Sahel Rainfall Variability Research Articles

Twentieth century Sahel rainfall variability as simulated by the ARPEGE AGCM, and future changes

The ability of the ARPEGE AGCM in reproducing the twentieth century Sahelian drought when only forced by observed SST time evolution has been characterized. Atmospheric internal variability is shown to have a strong contribution in driving the simulated precipitation variability over the Sahel at decadal to multi-decadal time scales. The simulated drought is associated with a southward shift of the continental rainbelt over central and eastern Sahel, associated with an inter-hemispheric SST mode (the southern hemisphere oceans warming faster than the northern ones after 1970). The analysis of idealized experiments further highlights the importance of the Pacific basin. The related increase of the tropospheric temperature (TT) over the tropics is then suggested to dry the margin of convection zones over Africa, in agreement with the so-called “upped-ante” mechanism. A simple metric is then defined to determine the ability of the CMIP3 coupled models in reproducing both the observed Sahel drying and these mechanisms, in order to determine the reliability of the twenty-first century scenarios. Only one model reproduces both the observed drought over the Sahel and consistent SST/TT relationships over the second half of the twentieth century. This model predicts enhanced dry conditions over the Sahel at the end of the twenty-first century. However, as the mechanisms highlighted here for the recent period are not stationary during the twenty-first century when considering the trends, similarities between observed and simulated features of the West African monsoon for the twentieth century are a necessary but insufficient condition for a trustworthy prediction of the future.

SST Forcings and Sahel Rainfall Variability in Simulations of the Twentieth and Twenty-First Centuries

Abstract The outlook for Sahel precipitation in coupled simulations of the twenty-first century is very uncertain, with different models disagreeing even on the sign of the trends. Such disagreement is especially surprising in light of the robust response of the same coupled models to the twentieth-century forcings. This study presents a statistical analysis of the preindustrial, twentieth-century and twenty-first-century A1B scenario simulations in the latest Coupled Model Intercomparison Project 3 (CMIP3) dataset; it shows that the relationship that links Sahel rainfall anomalies to tropical sea surface temperature (SST) anomalies at interannual time scales in observations is reproduced by most models, independently of the change in the basic state as the world warms. The same SST–Sahel relationship can be used to predict the simulated twentieth-century changes in Sahel rainfall from each model’s simulation of changes in Indo-Pacific SST and Atlantic SST meridional gradient, although the prediction overestimates the simulated trends. Conversely, such a relationship does not explain the rainfall trend in the twenty-first century in a majority of models. These results are consistent with there being, in most models, a substantial direct positive effect of atmospheric greenhouse gases on Sahel rainfall, not mediated through SST.

Spatial distribution and temporal variability of Harmattan dust haze in sub-Sahel West Africa

The distributions of frequency of occurrence (FOO) of ‘thick dust haze’ (TDH) and of ‘light dust haze’ (LDH) with respect to latitude, longitude, elevation and distance from source during Harmattan season (from November to February) are investigated, using 30-year visibility data collected from 27 synoptic stations located in Nigeria between the West African Sahel and the coasts of Gulf of Guinea. This region lies along the Harmattan season trajectory of Saharan dust. Also investigated is the temporal variability of TDH and LDH on intra-seasonal and inter-annual timescales spanning three decades from 1971 to 2000. Dust haze distribution over the region is found to correlate with the four geographical variables to different extents, with latitude, distance from source and elevation showing strong correlations with both TDH and LDH. The investigation also shows that while FOO of TDH days increases with latitude (correlation coefficient, r lat,FOO_TDH=0.88), FOO of LDH days decreases with latitude over the region (correlation coefficient, r lat,FOO_LDH=−0.74). The correlation coefficients with longitude over the region are r lon,FOO_TDH=0.44 and r lon,FOO_LDH=−0.25, respectively, indicating weaker dependence of TDH and LDH on longitude. The average number of TDH days per month during the Harmattan season in the region ranges from about 0.5 in the humid coastal zone near the Gulf of Guinea in the south, to approximately 6 in the dry semi-arid zone near the West African Sahel. An empirical equation which shows that FOO of TDH increases exponentially with latitude over the region is derived. The FOO of TDH is found to be most variable in or near the Sahel zone and decreases southwards towards the Gulf of Guinea. The average of the standard deviations is 1.13 for the six northernmost synoptic stations and decreases to 0.51 over the six coastal and southernmost locations. In contrast, the FOO of LDH is most variable in the south (with standard deviation of 2.11 (over the six southernmost and coastal stations) and decreases to an average of 0.30 (over six of the extreme northern stations). The observed patterns of TDH and LDH distribution are attributed to distance from dust source region, decrease in dust particle size and wind speed from north to south, increase in atmospheric humidity and vegetation cover from the Sahel in the north to the coastal zone in the south, as well as inter-annual variability of Sahel rainfall.

