West African Rainfall Research Articles

A multi-model approach to the Atlantic Equatorial mode: impact on the West African monsoon

This paper is focused on the West African anomalous precipitation response to an Atlantic Equatorial mode whose origin, development and damping resembles the observed one during the last decades of the XXth century. In the framework of the AMMA-EU project, this paper analyses the atmospheric response to the Equatorial mode using a multimodel approach with an ensemble of integrations from 4 AGCMs under a time varying Equatorial SST mode. The Guinean Gulf precipitation, which together with the Sahelian mode accounts for most of the summer West African rainfall variability, is highly coupled to this Equatorial Atlantic SST mode or Atlantic Nino. In a previous study, done with the same models under 1958–1997 observed prescribed SSTs, most of the models identify the Equatorial Atlantic SST mode as the one most related to the Guinean Gulf precipitation. The models response to the positive phase of equatorial Atlantic mode (warm SSTs) depicts a direct impact in the equatorial Atlantic, leading to a decrease of the local surface temperature gradient, weakening the West African Monsoon flow and the surface convergence over the Sahel.

An interdecadal American rainfall mode

Low‐frequency climate variability across the American continents and surrounding oceans is analyzed by application of singular value decomposition (SVD) to gauge‐based rainfall and environmental anomaly fields in the period 1901–2002. A 5‐year filter is used to maintain a focus on interdecadal cycles. The rainfall regime of particular interest (mode 1) is when West Africa and the Caribbean share positive loading and North and South America share negative loading. Wavelet cospectral energy is found at ∼8, 24, and 50 years for Caribbean/West African zones and 16 and 32 years for North/South America. West Africa and South America exhibit antiphase multidecadal variability, while North America and the Caribbean rainfall exhibit quasi‐decadal cycles. The rainfall associations are nonstationary. In the early 1900s, Caribbean and South American rainfall were antiphase. Since 1930 low‐frequency oscillations of North American (West African) rainfall have been positively (negatively) associated with South America. Low‐frequency oscillations of North American rainfall have been consistently antiphase with respect to Caribbean rainfall; however, West Africa rainfall fluctuations have been in phase with the Caribbean more in the period 1920–1950 than at other times. Hemispheric‐scale environmental SVD patterns and scores were compared with the leading rainfall modes. The north‐south gradient modes in temperature are influential in respect of mode 1 rainfall, while east‐west gradients relate to mode 2 (northern Brazil) rainfall. The ability of the GFDL2.1 coupled (ocean‐atmosphere) general circulation model to represent interdecadal rainfall modes in the 20th century was evaluated. While mode 2 is reproduced, mode 1 remains elusive.

Radar‐observed squall line propagation and the diurnal cycle of convection in Niamey, Niger, during the 2006 African Monsoon and Multidisciplinary Analyses Intensive Observing Period

Surface radar observations near Niamey, Niger, during the African Monsoon Multidisciplinary Analyses (AMMA) campaign in 2006 documented the structure, motion, and precipitation of cloud systems during the monsoon season. These unique observations for that part of the Sahel were combined with satellite rain estimates and infrared satellite imagery to study the diurnal cycle of rainfall in Niamey, Niger. This study confirms the bimodal structure of the diurnal rainfall cycle in Niamey during AMMA, seen by previous studies of West African rainfall. Radar analysis of squall line mesoscale convective systems (SLMCS) and non‐MCS isolated convection clearly demonstrated that the nocturnal maximum was associated with the observed arrival time of westward propagating SLMCS. Satellite imagery suggested that these SLMCS formed in elevated terrain to the east of Niamey the prior afternoon. Radar observations showed that local isolated convection produced the smaller afternoon maximum. Early in the monsoon season, locally generated convection produced an afternoon diurnal rainfall maximum that was delayed by several hours compared to midseason when African easterly wave (AEW) activity was much greater. We suggest that the observed greater mean convective inhibition early in the season, perhaps tied to the absence of large‐scale forcing from AEW, played a role in the delayed initiation time.

