently, another limitation of simulations of Holocene climates has been

that the role of ocean dynamics, which is of importance for understanding changes of

tropical upwelling and sea surface temperatures, has been ignored. This situation is now

changing as several studies report simulations of mid-Holocene climate with coupled

dynamical atmosphere/ocean models (Hewitt and Mitchell,1998; Liu et al, 1999;

Bush,1999; Braconnot et al 2000; Liu et al, 2002). Harrison et al (2002) have compared

simulated monsoon changes in the Americas with observations; the results show the

combined importance of changes in land/ocean temperature contrasts (not as large for the

American continents as for African and Asia) and changes in tropical SSTs, and the

location of the ITCZ, for influencing the monsoonal response of the northern tropical

Americas to orbital forcing.

This paper uses results from two different kinds of climate models to study the

response of summertime precipitation in the Americas to orbital forcing: (a) dynamical

climate models, which calculate climate variables of atmosphere and ocean on a three-

dimensional global grid (latitude, longitude, elevation), and (b) a macrophysical climate

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model, which uses energy budget equations to calculate the location of key features of the

general circulation (latitude of jet streams, subtropical highs, and ITCZ) and then uses

synoptic climatology to relate these features to local precipitation. The four models are

described in the Appendix. The dynamical models have the advantage of using a fairly

complete set of physical/dynamical equations solved on a regular latitude/longitude

global grid (Figure 1), but have the disadvantage of using a relatively coarse spatial

resolution which prevents accurate simulations in regions of complicated topography

(Figure 1). The macrophysical model has the advantage of providing climate estimates on

a fine spatial scale and incorporating topographic effects through its synoptic climatology

model of modern-day circulation/precipitation relationships at selected stations (Figure

1), but calculates the location of general circulation features at a simpler level (Figure 1).

By presenting a mapping of the observed climate of the northern tropical

Americas for the mid-Holocene (6000 years BP), compared to present (Section 2), and

by comparing these observations to simulations from the two different types of climate

models (Sections 3 and 4), we will assess the strengths and weaknesses of both modeling

approaches for this region. We will also provide a brief overview of changes in climate

and environment over the past 11,000 years in Central Mexico (Section 5).

2 : Mid-Holocene paleoclimatic proxy

We have compiled biotic and geological records from the region to evaluate the

accuracy of the retrodiction for the various climate models. These included studies of

pollen, diatoms geochemistry and isotopes from lacustrian and marine sediments, along

with macrobotanical studies of packrat middens. These studies were selected to

adequately survey the study region during the mid-Holocene but the coverage is neither

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uniform nor comparable in terms of availability or type of proxy. Central and southern

Mexico and the Southeast United States have better coverage than Central America,

South America or the Caribbean. We have followed the author’s interpretations for the

specific sites used to evaluate the model. Our regional generalizations follow those of

Metcalfe et al (2000), Marchant et al. (2002a, 2002b, 2001) and Curtis et al. (1995) and

Hodell et al. (1991) for Mexico, Colombia and the Caribbean basin respectively. Recent

PEP surveys (Behling and Hooghiemstra 2001, Fritz et al. 2001 and Grim et al. 2001)

have also been consulted. Table 1 lists the studies, their locations and the method of

analysis. Table 2 provides a regional paleoclimatic summary of the study regions set

against both a calibrated timescale in years BP as well as a radiocarbon year scale to

facilitate comparison between the GCM models (given in calendar years) and the

chronologies for the Macrophysical models, geological and biotic proxy (given as

radiocarbon years).

In North America the compiled proxy suggests that parts of the southern high

plains (USA) were significantly dryer during the mid-Holocene (Holliday 1988). Studies

from east Texas and the interior Southeast show little biotic adjustment to the mid-

Holocene and apparently experienced little change (Bryant 1977, Bryant and Holloway

1985). Florida and the Bahamas were far dryer, becoming mesic only after about 5000

BP (Watts 1971,1975). In contrast, some regions of the Southwestern US and the

Sonoran Desert of Northern Mexico characterized by a strong monsoon in the modern

period may have had greater effective summer precipitation at 6000 BP (Van Devender

1990, Metcalf et. al. 1991).

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In Central Mexico the predominant early to mid-Holocene pattern is one of higher

lake levels with effective precipitation exceeding that of the present following a relatively

dry period at the beginning of the Holocene; however, at least one site indicates less

precipitation at this time (Watts and Bradbury, 1982; Bradbury, 2000 - see Section 5).

The generally mesic phase ended by about 6000 BP as lake levels dropped. However the

onset of this arid interval is difficult to date and difficult to correlate between other

studies in close proximity partly due to problematic chronologies. Other sources of

discrepancy could include tectonic and related volcanic processes, which may have

affected local hydrology. The modern practice of irrigating from aquifers has drawn

down water tables so modern lakes in the region no longer reflect effective precipitation

(Fritz et al 2001). A greater sensitivity to changes in circulation due to the northwest

trending mountain ranges and rugged local topography may be another important factor

in Central Mexico. Because local hydrology is variable lake studies may not be easily

generalized to the region at large (Grimm et al 2001).

Further south, tropical hardwood forests were fully established in the Peten region

of Guatemala between 9000 - 6800 BP (Isleb et al. 1996). Leyden et al. (1996) infer a

period of wetter than modern conditions from 6150 BC (approximately 7200 BP) to the

Preclassic Period 1800 BC (approximately 3300 BP) for the southern Maya Lowlands on

the basis of phytoliths and pollen record from the San Jose Chulchaca Cenote. A decline

in the climax forest taxa after 5700 BP is interpreted as at least in part anthropogenic

(Curtis et al. 1998). Though the intensification of agriculture in the late Holocene was

significant on a local scale the biotic response observed throughout tropical Mexico and

Central America (with the possible exception of central Mexico) was primarily to

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decreased precipitation or generally greater variability in precipitation beginning after the

mid-Holocene (Curtis et al. 1996).

In general, Mexico, Central America and the Western and Southern Caribbean

region received greater effective precipitation during the mid-Holocene relative to the

present, with the possible exception of at least one site in Central-Mexico. In the Llanos

Orientales of Colombia and the Grand Sabana of Venezuela well into the South

American interior, multiple studies attest to consistently dryer than present conditions at

6000 BP, which became increasing more mesic during the late Holocene (Behling and

Hooghiemstra 2001, 1999, Marchant et al. 2002). Lakes in the Galapagos (Colinvaux

1972) and Panama (Bush et al. 1992) along the Eastern Pacific also record dryer

conditions relative to the present at 6000 BP.

