scholarly journals Understanding the variability of the rainfall dipole in West Africa using the EC-Earth last millennium simulation

2021 ◽  
Author(s):  
Qiong Zhang ◽  
Ellen Berntell ◽  
Qiang Li ◽  
Fredrik Charpentier Ljungqvist
Keyword(s):  
West Africa ◽  
Model Simulation ◽  
Rainfall Variability ◽  
Low Pressure ◽  
Last Millennium ◽  
Tropical Atlantic ◽  
Gulf Of Guinea ◽  
Dipole Pattern ◽  
Sahel Region ◽  
Heat Low

AbstractThere is a well-known mode of rainfall variability associating opposite hydrological conditions over the Sahel region and the Gulf of Guinea, forming a dipole pattern. Previous meteorological observations show that the dipole pattern varies at interannual timescales. Using an EC-Earth climate model simulation for last millennium (850–1850 CE), we investigate the rainfall variability in West Africa over longer timescales. The 1000-year-long simulation data show that this rainfall dipole presents at decadal to multidecadal and centennial variability and long-term trend. Using the singular value decomposition (SVD) analysis, we identified that the rainfall dipole present in the first SVD mode with 60% explained variance and associated with the variabilities in tropical Atlantic sea surface temperature (SST). The second SVD mode shows a monopole rainfall variability pattern centred over the Sahel, associated with the extra-tropical Atlantic SST variability. We conclude that the rainfall dipole-like pattern is a natural variability mode originated from the local ocean–atmosphere-land coupling in the tropical Atlantic basin. The warm SST anomalies in the equatorial Atlantic Ocean favour an anomalous low pressure at the tropics. This low pressure weakens the meridional pressure gradient between the Saharan Heat Low and the tropical Atlantic. It leads to anomalous northeasterly, reduces the southwesterly moisture flux into the Sahel and confines the Gulf of Guinea's moisture convergence. The influence from extra-tropical climate variability, such as Atlantic multidecadal oscillation, tends to modify the rainfall dipole pattern to a monopole pattern from the Gulf of Guinea to Sahara through influencing the Sahara heat low. External forcing—such as orbital forcing, solar radiation, volcanic and land-use—can amplify/dampen the dipole mode through thermal forcing and atmosphere dynamical feedback.

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

2006 ◽  
Vol 33 (21) ◽  
Author(s):  
Philippe Rogel ◽  
Yves M. Tourre ◽  
Vincent Benoit ◽  
Lionel Jarlan
Keyword(s):  
West Africa ◽  
Tropical Atlantic ◽  
Moisture Availability

scholarly journals Simulated Dust Aerosol Impacts on Western Sahelian Rainfall: Importance of Ocean Coupling

2018 ◽  
Vol 31 (22) ◽  
pp. 9107-9124 ◽  
Author(s):  
Asha K. Jordan ◽  
Anand Gnanadesikan ◽  
Benjamin Zaitchik
Keyword(s):  
Atlantic Ocean ◽  
North Africa ◽  
Negative Impact ◽  
Positive Impact ◽  
Geophysical Fluid Dynamics Laboratory ◽  
Tropical Atlantic ◽  
Tropical Atlantic Ocean ◽  
Ocean Atmosphere ◽  
Sahel Region ◽  
Dust Precipitation

North Africa is the world’s largest source of mineral dust, and this dust has potentially significant impacts on precipitation. Yet there is no consensus in published studies regarding the sign or magnitude of dust impacts on rainfall in either the highly climate-sensitive Sahel region of North Africa or the neighboring tropical Atlantic Ocean. Here the Geophysical Fluid Dynamics Laboratory (GFDL) Climate Model 2 (GFDL CM2.0) with Modular Ocean Model, version 4.1 (MOM4.1), run at coarse resolution (CM2Mc) is applied to investigate one poorly characterized aspect of dust–precipitation dynamics: the importance of sea surface temperature (SST) changes in mediating the atmospheric response to dust. Two model experiments were performed: one comparing Dust-On to Dust-Off simulations in the absence of ocean–atmosphere coupling, and the second comparing Dust-On to Dust-Off with the ocean fully coupled. Results indicate that SST changes in the coupled experiment reduce the magnitude of dust impacts on Sahel rainfall and flip the sign of the precipitation response over the nearby ocean. Over the Sahel, CM2Mc simulates a net positive impact of dust on monsoon season rainfall, but ocean–atmosphere coupling in the presence of dust decreases the inflow of water vapor, reducing the amount by which dust enhances rainfall. Over the tropical Atlantic Ocean, dust leads to SST cooling in the coupled experiment, resulting in increased static stability that overrides the warming-induced increase in convection observed in the uncoupled experiment and yields a net negative impact of dust on precipitation. These model results highlight the potential importance of SST changes in dust–precipitation dynamics in North Africa and neighboring regions.

scholarly journals Impact of intraseasonal wind bursts on sea surface temperature variability in the far eastern tropical Atlantic Ocean during boreal spring 2005 and 2006: focus on the mid-May 2005 event

