GEOPHYSICAL RESEARCH LETTERS, VOL. ???, XXXX, DOI:10.1029/,

The impact of dune mobilization on North American1

climate during the late Medieval Climate Anomaly2

B. I. Cook,1,2

R. Seager,2and R. L. Miller,

1

1NASA Goddard Institute for Space

Studies, New York, New York, USA.

2Lamont-Doherty Earth Observatory,

Palisades, New York, USA.

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During the Medieval Climate Anomaly, North America experienced severe3

droughts and widespread mobilization of dune fields that persisted for decades.4

We use an atmosphere general circulation model forced by a tropical Pacific5

sea surface temperature reconstruction and changes in the land surface con-6

sistent with estimates of surface dune mobilization (conceptualized as par-7

tial devegetation), to investigate whether the devegetation exacerbated the8

medieval droughts. Presence of devegetated dunes in the model significantly9

increases surface temperatures, but has little impact on precipitation or the10

severity of the medieval droughts. Results are similar to recent studies of the11

1930s Dust Bowl drought, suggesting bare soil associated with the dunes, in12

and of itself, is not sufficient to amplify droughts over North America.13

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1. Introduction

During the Medieval Climate Anomaly (MCA), North America (NA) experienced14

‘megadrought’ events that often persisted for periods (≥10 years) that exceeded even15

the worst droughts observed during the instrumental record [Cook et al., 2010]. These16

droughts were embedded within a global pattern of hydroclimate variability during the17

MCA that included widespread drying in the subtropics and mid-latitudes of both hemi-18

spheres [Burgman et al., 2010; Graham et al., 2007; Seager et al., 2007], potentially a19

consequence of persistently cold sea surface temperatures (SST) in the eastern tropical20

Pacific [Graham et al., 2007; Seager et al., 2007]. Over NA, these megadroughts caused21

vegetation die-off in the central and southern Great Plains region, leading to large scale22

dune mobilization [Forman et al., 2001; Hanson et al., 2010]. The role dune mobilization23

may have played in exacerbating the droughts during the MCA has been conjectured [e.g.,24

Hanson et al., 2010], but not explicitly investigated within a modeling framework.25

Currently, GCMs forced only by either idealized [Feng et al., 2008] or reconstructed26

[Seager et al., 2008] SSTs have difficulty reproducing the magnitude, duration, and spa-27

tial pattern of the megadroughts, pointing to a possible additional influence from the land28

surface changes at the time. We use an atmosphere GCM, forced by a tropical Pacific SST29

reconstruction and estimates of ‘dune mobilization’ during the late MCA to investigate the30

potential for devegetation over the Great Plains to influence temperature, precipitation,31

and drought over central NA. Our goal is to determine whether devegetation associated32

with dune mobilization significantly modified the character of the megadroughts beyond33

what would be expected from SST forcing alone.34

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2. Data and Methodology

2.1. SST Reconstruction

The tropical Pacific SST reconstruction is the same as that used by Seager et al. [2008]35

and Burgman et al. [2010] and is based on sub-annual resolution coral oxygen isotope data36

from modern (1886-2008) and fossil (1320-1462) corals on Palmyra Atoll (6◦N, 160◦W ) as37

reported by Cobb et al. [2003]. Details can be found in the Supplemental Material.38

2.2. Dune Area

Geomorphological evidence indicates that there were numerous dune mobilization events39

over the Great Plains during the MCA [Forman et al., 2001; Hanson et al., 2010], most40

recently in the late 14th and early 15th centuries, coinciding with our SST reconstruction.41

For our experiments, we conceptualize dune mobilization in the model as bare, deveg-42

etated soil by converting part of the Great Plains (105oE-95oE, 32oE-44oE) from 100%43

vegetation cover to a 50% vegetated/bare soil mix. The region chosen is based on maps44

from Hanson et al. [2010]. We realize this is a relatively coarse approximation; in reality,45

the dune mobilization was a consequence of dynamical interactions between drought, veg-46

etation dynamics, and wind. Despite these caveats, our approach should still provide some47

useful insights and serve as a reasonable baseline for future and possibly more complex48

implementations that, for example, also account for dust aerosol fluxes from the dunes.49

2.3. Model Description and Setup

For these experiments, we use the latest version of the Goddard Institute for Space Studies50

atmosphere general circulation model, ‘ModelE’ [Schmidt et al., 2006], run at F40 horizon-51

tal resolution (roughly 2ox2.5o) with 40 vertical atmospheric layers. This model has been52

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used previously to investigate land surface feedbacks to drought during the Dust Bowl53

drought of the 1930s with good success [Cook et al., 2008, 2009]. We conduct three, five54

member ensemble experiments. Our baseline run is MODC (‘Modern Control’), forced55

by the modern Kaplan SST dataset [Kaplan et al., 1998] and transient forcings (solar,56

volcanic, and greenhouse gas concentration) from 1858-2005. Both MCA ensemble exper-57

iments, MCAC (‘Medieval Control’) and MCAD (‘Medieval with Dunes’), are forced with58

the aforementioned SST reconstruction for years 1321-1461; in addition, MCAD includes59

the implementation of the dune mobilization described previously. We note that we explic-60

itly do not allow for surface dust fluxes or dust aerosols as part of the dune mobilization61

in these experiments since little or no data is currently available to constrain these. For62

our analyses, we focus on the boreal summer season (June-July-August, JJA), when we63

expect land surface processes to exert the strongest impact on the overlying atmosphere.64