3: Model Simulations for the mid-Holocene

a) model boundary conditions and simulated large-scale circulation

We have simulated the climate of 6000 years ago, and the modern climate, using

three global climate models (Figure 2) with either atmospheric dynamics and mixed-layer

ocean (GENESIS) or fully coupled atmosphere/ocean dynamics (FOAM, paleoCSM),

and using a macrophysical climate model (Figure 3- MCM); see the Appendix for details

of the models. The comparisons of the simulations with observations (Figure 4) is

described in Section 4. All of the models employ the changed seasonal cycle of solar

radiation appropriate for 6000 years ago: namely, increased insolation in northern

hemisphere summer/fall and decreased insolation in northern hemisphere winter/spring

(Figure 5). The MCM simulations also include volcanic forcing using a prescribed global

volcanic index for 200-year intervals derived from radiocarbon-dated eruption events

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(Figure 5); see Bryson (1988) and Bryson and Bryson (1997), and the Appendix for

details. For purposes of comparing the two kinds of models, we have averaged the MCM

output over three simulations within plus or minus 200 years of 6000 years ago, in order

to “average over” the effect of particular centuries of changed volcanicity and retain

primarily the MCM’s response to orbital forcing and be comparable to lower time

resolution of GCMs.

To provide a context for discussion of the simulated climate over the limited

spatial domain of the northern tropical americas, we first summarize the large-scale

structure of the climatic response to the changed orbital forcing for 6000 years ago. The

combination of the enhanced seasonal cycle of insolation in the northern hemisphere and

the different thermal response of land, relative to ocean, causes the continents to warm

more than the oceans in northern summer. This enhanced land-ocean temperature contrast

causes enhanced summer monsoon circulations, enhanced low-level inflow of air from

ocean to land, and enhanced summer precipitation (Kutzbach and Otto-Bliesner, 1982),

with the most pronounced enhancement of precipitation occurring on the largest land

masses of North Africa and South and East Asia (Kutzbach and Otto-Bliesner,1982;

Kutzbach et al, 1998), and, to a lesser extent, on the smaller North American continent

(Harrison et al, 2003). Enhanced heating, upward vertical motion, and lower surface

pressure over the continents is linked to enhanced downward vertical motion and higher

surface pressure over the oceans – generally in the form of intensified and northward-

shifted subtropical oceanic anticyclones (Kutzbach and Otto-Bliesner, 1982; Kutzbach et

al, 1998; Whitlock et al 2001).

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In the oceanic tropics, there is an additional important response to orbital forcing

because the insolation at 6000 years ago, compared to modern, increases more in the

northern hemisphere tropics than in the southern hemisphere tropics. This change in the

latitudinal gradient of insolation favors a slightly larger increase of SSTs north of the

equator, relative to south of the equator. This change in latitudinal temperature gradient is

one primary factor, working along with changes in winds, that contributes to a simulated

northward shift of the ITCZ in the mid-Holocene in both the Atlantic and Pacific sectors

(Kutzbach and Liu, 1997; Liu et al, 2003; Harrison et al, 2003). It is this combination of

enhanced monsoon precipitation over the northern continents, intensified and northward-

shifted subtropical highs, and a northward-shifted ITCZ with its associated precipitation

that seem to be common features of the climate response to mid-Holocene orbital forcing.

In the following paragraphs, the expression of these results in the northern

tropical Americas is described for the various kinds of climate models that we employed.

Our paper focuses primary attention on the changes in June-July-August (JJA)

precipitation, because changes in annual precipitation (not shown) reflect primarily the

changes in JJA.

b) simulations from global climate models with atmospheric dynamics or

atmosphere/ocean dynamics.

Figure 2 shows the simulated precipitation and surface wind changes for JJA

(6000 years ago compared to the modern control) for FOAM, paleoCSM, and GENESIS.

All three simulations indicate strengthened summer monsoon precipitation, but with

considerable regional differences. All three simulations show a tendency for increased

precipitation in the USA Southwest and decreased precipitation along the USA Gulf

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Coast and over the USA Southeast. In earlier work (Harrison et al, 2002), we showed that

this west-to-east difference at around 30N is consistent with modern (instrumental

record) observational studies of composite precipitation patterns keyed to enhanced

southwest monsoons (Higgins et al, 1997). The three models also indicate increased

precipitation in portions of Mexico and Central America, but with considerable

differences in spatial detail. In FOAM the increase is in west and central Mexico and

extends north into the USA Southwest. In the paleoCSM and in GENESIS, the increase

covers most of Mexico and connects with the increase in the USA Southwest. The grid

resolution of all three models is relatively coarse (Figure 1) and results in a prescribed

model topography that is smoother, lower, and broader than the actual topography over

most of this region. The exaggerated breadth of the plateau may contribute to the bias of

all three models toward excessive summer monsoon precipitation in this region in the

modern control simulation (not shown). Nevertheless, the paleoCSM and GENESIS

models, those with the highest spatial resolution, show the largest precipitation increases

at 6000 BP in Mexico and the USA Southwest. All three models show increased

precipitation in parts of Central America, but only GENESIS shows a large increase over

Yucatan. The FOAM and paleoCSM simulations, but not GENESIS, show increased

precipitation in northern South America.

