Mouhamadou Bamba Sylla

Consistency of projected drought over the Sahel with changes in the monsoon circulation and extremes in a regional climate model projections

Received 9 August 2009; revised 7 January 2010; accepted 15 February 2010; published 21 August 2010. [1] As a step toward an increased understanding of climate change over West Africa, in this paper we analyze the relationship between rainfall changes and monsoon dynamics in high‐resolution regional climate model experiments performed using the Regional Climate Model (RegCM3). Multidecadal simulations are carried out for present‐day and future climate conditions under increased greenhouse gas forcing driven by the global climate model European Center/Hamburg 5 (ECHAM5). Compared to the present day, the future scenario simulation produces drier conditions over the Sahel and wetter conditions over orographic areas. The Sahel drying is accompanied by a weaker monsoon flow, a southward migration and strengthening of the African Easterly Jet, a weakening of the Tropical Easterly Jet, a decrease of the deep core of ascent between the jets, and reduced African Easterly Wave activity. These circulation changes are characteristics of dry periods over the Sahel and are similar to the conditions found in the late twentieth century observed drought over the region. Changes in extreme events suggest that the drier conditions over the Sahel are associated with more frequent occurrences of drought periods. The projected drought over the Sahel is thus physically consistent with changes in the monsoon circulation and the extreme indices (maximum dry spell length and 5 day precipitation).

Projected robust shift of climate zones over West Africa in response to anthropogenic climate change for the late 21st century

The response of West African climate zones to anthropogenic climate change during the late 21st century is investigated using the revised Thornthwaite climate classification applied to ensembles of CMIP5, CORDEX, and higher-resolution RegCM4 experiments (HIRES). The ensembles reproduce fairly well the observed climate zones, although with some notable discrepancies. CORDEX and HIRES provide realistic fine-scale information which enhances that from the coarser-scale CMIP5, especially in the Gulf of Guinea encompassing marked landcover and topography gradients. The late 21st century projections reveal an extension of torrid climates throughout West Africa. In addition, the Sahel, predominantly semi-arid in present-day conditions, is projected to face moderately persistent future arid climate. Similarly, the Gulf of Guinea shows a tendency in the future to experience highly seasonal semi-arid conditions. Finally, wet and moist regions with an extreme seasonality around orographic zones become less extensive under future climate change. Consequently, West Africa evolves towards increasingly torrid, arid and semi-arid regimes with the recession of moist and wet zones mostly because of the temperature forcing, although precipitation can be locally an important factor. These features are common to all multimodel ensembles, a sign of robustness, with few disagreements in their areal extents, and with more pronounced changes in the higher-resolution RCM projections. Such changes point towards an increased risk of water stress for managed and unmanaged ecosystems, and thus add an element of vulnerability to future anthropogenic climate change for West African water management, ecosystem services and agricultural activities.

Projected Changes in the Annual Cycle of High-Intensity Precipitation Events over West Africa for the Late Twenty-First Century*

AbstractIn this study, the response of the annual cycle of high-intensity daily precipitation events over West Africa to anthropogenic greenhouse gas for the late twenty-first century is investigated using an ensemble of high-resolution regional climate model experiments. For the present day, the RCM ensemble substantially improves the simulation of the annual cycle for various precipitation statistics compared to the driving Earth system models. The late-twenty-first-century projected changes in mean precipitation exhibit a delay of the monsoon season, consistent with previous studies. In addition, these projections indicate a prevailing decrease in frequency but increase in intensity of very wet events, particularly in the premonsoon and early mature monsoon stages, more pronounced over the Sahel and in RCP8.5 than the Gulf of Guinea and in RCP4.5. This is due to the presence of stronger moisture convergence in the boundary layer that sustains intense precipitation once convection is initiated. The prem...

