Assessment of uncertainties in the response of the African monsoon precipitation to land use change simulated by a regional model
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- Hagos, S., Leung, L.R., Xue, Y. et al. Clim Dyn (2014) 43: 2765. doi:10.1007/s00382-014-2092-x
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Land use and land cover (LULC) over Africa have changed substantially over the last 60 years and this change has been proposed to affect monsoon circulation and precipitation. This study examines the uncertainties of model simulated response in the African monsoon system and Sahel precipitation due to LULC change using a set of regional model simulations with different combinations of land surface and cumulus parameterization schemes. Although the magnitude of the response covers a broad range of values, most of the simulations show a decline in Sahel precipitation due to the expansion of pasture and croplands at the expense of trees and shrubs and an increase in surface air temperature. The relationship between the model responses to LULC change and the climatologists of the control simulations is also examined. Simulations that are climatologically too dry or too wet compared to observations and reanalyses have weak response to land use change because they are in moisture or energy limited regimes respectively. The ones that lie in between have stronger response to the LULC changes, showing a more significant role in land–atmosphere interactions. Much of the change in precipitation is related to changes in circulation, particularly to the response of the intensity and latitudinal position of the African Easterly Jet, which varies with the changes in meridional surface temperature gradients. The study highlights the need for measurements of the surface fluxes across the meridional cross-section of the Sahel to evaluate models and thereby allowing human impacts such as land use change on the monsoon to be projected more realistically.
KeywordsAfrican monsoonLand use changeLand cover changeAfrican Easterly JetLand degradationCrop landPasture landRegional model simulationsLand surface models
The African Sahel has experienced major decadal climatic swings since the middle of the twentieth century with lasting socio-economic consequences. These variations have been attributed primarily to global sea surface temperature (SST) variations (Giannini et al. 2003; Hagos and Cook 2008). However, they also coincide with a period of rapid population growth and associated changes in land use. The United Nations Environmental Program (Kandji 2006) estimated that the population of the region has been doubling every 20 years. This 3 % per year increase outstrips the annual rate of increase of food production (2 %), which is manifested by the doubling of harvested area over the last 60 years. Furthermore poor land management practices such as overgrazing and cutting of trees for firewood are known to have led to soil erosion and further land surface degradation (Reynolds et al. 2007).
The potential feedback of land-use change on the African monsoon precipitation has been an area of active research since the mid 1970s. Charney (1975) suggested that the African monsoon circulation is closely tied to the albedo gradient between vegetation and bare soil. He argued that a reduction in the coverage of the former in favor of the latter would increase atmospheric subsidence and weaken the monsoon precipitation, which in turn could lead to further land degradation. Other modeling studies indicate that such mechanism might even be responsible for the relatively rapid transition of the Sahara from a savanna to desert during the mid-Holocene period (Claussen et al. 1999; Patricola and Cook 2008). Given the rapid expansion of agriculture over the last 60 years, a similar interaction of human induced land degradation with monsoon precipitation is not inconceivable. In fact, several model simulation studies that prescribe artificially degraded land-cover generally show decreased precipitation. The extent of the response in models, however, is quite sensitive to the treatment of land surface processes in the models and the estimate of the degradation, which itself is uncertain. Many of the earlier studies tended to be more idealized or the imposed land-cover changes were generally idealized to provide qualitative estimates of the precipitation response (e.g. Zheng and Eltahir 1997; Xue 1997). More recently however the need for quantitatively assessing the impact of land-use change to better understand the relative role of SST and land cover/land use change on the African climate has been recognized.
A GCM study involving a realistic estimate of land use change showed that in comparison to the 1,961 conditions, the simulated rainfall decreases by 4.6 % (1996) and 8.7 % (2015) in response to land degradation (Taylor et al. 2002). They found that the decreases are related to the late onset of the monsoon. In a similar study, using the International Center for Theoretical Physics (ICTP) regional climate model RegCM, Abiodun et al. (2008) showed that extreme deforestation and desertification reduced precipitation because the easterlies that drove moisture away from the Sahel region were enhanced by the increased surface temperature. While their study provides insight into the mechanism of the interaction of land-use change with the African monsoon, they acknowledge that the prescribed land use change and hence the precipitation response they found are likely exaggerated. A multi-model study of attribution of variability of precipitation in the late twentieth century by Lau et al. (2006) showed that models that capture the observed precipitation variability also showed robust land surface feedbacks with strong sensitivity of precipitation and land evapotranspiration (ET) to soil moisture. Another GCM study (Kucharski et al. 2013) suggests that the decadal variability in precipitation, which is primarily driven by SST variability, is significantly enhanced by land surface albedo feedbacks through the mechanisms similar to that proposed by Charney et al. (1977).
In this study, uncertainty in regional model simulated monsoon precipitation to land-use change over the last 60 years is examined. We used ensembles of regional model experiments with various land surface and cumulus parameterization schemes to explore model uncertainties. The relationships between the responses of the ensemble members and their respective climatologists (in the control simulations) are assessed.