The FOAM and paleoCSM simulations, the two models with interactive ocean

dynamics, also indicate a northward shift of the ITCZ in the eastern Pacific (wind

changes favoring more southerly components in low northern latitudes of the eastern

Pacific) and a tendency for slightly decreased precipitation between the equator and about

10 N, and slightly increased precipitation north of 10N (although changes in coastal SSTs

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in paleoCSM are associated with decreased precipitation along the west coast of Central

America). The GENESIS simulation, lacking an interactive dynamical ocean, fails to

indicate the shift of the ITCZ. All three models have a tendency for increased onshore

flow in parts of Mexico, Central America and along parts of the Gulf Coast. The

GENESIS and FOAM models simulate the enhanced Atlantic trades associated with a

strengthened subtropical anticyclone in the North Atlantic, but this feature is absent in

paleoCSM. In all three models, the strengthened northerly flow along the eastern flank of

the North Pacific subtropical high (COHMAP,1988; Whitlock et al, 2001) occurs to the

north of the domain shown here.

c) Simulations from the macrophysical climate model

The simulation of precipitation for JJA using the MCM (Figure 3) indicates

slightly increased precipitation in the USA Southwest, decreased precipitation in the USA

Gulf Coast and Southeast, decreased precipitation in much of Mexico, increased

precipitation in the eastern part of Yucatan and adjacent regions to the south along the

Atlantic coast of Central America, decreased precipitation along the Pacific coast of

Central America, and decreased precipitation in northern South America except along the

extreme northern coast, where precipitation is increased.

d) model/ model comparisons

Comparing the models (Figures 2 and 3) we find that the models are most

consistent in their simulations over the southern USA and Northern Mexico, where they

simulate generally increased precipitation in parts of the west, and decreased precipitation

over most of the Gulf Coast and the Southeast. The models differ considerably in their

simulations over Central and Southern Mexico, with GENESIS, FOAM and paleoCSM

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indicating increased precipitation and the MCM indicating reduced precipitation except

along parts of the west coast. FOAM and paleoCSM simulate increased precipitation in

large parts of northern South America, and GENESIS and the MCM simulate decreased

precipitation in much of this region. The three dynamical models exhibit rather large

model/model differences in Yucatan, Central America and the western Caribbean islands.

The GENESIS model and the MCM are in relatively close agreement in these regions.

The models are in best agreement in the subtropics where the land mass is largest

and, consequently, where the increase of land-sea temperature contrast and increase of

monsoon precipitation is the expected consequence of the increased summertime

insolation associated with the orbital forcing. The enhancement of the summer monsoon

in the west and the drying in the east is a pattern that appears in modern-day

teleconnections (Higgins et al,1997) and is consistent with dynamical theory (Rodwell

and Hoskins, 1996, 2001). The differences between simulations are greatest in the

tropics, due presumably to different model physics, different resolutions of model

dynamics (see Figure 1), different degrees of atmosphere/ocean coupling, and different

treatments of steep gradients in topography. Indeed, it is very likely that the details of

these simulations of the climate’s response to orbital forcing will change when these

experiments are repeated with models of higher resolution and improved physical

parameterizations.

The differences in simulated precipitation can in some cases be related to

differences in the simulation of large-scale circulation features. The global dynamic

models simulate the three-dimensional fields of pressure and wind over the entire globe,

whereas the MCM simulates circulation indices at selected longitudes (in this case, at

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120W, 90W, and 0W) . In comparing the latitudinal positioning of the MCM circulation

indices at these longitudes with the location of circulation features simulated in the fully

dynamical models (Table 3), we note a tendency for the dynamical models to simulate

somewhat larger northward displacements of the subtropical highs at 6000 years BP,

compared to modern, relative to shifts of these features in MCM. Moreover, FOAM and

paleoCSM simulate northward shifts of the eastern Pacific ITCZ and the Atlantic ITCZ

(east of 55W, Harrison et al, 2003), whereas GENESIS , without a dynamical ocean,

simulates no change and the MCM simulates a more southward (northward) location of

the ITCZ at 90W (120W) at 6000 years ago, compared to present. These shifts in

circulation features are consistent with the general tendency for the dynamical models to

simulate more JJA precipiatation than MCM in parts of Mexico and Central America.

However, the models differ in so many respects that is impossible to pinpoint the exact

cause of model-model differences.

4: Model/Data Comparisons for the mid-Holocene

We have compared the observations of climatic differences between 6000 years

ago and present with the simulations of JJA precipitation from the four climate models;

we focus on JJA because much of the observational evidence has a largely summertime

imprint. The discussion is organized by regions, starting in the north, and the

comparisons are summarized in Table 1 and Figure 4. We have adopted a rather stringent

condition for indicating a “significant” change in the simulated climate; i.e., the

simulated change at 6000BP, compared to present, had to be 10% (or larger). Given this

criterion, agreement, as here defined, depended not only on the model having the same

direction of departure (wetter or drier) as the observation, but also that the simulated

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change was at least 10%. Figure 4 indicates the observed changes as + ,-, or 0, and then

indicates agreement or disagreement of model and data by drawing circles around the

observed change (see Figure 4 caption). Whereas the individual models represented by

the circles are not identified in Figure 4, this information is summarized in Table 1.

Scoring only the instances with simulated changes greater than 10% (and the two sites

with “no change” observed, see Table 1 caption), the model agreement with the observed

changes ranged from 45% to 69% (Table 1), with a pooled score of 61%. Using a Chi-

squared test on a contingency table summary of simulated versus observed changes (not

shown), we find that the results exceed random expectations at the 95% level of

confidence. Although this result is somewhat encouraging, in the following discussion we

choose to emphasize that this very considerable spread in model results, with

disagreement between models and data in 39% of the comparisons, is a reminder of the

importance of using more than one model in model/data comparisons, whenever possible.

In the northern subtropics, the observations indicate that conditions at 6000 years

ago were wetter in the west, and drier in the east, compared to present (Figure 4), with the

crossover from wetter to drier being around 95W-105W longitude. As discussed in

Section 3 and in Harrison et al (2003) this spatial pattern is expected on physical grounds,

given that a stronger summer monsoon circulation (caused by orbital forcing) will induce

a downstream ridging and sinking motion to the north and east. To varying degrees all of

the models indicate this dipole structure (wetter in the west, drier in the east)- see Figures

2 and 3. Thus the response is relatively robust across models, and generally consistent

with the observations (Figure 4), although the exact longitude of the crossover is

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somewhat different for each model. The best fit between models and data is in the USA

Southeast.

In Mexico, observations (Figure 4) indicate generally wetter conditions from the

west coast to the western and central highlands (at 6000 years ago), which is to be

expected on physical grounds as a direct monsoonal response to increased summertime

insolation. GENESIS, FOAM, and paleoCSM simulate a significant large-scale

enhancement of summertime rainfall both in the northwest of Mexico and over a large

area of central Mexico (in agreement with most observations). MCM simulates this

summer rainfall enhancement only in the northwest corner of Mexico and, in contrast to

the other models, simulates decreased precipitation over much of central Mexico. The

smoothed topography of the dynamical models (Section 3) may influence the magnitude

and location of the monsoon-like response shown by FOAM, paleoCSM, and GENESIS

(Figure 4), and, as described in Section 3, the dynamical models tended to predict greater

northward shifts of the STHs and ITCZ than did MCM.