Climate Change over West Africa: Recent Trends and Future Projections

The West African climate has evolved in recent decades to respond to elevated anthropogenic greenhouse gas (GHG) forcing. An assessment of its recent trends and future changes is presented here based on various data sources (observations and models), along with an extensive review of recent literature including the latest Intergovernmental Panel on Climate Change report. A gradual warming spatially variable reaching 0.5 °C per decade in recent years is observed. In addition, the Sahel has recovered from the previous drought episodes (i.e., 1970s and 1980s); however, the precipitation amount is not at the level of the pre-drought period. Although these features are common across the different data sources, their magnitudes differ from one source to the other due to a lack of reliable observation systems. Projected climate change indicates continuous and stronger warming (1.5–6.5 °C) and a wider range of precipitation uncertainty (roughly between −30 and 30 %) larger in the Sahel and increasing in the farther future. However, the spatial distribution unveils significant precipitation decrease confined to the westernmost Sahel and becoming greater and more extensive in the high level GHG forcing scenario by the end of the 21st century. This coexists with a substantial increase in both dry spell length and extreme precipitation intensity. West Sahel is thus the most sensitive region to anthropogenic climate change. The rest of West Africa also experiences more intense extremes in future climate but to a lesser extent. It is also reported from other previous studies that the projected rainy season and the growing season will become shorter while the torrid, arid and semi-arid climate conditions will substantially extend. It is thus evident that in a “business as usual” World, most countries in West Africa will have to cope with shorter rainy seasons, generalized torrid, arid and semi-arid conditions, longer dry spells and more intense extreme precipitations. Such conditions can produce significant stresses on agricultural activities, water resources management, ecosystem services and urban areas planning. However, some GHG mitigation (i.e., a mid-level forcing) could help to reduce the stress.

Trends and projections of climate extremes in the Black Volta River Basin in West Africa

This study used the RClimDex software to examine trends in extreme air temperature and rainfall in the Black Volta River Basin (BVRB) for the present (1981–2010) and future 2051–2080 (late twenty-first century) horizons. The analysis of the future extreme events was conducted using data output of four ensemble models for two IPCC emission scenarios, Representative Concentration Pathways (RCPs) 4.5 and 8.5. A bias correction method, the quantile-quantile (Q-Q) transformation technique, was applied to all the modelled temperature and rainfall data set prior to the index calculation. The results of analysis of the present-day climate indicate warming and wetting of the BVRB. Increasing trends were seen in the extreme warm indices while the extreme cold indices showed mostly decreasing trends. Majority of the trends observed in the indices were statistically significant (95% confidence level). The extremes in rainfall also showed increasing trends in amounts and intensity of rainfall events (majority of increasing trends were statistically insignificant). Projected temperatures for the late twenty-first century showed decreasing and increasing trends in the cold and warm indices respectively, suggesting warming during the period. Trend analysis of future rainfall projections mostly showed a mix of positive and negative trends offering no clear indication of the direction of change in majority of the extreme rainfall indices. An increase in extremely wet day events is however projected for the period. The results from this study could inform climate change adaptation strategies targeted at reducing vulnerability and building resilience to extreme weather events in the BVRB.

Projected Heat Stress Under 1.5 °C and 2 °C Global Warming Scenarios Creates Unprecedented Discomfort for Humans in West Africa

Heat and discomfort indices are applied to the multimodel ensemble mean of COordinated Regional climate Downscaling EXperiment‐Africa regional climate model projections to investigate future changes in heat stress and the proportion of human population at risk under 1.5 °C and 2 °C global warming scenarios over West Africa. The results show that heat stress of category Extreme Caution is projected to extend spatially (up to 25%) over most of the Gulf of Guinea, Sahel, and Sahara desert areas, with different regional coverage during the various seasons. Similarly, the projected seasonal proportion of human population at discomfort substantially increases to more than 50% over most of the region. In particular, in June–August over the Sahel and the western Sahara desert, new areas (15% of West Africa) where most of the population is at risk emerge. This indicates that from 50% to almost everyone over most of the Sahel countries and part of the western Sahara desert is at risk of possible heat cramp, heat exhaustion, and heat stroke in future climate scenarios. These conditions become more frequent and are accompanied by the emergence of days with dangerous heat stress category during which everyone feels discomfort and is vulnerable to a likely heat cramp and heat exhaustion. In general, all the above features are more extended and more frequent in the 2 °C than in the 1.5 °C scenario. Protective measures are thus required for outdoor workers, occupational settings in hot environments, and people engaged in strenuous activities.