2 Model and simulation design
2.1 Estimate of land-use change and application to the regional model
Common to all simulations
January 1 2001 to December 31 2001
Lateral and surface forcing
NCEP-DOE reanalysis (Kanamitsu et al. 2002)
27,750 m (=0.25°)
Long wave radiation
RRTMG Mlawer (1997)
Short wave radiation scheme
Morrison et al. (2005)
Shortwave radiation scheme
RRTMG Mlawer (1997)
Parameterizations for ensemble members
Land surface schemes
PLEIM-XIU (Pleim and Xiu 2003)
Simplified Arakawa-Schubert (Grell 1993)
3 Response of precipitation to land use change
3.1 Relationship to model climatology
3.2 Mechanism of the response
This study examines the uncertainties in model response of the West African Monsoon to changes in land use that are estimated to have occurred over the last 60 years in the African Sahel using two groups of ensemble regional model simulations. In the control (CTRL) simulations, potential vegetation is prescribed, while in the land use change (LULC) simulations, the estimated land degradation associated with the observed increase in crop and pasture land is prescribed. For each case, sixteen simulations that correspond to combinations of six cumulus parameterizations and three land surface schemes are performed (two simulations with unrealistic climatology are discarded). All simulations have the same prescribed changes in surface albedo, green fraction and emissivity. Observations of precipitation and reanalysis of wind, water vapor mixing ratio, surface fluxes are used to evaluate the ensemble members and provide the constraints for a realistic estimate of the impact of land-surface degradation on Sahel precipitation.
In general, land surface degradation reduces surface evaporation in favor of surface sensible heating and increases surface temperature. The resulting increase in meridional surface temperature gradient across the Sahel (Fig. 11) enhances the AEJ (Fig. 10), which transports more moisture out of the Sahel region and reduces precipitation (Fig. 4). This is in agreement with the findings of Abiodun et al. (2008). The contribution of reduced ET to the change in precipitation is comparable to that by the change in circulation. The results are consistent among ensemble members with different cumulus parameterizations, but are very sensitive to the land surface scheme used. Two mechanisms play a role in this model sensitivity. First, different land surface schemes simulate different ET response to land use change. This leads to differences in the precipitation response through differences in local precipitation recycling. Second, land surface schemes determine the meridional structure of the evaporative fraction (or equivalently Bowen ratio, the partitioning of surface fluxes between latent and sensible heat fluxes), which shows a steep meridional gradient across the Sahel (Fig. 6). This modulates the meridional location of the model peak temperature response, leading to differences in the response of the model AEJ and precipitation.
Our finding is similar to those of the Global Land–Atmosphere Coupling Experiment (Guo and et al. 2006), where comparison of various combinations of atmospheric and land models revealed that the much of the differences in precipitation can be explained by the sensitivity of evaporation and hence surface temperature to soil moisture in the land models and the direct sensitivity of precipitation to evaporation through local precipitation recycling plays a smaller role in the inter-model difference. In this context, much of the uncertainty in the model response lies in the meridional structure of evaporation/temperature and how it responds to changes in soil moisture. As suggested by Guo et al. 2006, details of the land model scheme, particularly those associated with transpiration, bare soil evaporation and canopy interception loss, likely explain this uncertainty. Consistent with this, the response of the atmosphere to the changes in evaporation (modification to surface temperature, changes in AEJ and moisture transport and precipitation) is fairly consistent among the cumulus schemes used. Therefore accurate depiction of the impact of land use change on monsoon precipitation depends on representation of land surface processes and surface fluxes by the models. To this end, accurate in situ or remotely-sensed measurements of surface fields over the broad latitudinal cross section of the Sahel might be necessary to provide constraints on land surface parameterizations to realistically simulate the response to land use change.
This study does not explore the full range of uncertainty in how the Sahel climate responds to LULC. Rather we address specifically uncertainty in LULC response due to parameterization uncertainty and elucidate the physical and dynamical mechanisms for how sensitivity to parameterizations translates to uncertainty in LULC response. The latter improves understanding of how LULC influences the Sahel precipitation, and similar mechanisms should apply whether the uncertainty arises due to parameterizations, LULC scenarios, or soil moisture initialization. Providing a comprehensive assessment of uncertainty in model response and how uncertainty from each factor compares are beyond the scope of this study. Land degradation in the African Sahel has also resulted in soil texture change. Feddema (1998) has shown that variations in soil water holding capacity associated with land degradation can have important effects on evapotranspiration and local water balances. The relative contributions of LULC change and soil texture change and the degree to which uncertainty in model response to these change feeds back to the monsoon dynamics is a subject of future study.
The authors thank Dr. Yun Qian for his comments and suggestions. This research was supported by the Office of Science of the U.S. Department of Energy as part of the Regional and Global Climate Modeling Program and Earth System Modeling Program. Computing resources for the simulations are provided by the National Energy Research Scientific Computing Center (NERSC) and Oak Ridge Leadership Computing Facility (OLCF). The Pacific Northwest National Laboratory is operated for DOE by Battelle Memorial Institute under Contract DE-AC05-76RLO1830. The UDel_AirT_Precip data are provided by the NOAA/OAR/ESRL PSD, Boulder, Colorado, USA, from their Web site at http://www.esrl.noaa.gov/psd/.