Amadou Thierno GAYE | Ecole Supérieure Polytechnique (original) (raw)

Papers by Amadou Thierno GAYE

Atmosphere

The tropical Atlantic Warm Pool is one of the main drivers of the marine intertropical convergenc... more The tropical Atlantic Warm Pool is one of the main drivers of the marine intertropical convergence zone and the associated coastal Northeast Brazilian and West-African monsoons. Its meridional displacement is driven by the solar cycle, modulated by the atmosphere and ocean interactions, whose nature and respective proportions are still poorly understood. This paper presents a climatological study of the upper ocean and lower atmosphere contributions to the warm pool seasonal migration, using an Ocean General Circulation Model (OGCM). First, we provide quantitative, albeit simple, pieces of evidence on how the large amplitude of migration in the west, compared to the east, is mainly due to the strong east–west contrast of the background meridional SST gradient intensities, which is maintained by equatorial and eastern tropical upwellings. Our main results consist first in identifying a diagnostic equation for the migration speed of the two meridional boundary isotherms of the Warm Po...

The American Journal of Tropical Medicine and Hygiene

Open Journal of Air Pollution

The Westward transport of mineral dust from the North Africa continent to Atlantic Ocean can prod... more The Westward transport of mineral dust from the North Africa continent to Atlantic Ocean can produce poor air quality, low visibilities, and negatively impacting respiratory and cardiac health due to the optical and physical properties of aerosols. The dynamical impact of the sea-breeze on the dust vertical distribution in West Africa remains unknown. To investigate this issue, we have used in-situ measurements from lidar. We have focused on the attenuated backscatter of aerosols to study the effect of the local circulation on the vertical profile of mineral dust at land-sea transition. The results highlight a strong diurnal cycle of mineral dust associated with the nocturnal low-level jet (NLLJ). The jet is located between 500 m and 1000 m and crucially affected by the dynamic of the sea-breeze circulation.

One Earth

The health of the ocean, central to human well-being, has now reached a critical point. Most fish... more The health of the ocean, central to human well-being, has now reached a critical point. Most fish stocks are overexploited, climate change and increased dissolved carbon dioxide are changing ocean chemistry and disrupting species throughout food webs, and the fundamental capacity of the ocean to regulate the climate has been altered. However, key technical, organizational, and conceptual scientific barriers have prevented the identification of policy levers for sustainability and transformative action. Here, we recommend key strategies to address these challenges, including (1) stronger integration of sciences and (2) ocean-observing systems, (3) improved science-policy interfaces, (4) new partnerships supported by (5) a new ocean-climate finance system, and (6) improved ocean literacy and education to modify social norms and behaviors. Adopting these strategies could help establish ocean science as a key foundation of broader sustainability transformations.

We examine the role of zonal Ekman transport along the coast of Senegal on 30 August, 2015 when t... more We examine the role of zonal Ekman transport along the coast of Senegal on 30 August, 2015 when the tropical disturbance associated with Tropical Cyclone Fred was located to the west of Senegal causing considerable coastal damage to coastal areas south of Dakar, Senegal. Ten-meter winds from three Weather Research and Forecast model simulations were used to estimate zonal Ekman transport, with the largest values found during the 30 August. The simulations are in agreement with limited coastal observations showing increasing southerly wind speeds during 30 August but are overestimated relative to the 3 coastal stations. The strong meridional winds translate into increased zonal Ekman transport to the coast of Senegal on 30 August. The use of a coupled ocean model will improve the estimates of Ekman transport along the Guinea-Senegalese coast. The observed damage suggests that artificial and natural barriers (mangroves) should be strengthened to protect coastal communities in Senegal.

