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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
D R A F T January 28, 2011, 5:09pm D R A F T
COOK ET AL.: DUNES AND MEDIEVAL CLIMATE X - 3
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
D R A F T January 28, 2011, 5:09pm D R A F T
COOK ET AL.: DUNES AND MEDIEVAL CLIMATE X - 5
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
D R A F T January 28, 2011, 5:09pm D R A F T
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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
D R A F T January 28, 2011, 5:09pm D R A F T
COOK ET AL.: DUNES AND MEDIEVAL CLIMATE X - 7
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
D R A F T January 28, 2011, 5:09pm D R A F T
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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
References
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L06,705, 2010.125
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palaeoclimate context, Journal of Quaternary Science, 25 (1), 48–61, 2010.135
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ences on Medieval drought in North America simulated by the Community Atmospheric137
Model, Journal of Geophysical Research, 113 (D11), 2008.138
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activity on the Great Plains of North America: megadroughts and climate links, Global140
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matic Change, 83 (1), 241–285, 2007.143
Hanson, P., A. Arbogast, W. Johnson, R. Joeckel, and A. Young, Megadroughts and144
late Holocene dune activation at the eastern margin of the Great Plains, north-central145
Kansas, USA, Aeolian Research, 1 (3-4), 101–110, 2010.146
Kaplan, A., M. Cane, Y. Kushnir, A. Clement, M. Blumenthal, and B. Rajagopalan,147
Analyses of global sea surface temperature 1856-1991, Journal of Geophysical Research,148
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to in situ, satellite, and reanalysis data, Journal of Climate, 19 (2), 153–192, 2006.151
Seager, R., N. Graham, C. Herweijer, A. Gordon, Y. Kushnir, and E. Cook, Blueprints152
for Medieval hydroclimate, Quaternary Science Reviews, 26 (19-21), 2322–2336, 2007.153
Seager, R., R. Burgman, Y. Kushnir, A. Clement, E. Cook, N. Naik, and J. Miller, Tropical154
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180oW 120
oW 60
oW 0
o 60
oE 120
oE 180
oW
60oS
30oS
0o
30oN
60oN
180oW 120
oW 60
oW 0
o 60
oE 120
oE 180
oW
60oS
30oS
0o
30oN
60oN
MCAC/MCAD SST Anomaly, K (1321−1461)
−1 −0.8 −0.6 −0.4 −0.2 0 0.2 0.4 0.6 0.8 1
NINO 3.4 SST Anom, ANN (K)
1320 1340 1360 1380 1400 1420 1440 1460−1.5
−1
−0.5
0
0.5
1
1.5
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.
D R A F T January 28, 2011, 5:09pm D R A F T
COOK ET AL.: DUNES AND MEDIEVAL CLIMATE X - 11
140 oW
120oW
100oW
80oW
60o W
20 oN
30 oN
40 oN
50 oN
60 oN
70 oN
140 oW
120oW
100oW
80oW
60o W
20 oN
30 oN
40 oN
50 oN
60 oN
70 oN
140 oW
120oW
100oW
80oW
60o W
20 oN
30 oN
40 oN
50 oN
60 oN
70 oN
140 oW
120oW
100oW
80oW
60o W
20 oN
30 oN
40 oN
50 oN
60 oN
70 oN
140 oW
120oW
100oW
80oW
60o W
20 oN
30 oN
40 oN
50 oN
60 oN
70 oN
140 oW
120oW
100oW
80oW
60o W
20 oN
30 oN
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50 oN
60 oN
70 oN
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80oW
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20 oN
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50 oN
60 oN
70 oN
140 oW
120oW
100oW
80oW
60o W
20 oN
30 oN
40 oN
50 oN
60 oN
70 oN
JJA Prec Anom, MCAC JJA Prec Anom, MCAD
−1.1
−0.9
−0.7
−0.5
−0.3
−0.1
0.1
0.3
0.5
0.7
0.9
1.1
JJA Temp Anom, MCAC
JJA Temp Anom, MCAD
−2.2
−1.8
−1.4
−1
−0.6
−0.2
0.2
0.6
1
1.4
1.8
2.2
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
MODC
MCAC
MCAD
−4 −2 0 2 40
0.05
0.1
0.15
−2 −1 0 1 20
0.05
0.1
0.15
Figure 3. Histograms of full ensemble temperature and precipitation anomalies over the region
of dune mobilization 105oE-95oE, 32oE-44oE).
D R A F T January 28, 2011, 5:09pm D R A F T
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1320 1340 1360 1380 1400 1420 1440 1460−0.5
0
0.5
1
1.5
Temp Anom, JJA (K)
MCAC
MCAD
1320 1340 1360 1380 1400 1420 1440 1460
−0.4
−0.2
0
0.2
Prec Anom JJA (mm day−1)
−0.6
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).
D R A F T January 28, 2011, 5:09pm D R A F T