Tropical Atlantic Variability Modes (1979–2002). Part I: Time-Evolving SST Modes Related to West African Rainfall

AbstractThis work presents a description of the 1979–2002 tropical Atlantic (TA) SST variability modes coupled to the anomalous West African (WA) rainfall during the monsoon season. The time-evolving SST patterns, with an impact on WA rainfall variability, are analyzed using a new methodology based on maximum covariance analysis. The enhanced Climate Prediction Center (CPC) Merged Analysis of Precipitation (CMAP) dataset, which includes measures over the ocean, gives a complete picture of the interannual WA rainfall patterns for the Sahel dry period. The leading TA SST pattern, related to the Atlantic El Niño, is coupled to anomalous precipitation over the coast of the Gulf of Guinea, which corresponds to the second WA rainfall principal component. The thermodynamics and dynamics involved in the generation, development, and damping of this mode are studied and compared with previous works. The SST mode starts at the Angola/Benguela region and is caused by alongshore wind anomalies. It then propagates westward via Rossby waves and damps because of latent heat flux anomalies and Kelvin wave eastward propagation from an off-equatorial forcing. The second SST mode includes the Mediterranean and the Atlantic Ocean, showing how the Mediterranean SST anomalies are those that are directly associated with the Sahelian rainfall. The global signature of the TA SST patterns is analyzed, adding new insights about the Pacific–Atlantic link in relation to WA rainfall during this period. Also, this global picture suggests that the Mediterranean SST anomalies are a fingerprint of large-scale forcing.This work updates the results given by other authors, whose studies are based on different datasets dating back to the 1950s, including both the wet and the dry Sahel periods.

How certain is desiccation in west African Sahel rainfall (1930–1990)?

Hypotheses for the late 1960s to 1990 period of desiccation (secular decrease in rainfall) in the west African Sahel (WAS) are typically tested by comparing empirical evidence or model predictions against “observations” of Sahelian rainfall. The outcomes of those comparisons can have considerable influence on the understanding of regional and global environmental systems. Inverse‐distance squared area‐weighted (IDW) estimates of WAS rainfall observations are commonly aggregated over space to provide temporal patterns without uncertainty. Spatial uncertainty of WAS rainfall was determined using the median approximation sequential indicator simulation. Every year (1930–1990) 300 equally probable realizations of annual summer rainfall were produced to honor station observations, match percentiles of the observed cumulative distributions and indicator variograms and perform adequately during cross validation. More than 49% of the IDW mean annual rainfall fell outside the 5th and 95th percentiles for annual rainfall realization means. The IDW means represented an extreme realization. Uncertainty in desiccation was determined by repeatedly (100,000) sampling the annual distribution of rainfall realization means and by applying Mann‐Kendall nonparametric slope detection and significance testing. All of the negative gradients for the entire period were statistically significant. None of the negative gradients for the expected desiccation period were statistically significant. The results support the presence of a long‐term decline in annual rainfall but demonstrate that short‐term desiccation (1965–1990) cannot be detected. Estimates of uncertainty for precipitation and other climate variables in this or other regions, or across the globe, are essential for the rigorous detection of spatial patterns and time series trends.

Revised Prediction of Seasonal Atlantic Basin Tropical Cyclone Activity from 1 August

Abstract Predictions of the remainder of the season’s Atlantic basin tropical cyclone activity from 1 August have been issued by Gray and his colleagues at the Tropical Meteorology Project at Colorado State University since 1984. The original 1 August prediction scheme utilized several predictors, including measures of the stratospheric quasi-biennial oscillation (QBO), West African rainfall, El Niño–Southern Oscillation, and the sea level pressure anomaly and upper-tropospheric zonal wind anomalies in the Caribbean basin. The recent failure of the West African rainfall and QBO relationships with Atlantic hurricanes has led to a general degradation of the original 1 August forecast scheme in recent years. It was decided to revise the scheme using only surface data. The development of the National Centers for Environmental Prediction–National Center for Atmospheric Research reanalysis has provided a vast wealth of globally gridded meteorological and oceanic data from 1948 to the present. In addition, other datasets have been extended back even further (to 1900), which allows for a large independent dataset. These longer-period datasets allow for an extended period of testing of the new statistical forecast scheme. A new prediction scheme has been developed on data from 1949 to 1989 and then tested on two independent datasets. One of these datasets is the 16-yr period from 1990 to 2005, and the other dataset is from 1900 to 1948. This allows for an investigation of the statistical significance over various time periods. The statistical scheme shows remarkable stability over an entire century. The combination of these four predictors explains between 45% and 60% of the variance in net tropical cyclone activity over the following separate time periods: 1900–48, 1949–89, 1949–2005, and 1900–2005. The forecast scheme also shows considerable skill as a potential predictor for giving the probabilities of United States landfall. Large differences in U.S. major hurricane landfall are also observed between forecasts that call for active seasons compared with those that call for inactive seasons.

