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The 2nd WAMME Workshop |
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Workshop Abstracts
1. Past WAM studies and new horizons in WAM researches (Invited)
The second Phase of the AMMA program. New challenges and first results.
Serge Janicot1, Christopher D. Thorncroft2, Doug J. Parker3, Arona Diedhiou4, Ernest A. Afiesimama5
1LOCEAN-IPSL, IRD, University Pierre et Marie Curie, 75252 Paris cedex 05, France 2DAES, State University of New York, Albany, NY 12222, USA 3ICAS, School of Earth and Environment, University of Leeds, Leeds, LS2 9JT, UK 4LTHE, IRD-CNRS-University of Grenoble, BP53, 38041 Grenoble cedex 9, France 5Nigerian Meteorological Agency, Maitama District, Abuja, Nigeria
AMMA, African Monsoon Multidisciplinary Analyses (http://www.amma-international.org/), launched in 2002, is an international interdisciplinary research program concerned with the variability of the West African Monsoon (WAM) and its impacts on communities in that region. The need to improve weather and climate forecasting for implementation of early warning systems motivated the scientific community to define three major objectives for AMMA: 1.To improve our understanding of the WAM and its influence on the physical, chemical and biological environment regionally and globally. 2.To provide the underpinning science that relates variability of the WAM to issues of health, water resources and food security for West African nations and defining and implementing relevant monitoring and prediction strategies. 3.To ensure that the multidisciplinary research carried out in AMMA is effectively integrated with prediction and decision making activity. In Phase 1 (2002-2009), thanks to a strong international coordination, AMMA has achieved a lot and is now recognized as a flagship research program on WAM. For Phase 2 (2010-2020) AMMA’s 2nd International Scientific Plan (http://www.amma-international.org/IMG/pdf/ISP2_v2.pdf) aims at better addressing the third objective while still improving our knowledge related to the first two objectives. Hence it hinges on 3 key interacting research themes: (i) interactions between society, environment and climate, which necessitates (ii) the study of predictability and improvement of meteorological, seasonal and climate forecasting, which itself requires (iii) a continued effort to enrich our knowledge of the monsoon system. This will be illustrated through examples of new research results and new incoming research activities.
The Dynamics-Aerosol-Chemistry-Cloud Interactions in West Africa (DACCIWA) project
Andreas H. Fink1 and Peter Knippertz1
1Institute for Meteorology and Climate Research, Karlsruhe Institute of Technology (KIT), D-76128 Karlsruhe, Germany
Massive economic and population growth and urbanisation are expected to lead to a tripling of anthropogenic emissions in southern West Africa (SWA) between 2000 and 2030, the impacts of which on human health, ecosystems, food security and the regional climate are largely unknown. An assessment of these impacts is complicated by (a) a superposition with effects of global climate change, (b) the strong dependence of SWA on the sensitive West African monsoon, (c) incomplete scientific understanding of interactions between emissions, clouds, radiation, precipitation and regional circulations and (d) by a lack of observations to advance our understanding and improve predictions. The DACCIWA project, funded under the 7th Framework Programme of the European Union from December 2013 to May 2018 and involving 16 European and African partners, will conduct extensive fieldwork in SWA to collect high-quality observations, spanning the entire process chain from surfacebased natural and anthropogenic emissions to impacts on health, ecosystems and climate. The backbone of the fieldwork will be a four-week surface, upper-air, and aircraft campaign in June or July 2015 with sites in Ghana, Benin, and Nigeria. Combining the resulting benchmark dataset with a wide range of modelling activities will allow (a) to assess all relevant physical and chemical processes, (b) to improve the monitoring of climate and compositional parameters from space and (c) to develop the next generation of weather and climate models capable of representing coupled cloud-aerosol interactions, which will ultimately lead to reduced uncertainties in climate predictions. SWA with its rich mix of emissions and diverse clouds is ideal for such a study and many findings and technical developments will be applicable to other monsoon regions. Using a targeted dissemination strategy, DACCIWA will deliver a comprehensive scientific assessment and actively guide sustainable future planning and policy-making for West Africa and beyond. The interdisciplinary and experienced DACCIWA team will build on the scientific and logistical foundations established by AMMA (EU FP6) and collaborate closely with operational centres, international programs (e.g. WCRP, IGBP), policy-makers and users to maximise impact.