Journal of Geophysical Research: Atmospheres

Sécheresse

Comparison of mean climate and rainfall interannual variability simulated from regional climate m... more Comparison of mean climate and rainfall interannual variability simulated from regional climate models over the Sahel The purpose of this study is to evaluate the performance of a set of regional climate models (RCMs) in simulating the mean climatology and the interannual variability of rainfall over the Sahel. We analyzed and compared RCMs outputs within the framework of the ENSEMBLES project. The RCMs are in the same horizontal resolution and are driven by the same lateral boundary condition, the ERA-Interim, which is the latest version of the European Centre for Medium Range Weather Forecasts (ECMWF) reanalysis. The analysis focuses on the period of the monsoon peak (June-July-August [JJA]) over the Sahel. Temperature and precipitation distributions are reasonably well simulated although a few cold and dry biases are present around the Guinean coast and regions of steep topography. All RCMs, except HIRHAM, replicate reasonably well the interannual rainfall anomalies over the Sahel in both sign and magnitude.

Natural Hazards and Earth System Sciences Discussions

In this study, wet and dry spells over Senegal provided by four datasets based on satellite data ... more In this study, wet and dry spells over Senegal provided by four datasets based on satellite data (TRMM-3B42 V7, TAMSAT V3, CMORPH V1.0, CHIRPS V2.0), two fully based on (re)analyses (NCEP-CFSR, ERA5) and one was fully based on gauge observations (CPC Unified V1.0/RT) are compared with respect to observation datasets derived from 65 rain gauge network. All datasets were converted to the same temporal and spatial scales with 0.25 • × 0.25 • as resolution. Ordinary kriging (OK) and block kriging (BK) were used for the spatial interpolation of the gauge data. Despite a spatial coherence of the seasonal rainfall accumulation between all products, more variability with intra-seasonal features are shown in this paper. The seasonal cycle of dry days shows that TRMM, CPC, ERA5, NCEP and OK record more dry days (from 45% to 55% of dry days in August) while TAMSAT, CHIRPS, CMORPH and BK record less dry day (from 40% to 30% of dry days in August). All datasets highlighted an agreement that dry spell indicator underscore often false start and early cessation of the rainy Season in Senegal. Although, it can rarely occurs during intensification of West African monsoon (August-September). The most contrast is found on the detection of wet indicators intensity. Wet spell (defined as period with precipitation higher than a certain percentile of historical precipitation) are more severe in OK and TRMM than in other datasets. However, a great similarity is shown on their temporal frequencies. An increase of extreme events is one of the main phenomena accompanying the rainfall recovery in Sahel (Alhassane et al.,

This study analyses uncertainties associated with the annual cycle of West African rainfall chara... more This study analyses uncertainties associated with the annual cycle of West African rainfall characteristics in 15 simulations of the Coupled Model Intercomparison Project phase 5 (CMIP5) over the Sahel and Guinean regions. Indices based on daily rainfall such as the frequency and the ntensity of wet days, the consecutive dry days (CDD) and wet days (CWD), the 95th percentile of daily rainfall (R95) and its contribution to the umulative monsoon rainfall (R95PTOT) have been assessed. Over both regions, TRMM, GPCP and CHIRPS observational datasets provide very consistent results on the annual cycle of precipitation but less so on the frequency of wet days. Conversely, higher uncertainties are noted on the intensity of wet days over both study areas, particularly over the Guinean region. Overall, CMIP5 simulations present much higher uncertainties in the representation of the mean precipitation climatology, often provide too early (late) onset dates over the Sahel (the Guinean region) a...

Sustainability

Land Use Land-Cover Change (LULCC), such as deforestation, affects the climate system and land-at... more Land Use Land-Cover Change (LULCC), such as deforestation, affects the climate system and land-atmosphere interactions. Using simulations carried out within the LUCID (Land Use and Climate, IDentification of robust Impacts) project framework, we first quantify the role of historical land-cover change induced by human activities on surface climate in West Africa. Focusing on two contrasted African regions, we find that climate responses of land-use changes are small but they are still statistically significant. In Western Sahel, a statistically significant near-surface atmospheric cooling and a decrease in water recycling are simulated in summer in response to LULCC. Over the Guinean zone, models simulate a significant decrease in precipitation and water recycling in autumn in response to LULCC. This signal is comparable in magnitude with the effect induced by the increase in greenhouse gases. Simulated climate changes due to historical LULCC could however be underestimated because: (i) the prescribed LULCC can be underestimated in those regions; (ii) the climate models underestimate the coupling strength between West African surface climate and leaf area index (LAI) and (iii) the lack of interactive LAI in some models. Finally, our study reveals indirect atmospheric processes triggered by LULCC. Over the Western Sahel, models reveal that a significant decrease in solar reflection tends to cool down the surface and thus counteracts the atmospheric feedback. Conversely, over the Guinea zone, models reveal that the indirect atmospheric processes and turbulent heat fluxes dominate the climatic responses over the direct effects of LULCC.