The variation of discharge entering the Niger Delta system, 1951–2000, and estimates of change under global warming

AbstractA simple and effective statistical scheme for estimating river discharges on the River Niger has been derived using catchment rainfall data. Observed station annual average discharges at two locations were correlated with averaged annual rainfall data in recent years (1951–2000). A comparison of the estimated discharge and the observed discharge showed good agreement. The upstream station of Gaya on the border of Niger and Nigeria, with more extensive records, was used to validate the technique. A more limited discharge record at Lokoja, at the focal point of the Niger Delta, is used to reconstruct the discharge over 1951–2000. Despite the extensive dam construction on the Upper Niger during this period the main determinant of flow into the Delta has remained upstream rainfall. The relationship between the past rainfall data and river discharges at Lokoja are used to predict flows in the 2070s using rainfall values derived from various scenarios of a set of coupled climate model simulations. While there is uncertainty even over the sign of future change in west African rainfall, the river flow inferred from those models predicting increased rainfall over the coming century is at a level consistent with rapid change in channel position, as observed during the 1990s. This technique could be applied more widely to forecast future discharges from major river systems. Copyright © 2007 Royal Meteorological Society

West African Storm Tracks and Their Relationship to Atlantic Tropical Cyclones

Abstract The automatic tracking technique used by Thorncroft and Hodges has been used to identify coherent vorticity structures at 850 hPa over West Africa and the tropical Atlantic in the 40-yr ECMWF Re-Analysis. The presence of two dominant source regions, north and south of 15°N over West Africa, for storm tracks over the Atlantic was confirmed. Results show that the southern storm track provides most of the storms that reach the main development region where most tropical cyclones develop. There exists marked seasonal variability in location and intensity of the storms leaving the West African coast, which may influence the likelihood of downstream intensification and longevity. There exists considerable year-to-year variability in the number of West African storm tracks, both in numbers over the land and continuing out over the tropical Atlantic Ocean. While the low-frequency variability is well correlated with Atlantic tropical cyclone activity, West African rainfall, and SSTs, the interannual variability is found to be uncorrelated with these. In contrast, variance of the 2–6-day-filtered meridional wind, which provides a synoptic-scale measure of African easterly wave activity, shows a significant, positive correlation with tropical cyclone activity at interannual time scales.

Dynamics of the West African monsoon

A review is given of the dynamical mechanisms responsible for the monsoon circulation over West Africa. Features of the circulation are first described, including the seasonal displacement of the rain bands, the structure of the heat low over the Sahara, the meridional circulation to the south and the associated zonal jets. Simple theories for the zonal-mean meridional circulation are then presented, using the principles of angular momentum conservation, thermal wind balance and moist convective equilibrium. The application of these theories to the West African monsoon reveals a sensitivity to the low-level meridional gradient of equivalent potential temperature, which helps explain observed variability in the monsoon onset. Processes leading to east-west asymmetries in the circulation are also described, and mechanisms linking West African rainfall anomalies with remote events in the tropics are discussed. These dynamical considerations are then placed in the broader context of the ongoing AMMA research program.

Tropical Atlantic moisture availability and precipitation over West Africa: Application to DEMETER hindcasts

[1] From rainfall analysis over West Africa (rotated empirical orthogonal functions, Varimax criterion), significant leading modes are identified: i.e., along the Gulf of Guinea, the Senegal/west Mali area, and the central Sahel. Using the moisture convergence/divergence (MCD) parameter derived from ERA-40, three leading empirical orthogonal function modes are extracted. They are respectively linked to the Intertropical Convergence Zone position, the equatorial Atlantic upper-ocean thermal conditions (center and west), and significantly correlated with identified rainfall modes. Linkages between temporal variability of the above modes and that of the tropical Atlantic upper-ocean thermal are highlighted. A new statistico-dynamical probabilistic model for West African rainfall prediction is thus built using MCD as predictors diagnosed from the DEMETER hindcasts. The model takes advantage of both identified statistical linkages, and conditional probabilities between precipitation occurrence and MCD values. It is found that new probabilistic scores for precipitation predictions, are significantly improved when compared to those which were obtained directly from DEMETER precipitation forecasts.