The Weather-to-Climate Continuum and Sahel Rainfall
Peter Lamb
University of Oklahoma
This presentation will summarize the most important results of several papers published during the last few years. Daily weather data were analyzed to document the intraseasonal-to-multidecadal variability of West African mesoscale convective weather systems and the InterTropical Front (ITF) that is the leading edge of the southwesterly monsoon flow from the tropical Atlantic. Associations between these phenomena were quantified statistically with a view to underpinning seasonal prediction, and applications will be presented that illustrate the potential for real-time monitoring of monsoon season quality.
West Africa
Monsoon Changes in the CMIP5 Projections:
Lessons from Idealized Simulations. Michela Biasutti1
The simulations of the fifth Coupled Models Intercomparison Project (CMIP5) confirm the CMIP3 results in many details. Projections of 21st century seasonal mean totals are still uncertain with outlier models disagreeing with the multi-model mean. Yet, CMIP5 also reaffirms the prediction of a rainy season that is more feeble at its start, especially in West Africa, and more abundant at its core across the entire Sahel. Idealized simulations from a sub-set of the CMIP5 ensemble --- simulations designed to separate the fast land-atmosphere response to increased greenhouse gases from the slow response mediated through changes in sea surface temperature (SST) --- confirm that the direct effect of CO2 is to enhance the monsoon throughout the year, while warmer SST induce drying over the Sahel throughout the year. This is in contrast to idealized simulation of the zonal-mean precipitation response, which suggest that both an amplification and a delay of summer rainfall can be reproduced as responses to a uniform warming. These simulations indicate (i) that the seasonal evolution of the Sahel rainfall trends in the scenario simulations, spring drying and fall wetting, is an inherently coupled response, (ii) that regional changes in circulation are key determinants of the precipitation response in the Sahel, and (iii) that the circulation changes are sensitive to the interaction of the response to CO2 of the ocean and land surfaces.
How well do land surface models reproduce the water and energy
cycles in the West African
1 HSM, IRD, Montpellier, France. Land surface models (LSMs) are widely used in environmental and climate sciences to simulate matter (water, carbon) and energy exchanges between the continental surface and the atmosphere. They are also increasingly used in a wide range of applications such as impact studies on land use management or climate change. The AMMA project and the associated observation campaigns provided unique data-sets to drive land surface models and to evaluate their results over the West African region, where such high added-value information has been lacking.
In the framework of the ALMIP2 project (AMMA Land surface Model Inter-comparison Project – phase 2), simulations from about 20 LSMs were done on three contrasted meso-scale domains in Mali, Niger and Benin, over the period 2005-2008 using the same forcing data sets at 0.05 degree and 30 minutes space-time resolution. This talk analyses the simulated water and energy budget components on the sub-humid upper Ouémé basin (10,000 km2, Benin site), where the high-quality rainfall and runoff datasets allow a detailed and original hydrological evaluation of these meso-scale simulations.
As expected, the model inter-comparison shows large differences in the water and energy partitioning, either at the annual (see figure) or intra-seasonal time scale. Most of them do not reproduce the observed runoff, with annual biases ranging from -100% to 200 %. The multi-model mean Evapotranspiration (ET) correctly matches the observations, specially in the rainy season, with contrasted simulations of evaporation vs. transpiration from one model to the other. The identification of the more realistic water and energy partitioning is a key issue addressed by the ALMIP2 project.
Previous studies performed in the AMMA framework on the Oueme basin highlighted the key role of belowground water dynamics in the hydrological cycle (lateral transfer to rivers, groundwater seasonal rechargedischarge, ...). Moreover, field evidences suggested that groundwater uptake by deep rooted trees supply transpiration almost all year long, including the dry season. We postulate that a realistic representation of these processes would better constrain land surface modelling in the sub-humid West African tropics. As a conclusion, we discuss future research directions to test this assumption. Fig 1. Annual water budget (2007) relative to the rainfall depth as simulated by each model (labelled A to R), and superimposed observed runoff.