In the Yucatan peninsula, observations indicate generally wetter-than-present

conditions around 7000 and 6000 years ago (Figure 4). FOAM and paleoCSM do not

simulate this condition, whereas GENESIS and MCM indicate wetter conditions in at

least parts of the Yucatan. The Mexican land mass is relatively narrow to the south, so

we would expect that a direct monsoonal response to the increased insolation might be

rather weak. Indeed, even the two models indicating the strongest summer monsoon

enhancement (GENESIS and paleoCSM) simulate the maximum increase of precipitation

well to the north of 20N with the degree of enhancement falling off rapidly toward the

south. It is possible that the tendency for drier conditions simulated in some of the models

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is caused by enhanced subsidence in the region surrounding the enhanced upward motion

– if so, this response would be analogous to that found at higher latitudes (30N).

In Central America, the observations are sparse and equivocal. Two sites in Costa

Rica indicate that conditions 6000 years ago were either wetter than present or

unchanged, while one site in Panama indicates drier. On physical grounds (Section 3) we

argued that orbital forcing would tend to shift the ITCZ precipitation farther north in this

region, and indeed, the two models with interactive dynamical oceans (FOAM and

especially paleoCSM) indicate winds with a more southerly component in the eastern

Pacific- and increased precipitation in parts of Central America, but north of the site of

the proxy record. MCM, lacking an interactive dynamical ocean, indicates drier

conditions. GENESIS, also lacking a dynamical ocean, simulates drier in the north and

wetter in the south; however, most of these simulated changes are relatively small (Figure

4).

The only observational site in the Caribbean, in western Haiti, indicates wetter

conditions at 6000 years ago, relative to present. All four models simulate wetter

conditions in this area (Figure 4). In FOAM and GENESIS, the wetter conditions appear

to be associated with stronger Trades to the east of Haiti, but not to the west, and hence

increased low-level convergence and upward vertical motion. Nevertheless, much of the

Gulf of Mexico is simulated to be drier than present in all four models (related to

enhanced subsidence in the dynamical models) so the indication of wetter in western

Haiti should not be considered to be a general feature of the Gulf and the Caribbean.

In South America, we have charted two observational sites along the northern

coast of Venezuela, both indicating wetter than present 6000 years ago (one of these sites

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represents increased river discharge from near-coastal mountains) (Figure 4). We have

also charted three sites farther inland, one of which indicates wetter and two drier than

present (Figure 4). FOAM and paleoCSM both simulate significantly increased

precipitation over most of northern South American (north of the equator), as would be

expected over this broad land mass as a direct response to increased JJA insolation north

of the equator (Section 3). However, neither the MCM nor GENESIS simulate this broad

enhancement of precipitation. The MCM simulation, with its more detailed representation

of the topographic controls on precipitation, agrees with the observed dryness in the

upper Orinoco valley.

The observation of drier conditions on the Galapagos at 6000 years ago, relative

to the present, was associated by the investigators (Section 2) with a northward shifted

ITCZ. The two models with interactive dynamical oceans, FOAM and paleoCSM, both

simulate a relatively broad longitudinal band of decreased precipitation between the

equator and about 10N – thus, these models agree with the observation and provide a

plausible explanation of the causal mechanism—i.e., a northward shifted ITCZ which

shifts the precipitation to the north of the Galapagos (Figure 4). MCM, which predicted a

southward-shifted ITCZ at 6000 years ago at 90W, but a northward shift at 120W (see

Section 3), also has drier conditions across a broad longitudinal sector including the

Galapagos. GENESIS, using a mixed-layer ocean, simulates alternating regions of wetter

and drier with no indication of a systematic northward shift of the ITCZ precipitation.

In summary, the comparison of observations and simulations yields areas of

agreement and areas of disagreement. No one model comes close to fitting all the

observations. FOAM, GENESIS, and MCM indicate agreement with observations at 65-

21

69% of the sites, and the pooled score of all models, a 61% agreement, is highly

significant statistically compared to random expectations. Although the paleoCSM’s

agreement with observations is slightly lower than that of the other models, the general

patterns of climate simulation are similar for the three dynamical models and the

differences in scoring arise from relatively small displacements of precipitation features.

The model/data agreement is highest in the subtropics (79% at the 12 subtropical sites)

where the relatively large northern land mass produces the most direct response of

land/sea temperature contrast to the enhanced insolation. The model/data agreement is

lowest in the tropics (49% at the 24 tropical sites) where factors such as the small land

mass (but steep topographic relief) and the different degrees of atmosphere/ocean

coupling where relatively more important.

We note six points that relate to the kinds of comparisons made here. First, there

are still significant areas of the northern tropical Americas that lack the kind of

observational studies required to establish what the climate was like in the mid-Holocene,

compared to present. This lack of an adequate and spatially-dense observational record

limits the kinds of conclusions we can reach on the level of agreement/disagreement

between observations and models. Second, while the models differ in important details, it

is encouraging that in some regions they simulate conditions that agree with well-

understood physical concepts of the linkages between seasonality of insolation and

seasonality of climate. Third, in some regions we have been unable to explain the cause

of some of the model/model differences and some of the differences between simulations

and observations. Fourth, just as we need more and better observations, it is equally

important to develop dynamical models with higher resolution, improved physical

22

parameterizations including atmosphere/ocean coupling, and, for the dynamical models,

improved specification of topography . The approach of MCM, to incorporate synoptic

climatological information involving local topography, provides a means of estimating

changes at higher spatial-resolution than is possible currently with global dynamical

models. Fifth, the estimates of “agreement” (Table 1 and Figure 4) would no doubt be

different if we had picked a different criterion for defining a “significant” change in the

simulation. Sixth, and very important, although showing skill compared to random

expectations, all models leave considerable room for improvement; moreover, the

considerable degree of model/model variance suggests to us the desirability of using

more than a single model in data/model comparison, as one means of gauging the

robustness of model results.