Filtering of GCM simulated Sahel precipitation

Atmospheric general circulation models (GCMs) forced with observed sea surface temperature (SST) reproduce some aspects of observed Sahel rainfall variability, particularly decadal variability. Here a filter based on signal‐to‐noise (S/N) EOFs is applied to seven GCM simulations of Sahel precipitation to extract SST‐forced variability. Using filter coefficients based on GCM estimates of internal variability has limited, though positive, impact on simulation skill. Additional removal of empirically identified model error improves the representation of both decadal and interannual variability. The model error shows some coherence across the seven GCMs and correlates with local Atlantic SST. We hypothesize that the model error is related to the representation of ocean‐atmosphere interactions in the SST‐forced GCM simulations.

Sahel rainfall variability and response to greenhouse warming

The NCEP/NCAR re‐analyses as well as ensemble integrations with an atmospheric GCM indicate that interannual variations in Sahel rainfall are related to variations in the mean sea level pressure (MSLP) over the Sahara. In turn the MSLP variations are related to the global distribution of surface air temperature (SAT). An increase in SAT over the Sahara, relative to the surrounding oceans, decreases the MSLP over the Sahara, thereby increasing the Sahel rainfall. We hypothesize that through this mechanism greenhouse warming will cause an increase in Sahel rainfall, because the warming is expected to be more prominent over the summer continents than over the oceans. This has been confirmed using an ensemble of 62 coupled model runs forced with a business as usual scenario. The ensemble mean increase in Sahel rainfall between 1980 and 2080 is about 1–2 mm day−1 (25–50%) during July–September, thereby strongly reducing the probability of prolonged droughts.

Climate Research and Seasonal Forecasting for West Africans: Perceptions, Dissemination, and Use?

Beginning in response to the disastrous drought of 1968–73, considerable research and monitoring have focused on the characteristics, causes, predictability, and impacts of West African Soudano–Sahel (10°–18°N) rainfall variability and drought. While these efforts have generated substantial information on a range of these topics, very little is known of the extent to which communities, activities at risk, and policy makers are aware of, have access to, or use such information. This situation has prevailed despite Glantz's provocative BAMS paper on the use and value of seasonal forecasts for the Sahel more than a quarter century ago. We now provide a systematic reevaluation of these issues based on questionnaire responses of 566 participants (in 13 communities) and 26 organizations in Burkina Faso, Mali, Niger, and Nigeria. The results reveal that rural inhabitants have limited access to climate information, with nongovernmental organizations (NGOs) being the most important source. Moreover, the pathways for information flow are generally weakly connected and informal. As a result, utilization of the results of climate research is very low to nonexistent, even by organizations responsible for managing the effects of climate variability. Similarly, few people have access to seasonal climate forecasts, although the vast majority expressed a willingness to use such information when it becomes available. Those respondents with access expressed great enthusiasm and satisfaction with seasonal forecasts. The results suggest that inhabitants of the Soudano–Sahel savanna are keen for changes that improve their ability to cope with climate variability, but the lack of information on alternative courses of action is a major constraint. Our study, thus, essentially leaves unchanged both Glantz's negative “tentative conclusion” and more positive “preliminary assessment” of 25 years ago. Specifically, while many of the infrastructural deficiencies and socioeconomic impediments remain, the great yearning for climate information by Soudano–Sahalians suggests that the time is finally ripe for fostering increased use. Therefore, a simple model for improved dissemination of climate research and seasonal climate forecast information is proposed. The tragedy is that a quarter century has passed since Glantz's clarion call.