2. WAM research in Africa (Invited)
Current Interseasonal and Interannual Variabilities of West African Climate
Ernest A. Afiesimama and Eniola A. Olaniyan
Numerical Weather and Climate Predictions National Weather Forecasting and Climate Research Centre Nigerian Meteorological Agency Abuja, Nigeria e-mail: e.afiesimama@nimet.gov.ng
The climate of West Africa has been with so much variability that considerable concern for climatic variability and change studies over the last half-century. The 1930-1960 wet period, the 1970-1980 droughts and the apparent return of rainfall over some parts of the Sahel in the last decade illustrate this clearly and also demonstrated the population’s vulnerability to these extreme climate events. Consequently, the characteristics of variabilities and change of climate in the region has thus been one of the most engaging subjects of interest.
This paper examines the current interseasonal and interannual variabilities of West African climate from 1981 – 2010 in order to determine the extent of the meridional oscillation of the intertropical discontinuity (ITD), which is a key driver of the regional climate. The paper also attempts to provide spatio-temporal distributions of key climate characteristics of the region. To a larger extent, these spatio-temporal variations have been critical factors influencing water resources and agricultural productivity and other related activities. Also, they offer insight into the extreme climate events such as flooding and droughts and such other environmental issues of climate variability affecting the region.
The data for this study was obtained from West African meteorological stations through regional workshops on climate studies and reanalyses data from NCEP–NCAR for the period 1981–2010. The reanalyses datasets were evaluated with observations and proved useful for the study. The datasets were subsequently blended and subjected to statistical analyses.
The analyses show that the ITD which lies between latitudes 6oN and 22oN in the mean, before the 1980s, at the peaks of the dry and rainy seasons respectively, maintaining about 16o of latitudes, currently lies between latitude 8oN and 20oN, retaining only 12o of latitudes. This is evident in the less severity of the harmattan dust haze in the coastal region and the desert encroachment in the Sahel and by extension, the implication is that although the rains may increase in the Sahel but not sufficient to return pre-1960 while the coast will experience more rainfall. The meridional oscillation of the temperature dipole over tropical West Africa is inhibited by temperature reversal between latitudes 17oN (29oC) and 25oN (21oC). The rainfall regime advances from the coast to the inland areas in 6months (from March – August) and retreats in 3months (September – November) with 3months (December – February) of no significant rainfall. Temperature ranges from 25.4oC in 1974 to 27.4oC in 2010. Except in 1970s and 1980s, there is clear evidence of increasing temperatures over West Africa. Similarly, the rainfall trend shows decreasing rainfall from late 1960s to a minimum in early 1980s and increasing values to the present day except in early 2000s.
Realistic land use change effects on the West African Climate using a regional climate Model.
Ibrah SEIDOU SANDA1 and Yongkang XUE 2
1 Centre
Régional AGRHYMET/CILSS, Niamey, Niger The Sahel area with the largest percentage of population relying on rainfed agriculture is subject to strong inter-annual variability with important human, social and economic consequences. Following the severe drought of the early seventies, the Sahel countries has established in 1973 the Permanent Interstate Committee for Drought Monitoring (CILSS) with the mandate to contribute to achieve sustainable food security and rational natural resource management. The AGRHYMET Regional Center is the specialized institution of CILSS in charge of producing and disseminating climate information among member countries for decision support.
As part of its activities AGRHYMET is taking part to West African Monsoon Modeling and Evaluation project (WAMME) which aims to better understand the complex interactions between land-surface and the West African Monsoon (WAM). Here we used a regional climate model to assess the impact of a realistic land use change on the WAM. We discuss the results obtained from two sets of six years long experiments done with the fourth generation ICTP regional climate model RegCM driven by the NCEP reanalysis data. The results show that there is a significant change in the monsoon circulation with a decrease of the JJAS rainfalls over the Sahel area above 10 degree north.