Climate Dynamics

Upwelling processes bring nutrient-rich waters from the deep ocean to the surface. Areas of upwel... more Upwelling processes bring nutrient-rich waters from the deep ocean to the surface. Areas of upwelling are often associated with high productivity, offering great economic value in terms of fisheries. The sensitivity of spring/summer-time coastal upwelling systems to climate change has recently received a lot of attention. Several studies have suggested that their intensity may increase in the future while other authors have shown decreasing intensity in their equatorward portions. Yet, recent observations do not show robust evidence of this intensification. The senegalo-mauritanian upwelling system (SMUS) located at the southern edge of the north Atlantic system (12°N-20°N) and most active in winter/spring has been largely excluded from these studies. Here, the seasonal cycle of the SMUS and its response to climate change is investigated in the database of the Coupled Models Inter comparison Project Phase 5 (CMIP5). Upwelling magnitude and surface signature are characterized by several sea surface temperature and wind stress indices. We highlight the ability of the climate models to reproduce the system, as well as their biases. The simulations suggest that the intensity of the SMUS winter/spring upwelling will moderately decrease in the future, primarily because of a reduction of the wind forcing linked to a northward shift of Azores anticyclone and a more regional modulation of the low pressures found over Northwest Africa. The implications of such an upwelling reduction on the ecosystems and local communities exploiting them remains very uncertain.

Theoretical and Applied Climatology

This study focuses on the ability of the global land data assimilation system LDAS-Monde to impro... more This study focuses on the ability of the global land data assimilation system LDAS-Monde to improve the representation of land surface variables (LSVs) over Burkina Faso through the joint assimilation of satellite derived Surface Soil Moisture (SSM) and Leaf Area Index (LAI) from January 2001 to June 2018. The LDAS-Monde offline system is forced by the latest European Centre for Medium-Range Weather Forecasts (ECMWF) atmospheric reanalysis ERA5, leading to a 0.25° x 0.25° spatial resolution reanalysis of the LSVs. Within LDAS-Monde, SSM and LAI observations from the Copernicus Global Land Service (CGLS) are assimilated using the CO2-responsive version of the ISBA (Interactions between Soil, Biosphere and Atmosphere) land surface model (LSM). First, it is shown that ERA5 better represents precipitation and incoming solar radiation than ERA-Interim former reanalysis from ECMWF. Results of two experiments are compared: open-loop simulation (i.e. no assimilation) and analysis (i...

Fisheries Oceanography

We investigate Sardinella aurita and Sardinella maderensis recruitment success relative to the va... more We investigate Sardinella aurita and Sardinella maderensis recruitment success relative to the variability of oceanographic conditions in Senegalese waters using generalized additive models (GAM). Results show that recruitment of both species is marked by a strong intra-annual (seasonal) variation with minimum and maximum in winter and summer, respectively. Their interannual variations are synchronous until 2006 (recruitment decreasing), while from 2007 there is no synchrony. The model developed shows that sardinella recruitment variability is closely related to the tested environmental variables in the study area. However, the key environmental variables influencing the recruitment success are different for both species: the Coastal Upwelling Index and the sea surface temperature for S. aurita and S. maderensis, respectively. We report that recruitment success of S. aurita and S. maderensis are associated with distinct ranges of sea surface temperature, upwelling intensity, wind-induced turbulence, concentration of chlorophyll-a and north Atlantic oscillation index. Considering food security and socioeconomic importance of both stocks, we recommend that consideration is given to the environmental variability in the small pelagic fish national management plans, particularly in the context of climate change.