Sea surface temperature variability in the tropical southeast Atlantic Ocean and West African rainfall

Evidence is presented of a connection between sea surface temperature (SST) variability in the tropical southeast Atlantic Ocean off Angola and boreal summer rainfall over coastal West Africa. The strongest relationships exist between April–May SST in the Angola‐Benguela Frontal Zone area, May–June latent heat fluxes in this area, and July–August rainfall over the region 5°W–5°E, 5°N–10°N. Anomalously wet summers in this region are also characterised by a weaker African Easterly Jet, a stronger Tropical Easterly Jet and, in many cases, reduced rainfall further north over the Southern Sahel.

Leading Tropical Modes Associated with Interannual and Multidecadal Fluctuations in North Atlantic Hurricane Activity

Abstract Interannual and multidecadal extremes in Atlantic hurricane activity are shown to result from a coherent and interrelated set of atmospheric and oceanic conditions associated with three leading modes of climate variability in the Tropics. All three modes are related to fluctuations in tropical convection, with two representing the leading multidecadal modes of convective rainfall variability, and one representing the leading interannual mode (ENSO). The tropical multidecadal modes are shown to link known fluctuations in Atlantic hurricane activity, West African monsoon rainfall, and Atlantic sea surface temperatures, to the Tropics-wide climate variability. These modes also capture an east–west seesaw in anomalous convection between the West African monsoon region and the Amazon basin, which helps to account for the interhemispheric symmetry of the 200-hPa streamfunction anomalies across the Atlantic Ocean and Africa, the 200-hPa divergent wind anomalies, and both the structure and spatial scale of the low-level tropical wind anomalies, associated with multidecadal extremes in Atlantic hurricane activity. While there are many similarities between the 1950–69 and 1995–2004 periods of above-normal Atlantic hurricane activity, important differences in the tropical climate are also identified, which indicates that the above-normal activity since 1995 does not reflect an exact return to conditions seen during the 1950s–60s. In particular, the period 1950–69 shows a strong link to the leading tropical multidecadal mode (TMM), whereas the 1995–2002 period is associated with a sharp increase in amplitude of the second leading tropical multidecadal mode (TMM2). These differences include a very strong West African monsoon circulation and near-average sea surface temperatures across the central tropical Atlantic during 1950–69, compared with a modestly enhanced West African monsoon and exceptionally warm Atlantic sea surface temperatures during 1995–2004. It is shown that the ENSO teleconnections and impacts on Atlantic hurricane activity can be substantially masked or accentuated by the leading multidecadal modes. This leads to the important result that these modes provide a substantially more complete view of the climate control over Atlantic hurricane activity during individual seasons than is afforded by ENSO alone. This result applies to understanding differences in the “apparent” ENSO teleconnections not only between the above- and below-normal hurricane decades, but also between the two sets of above-normal hurricane decades.

Regional dynamical downscaling over West Africa: model evaluation and comparison of wet and dry years

In this study, a 25-year regional climate model run over West Africa is evaluated and examined with respect to causes of interannual rainfall variability related to the West African Monsoon. West African rainfall has been subject to strong interannual and decadal variability throughout the past 50 years. Known driving forces for this variability are large-scale changes in Atlantic sea surface temperatures (SSTs), variability due to global atmospheric circulation changes, like for instance variability related to El Nino-Southern Oscillation, but also regional and local-scale changes in land use and vegetation cover. The interaction of these impact factors with West African synoptic and subsynoptic processes is still not completely understood. One reason for this lack of knowledge is that basic features of West African climate, including the African Easterly Jet (AEJ), African Easterly Waves (AEWs) as well as monsoon dynamics, are very complex multiscale phenomena. Climate modeling in West Africa requires the ability to simulate these effects, which can only be achieved by mesoscale atmospheric models. Using the regional climate model REMO from the Max-Planck Institute for Meteorology in Hamburg, a 25-year dynamical downscaling study was undertaken in order to evaluate a tool, which will then be used for the examination of causes of rainfall variability in West Africa. The model was used on a 0.5° grid over North Africa northward of 15°S. The model evaluation leads to some confidence in the reliability of the modeled climate. A detailed examination of composites of selected wet and dry years in the Guinean coast region elucidates the role of SST forcing and external atmospheric forcing for interannual rainfall variability. In general, abundant monsoonal rainfall comes along with warm tropical Atlantic SSTs, enhanced latent heat fluxes from the ocean to the atmosphere and stronger surface wind convergence near the Guinean Coast. This is accompanied by large-scale dynamical changes in strength and direction of both the Tropical Easterly Jet (TEJ) over the Indian Ocean and the Subtropical Jet (STJ) over the Near East and the Caucasian region.