Skill of Sahel rainfall variability in four atmospheric GCMs forced by prescribed SST

The inter‐annual and long‐term variability of July–September regional rainfall over the central and western Sahel (16°W–20°E; 11.25°–18.75°N) is evaluated using 21 runs from four atmospheric general circulation models (AGCMs) integrated from 1948. The skill is stronger at decadal time scales: each AGCM simulates quite successfully the rainfall decrease from the wet 1950–60s toward the dry 1970–90s although its amplitude is lower than observed. At inter‐annual time scales, the skill is smaller and decreases over time between the 1950–70s and the recent dry period; all AGCMs have a close to zero skill around the 1980s. This is not linked to a reduced AGCM sensivity to global SST but to their failure to reproduce the observed SST‐rainfall teleconnection changes, in particular the weakening (strengthening) linear relationship with the equatorial and southern Atlantic (equatorial Pacific).

Summer Sahel-ENSO teleconnection and decadal time scale SST variations

The correlation between Sahel rainfall and El Nino–Southern Oscillation (ENSO) in the northern summer has been varying for the last fifty years. We propose that the existence of periods of weak or strong relationship could result from an interaction with the global decadal scale sea surface temperature (SST) background. The main modes of SST variability have been extracted through a principal component analysis with Varimax rotation. The correlations between a July-September Sahel rainfall index and these SST modes have been computed on a 20-year running window between 1945 and 1993. The correlations with the interannual ENSO-SST mode are negative, not significant in the 1960s during the transition period from the wet climate phasis to the long-running drought in the Sahel, but then were significant since 1976. During the former period, the correlations between the Sahel rainfall index and the other SST modes (expressing mostly on quasi and multi-decadal scales) are the highest, in particular correlations with the tropical Atlantic “dipole”. Correlations between Sahel and Guinea Coast rainfall are also significantly negative. After 1970, the Sahel-Guinea Coast rainfall correlations are no longer significant, and the ENSO-SST mode becomes the only one significantly correlated with Sahel rainfall, especially due to the impact of warm events. The partial correlations between the ENSO-SST mode and the Sahel rainfall index, when the influence of the other SST modes are eliminated, are significant over all the 20-year running periods between 1945 and 1993, suggesting that this summer teleconnection could be modulated by the decadal scale SST background. The NCEP/NCAR reanalyses reproduce accurately the interannual variability of the atmospheric circulation after 1968. In particular a regional West African Monsoon Index (WAMI), combining wind speed anomalies at 925 and 200 hPa, is highly correlated with the July-September Sahel rainfall index. A warm ENSO event is associated both with an eastward mean sea level pressure gradient between the eastern tropical Pacific and the tropical Atlantic and with a northward pressure gradient along the western coast of West Africa. This pattern leads to enhanced trade winds over the tropical Atlantic and to weaker moisture advection over West Africa, consistent with a weaker monsoon system strength and a weaker Southern Hemisphere Hadley circulation. The NCEP/NCAR reanalyses do not reproduce accurately the decadal variability of the atmospheric circulation over West Africa because of artifical biases. Therefore the impact of the decadal scale pattern of the atmospheric circulation has been investigated with atmospheric general circulation model (AGCM) sensitivity experiments, by forcing the ARPEGE-Climat model with different combinations of an El Nino-like SST pattern with the pattern of the main mode of decadal scale SST variability where the hightest weights are located in the Pacific and Indian basins. AGCM outputs show that the decadal scale SST variations weakly affect Sahel rainfall variability but that they do induce an indirect effect on Sahel rainfall by enhancing the impact of the warm ENSO phases after 1980, through an increase in the fill-in of the monsoon trough and a moisture advection deficit over West Africa.