Advance analyses of temperature trends in the Sahel, and their relation to precipitation trends
Mouhamed Ly
Universté Cheick Anta Diop, Senegal/Centre Régional AGRHYMET, Niger
Climate variability and change affect most socioeconomic sectors in West Africa. It is now admitted that the variability of climate has increased since the 1950s mainly because of the increased concentration of anthropogenic greenhouse gases in the atmosphere. In this study, we analyse the evolution of some extreme temperature and precipitation indices over a large area of West Africa. Prior results show a general warming trend at individual stations throughout the region during the period from 1960 to 2010, namely negative trends in the number of cool nights, and positive trends in the number of warm days and length of warm spells. Trends in rainfall-related indices are not as uniform as the ones in temperatures, rather they display marked multi-decadal variability, as expected. To refine analyses of temperature variations and their relation to precipitation we investigated on cluster analysis aimed at distinguishing different sub-regions, such as continental and coastal, and relevant seasons, such as wet, dry/cold and dry warm. The evolution of the climate extreme and variability and the relationship of global warming and precipitation over West Africa are still not well understood. To study this evolution, long climate records are essential to describe the climate of the recent past and the observed tendency will be explained in evaluating the climate model in the region. Here we investigate a method of composite through the dynamic namely wind fields (u,v) of the Regional of Climate Model (RCM) in the new CMIP5 experiment to aid in detection and attribution of anthropogenic climate change at a regional scale. The issue of this work will be further discussed at the workshop.
Prediction of meningococcal meningitis epidemics in western Africa by using climate information.
Dieudonne Pascal Alda YAKA1, Benjamin Sultan2, Felix Tarbangdo1, Wassila Mamadu Thiaw3
1 NMS, Ouagadougou, Burkina Faso. 2LOCEAN, Paris, France. 3NCEP, Washington, WA, United States.
The variations of certain climatic parameters and the degradation of ecosystems, can affect human’s health by influencing the transmission, the spatiotemporal repartition and the intensity of infectious diseases. It is mainly the case of meningococcal meningitis (MCM) whose epidemics occur particularly in Sahelo-Soudanian climatic area of Western Africa under quite particular climatic conditions. Meningococcal Meningitis (MCM) is a contagious infection disease due to the bacteria Neisseria meningitis. MCM epidemics occur worldwide but the highest incidence is observed in the "meningitis belt" of sub-Saharan Africa, stretching from Senegal to Ethiopia.
In spite of standards, strategies of prevention and control of MCS epidemic from World Health Organization (WHO) and States, African Sahelo-Soudanian countries remain frequently afflicted by disastrous epidemics. In fact, each year, during the dry season (February-April), 25 to 250 thousands of cases are observed. Children under 15 are particularly affected. Among favourable conditions for the resurgence and dispersion of the disease, climatic conditions may be important inducing seasonal fluctuations in disease incidence and contributing to explain the spatial pattern of the disease roughly circumscribed to the ecological Sahelo-Sudanian band.
In this study, we tried to analyse the relationships between climatic factors, ecosystems degradation and MCM for a better understanding of MCM epidemic dynamic and their prediction. We have shown that MCM epidemics, whether at the regional, national or local level, occur in a specific period of the year, mainly from January to May characterised by a dry, hot and sandy weather. We have identified both in situ (meteorological synoptic stations) and satellitales climatic variables (NCEP reanalysis dataset) whose seasonal variability is dominating in MCM seasonal transmission. Statistical analysis have measured the links between seasonal variation of certain climatic parameters (particularly the meridional wind components) and seasonal recrudescence of MCM cases in order to elaborate seasonal occurrence of MCM epidemics prediction models on different spatiotemporal scales.
These predictions have been experienced and evaluated by Burkina Meteorological Authority and Health Protection General Direction since 2009. The encouraging results from the monitoring and evaluation of these predictions given by such simple models enable the development of a monitoring and an early warning integrated system of MCM epidemics in Burkina Faso. This experience could be implemented in others African Sahelo-Soudanian countries.