Variabilités interannuelle et intra-saisonnière des pluies aux échelles hydrologiques. La mousson ouest-africaine en climat soudanien / Seasonal cycle and interannual variability of rainfall at hydrological scales. The West African monsoon in a Sudanese climate

West African rainfall is characterized by a strong variability, both at decadal and interannual scales. In order to quantify the hydrological impacts of such a variability, analysis of rainfall patterns at fine scales is highly essential. This diagnostic study aims to characterize the Sudanese rainfall regime at hydrological scales, using a raingauge data set collected on the upper Oueme River catchment (Benin) between 1950 and 2002. A long-term drought is observed during the 1970s and 1980s, as in the Sahel. However, the interannual variability remains significant in the Sudanese region. The study of the seasonal cycle, based on the distinction between the oceanic and continental monsoon regimes, shows that the majority of rainfall changes occur in the continental regime. On the one hand, the rainfall peak associated with this regime that has been observed for the last 50 years has occurred increasingly earlier in the season. On the other hand, the annual rainfall deficit is mainly linked to the decrease in the number of large events during the continental part of the season.

Updated 6–11-Month Prediction of Atlantic Basin Seasonal Hurricane Activity

An updated statistical scheme for forecasting seasonal tropical cyclone activity in the Atlantic basin by 1 December of the previous year is presented. Previous research by Gray and colleagues at Colorado State University showed that a statistical forecast issued on 1 December of the previous year could explain up to about 50% of the jackknife hindcast variance for the 1950–90 time period. Predictors utilized in the original forecast scheme included a forward extrapolation of the quasi-biennial oscillation (QBO) and two measures of West African rainfall. This forecast has been issued since 1991 but has shown little skill because of the as yet unexplained failure of the West African rainfall predictors during the 1990s. The updated scheme presented in this paper does not utilize West African rainfall predictors. It employs the new NCEP–NCAR reanalysis data and involves predictors that span the globe. Much experimentation has led to the choosing of conditions associated with the El Niño-Southern Oscillation (ENSO), the Arctic Oscillation (AO), the North Atlantic Oscillation (NAO), the Pacific–North American pattern (PNA), and the QBO. This new statistical scheme shows similar cross-validated (jackknifed) hindcast skill to the original scheme (explaining up to about 50% of the variance) but is developed over a decade longer time period (1950–2001). In addition, since all predictors are taken directly from the NCEP–NCAR reanalysis, one does not need to consider rainfall collection issues that have caused major problems with the West African rainfall data since 1995. Hypothetical physical linkages between the predictors and the following year's hurricane activity are also presented. Based on the net tropical cyclone (NTC) activity prediction and a weighted average of North Atlantic sea surface temperatures, forecasts of U.S. hurricane landfall probability that showed considerable hindcast skill over the 52-yr period of 1950–2001 can also be issued.