Modern Environmental Science and Engineering

In this study, daily streamflow data of 35 years records and water quality data were investigated... more In this study, daily streamflow data of 35 years records and water quality data were investigated to determine Lake Guiers hydrologic alteration within its ecosystem. The Range of Variability Approach (RVA) was used to quantify the modification of the Indicators of Hydrologic Alteration (IHA) and the trophic State Indices (TSIs) was calculated to characterize the eutrophication level. Results show that the flow pattern at Lake Guiers is reversed from the pre-impact with high alteration on most of indicators. These changes on flow have greater impacts on the lake ecological functioning and on water quality. The trophic State Indices (TSIs) of Nitrogen (N) and Phosphorus (P) indicate respectively a eutrophic and hypereutrophic state. Lake Guiers hydrological alteration endangers its ecological integrity. Its restoration requires more than ever reconciling economic needs and ecological requirement.

Atmospheric Chemistry and Physics

The impact of desert aerosols on climate, atmospheric processes, and the environment is still deb... more The impact of desert aerosols on climate, atmospheric processes, and the environment is still debated in the scientific community. The extent of their influence remains to be determined and particularly requires a better understanding of the variability of their distribution. In this work, we studied the variability of these aerosols in western Africa using different types of satellite observations. SeaWiFS (Sea-Viewing Wide Field-of-View Sensor) and OMI (Ozone Monitoring Instrument) data have been used to characterize the spatial distribution of mineral aerosols from their optical and physical properties over the period 2005–2010. In particular, we focused on the variability of the transition between continental western African and the eastern Atlantic Ocean. Data provided by the lidar scrolling CALIOP (Cloud-Aerosol Lidar with Orthogonal Polarization) onboard the satellite CALIPSO (Cloud Aerosol Lidar and Infrared Pathfinder Satellite Observations) for the period 2007–2013 were th...

La Météorologie

Cette étude vise à utiliser le code de transfert radiatif Streamer pour l'évaluation de l'impact ... more Cette étude vise à utiliser le code de transfert radiatif Streamer pour l'évaluation de l'impact des aérosols sur le cycle diurne du rayonnement solaire, afin d'estimer le potentiel d'énergie solaire pour un meilleur dimensionnement des modules solaires à Mbour (16,95°W-14,40°E) au Sénégal. Avec le manque de données observées qui caractérise cette partie du monde, l'idée de cet article était de valider les potentiels direct et diffus avec les mesures disponibles à Mbour, afin de pouvoir utiliser ce modèle dans d'autres études de cas dans cette région. Les mesures in situ (fluxmètre et photomètre) ont été utilisées pour, d'une part, caractériser l'aérosol et, d'autre part, valider les simulations du code Streamer. Cette étude a montré que les aérosols réduisent le potentiel d'énergie solaire de près de 80 % pour les journées très polluées en aérosols. La définition de modèles d'aérosols spécifiques, basés sur les observations AERONET, a permis d'améliorer la simulation du rayonnement solaire de près de 15 %.

International Journal of Environmental Research and Public Health

The analysis of the spatial and temporal variability of climate parameters is crucial to study th... more The analysis of the spatial and temporal variability of climate parameters is crucial to study the impact of climate-sensitive vector-borne diseases such as malaria. The use of malaria models is an alternative way of producing potential malaria historical data for Senegal due to the lack of reliable observations for malaria outbreaks over a long time period. Consequently, here we use the Liverpool Malaria Model (LMM), driven by different climatic datasets, in order to study and validate simulated malaria parameters over Senegal. The findings confirm that the risk of malaria transmission is mainly linked to climate variables such as rainfall and temperature as well as specific landscape characteristics. For the whole of Senegal, a lag of two months is generally observed between the peak of rainfall in August and the maximum number of reported malaria cases in October. The malaria transmission season usually takes place from September to November, corresponding to the second peak of temperature occurring in October. Observed malaria data from the Programme National de Lutte contre le Paludisme (PNLP, National Malaria control Programme in Senegal) and outputs from the meteorological data used in this study were compared. The malaria model outputs present some consistencies with observed malaria dynamics over Senegal, and further allow the exploration of simulations performed with reanalysis data sets over a longer time period. The simulated malaria risk significantly decreased during the 1970s and