Unless otherwise indicated, all anomalies are expressed relative to the 1961-1990 baseline65

from the MODC ensemble mean.66

3. Results

Boundary conditions for the MCAC and MCAD experiments are shown in Figure 1. The67

average SSTs are indicative of a mean ‘La Nina’ like state, characterized by cool SSTs in68

the eastern tropical Pacific and along the west coast of NA and warm conditions in the69

central extratropical Pacific (top panel). The NINO3.4 index, calculated from the SST70

field, shows persistent cold periods, with anomalies often exceeding -1 K. The region of71

devegetation for our dune mobilization experiment, MCAD, is outlined in brown.72

In the ensemble mean, both Medieval runs are characterized by warm and dry conditions73

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in central and eastern NA (FIgure 2). The spatial patterns for these anomalies generally74

compare favorably with previous GCM experiments [Burgman et al., 2010; Seager et al.,75

2008]. The warming intensifies in the MCAD ensemble, primarily over the region of76

dune mobilization, but the addition of dunes has little impact on the mean precipitation77

anomaly pattern. Averaged over the region of dune mobilization, both the MCAC and78

MCAD ensembles are significantly warmer (Wilcoxon rank sum test) than the ensemble79

median calculated for the full MODC ensemble (1858-2005): +0.27 K in MCAC and +0.8880

K in MCAD (Figure 3, left panel). The MCAC and MCAD simulations are also signifi-81

cantly drier than MODC (Figure 3, right panel), by -0.22 mm day-1 and -0.25 mm day-1,82

respectively. The reduced precipitation in the MCAC ensemble comes entirely from the83

anomalous SST forcing, which will tend to dry out the midlatitudes in both hemispheres,84

including over central and southern NA. This drying also explains the modest warming85

in MCAC because temperature and precipitation are tightly and inversely coupled over86

this region during boreal summer. For example, JJA correlation (Spearman’s Rank) be-87

tween temperature and precipitation in the MODC ensemble is -0.74. The exceptional88

warmth in the MCAD simulations is not only significantly warmer compared to MODC,89

but is also significantly warmer than the MCAC ensemble, implying a significant impact90

of the dune mobilization. This additional warming comes from an increase in the Bowen91

ratio: removal of vegetation in MCAD reduces evapotranspiration and total latent heat92

fluxes, which is compensated for by increased sensible heating and and longwave radiative93

cooling required by increased surface temperatures. Precipitation is not significantly re-94

duced compared to MCAC, implying little aggregate impact of the dunes on precipitation95

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above and beyond the influence of the SST forcing. The ensemble median time series of96

temperature and precipitation anomalies from the MCAC and MCAD ensembles show97

that the MCAD simulation is warmer over almost the entire Medieval interval (Figure 4),98

although with large variability. As expected, the precipitation differences between the two99

runs are on the whole negligible, with certain periods drier in the MCAD ensemble (e.g.,100

1321-1360 and 1430-1461) and others wetter (e.g., 1400-1420), compared to MCAC. An101

integrative drought index calculated from the model runs, the Palmer Drought Severity102

Index (PDSI), also indicated little impact on mean drought conditions from the dune103

mobilization (Supplemental Figure 1).104

4. Discussion and Conclusion

Our experiments demonstrate that dune mobilization over central NA can exert a signifi-105

cant influence on surface temperatures during the MCA beyond the influence of the SST106

forcing. However, the impact on precipitation and drought (as defined by the PDSI) was107

found to be negligible. This is largely consistent with the results of Cook et al. [2009], who108

found that increased bare soil during the Dust Bowl could explain the warm temperature109

anomalies over central NA, but had little impact on precipitation. In that study, the110

authors found it was necessary to include wind erosion and dust aerosols in order to get a111

significant precipitation response from land surface changes. Dust forcing may have played112

a role during the MCA droughts; however, there is even less quantitative information on113

dust fluxes and aerosol loadings during the MCA than dune mobilization. Detection of a114

significant dune impact on precipitation in our experiments may also be hampered by a115

low signal to noise ratio: our forcing is relatively modest (only 50% devegetated over our116

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region of dune mobilization) and our ensemble size is relatively small (only five members).117

Regardless, results from these experiments should provide another point of comparison to118

the already existing model studies of the MCA megadroughts [Feng et al., 2008; Seager119

et al., 2008].120

Acknowledgments. The authors acknowledge the support of NSF grant ATMO9-121

02716 and NOAA grant NA100AR-4310137. Lamont contribution XXXX.122

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Figure 1. Mean annual global SST anomalies for the MCAC and MCAD ensemble runs

(top) and the NINO3.4 SST anomaly calculated from this reconstruction (bottom). The brown

rectangle indicates the region of 50% devegatation, used to simulate dune mobilization in the

MCAD run.

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140 oW

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JJA Prec Anom, MCAC JJA Prec Anom, MCAD

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Figure 2. Ensemble mean maps of JJA temperature (K, top row) and precipitation (mm

day-1) anomalies from the MCAC and MCAD runs, calculated from 1321-1461. The region of

dune mobilization (105oE-95oE, 32oE-44oE) is outlined in brown.

Temp Anom (K), Hist

Prec Anom (mm day ), Hist-1

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Figure 3. Histograms of full ensemble temperature and precipitation anomalies over the region

of dune mobilization 105oE-95oE, 32oE-44oE).

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1320 1340 1360 1380 1400 1420 1440 1460−0.5

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Figure 4. Smoothed (10 year lowess filter) time series of ensemble mean temperature and

precipitation anomalies over the dune mobilization area (105oE-95oE, 32oE-44oE).

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