5: Overview of changes in climate and environment over the past 11,000 years in

Central Mexico

Although the period around 6000 BP was the primary focus of this study, the

paleoecological data (Table 2) show what the changing climate controls and the models

imply; namely, that climate change was a process ongoing through the Holocene (Table 2

and Figure 5). First, we provide a broader perspective of the possible causal mechanisms

influencing the climate since late glacial time, and illustrate climate simulations covering

the past 14000 years (MCM) and five “snapshot” simulations, each 150 years in length, at

11-, 8-, 6-, and 3000 years BP (FOAM) for the region of the Central Mexican Volcanic

Belt (about 20N) (Figure 5c). We pick this region because it is one with considerable

paleoecological and archaeological data. Second, we compare the modeled trends of

climate change in this region with the regional paleoecological data (Table 2) and with

23

one local and continuous environmental record (Figure 5c). During the period since the

late glacial, the North American ice sheet was melting, but remained relatively large and

capable of influencing the climate of our region of interest until about 8000BP. (The

MCM includes ice sheet forcing in the late glacial and early Holocene, whereas the

earliest FOAM simulation (11000BP) does not.) During this same period, CO2 levels

were rising toward pre-industrial levels, and sea level was rising. The orbital parameter

changes that led to enhanced summertime insolation in the NH (see Section 1) caused the

maximum JJA insolation around 11000BP (Figure 5a). By 6000 BP, the focus of much of

our study, the insolation was still greater than present, but was decreasing rapidly toward

present-day conditions (Figure 5a). As elaborated in earlier sections, the enhanced

summertime insolation is a major forcing mechanism for the enhanced monsoons. The

paleoecological data indicates a trend toward drier conditions after the mid- Holocene,

and, in particular, the region of the Central Mexican Volcanic Belt indicates generally

drier conditions after about by 6000 BP, compared to the period before (Table 2). As seen

in Figure 5c, both the MCM and the FOAM indicate a general drying trend from the late

glacial to the mid Holocene.

While agreeing in the general drying trend until the mid Holocene, the two

models diverge in their simulations thereafter (Figure 5c), with the MCM simulating

driest conditions around 7000-6000 BP and then a return to conditions as wet or wetter

than the early Holocene (but with considerable century-scale variability), and FOAM

simulating driest conditions around 3000 BP followed by a slight recovery, but remaining

far drier than in the early Holocene. A record of salinity (here indicated as relative

wetness and treated as a proxy for precipitation as reflected by changes in the proportion

24

of planktic taxa) from Lake Patzcuaro in Central Mexico (Watts and Bradbury, 1982;

Bradbury, 2000) indicates the drying trend of the late glacial to mid Holocene, and then a

highly- variable (but trending wetter) record thereafter, Some of this variability is

indicated by MCM, which includes the volcanicity forcing (Figure 5b) that modulates the

insolation and therefore the climate (see Section 3a and Appendix). However, as

mentioned in Section 2, this record from Lake Patzcuaro is somewhat exceptional in the

region in suggesting dryer conditions in the mid Holocene relative to modern. Most of the

surrounding sites (See Table 2 and Figure 4) evidence more mesic conditions than

modern (although not as wet as in the early Holocene). This mesic period was followed

by generally drier conditions, continuing to the present, although there is considerable

spatial variability of moisture conditions in the late Holocene. Thus while both FOAM

and MCM agree with the observations of a drying trend from the early to the mid-

Holocene (Table 2 and Figure 5), the two models differ in their representation of the late

Holocene, with MCM agreeing best with the local record from Lake Patzcuaro (Figure

5c), and FOAM agreeing best with the general regional records (Table 2 and Figure 4).

We have been unable to isolate the cause of this late-Holocene divergence of the

simulations of the two models for this particular site in Central Mexico.

To summarize, the period around 7000-6000 BP (Table 2) was a period of

transition from wetter to drier in many, but not all of the regions we studied. Our

simulations at 6000 BP are near this transition, perhaps explaining in part some of

disagreement between the models and the data, and also among various environmental

records where accurate dating is required to determine timing relative to the transition

that was underway.

25

6: Summary

This paper has focused on the climate and environmental history of the region of

the Americas extending from the equator to about 30N, and spanning the past 11000

years, but with most emphasis on the period around 6000 years ago. In general, the

observations indicate that Northern and Central Mexico, Central America and the

Yucatan received greater effective precipitation during the early and mid-Holocene

relative to the present, with the exception of at least one site in Central-Mexico. The

southeast USA was drier during this period. Farther south, conditions at 6000 BP were

wetter along the Venezuelan coast. In the Llanos Orientales of Colombia and the Grand

Sabana of Venezuela, well into the South American interior, conditions were dryer than

present at 6000 BP and became increasing more mesic during the late Holocene. Lakes

in the Galapagos and in Panama along the Eastern Pacific coast also record dryer

conditions at 6000 BP relative to the present.

For comparison with the above field data, we used four different climate models

to simulate the climate of 6000 years ago. These models all showed a response of the

climate to the large changes in insolation. The models generally agreed, and they also

agreed with observations, in simulating increased summertime precipitation at 6000 BP,

relative to modern, in parts of the USA southwest and Northern Mexico, and decreased

precipitation in the USA southeast. The increases in the west were a direct response to the

enhanced summertime insolation, whereas the decreases in the east were linked

dynamically to the changes in the west, as in modern-day teleconnections. In the tropics,

the various simulations differed significantly from one another, and with observations,

and these differences appear related to differences in the degree of atmosphere/ocean

26

coupling, model resolution, and the treatment of topography. In scoring model/data

agreement primarily for those sites where the simulated precipitation changes were large

(greater than 10%), the pooled results showed an agreement of 61% and a level of

accuracy that was significant at the 95% level relative to random expectations. Although

somewhat encouraging, these results also show that model accuracy leaves considerable

room for improvement and that, where possible, it is useful to compare observations with

more than one model simulation in order to assess the robustness of model results.

Then examining the simulations and the observations from Central Mexico over

a broader span of time, from 11000 years ago to present, we noted the general trend from

wetter to dryer in the Holocene, with many sites switching from wetter-than- present to

drier-than-present shortly after 6000 years ago. The fact that this major transition

occurred relatively close to the time of our central modeling focus of 6000 years BP

complicates our data/model comparison, and suggests to us that a future study might be

focussed somewhat earlier in the Holocene in the period before the transition began.

APPENDIX- short descriptions of the four models used in this paper.