3. New WAM Research Highlight
Effect of multi-decadal variation of SST on rainfall and dust transport over West Africa and northern Atlantic Ocean
William K. Lau, Kyu-Myong Kim and Peter Colarco
Division of Earth Science, NASA/Goddard Space Flight Center
Effects of multi-decadal variation of SST on the West African monsoon rainfall and circulation, and consequent change in dust emission and transport are examined based on model experiments with NASA GEOS-5 GCM with prescribed SST. SST patterns representing 1950’s and 1980’s, derived from SVD between Sahel rainfall and global SST distribution, are selected and prescribed for 7-year simulation for each experiment. With 1950’s SST, the model produced weakened subtropical high, stronger cross equatorial flows, and enhanced rainfall in ITCZ and Sahel. Due to weakened surface winds, dust emission is reduced over the most of Sahel. Increased wet deposition due to increase rainfall in ITCZ and reduced easterly due to weakening of subtropical high, reduced dust transport in to the deep tropics, while dust transport increases between 20N-25N. Comparing the control with experiments in which dust radiative feedback is disabled, we find that radiative effects of dust lead to a) stronger dust emission over the Saharan dust through increased surface wind, b) ejection of dust to higher elevations, reaching further west over the Caribbean, and c) increased major Saharan dust outbreak events over the North Atlantic. Possible impact of radiative effects of dust aerosol on the Atlantic ITCZ convection will also be discussed.
On the connection between continental-scale land surface processes and the tropical climate in a coupled ocean-atmosphere-land system
Hsi-Yen Ma*, Carlos Roberto Mechoso Yongkang Xue, Heng Xiao, J. David Neelin, and Xuan Ji *Program for Climate Model Diagnosis and Intercomparison, Lawrence Livermore National Laboratory, Livermore, California, USA Department of Atmospheric and Oceanic Sciences, University of California, Los Angeles, Los Angeles, California, USA Department of Geography, University of California, Los Angeles, Los Angeles, California, USA Atmospheric Sciences & Global Change Division, Pacific North National Laboratory, Richland, Washington, USA
Abstract
An evaluation is presented of the impact on tropical climate of continental-scale perturbations given by different representations of land surface processes (LSP) in a general circulation model that includes atmosphere-ocean interactions. One representation is a simple land scheme, which specifies climatological albedos and soil moisture availability. The other representation is the more comprehensive Simplified Simple Biosphere Model, which allows for interactive soil moisture and vegetation biophysical processes. The results demonstrate that such perturbations have strong impacts on the mean states and variability of global precipitation, clouds, and surface air temperature. The impact is especially significant over the tropical Pacific Ocean. To explore the mechanisms for such impact, model experiments are performed with different LSP representations confined to selected continental-scale regions where strong interactions of climate-vegetation biophysical processes are present. The largest impact found over the tropical Pacific is mainly from perturbations in the tropical African continent where convective heating anomalies associated with perturbed surface heat fluxes trigger global teleconnections through equatorial wave dynamics. In the equatorial Pacific, the remote impacts of the convection anomalies are further enhanced by strong air-sea coupling between surface wind stress and upwelling, as well as by the effects of ocean memory. LSP perturbations over South America and Asia-Australia have much weaker global impacts. The results further suggest that correct representations of LSP, land use change, and associated changes in the deep convection over tropical Africa are crucial to reducing the uncertainty of future climate projections with global climate models under various climate change scenarios. (This work is performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344. LLNL-ABS-641339)
Multi-decadal dust aerosol trends in North Africa and tropical North Atlantic: What are the driving forces? Mian Chin, Dongchul Kim, Thomas Diehl We present a global model analysis of dust aerosol trends over Sahara, Sahel, and the tropical North Atlantic from 1980 to 2009 using the GOCART model and multiple satellite data of AVHRR, TOMS, SeaWiFS, MISR, and MODIS. We examine several key parameters determining the dust emission, removal, and transport in North Africa and tropical Atlantic, and their relationship with climate variability. We found that the dust emission over Sahel has decreased since the mid-1980s, mainly driven by the weakening winds near the surface. The decrease of the dust in the tropical North Atlantic in the past three decades is mostly correlated with the decrease of Sahel dust emission. Investigating the changes of sea surface temperature (SST), North Atlantic Oscillation (NAO), and other climate index, we have found that the reduction of Sahel dust emission and dust amount over the tropical North Atlantic is mostly driven by the increase of SST in the past three decades. This has implied the consequences of climate change on the dust emission and transport in this dust belt of North Africa and tropical North Atlantic.
