Detecting vegetation-precipitation feedbacks in mid-Holocene North Africa from two climate models Page: 1 of 9
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Clim. Past, 4, 59-67, 2008
Author(s) 2008. This work is distributed under
the Creative Commons Attribution 3.0 License.
of the Past
Detecting vegetation-precipitation feedbacks in mid-Holocene North
Africa from two climate models
Y. Wang',*, M. Notaro', Z. Liu', R. Gallimorel, S. Levis2, and J. E. Kutzbachl
1Center for Climatic Research, University of Wisconsin-Madison, 1225 West Dayton Street, Madison, WI 53706, USA
2National Center for Atmospheric Research, PO BOX 3000, Boulder, CO 80307, USA
*now at: Pacific Northwest National Laboratory, PO BOX 999, MSIN K9-24, Richland, WA 99352, USA
Received: 13 July 2007 - Published in Clim. Past Discuss.: 25 July 2007
Revised: 7 February 2008 - Accepted: 10 March 2008 - Published: 31 March 2008
Abstract. Using two climate-vegetation model simulations
from the Fast Ocean Atmosphere Model (FOAM) and the
Community Climate System Model (CCSM, version 2), we
investigate vegetation-precipitation feedbacks across North
Africa during the mid-Holocene. From mid-Holocene snap-
shot runs of FOAM and CCSM2, we detect a negative feed-
back at the annual timescale with our statistical analysis. Us-
ing the Monte-Carlo bootstrap method, the annual negative
feedback is further confirmed to be significant in both sim-
ulations. Additional analysis shows that this negative inter-
action is partially caused by the competition between evap-
oration and transpiration in North African grasslands. Fur-
thermore, we find the feedbacks decrease with increasing
timescales, and change signs from positive to negative at
increasing timescales in FOAM. The proposed mechanism
for this sign switch is associated with the different persistent
timescales of upper and lower soil water contents, and their
interactions with vegetation and atmospheric precipitation.
Vegetation interactions/feedbacks have received tremendous
attention in modern climate (Charney et al., 1975, 1977;
Schlesinger et al., 1990; Pielke et al., 1998; Brovkin 2002)
and paleoclimate (Kutzbach, 1981; Kutzbach et al., 1996;
Ganopolski et al., 1998; Claussen et al., 1999, 2003; Fo-
ley et al., 2003; Wang et al., 2005a, b; Wang and Mysak,
2005) studies. Previous understanding (Charney et al., 1975,
1977; Woodward et al., 1998; Box 2-6 in Ruddiman, 2001)
largely emphasized that those interactions played an impor-
tant role in amplifying initial climate perturbations (i.e., pos-
itive feedbacks). Using a statistical method (see Liu et al.,
LC Correspondence to: Y. Wang
2006a, Notaro et al., 2006 for detailed methodology), we
present a negative vegetation-precipitation feedback at the
annual timescale from two mid-Holocene simulations with
FOAM (Gallimore et al., 2005), and CCSM2 (Levis et al.,
2004a), both of which are coupled with the Lund-Potsdam-
Jena Dynamic Global Vegetation Model (LPJ-DGVM, Sitch
et al., 2003).
In semiarid areas, the dynamics of the simulated hydro-
logic cycle is partially governed by the interplay between
transpiring water in vegetated areas and surface evapora-
tion from bare soils (Dirmeyer, 1994; Sellers et al., 1997).
One of the two underlying processes is that when vegetation
cover increases, ground evaporation decreases (mainly be-
cause there is less energy reaching the soil) and transpiration
increases (because there is more vegetation). In our experi-
ments, the imbalance of a large bare ground evaporation over
transpiration under fully wet soil conditions in mid-Holocene
can produce a local enhancement of rainfall for bare soil con-
dition compared to vegetated condition (i.e., negative feed-
back). A previous study (Doherty et al., 2000) with the GEN-
ESIS climate model (Thompson and Pollard, 1997) detected
weak or insignificant vegetation feedback in amplifying pre-
cipitation in eastern North Africa. Furthermore, Levis et
al. (2004a) also mentioned in their 50-year "6K6V" simu-
lation that there may have been a weak negative precipita-
tion feedback in North Africa. Furthermore, a recent ob-
servational study (Wang et al., 2006) detected that at differ-
ent timescales, the sign of vegetation-precipitation interac-
tions may change with their statistical model. Previous mid-
Holocene studies (Cooperative Holocene Mapping Project
(COHMAP), 1988; BIOME 6000, Prentice and Webb, 1998)
indicated that soil in mid-Holocene North Africa was wetter
and darker than that in pre-industrial and present-day condi-
tions. Vegetation, mainly grassland, extended farther north
into the present-day Sahara Desert (Gasse, 2000, 2002).
Small and Kurc (2001) find that observed surface albedo is
lower under wet conditions in semiarid areas. Under such a
Published by Copernicus Publications on behalf of the European Geosciences Union.
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Wang, Yi; Notaro, Michael; Liu, Zhengyu; Gallimore, Robert; Levis, Samuel & Kutzbach, John E. Detecting vegetation-precipitation feedbacks in mid-Holocene North Africa from two climate models, article, March 31, 2008; Richland, Washington. (digital.library.unt.edu/ark:/67531/metadc896302/m1/1/: accessed June 20, 2018), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.