Ocean Science ◽  
2018 ◽  
Vol 14 (4) ◽  
pp. 849-869 ◽  
Author(s):  
Gaëlle Herbert ◽  
Bernard Bourlès
Keyword(s):  
Atlantic Ocean ◽  
Surface Temperature ◽  
Kelvin Waves ◽  
Sea Surface ◽  
Tropical Atlantic ◽  
Gulf Of Guinea ◽  
Atmospheric Conditions ◽  
Tropical Atlantic Ocean ◽  
Boreal Spring ◽  
Wind Bursts

Abstract. The impact of boreal spring intraseasonal wind bursts on sea surface temperature variability in the eastern tropical Atlantic Ocean in 2005 and 2006 is investigated using numerical simulation and observations. We especially focus on the coastal region east of 5° E and between the Equator and 7° S that has not been studied in detail so far. For both years, the southerly wind anomalies induced cooling episodes through (i) upwelling processes, (ii) vertical mixing due to the vertical shear of the current, and for some particular events (iii) a decrease in incoming surface shortwave radiation. The strength of the cooling episodes was modulated by subsurface conditions affected by the arrival of Kelvin waves from the west influencing the depth of the thermocline. Once impinging the eastern boundary, the Kelvin waves excited westward-propagating Rossby waves, which combined with the effect of enhanced westward surface currents contributed to the westward extension of the cold water. A particularly strong wind event occurred in mid-May 2005 and caused an anomalous strong cooling off Cape Lopez and in the whole eastern tropical Atlantic Ocean. From the analysis of oceanic and atmospheric conditions during this particular event, it appears that anomalously strong boreal spring wind strengthening associated with anomalously strong Hadley cell activity prematurely triggered the onset of coastal rainfall in the northern Gulf of Guinea, making it the earliest over the 1998–2008 period. No similar atmospheric conditions were observed in May over the 1998–2008 period. It is also found that the anomalous oceanic and atmospheric conditions associated with the event exerted a strong influence on rainfall off northeast Brazil. This study highlights the different processes through which the wind power from the South Atlantic is brought to the ocean in the Gulf of Guinea and emphasizes the need to further document and monitor the South Atlantic region.

scholarly journals Seasonal forecasts of the Saharan heat low characteristics: a multi-model assessment

2021 ◽  
Vol 2 (3) ◽  
pp. 893-912
Author(s):  
Cedric G. Ngoungue Langue ◽  
Christophe Lavaysse ◽  
Mathieu Vrac ◽  
Philippe Peyrillé ◽  
Cyrille Flamant
Keyword(s):  
West African Monsoon ◽  
Seasonal Forecast ◽  
Lead Times ◽  
Model Assessment ◽  
Seasonal Forecasts ◽  
Weather Forecasts ◽  
Modes Of Variability ◽  
Forecast Models ◽  
Sahel Region ◽  
Heat Low

Abstract. The Saharan heat low (SHL) is a key component of the West African Monsoon system at the synoptic scale and a driver of summertime precipitation over the Sahel region. Therefore, accurate seasonal precipitation forecasts rely in part on a proper representation of the SHL characteristics in seasonal forecast models. This is investigated using the latest versions of two seasonal forecast systems namely the SEAS5 and MF7 systems from the European Center of Medium-Range Weather Forecasts (ECMWF) and Météo-France respectively. The SHL characteristics in the seasonal forecast models are assessed based on a comparison with the fifth ECMWF Reanalysis (ERA5) for the period 1993–2016. The analysis of the modes of variability shows that the seasonal forecast models have issues with the timing and the intensity of the SHL pulsations when compared to ERA5. SEAS5 and MF7 show a cool bias centered on the Sahara and a warm bias located in the eastern part of the Sahara respectively. Both models tend to underestimate the interannual variability in the SHL. Large discrepancies are found in the representation of extremes SHL events in the seasonal forecast models. These results are not linked to our choice of ERA5 as a reference, for we show robust coherence and high correlation between ERA5 and the Modern-Era Retrospective analysis for Research and Applications (MERRA). The use of statistical bias correction methods significantly reduces the bias in the seasonal forecast models and improves the yearly distribution of the SHL and the forecast scores. The results highlight the capacity of the models to represent the intraseasonal pulsations (the so-called east–west phases) of the SHL. We notice an overestimation of the occurrence of the SHL east phases in the models (SEAS5, MF7), while the SHL west phases are much better represented in MF7. In spite of an improvement in prediction score, the SHL-related forecast skills of the seasonal forecast models remain weak for specific variations for lead times beyond 1 month, requiring some adaptations. Moreover, the models show predictive skills at an intraseasonal timescale for shorter lead times.