Impact of burned area on African seasonal climate in regional modeling
Fernando De Sales1, Yongkang Xue1, Greg S Okin1
1 Geography, UCLA, Los Angeles, CA, United States. PRESENTER: Fernando De Sales
The WRF/SSiB2 model has been employed on a series of regional simulations to investigate the impact of burned areas associated with wildfires on African seasonal climate and surface energy balances. Burned areas are characterized by deposits of charcoal and ash, removal of vegetation, and alteration of the vegetation structure. Burned area information for the experiments was based on the MODIS burn date maps with an 8-day interval on 500m spatial resolution. Monthly burned area maps averaged over 2000-2011, and aggregated from the MODIS resolution, were created and incorporated in the regional model (50km resolution), whereby vegetation was reduced according to the percentage of grid cell area burned and ground albedo was reduced to 0.1 for a 10-day period after burning to reproduce the ground darkening associated with the amount of grid cell burned. Control (unburned) and burned preliminary experiments were carried out between 01 Oct 2010 and 31 Sep 2011 and compared to examine the sensitivity of different wildfire parameters on precipitation and surface fluxes; including sensitivity to ground albedo recovery time and vegetation resistance to fire. Vegetation cover, greenness, and LAI information were taken from the Fourier-Adjusted, Sensor and Solar zenith angle corrected, Interpolated, Reconstructed data set. Analysis of annual burned area maps revealed extensive burning, especially in the Sahel and between latitudes 0° and 15°S (Central Africa), with both regions exhibiting 50% or more of the area of a grid cell burned. Most of burning in Sahel occurred between November and February, while in the southern hemisphere it took place between June and September. Extensive burning was also found along eastern South Africa and Mozambique between 25° and 40° W, where some grid cells were 10% to 30% burned in August and September. Preliminary results indicated that the WRF/SSiB2 is sensitive to the land degradation associated with the burned areas. Areas with widespread burning experienced a reduction in evapotranspiration in the Sahel especially during the pre-monsoon months, and during monsoon withdrawal. In the Central and Southern Africa, the largest difference took place between January and April, and later August. In general, the impact on sensible heat flux was of opposite sign and significantly weaker than that of evapotranspiration in the Sahel. Changes to surface albedo in the model depends on two distinct processes; ground darkening associated with burning (direct effect) and changes in soil moisture associated with precipitation variability (indirect effect). The patterns of precipitation change resulting from burn degradation were complex, with areas of positive and negative changes within both regions. On average, annual precipitation was reduced in the Sahel by approximately 3.5%. Most of the change occurred during the monsoon season. In contrast, Central and Southern Africa experienced increase in rainfall in the austral winter but decrease in the summer months, resulting in nearly no annual change. Results also revealed that vegetation resistance to fire and ground albedo recovery time are important factors that must be accounted for to realistically simulate the impact of burned area in Africa.