The West African dipole in rainfall and its forcing mechanisms in global and regional climate models

The leading mode of 20th century West African rainfall variability represents a well-documented drought tendency in the Sahel and the Guinea Coast region from the 1960s onward. The following three modes describe an out-of-phase relationship between the Sahel Zone (SHZ) and the Guinea Coast region (GCR) to the south, with positive rainfall anomalies in GCR being associated with even more severe drought conditions in SHZ between 1970 and 1998. This West African dipole in rainfall (WDR) has been of high relevance to migration processes in recent decades over the entire subsaharan region.
 WDR is equally revealed in long-term observational data and global climate model output. There is a clear scale-dependence of the anticorrelation, SHZ rainfall changes being decoupled from GCR ones in the low-frequency range. It is found that the high pass filtered interannual WDR fluctuations are closely tied to Atlantic sea surface temperatures (SSTs), particularly in the Gulf of Guinea. Soil moisture is not a dominant player in forcing the dipole anomalies.
 Sensitivity studies with a regional climate model support the physical link between tropical Atlantic SST and WDR. The simulated rainfall response to changing SST is non-linear and in the same order of magnitude as in the long-term observations. The SST impact accounts for up to 40% of total rainfall variability, particularly over the southernmost part of West Africa, and is statistically significant at the 1% level even with respect to the remarkable day-to-day variations. Prescribing late 21st century warmer tropical Atlantic SST as derived from global climate model experiments under increasing greenhouse gas (GHG) concentrations, leads to increasing rainfall amount in GCR (around+300mm) and a reduction in freshwater supply in SHZ (around -150mm) during the July-August main rainy season. Thus, the SHZ-GCR contrast in rainfall amount may rise in the future, inducing ongoing north-to-south migrations in whole subsaharan West Africa.

SST versus Climate Change Signals in West African Rainfall: 20th-Century Variations and Future Projections

Rainfall variability is a crucial factor in food production,water resource planning and ecosystems, especially in regions with scarce freshwaterresources. In West Africa rainfall has been subject to largedecadal and interdecadal variations during the 20th century. The most prominent feature is thereduction in rainfall amount throughout the second half of the century with somerecovery at the end. Among the conceivable mechanisms, which might inducesuch low-frequency variability in West African precipitation, this study isfocussed onsea surface temperature (SST) variations and increasing greenhouse gas (GHG)concentrations. A tool is presented to distinguish between both impacts bymeans of various climate model simulations, which are found to reproduce theobserved rainfall characteristics over West Africa reasonably well.Further, a multi-model approach is usedto evaluate the expected future greenhouse signal in West African rainfall with respect to natural variability and intermodel variations.It is found that observed SST fluctuations, forcing two different atmospheric climate models, are able to reproduce the main features ofobserved decadal rainfall anomalies in the southern part of West Africathroughout the second half of the 20th century. The seasonal response to varying SST isstrongest in summer when the region is undergoing intensive monsoondynamics. Whereas both atmospheric models simulate the observeddrought tendency,following the 1960s, there is some indication that the additional GHG forcing in one model inducessome significantly different rainfall anomalies in recent years, re-initiatingeven positive anomalies relative to the climatological mean which has alsobeen observed since the 1990s. However, thisresult is still subject to model uncertainty.Coupled climate model integrations with different climate change scenariosalsopredict that precipitation, particularly over the Guinea Coast and Sahelregion, will steadily increase into the 21st century. The model-comprehensive signal isstatistically significant with respect to natural variability and modeluncertainty, suggesting that the observed recovery of yearly rainfall overparts of West Africa might actually reflect the beginning impact of risinganthropogenic GHG. The physical mechanism, linking the radiative forcing tothe monsoonal rainfall, probably works via warming of the tropicalAtlantic Ocean.

Modelling climate change in West African Sahel rainfall (1931–90) as an artifact of changing station locations

AbstractSince the major droughts in the West African Sahel during the 1970s, it has been widely asserted that mean annual summer rainfall has declined since the late 1960s. Explanation of this persistent regional drying trend was important for famine early‐warning and global climate models. However, the network of rainfall stations changed considerably during that recent period of desiccation. Furthermore, it was difficult to reconcile the calculation of a simple mean value for a region known to have a complex spatial and temporal rainfall pattern.A simple model separated the Sahel into ‘wet’ and ‘dry’ regions. This model was inverted against mean annual summer rainfall for the Sahel between 1931 and 1990. Model predictions were found to be insensitive to initial starting conditions. The optimized parameters explained 87% of the variation in observed mean annual summer rainfall. The model predicted the mean annual rainfall in the wet ‘coastal’ and dry ‘continental’ regions of the Sahel to be 973 mm and 142 mm respectively. Consequently, the predicted long‐term mean annual summer rainfall was 558 mm, 15% greater than that of the observed long‐term mean (417 mm).The mean annual summer rainfall for the region was corrected by removing the influence of changing station locations over the study period. No persistent decline was found in mean annual summer rainfall, which suggested that the perceived drying trend was an artifact of the crude statistical aggregation of the data and historical changes in the climate station networks. The absence of a decline in rainfall questioned the validity of the hypotheses and speculations for the causes of the drying trend in the region and its effects on global climate change. It also increased the likelihood that changes over time in other regional and global climate station networks have influenced the performance and interpretation of global climate models. Copyright © 2004 Royal Meteorological Society