A conceptual model for understanding rainfall variability in the West African Sahel on interannual and interdecadal timescales

AbstractThis article describes and validates a new conceptual model for understanding Sahel rainfall variability. This conceptual model provides a framework that can readily incorporate and synthesize the roles played by the oceans, the African landmass and local meteorological factors. The most important ‘local’ factors are the location of the African Easterly Jet (AEJ) and the associated shears. The position of the AEJ helps to distinguish between a ‘wet mode’ and a ‘dry mode’ in the Sahel, while other factors determine which of two spatial patterns prevail during years of the dry regime. We test the paradigm by contrasting selected circulation parameters for the years 1958–1967 (representing the wet mode) and 1968–1997 (representing the dry mode). In doing so, we have identified several changes in the general atmospheric circulation that have accompanied the shift to drier conditions. The AEJ is further southward and more intense, the Inter‐tropical Convergence Zone (ITCZ) is further south, the Tropical Easterly Jet (TEJ) is weaker, the equatorial westerlies are shallower and weaker, the southwesterly monsoon flow is weaker, and the relative humidity is lower (but not consistently so).The results of this study suggest that the key factor controlling the occurrence of the ‘wet Sahel’ mode versus the ‘dry’ mode is the presence of deep, well‐developed equatorial westerlies. These displace the AEJ northward into Sahelian latitudes and increase the shear instabilities. The westerlies appear to be at least partially responsible for the well‐known association between a weaker AEJ and wetter conditions in the Sahel, because the thermal wind induced by the Sahara/Atlantic temperature gradient is imposed upon a westerly basic state. Since one of the strongest contrasts between the ‘wet Sahel’ and ‘dry Sahel’ modes is the strength of the TEJ, the TEJ probably also plays a pivotal role in rainfall variability. In the dry mode, the equatorial westerlies are poorly developed and the core of the AEJ lies well to the south of the Sahel. The dry mode consists of two basic spatial patterns, depending on whether the Guinea Coast Region is anomalously wet or dry (the well‐known dipole and no‐dipole patterns, respectively). Which occurs is determined by other factors acting to reduce the intensity of the rainbelt. One of the relevant factors appears to be sea‐surface temperatures (SSTs) in the Gulf of Guinea. Copyright © 2001 Royal Meteorological Society

Enhancement of Interdecadal Climate Variability in the Sahel by Vegetation Interaction.

The role of naturally varying vegetation in influencing the climate variability in the West African Sahel is explored in a coupled atmosphere-land-vegetation model. The Sahel rainfall variability is influenced by sea-surface temperature variations in the oceans. Land-surface feedback is found to increase this variability both on interannual and interdecadal time scales. Interactive vegetation enhances the interdecadal variation substantially but can reduce year-to-year variability because of a phase lag introduced by the relatively slow vegetation adjustment time. Variations in vegetation accompany the changes in rainfall, in particular the multidecadal drying trend from the 1950s to the 1980s.

Are revised models better models? A skill score assessment of regional interannual variability

Various skill scores are used to assess the performance of revised models relative to their original configurations. The interannual variability of all‐India, Sahel and Nordeste rainfall and summer monsoon windshear is examined in integrations performed under the experimental design of the Atmospheric Model Intercomparison Project. For the indices considered, the revised models exhibit greater fidelity at simulating the observed interannual variability. Interannual variability of all‐India rainfall is better simulated by models that have a more realistic rainfall climatology in the vicinity of India, indicating the beneficial effect of reducing systematic model error.