4. New WAM Research
Comparative Study of the West African Continental, Coastal, and Marine Atmospheric Profiles during the summer of 2006
Modeling the Impact of Land Cover Changes on West African Monsoon Using a Regional Climate Model: Sensitivity to Lateral Boundary Conditions
Guiling Wang1, Miao Yu1,2, Yongkang Xue3, Chenming Ji 1,4
1University of Connecticut, Storrs, CT, USA 2Nanjing University of Information Science &Technology, Nanjing, China 3University of California, Los Angeles, CA, USA 4Loyola Marymount University, Los Angeles, CA, USA
The impact of land use land cover changes on West African monsoon climate is studied based on numerical experiments using a regional climate model (RCM), the coupled RegCM4-CLM4 model. The scenario of land cover changes is constructed according to the WAMME2 guideline applied to the specific format of vegetation representation in CLM4. In each grid cell within the WAMME-defined areas of interest (including most of the Sahel and part of the Guinea Coast), land cover changes include primarily the conversation of up to 30% of grassland to desert and up to 30% of forest coverage to grassland. The climate differences between a model Control simulation (with MODIS-derived vegetation cover) and the corresponding experiment simulation LUC (with land cover changes applied) are used to quantify the impact of land cover changes. Three different types of Control-vs.-LUC simulations are conducted, each driven with a different type of lateral boundary conditions (LBCs). In Type 1, both Control and LUC simulations are driven by LBCs from the NCEP/NCAR Reanalysis data during the period 2001-2006, and model output from the last five years of the simulations is used. In Type 2, both Control and LUC are driven by LBCs derived from a same GCM control simulation that prescribes vegetation cover according to MODIS data and sea surface temperature according to the WAMME-2 SST climatology. Two GCMs are used to test the GCM-related uncertainties, the NCAR CAM5 and the UCLA GCM. Similar to Type 1, Control and LUC in Type-2 RCM runs share the same atmospheric lateral boundary conditions. The Control simulation in Type 3 is the same as that in Type 2, but LUC simulation in Type 3 is driven by LBCs derived from a corresponding GCM run that includes the same land cover changes applied to the RCM LUC simulation. Results from Type-1 simulations show a clear signal of precipitation decrease in the land cover change regions and some increase to the south; results from Type-2 simulations show a signal that is similar to but weaker than that in Type-1. Despite a decrease of precipitation over the land cover change zones simulated by the driving GCM and better consistency in experimental design between the RCM and its driving GCM in Type-3 runs, results from Type-3 simulations indicate a mixed response, with increase in some areas of local land cover changes and decrease in others. This discrepancy between GCM and RCM and the lack of clear signal might be due to the potential inconsistency in atmospheric circulations between RCM and the driving GCM.
Assessment of Uncertainties in the Response of the African Monsoon Precipitation to Land Use Change in Regional Model Simulations
Samson M Hagos1, Lai-Yung Leung1, Yongkang Xue2, Aaron Anthony Boone3, Maoyi Huang1, JinHo Yoon1
1 Pacific Northwest National Laboratory, Richland, WA, United States. 2University of California, Los Angeles, CA, United States. 3CNRM, Tulouse , France.
Land use and land cover over Africa have changed substantially over the last sixty years and this change has been proposed to affect monsoon circulation and precipitation. This study examines the uncertainties on the effect of these changes on the African Monsoon system and Sahel precipitation using an ensemble 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 land use change and the climatologies 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 and therefore land-atmosphere interactions play a more significant role have stronger response to the land use and land cover changes. 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.