Mapping of Sahelian vegetation parameters from ERS scatterometer data with an evolution strategies algorithm

The West African Sahel rainfall regime is known for its spatio-temporal variability at different scales which has a strong impact on vegetation development. This study presents results of the combined use of a simple water balance model, a radiative transfer model and ERS scatterometer data to produce map of vegetation biomass and thus vegetation cover at a spatial resolution of 25 km. The backscattering coefficient measured by spaceborne wind scatterometers over Sahel shows a marked seasonality linked to the drastic changes of both soil and vegetation dielectric properties associated to the alternating dry and wet seasons. For lack of a direct observation, METEOSAT rainfall estimates are used to calculate temporal series of soil moisture with the help of a water balance model. This a priori information is used as input of the radiative transfer model that simulates the interaction between the radar wave and the surface components (soil and vegetation). Then, an inversion algorithm is applied to retrieve vegetation aerial mass from the ERS scatterometer data. Because of the nonlinear feature of the inverse problem to be solved, the inversion is performed using a global stochastic nonlinear inversion method. A good agreement is obtained between the inverse solutions and independent field measurements with mean and standard deviation of −54 and 130 kg of dry matter by hectare (kg DM/ha), respectively. The algorithm is then applied to a 350,000 km 2 area including the Malian Gourma and Seno region and a Sahelian part of Burkina Faso during two contrasted seasons (1999 and 2000). At the considered resolution, the obtained herbaceous mass maps show a global qualitative consistency ( r 2=0.71) with NDVI images acquired by the VEGETATION instrument.

Seasonal forecast of sub-sahelian rainfall using cross validated model output statistics

Rainfall over sub-Saharan West Africa is subject to large interannual variations, strongly affecting agricultural planning. This study deals with the predictability of interannual rainfall anomalies during the rainy season (June-September) over sub-Sahelian West Africa. First, regional and global near-surface predictors for sub-Sahelian precipitation are derived from multi-model ensemble output. Second, a cross validated stepwise multiple regression model is developed, choosing physically motivated predictors. This statistical model is used as Model Output Statistics system (MOS) in order to forecast the rainy season. The forecast skill of the MOS system is found to be small but significantly larger than zero. A low-cost method for operational seasonal forecast is proposed. Sub-Saharan rainy seasonal precipitation (RSP) is closely connected with tropical Atlantic sea surface temperature (SST). However, there is also a significant teleconnection to El Nino and La Nina events in the eastern equatorial Pacific. In addition, RSP reveals a strong negative correlation with sea level pressure (SLP) over the tropical Atlantic and the West African subcontinent, indicating the West African monsoonal strength. Positive correlation occurs with the East Pacific SLP field, which is linked to the SST anomalies underneath and indicative of the coupled ocean-atmosphere El Nino-Southern Oscillation (ENSO) mode. The impact of extratropical variability or the Indian summer monsoon dynamics is minor with respect to West African rainfall. Applying stepwise multiple regression to simulated RSP, it is shown that the most (physically) relevant predictors in terms of SST and SLP are found in the Atlantic sector, accounting for 46% of interannual variations if the method is based on a multi-model ensemble. The most important predictors for observed precipitation are represented by the SLP variables, regional mean and gradient, rather than the simulated rainfall itself. Compared with the null-hypothesis based on randomized predictor chronology, the multi-model predictability is statistically significant. The validation reveals a significant forecast skill, especially with respect to small rainfall anomalies. Given the distinct autocorrelation of tropical SST, this study holds the prospect of operational seasonal forecast over a broad sub-Sahelian region with low-cost methods.