FOAM. FOAM (Fast Ocean Atmosphere Model) is a fully coupled

ocean/atmosphere model that operates without flux adjustments. The atmospheric model

is a fully parallel version of the NCAR CCM2, in which the atmosphere physics are

replaced by those of CCM3 (Jacob, 1997). The atmosphere is run here at R15 resolution

(about 4.4 degrees latitude by 7.5 degrees longitude with 18 vertical levels). The land

surface scheme operates on a 2 degree by 2 degree grid and uses a simple one-level soil

moisture parameter and prescribed vegetation characteristics. The ocean model was

developed following the Geophysical Fluid Dynamics Laboratory (GFDL) MOM model.

27

It has a resolution of about 1.4o in latitude, 2.8o in longitude, and 16 layers in the vertical.

FOAM has been integrated for 600 years without flux correction and shows no apparent

climate drift in temperatures. The model has a thermodynamic sea-ice scheme. The

simulation with presecribed insolation for 6000 BP was 150 years in duration, with the

last 120 years being used in the averages summarized here.

CSM. The CSM (Community Climate Model) is a fully coupled ocean-

atmosphere model without flux adjustments (Boville and Gent, 1998). It consists of an

atmospheric model (CCM3: Kiehl et al., 1998), an ocean model (NCOM: Gent et al.,

1998), a thermodynamic and dynamic sea ice model (Weatherly et al. 1998), and a land

surface biophysics model (LSM1.0: Bonan, 1998). The version of the CSM that we have

used has a resolution of T31 for the atmosphere and land surface components and a

variable 3-dimensional grid (25 vertical levels, 3.6° longitudinal grid spacing, and a

latitudinal spacing of 1.8° poleward of 30° decreasing to 0.9° within 10° of the equator)

for the ocean and sea ice models (Otto-Bleisner, 1999; Otto-Bleisner and Brady, 2001).

Compared with FOAM, the CSM-T31 has a higher atmospheric resolution (about 3.75

degrees latitude by 3.75 degrees longitude) (at the expense of longer computation time).

The paleoCSM ocean has a higher latitudinal resolution but a lower longitudinal

resolution than FOAM. The simulation with prescribed insolation for 6000 BP was 50

years in duration, with the last 30 years being used in the averages summarized here.

GENESIS GENESIS2 is a global climate model consisting of three components:

an atmospheric model, a static mixed-layer ocean, and a land-surface package that

contains sea ice code and prescribed vegetation (Thompson and Pollard 1997). The

atmospheric model employs a horizontal spectral truncation of T31 (approximately 3.75o

28

x 3.75o) and uses 18 vertical levels in a hybrid coordinate system, while the ocean and

land-surface are resolved on a 2o x 2o grid. The physical effects of vegetation are

accounted for by the land-surface transfer model (LSX), which exchanges energy, mass

and momentum between the atmosphere and vegetation but does not allow interactive

climate-vegetation feedbacks. The ocean component is a mixed-layer of fixed 50-m depth

with a prognostic meridional heat transport. The simulation with prescribed insolation

for 6000 BP was 20 years in duration, with the last 10 years being used in the averages

summarized here.

MCM. The MCM (Macrophysical Climate Model; Bryson and Bryson, 2000) is

forced by seasonal insolation, as are the three dynamic models, but the incoming

radiation is modulated by a variable atmospheric optical depth, at 200-year increments,

derived from the volcanic eruption history and adjusted for the observational bias of

greater reporting of eruptions in recent millennia, compared to earlier times

(Bryson,1988; Bryson and Bryson, 1997). MCM uses energy budget equations to

calculate the latitudinal and seasonal distribution of temperature for various longitudinal

sectors. The model then employs a dynamic criterion based upon baroclinic instability

theory to transform the calculated latitudinal (north-south) temperature gradients into

estimates of the latitude of the subtropical anticyclones and the latitude of the jetstream;

the location of the ITCZ is related, using synoptic climatology, to the latitude of the

subtropical anticyclones in both hemispheres. Based again upon modern-day synoptic

climatology, precipitation (or other parameters) is then calculated at particular stations,

by establishing the local response to the locations of the above-mentioned “centers of

action” (latitude of the jetstream, latitude of the subtropical highs, latitude of the ITCZ).

29

The calculations for each 200 year increment are done once for each longitudinal sector,

so that the computation of the local response is very fast on a personal computer. The

resolution of the output is determined only by the spacing of the modern climatology

network (Figure 1) (this feature of MCM is somewhat lost in interpolation of model

output to a grid).

Acknowledgements: JA and AR were provided financial support from the Bryson

Interdisciplinary Climate, People and Environment Program, an endowed program in

honor of Professor Reid Bryson. JK and SV received support from the National Science

Foundation, Climate Dynamics and Earth System History programs. The authors thank B.

Otto-Bliesner for allowing us to use results of a CSM experiment that was part of an

earlier joint publication (Harrison et al, 2003). We also wish to thank the two anonymous

reviewers who provided helpful suggestions for revising the article. The authors thank S.

Metcalfe her comments and suggestions, P. Behling for assistance with graphics, and M.

Marohl for assistance with preparing the manuscript and tables. This is CCR contribution

number 823.

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Metcalfe, S.E., Street-Perrott, F.L., O'Hara, S.L., Hales, P.E., Perrott, R.A., 1994. The palaeolimnological record of environmental change: Examples from the arid frontier of Mesoamerica. In: Millington, A.C., Pye, K. (Eds.), Environmental Change in Drylands: Biogeographical and Geomorphological Perspectives. Wiley, New York, pp. 131-145.

Metcalfe, S.E., Hales, P.E., 1994. Holocene diatoms from a Mexican crater lake - La Piscina de Yuriria. Proceedings of the 11th International Diatom Symposium, SanFrancisco, 1990. California Academy of Sciences 17, 505-515.

Metcalf, S.E., 1995. Holocene Environmental Change in the Zacapu basin: A diatombased record. The Holocene 5, 197-218.

Metcalf, S.E., O’Hara, S.L., Caballero, M., Davies, S.J., 2000. Records of Late Pleistocene – Holocene climatic change in Mexico – a review Quaternary ScienceReviews 19, 699-721.

Otto-Bliesner, B.L., 1999. El Nino/La Nina and Sahel precipitation during the middle Holocene. Geophysical Research Letters 26, 87-89.

Otto-Bliesner, B.L., Brady, E.C., 2001. Tropical Pacific variability in the NCAR Climate System Model. Journal of Climate 14, 3587-3607

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Rodwell, M.J., Hoskins, B.J., 2001. Subtropical anticyclones and summer monsoons. Journal of Climate 14, 3192-3211.