Dust Aerosol Impact on the Sahel Climate: A GCM Investigation of Aerosol-Radiation-Cloud-Precipitation Interactions
1Y. Gu and 2Y. Xue
1Joint Institute for Regional Earth System Science and Engineering and Department of Atmospheric and Oceanic Sciences, University of California, Los Angeles, CA 90095 2 University of California, Los Angeles, CA 90095
Absorbing aerosols, such as dust, play an important role in regional and global climate by heating the air column and modifying the horizontal and vertical temperature gradients, atmospheric stability, and convection strength. Dust is one of the most important aerosol sources, which cover the sky over Africa, the equatorial Atlantic Ocean, Middle East, and many other areas over the world. The climatic effects of dust aerosols in the Sahel region have been investigated using the atmospheric general circulation model (AGCM) developed at the University of California, Los Angeles (UCLA). The model includes an efficient and physically based radiation parameterization scheme developed specifically for application to clouds and aerosols. Parameterization of the effective ice particle size in association with the aerosol first indirect effect based on ice cloud and aerosol data retrieved from A-Train satellite observations have been employed in climate model simulations. The objective of our study is to investigate the impact of dust aerosols on regional climate, with a focus on the Sahel region where dust events frequently occur, by examining responses of the regional climate system to direct, semi-direct, and indirect aerosol radiative forcings in the UCLA AGCM in terms of the cloud, radiation, temperature, precipitation, and general circulation patterns. To examine the uncertainty regarding the role of dust in Sahel climate variability, the 3D monthly climatology of the GOCART and MATCH dust data has been employed, respectively, in two experiments. Offline simulations reveal that 1) dust direct effect could overshadow cloud forcing under heavily polluted case; 2) semi-direct effect may change the sign of dust radiative forcing, indicating that cloud forcing associated with aerosol semi-direct effect can exceed direct aerosol forcing; and 3) changes in radiative forcing associated with indirect effect are not very significant with increasing AOD. AGCM simulations show that aerosol indirect effect tends to act oppositely to the aerosol direct and semi-direct effects with a much smaller magnitude, and hence the direct and semi-direct effects play a dominant role in the overall climatic effect of dust aerosols over the Sahel region. Inclusion of the dust aerosols in the AGCM simulations reduces the precipitation in the normal rainfall band over West Africa, where precipitation is shifted to the south and produced a dipole anomaly, similar to the feature that we found in the Sahel land use land cover change studies. . Reduced precipitation is found in the Sahel Savanna area centered at 10 N; , and more precipitation is found to the south, (around equator. Differences in the simulated precipitation due to the use of different aerosol data reveal that the responses of the regional climate to the aerosol forcing are sensitive to the aerosol optical depth and properties.
Extratropical North Atlantic SST influence on Sahel rainfall
Yuwei Liu1, John C H Chiang1
1Univ. of California, Berkeley, Berkeley, CA, United States
We present evidence suggesting that the late 1960’s Sahel drought was linked to an abrupt cooling in the extratropical North Atlantic, whose influence was then propagated to the Sahel by atmospheric teleconnection. Such linkages have been observed in paleoclimate during abrupt climate changes of the last glacial period. They have also occurred in coupled model simulations of Atlantic meridional overturning circulation (AMOC) slowdown, the latter being the leading cause of said paleoclimate abrupt changes. The AMOC-slowdown simulations show a characteristic global pattern of climate changes, including a northern hemispheric-wide cooling and increased surface pressure, and weakening of the West African and Asian monsoons. We show that an observed northern-hemispheric pattern of changes, resembling the AMOC slowdown, occurred during the late 1960’s, co-incident with the Sahel drought. A combined principal component analysis of 20th century surface temperature, sea level pressure and precipitation extracts a leading mode whose spatial pattern closely resemble the impacts of AMOC slowdown. A similar analysis of AMIP-type simulations forced by 20th century observed forcings shows a similar result, suggesting that the origins of the climate change reside in SST changes, in particular over extratropical North Atlantic. Taken together, the results suggests the influence of extratropical North Atlantic cooling on the 20th century Sahel drought, and a teleconnection pathway through surface/tropospheric cooling. Motivated by our observational result, we investigated atmospheric teleconnection mechanisms of extratropical North Atlantic cooling in an atmospheric general circulation model (GCM) coupled with slab ocean. Our results indicate the central role of tropospheric cooling in communicating the influence on the Sahel. We explicitly show this using regional climate model simulation of the Sahel, with air temperature and associated humidity anomalies from the GCM simulation imposed as lateral boundary conditions. A moisture budget analysis of the GCM anomalies further suggests that reductions of humidity and monsoon flow are dominant processes through which the cooling reduces the Sahel rainfall. The strength of the teleconnection, inferred from air temperature changes due to radiative feedbacks diagnosed with a radiative kernel technique, is shown to be tied to low cloud cover over the Atlantic cooling region and the greenhouse effect of moisture over North Africa.
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