The role of synoptic systems in the interannual variability of Sahel rainfall

Components of the June-September climate over the Sahel are investigated in simulations with the GCM of the NASA/Goddard Institute for Space Studies, forced by SST observed during 1987 and 1988. The study analyzes the role of the synoptic patterns in determining precipitation differences between the two seasons, with special attention given to African wave disturbances (AWD). Emphasis is placed on deducing the characteristics of individual systems which may be missed by spectral and/or composite analyses. Results are derived from time-longitude cross-sections and spatial distributions of daily and weekly averages of key climatological variables. Despite the overall rainier season, rainless AWD are more prevalent in the simulations corresponding to June–September 1988 forcing than for 1987. Daily precipitation is shown to be highly correlated with mid-tropospheric vorticity, near surface convergence and 200 mb divergence. August daily rainfall was some-what better correlated with implied large scale vertical motion for 1988 forcing, emphasizing the dominance of broad circulation influences during that summer. While significant rainfall variability is attributed to AWD, quasistationary mechanisms cannot be ignored. In these simulations, upper tropospheric divergence modulated by changes in the Tropical Easterly Jet serves to both intensify the rainfall triggered by AWD and to sustain broader rainfall patterns during events of “massive uplift”.

Sea Surface Temperature Fields Associated with West African Rainfall Anomaly Types

Four West African rainfall anomaly types are defined in relation to the northern summer rainfall departure signs in the Sahel and in the Guinean region in order to investigate the statistical links between interannual variability of West African rainfall and sea surface temperature (SST) through the period 1950–90. Composite analysis depicts the setup of four different mean SST anomaly fields. Drought over all of West Africa is associated with the growth of positive SST anomalies in the eastern Pacific and in the Indian Ocean, and negative SST anomalies in the northern Atlantic and in the Gulf of Guinea. In contrast, drought limited to the Sahel corresponds mostly to a northward expansion of positive SST anomalies in the southern Atlantic, and negative SST anomalies in the northern Atlantic. Northward expansion of negative SST anomalies in the southern Atlantic, positive SST departures in the northern Atlantic, and development of negative SST anomalies in the eastern Pacific appear to be synchronous of flood limited to the Sahel. Flooding over all of West Africa is mainly associated with positive SST anomalies in the northern Atlantic. This approach complements previous papers dealing with rainfall anomalies located in the Sahel alone. In particular, it points out different associations between Atlantic/Pacific SST anomalies and Sahel rainfall variability. Individual cases are also discussed, especially the case of 1972 when a West African rainfall deficit was concomitant to a southward location of the intertropical convergence zone over the tropical Atlantic.

Surface conditions associated with anomalous rainfall in the guinea coastal region

AbstractThis paper examines surface atmospheric and oceanic conditions associated with anomalous rainfall at the Guinea coast. For time‐scales shorter than 5 years, rainfall in the Guinea coast is practically not correlated with rainfall in the Sahel, although both time series show a well‐defined drying trend in the last 30 years or so. While interannual variability of Sahel rainfall is associated with a sea‐surface temperature (SST) pattern anti‐symmetric about the Equator, anomalous rainfall near the Guinea coast in associated with an SST pattern roughly symmetric about the Equator. During wet years near the Guinea coast this SST pattern corresponds to a less developed cold tougue in the boreal summer.Using the Comprehensive Ocean‐Atmosphere Data Set surface fields we examine key terms of the oceanic heat budget. Anomalous shortwave radiation and latent heat fluxes seem to play an important role in the decaying of the SST anomaly. During wet years anomalous atmospheric winds act to relax the trade winds, which probably reduces the equatorial thermocline slope and contributes to the observed warming of the equatorial waters in the east.We also investigate the relationship between the surface winds and the underlying SST using numerical and analytical solutions of a simple model. The anomalous winds are shown to be consistent with the hydrostatic adjustment of the tropical boundary layer.

Modelling the influence of global sea surface temperatures on the variability and predictability of seasonal Sahel rainfall

An atmospheric general circulation model has been forced by observed global sea surface temperature data from ten individual years. The results support the idea that oceanic effects tend to dominate the forcing of both interannual and interdecadal variability of Sahel rainfall, with different local atmospheric mechanisms dominating in different years. Further experiments indicate that skillful predictions of July–September Sahel rainfall are possible using the persistence of June SST anomalies.