The ENSO Teleconnection to the Tropical Atlantic Ocean: Contributions of the Remote and Local SSTs to Rainfall Variability in the Tropical Americas*

2001 ◽  
Vol 14 (24) ◽  
pp. 4530-4544 ◽  
Author(s):  
Alessandra Giannini ◽  
John C. H. Chiang ◽  
Mark A. Cane ◽  
Yochanan Kushnir ◽  
Richard Seager
Keyword(s):  
Atlantic Ocean ◽  
Rainfall Variability ◽  
Tropical Atlantic ◽  
Tropical Atlantic Ocean

scholarly journals Eastern Tropical Atlantic Mixed Layer Depth: Assessment of Methods from In Situ Profiles in the Gulf of Guinea from Coastal to High Sea

2019 ◽  
Vol 36 (1) ◽  
pp. 201-212
Author(s):  
Benjamin Kouadio N’Guessan ◽  
Aka Marcel Kouassi ◽  
Albert Trokourey ◽  
Elisée Toualy ◽  
Desiré Kouamé Kanga ◽  
...  
Keyword(s):  
Mixed Layer ◽  
Mixed Layer Depth ◽  
Tropical Atlantic ◽  
Gulf Of Guinea ◽  
Layer Depth

scholarly journals Local air pollution from oil rig emissions observed during the airborne DACCIWA campaign

2019 ◽  
Vol 19 (17) ◽  
pp. 11401-11411 ◽  
Author(s):  
Vanessa Brocchi ◽  
Gisèle Krysztofiak ◽  
Adrien Deroubaix ◽  
Greta Stratmann ◽  
Daniel Sauer ◽  
...  
Keyword(s):  
West Africa ◽  
Marine Boundary Layer ◽  
Dispersion Model ◽  
Particle Dispersion ◽  
Gulf Of Guinea ◽  
Aerosol Chemistry ◽  
Oil Platforms ◽  
Sensitivity Tests ◽  
Local Air Pollution ◽  
Oil Rig

Abstract. In the framework of the European DACCIWA (Dynamics–Aerosol–Chemistry–Cloud Interactions in West Africa) project, the airborne study APSOWA (Atmospheric Pollution from Shipping and Oil platforms of West Africa) was conducted in July 2016 to study oil rig emissions off the Gulf of Guinea. Two flights in the marine boundary layer were focused on the floating production storage and offloading (FPSO) vessel operating off the coast of Ghana. Those flights present simultaneous sudden increases in NO2 and aerosol concentrations. Unlike what can be found in flaring emission inventories, no increase in SO2 was detected, and an increase in CO is observed only during one of the two flights. Using FLEXPART (FLEXible PARTicle dispersion model) simulations with a regional NO2 satellite flaring inventory in forward-trajectory mode, our study reproduces the timing of the aircraft NO2 enhancements. Several sensitivity tests on the flux and the injection height are also performed, leading to the conclusion that a lower NO2 flux helps in better reproducing the measurements and that the modification of the injection height does not impact the results of the simulations significantly.

scholarly journals Identification of Potential Drought Areas in West Africa Under Climate Change and Variability

2019 ◽  
Vol 3 (3) ◽  
pp. 429-444 ◽  
Author(s):  
Gandome Mayeul L. D. Quenum ◽  
Nana A. B. Klutse ◽  
Diarra Dieng ◽  
Patrick Laux ◽  
Joël Arnault ◽  
...  
Keyword(s):  
West Africa ◽  
Climate Models ◽  
Potential Evapotranspiration ◽  
Standardized Precipitation Index ◽  
Global Climate Models ◽  
Spatiotemporal Pattern ◽  
Historical Period ◽  
Gulf Of Guinea ◽  
Precipitation Concentration ◽  
The Impact

Abstract The study investigates how the rising global temperature will affect the spatial pattern of rainfall and consequently drought in West Africa. The precipitation and potential evapotranspiration variables that are obtained from the Rossby Centre regional atmospheric model (RCA4) and driven by ten (10) global climate models under the RCP8.5 scenario were used. The model data were obtained from the Coordinated Regional Climate Downscaling Experiment (CORDEX) and analyzed at four specific global warming levels (GWLs) (i.e., 1.5 °C, 2.0 °C, 2.5 °C, and 3.0 °C) above the pre-industrial level. This study utilized four (4) indices: the standardized precipitation index, the precipitation concentration index, the precipitation concentration degree, and the precipitation concentration period over West Africa to explore the spatiotemporal variations in the characteristics of precipitation concentrations. Additionally, studying the impact of the four GWLs on consecutive dry days, consecutive wet days, and frequency of the intense rainfall events led to a better understanding of the spatiotemporal pattern of extreme precipitation. The results show that, at each GWL studied, the onset of rainfall comes 1 month earlier in the Gulf of Guinea compared to the historical period (1971–2000) with increasing rainfall intensity in the whole study domain, and the northeastern part of the study area becomes wetter. The rainfall concentration is uniformly distributed over the Gulf of Guinea and the Savanna zone for both the historical period and RCP8.5 scenario, while the Sahel zone which has shown an irregular concentration of rainfall for the historical period shows a uniform concentration of rainfall under all four GWLs.

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