Climate change to severely impact West African basin scale irrigation in 2 °C and 1.5 °C global warming scenarios

West Africa is in general limited to rainfed agriculture. It lacks irrigation opportunities and technologies that are applied in many economically developed nations. A warming climate along with an increasing population and wealth has the potential to further strain the region’s potential to meet future food needs. In this study, we investigate West Africa’s hydrological potential to increase agricultural productivity through the implementation of large-scale water storage and irrigation. A 23-member ensemble of Regional Climate Models is applied to assess changes in hydrologically relevant variables under 2 °C and 1.5 °C global warming scenarios according to the UNFCCC 2015 Conference of Parties (COP 21) agreement. Changes in crop water demand, irrigation water need, water availability and the difference between water availability and irrigation water needs, here referred as basin potential, are presented for ten major river basins covering entire West Africa. Under the 2 °C scenario, crop water demand and irrigation water needs are projected to substantially increase with the largest changes in the Sahel and Gulf of Guinea respectively. At the same time, irrigation potential, which is directly controlled by the climate, is projected to decrease even in regions where water availability increases. This indicates that West African river basins will likely face severe freshwater shortages thus limiting sustainable agriculture. We conclude a general decline in the basin-scale irrigation potential in the event of large-scale irrigation development under 2 °C global warming. Reducing the warming to 1.5 °C decreases these impacts by as much as 50%, suggesting that the region of West Africa clearly benefits from efforts of enhanced mitigation.

Changes in climate extremes over West and Central Africa at 1.5 degrees C and 2 degrees C global warming

In this study, we investigate changes in temperature and precipitation extremes over West and Central Africa (hereafter, WAF domain) as a function of global mean temperature with a focus on the implications of global warming of 1.5 ◦C and 2 ◦C according the Paris Agreement. We applied a scaling approach to capture changes in climate extremes with increase in global mean temperature in several subregions within the WAF domain: Western Sahel, Central Sahel, Eastern Sahel, Guinea Coast and Central Africa including Congo Basin. While there are several uncertainties and large ensemble spread in the projections of temperature and precipitation indices, most models show high-impact changes in climate extremes at subregional scale. At these smaller scales, temperature increases within the WAF domain are projected to be higher than the global mean temperature increase (at 1.5 ◦C and at 2 ◦C) and heat waves are expected to be more frequent and of longer duration. The most intense warming is observed over the drier regions of the Sahel, in the central Sahel and particularly in the eastern Sahel, where the precipitation and the soil moisture anomalies have the highest probability of projected increase at a global warming of 1.5 ◦C. Over the wetter regions of the Guinea Coast and Central Africa, models project a weak change in total precipitation and a decrease of the length of wet spells, while these two regions have the highest increase of heavy rainfall in the WAF domain at a global warming of 1.5 ◦C. Western Sahel is projected by 80% of the models to experience the strongest drying with a significant increase in the length of dry spells and a decrease in the standardized precipitation evapotranspiration index. This study suggests that the ‘dry gets drier, wet gets wetter’ paradigm is not valid within the WAF domain.

Projected increased risk of water deficit over major West African river basins under future climates

Estimating climate change impacts on water resources in West Africa has been challenged by hydrological data scarcity and inconsistencies in the available climate projections. In this study, multi-model ensembles of the most recent global and regional climate models output are used to simulate the hydrologic impacts of climate change in five major river basins (i.e. Senegal, Gambia, Volta, Niger and Chad) that comprise most of West Africa. Under Representative Concentration Pathways 4.5 and 8.5, the results consistently project substantial decreases (10 to 40%) in potential water availability across the five major river basins. The largest changes are projected to occur in the Senegal basin, Gambia basin and the Sahelian part of the other river basins. The negative trends are steepest after 2050 and in the higher greenhouse gas forcing scenario. Therefore, in a business-as-usual world, reduced water availability combined with the region’s rapidly growing population will have West Africa facing an unprecedented water deficit during the second half of the twenty-first century. However, greenhouse gas mitigation can help reduce this deficit. In the Volta basin, although potential water availability declines considerably, precipitation exceeds potential evapotranspiration during the monsoon season in both forcing scenarios, suggesting opportunities for adaptation.