36

Rull, V., 1991. Contribución a la paleoecología de Pantepui la Gran Sabana (Guayana Venezolana): clima, biogeografía, y ecología. Scientia Guaianae 2, 1-333.

Thompson, S.L., Pollard, D., 1997. Greenland and Antarctic mass balances for present and doubled atmospheric CO2 from the GENESIS version 2 global climate model. Journal of Climate 10, 871-900.

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37

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MCM CSM FOAM GENESISTlapacoya + X OK OKChalco + X OK OKPatzcuaro - OK X X

Chiconahuapan + X OK OKZacapu + X OK OKLa Piscian de Yuriria + X OK OKLaguna San Pedro + OK OK

Cañon de la Fragua +? X OK OK OKPuerto de Ventanillas +? X OK OK OKSierra de la Misericordia + X OK OK OKHornoday Mountains + OK X XBabicora +? X OK OKSierra Bacha + OK X OK OK

Annie Lake, FL - OK OK OK OKScott Lake, FL - OK OK OK OKLake Louise, FL - OK OK OK OKGoshen Springs, AL - OK OK OK

Lubbock Lake/ Blackwater Draw - OK X XBoriack Bog, TX 0 OK X OK OK

San Jose Chulchaca + X XChichancanab + X XCoba + X X

Lake Yeguada, Panama - OKLake Miragoane, Haiti + OK OK OKLake Peten-Itza, Guatamala + OK XLago Chirripo Paramo, Costa Rica 0 XLa Chonta, Costa Rica + XCariaco Basin, Venezuela + OK X OK XLake Valencia, Venezuela + OK X OK XLake Moriru, Surinam +? OK OK OK

Laguna Angel- Llanos Orientales - OK X XLaguna Sardinas- Llanos Orientales - OK X XLaguna El Pinal- Llanos Orientales - OK X XLaguna Carimagua- Llanos Orientales - OK X XLaguna Divina Pastora, Gran Sabana - OK X XEl Junco Lake, Galapagos - OK X

MCM CSM FOAM GENESIS

Incorrectcorrect

Total

10 16 9 720 13 17 16

29 23263067 45 65 69% correct or near 0

26°2"N

30 °45"N

19°15"N

21°N

25°54"N

19°50"N

19°15"N

20°50"N

31°59"N

8°27"N18°24"N

29°N

16°55"N

20°13"N

9°29"N

29°50"N

9°41"N

19°55"N

10°43"N

27°12"N

10°10"N

21°12"N

3°54"N

27°58"N

4°28"N

19°15"N

4°58"N

30°43"N

4°8"N

26°39"N

4°4"N

31°43"N

4°42"N

19°8"N

1°20" S

34°N

102°44"W

97 °30"W

98°55"W

90°W

103°40"W

88°45"W

101°40"W

88°W

113°36"W

80°51"W

98°50"W

73°5"W

108°W

89°50"W

101°8"W

83°30"W

112°29"W

83°57"W

101°39"W

65°10"W

81°25"W

67°45"W

104°45"W

59°31"W

80°57"W

70°34"W69°28"W

83°15"W

71°23"W

102°45"W

70°14"W

86 °8"W

61°04"W

99°31"W

91°25"W

102 °W

Gonzales Quintero (1986), Lozano Garcia et al.Bradbury (1989), Gonzales Quintero (1986)

Bradbury (2000)Watts and Bradbury (1982), O'Hara (1991),(1991), Watts and Bradbury (1982)

Delcourt (1980)

Van Devender and Burgess (1985)

Leyden et al. (1996)

Watts (1975)

Hodell et al. (1995)

Van Devender (1990)

Leyden at al. (1998)

Bryant (1977)

Bush et al. (1992)

Brown (1985)

Higuera-Grundy (1999)

Anderson and Van Devender (1995)

Islebe et al. (1996)

Metcalfe et al. (1994)

Horn (1993)

Watts (1971)

Islebe and Hooghiemstra (1997)

Metcalfe (1995)

Haug et al. (2001)

Metcalf et al. (1991)

Curtis et al. (1999)

Metcalfe et al. (1991)

Wijmstra and Van der Hammen (1966)

Holliday (1988)

Behling and Hooghiemstra (1998)Behling and Hooghiemstra (1998)

Van Devender et al. (1990)

Behling and Hooghiemstra (1999)

Behling and Hooghiemstra (1999)

Watts (1971)

Rull (1991)

Colinvaux (1972)

Van Devender and Burgess (1985)

B, AA, B, CA, D

B, DB, A, DD, BA

A

EB, D

EEE

AAAA

A

AAA

A

AAAAA

F, G

A, HHB, H

A, BA, H

D, HD

A

Key:A = pollenB = diationsC = magneticsD = geochemistry

E = packrat middenF = geomorphologyG = pedologyH = isotopes

Basin of Mexico

Upper Lerma

Bolson de Mapimi, Southwest USA

Southeast USA

Yucatan

Caribbean Basin and Central America

South America

Southern Plains, USA

Site Name and Region Lat. Long.

Wetness(relativechange) Methods References to Site Studies

Table 1: Paleoecological studies (location, relative wetness at 6000 yr BP relative to present, observa-tional methods (see key at bottom), and reference - column on far right) and climate model scores ofagreement/disagreement with the paleoecological studies. The climate model scores are given for thefour models (see text), where “OK” indicates correct correspondence of model simulation with the obser-vation of relative wetness at 6000 yr BP (and a model departure of at least 10%), “X” indicates incorrectcorrespondence (and a model departure of at least 10%), and “~" indicates that the observation site wasin a region for which the simulated change in precipitation was less than 10%, and often near zero. Thescoring of the model simulations (see bottom of table) assigned a value of 1 for correct correspondenceand a value of 0 for the category less than 10% or “near zero.” Two sites indicating “no change” werescored correct if the simulated change was close to zero. For the purpose of this scoring, the questionmarks on the relative wetness categories were ignored.

Climate Model Scores

~~

~

~~~

~ ~

~~

~

~

~ ~~ ~~ ~

~~~~

~~OKOK X

~~~

~

~~~~~~~

CalibratedYearsBP

SoutheastUSASouthernHighPlains

USSouthwestandNorthernMexico

CentralMexicanVolcanicBelt

Yucatan,CentralAmericaandthe

CaribbeanVenezuelanCoast

Colombia-LlanosOrientalesand

Venezuela-GranSabana

RadiocarbonyearsBP

Dry;Dryerbutvariable;

Increasing Mauritia1000

Highsalinitysalinelakes

palm.1000

Lowlakelevels-arid

VerydrySignificantlocal

Veryhumid,2000

MesicintervalSonoranvegetation

Significantanthropogenicimpactanthropogenicimpacts

Highersalinity,AridForest/GalleryForest

2000IncreasedOak

spreadsnorthward.Drying

Highlakelevels3200

IncreasedmoistureVariableregionally

3000Pinesincrease,

increased4500

cypressbayheadsmoisture

Increasingmoisture4000

develop.Generallylowerlakelevels-

grass/shrubsavanna5900

HigherwatertableHighlakelevels

buthighlyvariable.Wet;

Higherlake5000

Maximumaridity

Strongermonsoon------Dryingbegins------

Highestlakelevelslevels,wetter

6950Dryer;

drylakesHighlakelevels

Shortdryinterval-

Brosimumdecline

increasederosion6000

dryoakshrub/Wet,risinglakelevels.

Highersalinity,7900

Bluestemprairie

FallinglakelevelsRisinglakelevels

AridDry:

7000Warm

anddrysavanna

8900Lowerwatertable

lakesdryingGlacialadvance?

8000

10000Veryaridandcold

Lowestlakelevels9000

HighestlakelevelsSalinelake,

11200Increasedwinterrain?

verydry10000

ReferencesWatts(1971,1975),Delcourt(1980)

BryantandHolloway(1985),Holiday(1988) VanDevenderandBurgess

(1985),VanDevender(1990),Metcalfetal.(1991),AndersonandVanDevender(1995)

WattsandBradbury(1982),Brown(1985),Bradbury(1989),Metcalfeetal.(1991),O'Hara(1991),LozanoGarciaetal.(1993),Lopez(1994),Metcalfeetal.(1994)Metcalf(1995),Heine(1988)

Bushetal.(1992),Curtisetal.(1993),,Horn(1993),Hodelletal.(1995),Leydenetal.(1996),,Islebeetal.(1996),IslebeandHooghiemstra(1997),Leydenetal.(1998),Higuera-Grundy(1999)

Bradburyetal.(1981),Haugetal.(2001),Curtisetal.(1999)

BehlingandHooghiemstra(1998,1999),Rull(1991),Wijmstra(1966)

decreasing

pine

wet;

low

salinity

Tab

le2

:Sum

mary

of

Holo

cenepaleo

climatic

pro

xyfo

rthe

study

regio

n.The

perio

dof

study

isind

icatedby

thehig

hlight.

Model N. Atlantic STH N. Pacific STH W. N. American Jet ITCZ

PaleoCSM 0 0 0 1 to 3FOAM 2 2 0 1 to 3GENESIS 2 2 4 0MCM -0.5 0.5 0 -2

Table 3: Changes in latitudes (positive for northward shift, negative forsouthward shift) of circulation features for the four climate models(paleoCSM, FOAM, GENESIS, MCM). Values are for 6000 years BP comparedto present. In MCM, these features are simulated at 0W (N. Atlantic STH),90W (ITCZ), and 120W (N. Pacific STH and W. N. American Jet).

Figure 1: Actual topography (meters) of the study domain (shaded). Superimposed arethe locations modeled by the MCM (dots) and a GCM model grid at T31 resolution.

GENESIS

CSM

FOAM

Figure 2: JJA precipitations (mm/month) and wind vector (m/s) anomaliesin the 6ka GCM simulations. Negative precipitation values are dashed.

2

MCM

120W 115W 110W 105W 100W 95W 90W 85W 80W 75W 70W 65W 60W

0

5

10

15

20

25

30

Figure 3: Macrophysical modeled JJA average precipitation in millimeters at 6kminus 0k. Arrows indicate the average difference in prevailing wind directionand intensity between 6k and 0k. Scale arrow is 2 meters/second.

++

++

++

+

++

++

o

--

---

-

+

o++ +

-

+

-

-

+

+

-

Figure 4: Summary of the model’s performance at each of the sites that have paleoenvironmental evi-dence for JJA precipitation changes at 6 ka. Each site is marked with a “+” (“-”) for wetter (drier) condi-tions relative to modern, according to the field evidence, or with a “0" for no apparent change. The col-ored rings around each site indicate the simulated sign of the precipitation anomaly and the agreementamong the models. Each ring represents one model simulation of either substantially (>10%) greaterprecipitation (blue) or less precipitation (yellow) compared with the model’s modern simulation. Forexample, two blue rings and two orange rings around a site indicate that two models simulate wetterconditions and two simulate drier conditions than present. If there are fewer than four rings, then not allof the models generated substantial precipitation changes from modern.

-14000-12000

-10000-8000

-6000-4000

-20000

RADIOCARBON DATE BP

150

250

350

450

550

L.P

AT

ZC

UA

RO

WE

TN

ES

S&

JJA

PR

EC

.mm

MC

M

700

750

800

850

900

JJAP

RE

C.m

mF

OA

M

MCM, JJA PATZCUARO FOAM, JJA

MODELED AND OBSERVED HISTORYMORELIA, MEXICO

-14000-12000

-10000-8000

-6000-4000

-20000

RADIOCARBON DATE BP

0

0.5

1

1.5

2

2.5

VO

LC

AN

ICIT

YIN

DE

X

SOURCE OF VOLCANIC MODULATION

-14000-12000

-10000-8000

-6000-4000

-20000

CALENDAR DATE BP

420

425

430

435

440

445

SU

MM

ER

WA

TT

S/M

2

240

245

250

255

260

265

WIN

TE

RW

AT

TS

/M2

SUMMER WINTER

MEAN NORTHERN HEMISPHERE IRRADIANCE

Figure 5: Forcing and response of climate models for precipitation, andobserved response at Lake Patzcuaro, Mexico. A: The orbital forcing, i.e.irradiance used in all four models; B: The volcanicity index used to derive theaerosol optical depth for the MCM; C: A comparison of Bradbury’s salinityproxy data inverted and treated as a proxy for precipitation (simple line),compared with the FOAM output (black triangles and scale on right), and theMCM output (open circles and line, left scale).