scholarly journals Wave Activity on the Tropical Easterly Jet

2007 ◽  
Vol 64 (7) ◽  
pp. 2756-2763 ◽  
Author(s):  
S. E. Nicholson ◽  
A. I. Barcilon ◽  
M. Challa ◽  
J. Baum
Keyword(s):  
West Africa ◽  
Asian Monsoon ◽  
Model Simulation ◽  
Reanalysis Data ◽  
West African ◽  
Planetary Scale ◽  
African Easterly Jet ◽  
Wave Disturbances ◽  
Tropical Easterly Jet ◽  
Wave Development

Abstract This article examines the question of the existence of waves on the tropical easterly jet (TEJ) over West Africa. The TEJ is a well-known feature of the Asian monsoon and waves on the jet have been implicated in various weather phenomena. Its role in West African meteorology has received little attention. A model simulation of wet and dry years over West Africa predicted wave development on the TEJ. NCEP reanalysis data confirmed the existence of these waves, using case studies in the dry year 1983 and the wet year 1950. Both the simulated and observed waves are of planetary scale, with a period of 5–6 days. Potential vorticity (PV) theory suggests that they develop via interactions between the surface and the TEJ. Overall, the results suggest that interactions between the TEJ and African easterly jet play an important role in the development of wave disturbances over West Africa.

scholarly journals An Analysis of West African Dynamics Using a Linearized GCM*

2008 ◽  
Vol 65 (4) ◽  
pp. 1182-1203 ◽  
Author(s):  
S. E. Nicholson ◽  
A. I. Barcilon ◽  
M. Challa
Keyword(s):  
Interannual Variability ◽  
Spherical Model ◽  
Planetary Scale ◽  
Wave Disturbances ◽  
Easterly Wave ◽  
Key Factor ◽  
Wave Development ◽  
Basic States ◽  
Vertical Development ◽  
The Waves

Abstract This study utilizes a linear, primitive equation spherical model to study the development and propagation of easterly wave disturbances over West Africa. Perturbations are started from an initial disturbance consisting of a barotropic vortex and the governing equations are integrated forward in time. The perturbations are introduced into basic states corresponding to the observed dynamical and thermodynamical characteristics of two wet years in the Sahel and two dry years. The model simulations show consistent contrasts in wave activity between the wet and dry years. The waves are markedly stronger in the wet years and show a barotropic structure throughout the troposphere. The waves tend to extend throughout the troposphere to the level of the tropical easterly jet (TEJ) in the wet years, but not in the dry years. The upper-tropospheric shear, which is stronger in wet years, appears to be a key factor in wave development. This shear is dependent on the intensity of the TEJ, suggesting that the TEJ is an important factor in interannual variability in the Sahel. When the overall shear is weak, vertical development is suppressed. Another contrast is that in the dry years the growth rates show a single maximum around 3000–4000 km, but in the wet years there is a second, around 6000–7000 km. This suggests that both synoptic-scale and planetary-scale waves are active in the rainy season of some wet years. Imposing considerations of potential vorticity, the generation of planetary-scale waves implies a strong link between the surface and the TEJ in wet years. Such a link is absent in the dry years. This is likely a major factor in the interannual variability of rainfall in the Sahel.

scholarly journals Assessing the Capabilities of Three Regional Climate Models over CORDEX Africa in Simulating West African Summer Monsoon Precipitation

2015 ◽  
Vol 2015 ◽  
pp. 1-13 ◽  
Author(s):  
A. A. Akinsanola ◽  
K. O. Ogunjobi ◽  
I. E. Gbode ◽  
V. O. Ajayi
Keyword(s):  
West Africa ◽  
Summer Monsoon ◽  
Climate Models ◽  
Regional Climate ◽  
Regional Climate Models ◽  
West African ◽  
Annual Rainfall ◽  
Climate Projections ◽  
Wind Fields ◽  
African Easterly Jet

This study evaluates the ability of three Regional Climate Models (RCMs) used in Coordinated Regional Climate Downscaling Experiment (CORDEX) to simulate the characteristics of rainfall pattern during the West Africa Summer Monsoon from 1998 to 2008. The seasonal climatology, annual rainfall cycles, and wind fields of the RCMs output were assessed over three homogenous subregions and validated using precipitation data from eighty-one (81) ground observation stations and TRMM satellite data. Furthermore, the ability of the RCMs to simulate response to El Nino and La Nina events was assessed. Results show that two of the RCMs (RCA and REMO) simulated the main features of the rainfall climatology and associated dynamics over the three subregions (Guinea Coast, Savannah, and Sahel) of West Africa. The RCMs also capture the African Easterly Jet (AEJ) and Tropical Easterly Jet (TEJ) with little variations in position and intensity. Analysis shows significant biases in individual models depending on subregion and season under consideration which may be attributed to strong cyclonic circulation observed at 850 mb pressure level. In general, the study shows RCA and REMO fairly simulate West Africa rainfall adequately and can therefore be used for the assessment of West African Summer Monsoon and future climate projections.

scholarly journals Black Carbon and West African Monsoon precipitation: observations and simulations

2009 ◽  
Vol 27 (11) ◽  
pp. 4171-4181 ◽  
Author(s):  
J. Huang ◽  
A. Adams ◽  
C. Wang ◽  
C. Zhang
Keyword(s):  
Black Carbon ◽  
Radiative Forcing ◽  
Large Scale ◽  
Model Simulation ◽  
West African Monsoon ◽  
Cloud Amount ◽  
Reanalysis Data ◽  
West African ◽  
African Monsoon ◽  
Surface Precipitation

Abstract. We have recently investigated large-scale co-variability between aerosol and precipitation and other meteorological variables in the West African Monsoon (WAM) region using long term satellite observations and reanalysis data. In this study we compared the observational results to a global model simulation including only direct radiative forcing of black carbon (BC). From both observations and model simulations we found that in boreal cold seasons anomalously high African aerosols are associated with significant reductions in cloud amount, cloud top height, and surface precipitation. These results suggest that the observed precipitation reduction in the WAM region is caused by radiative effect of BC. The result also suggests that the BC effect on precipitation is nonlinear.

scholarly journals Sahel Rainfall–Tropical Easterly Jet Relationship on Synoptic to Intraseasonal Time Scales

2019 ◽  
Vol 147 (5) ◽  
pp. 1733-1752 ◽  
Author(s):  
Alexander Lemburg ◽  
Jürgen Bader ◽  
Martin Claussen
Keyword(s):  
West Africa ◽  
Time Scales ◽  
West African Monsoon ◽  
Reanalysis Data ◽  
Limiting Factor ◽  
Convective Activity ◽  
Affect Intensity ◽  
Tropical Easterly Jet ◽  
Sahel Rainfall ◽  
Upper Level

Abstract The tropical easterly jet (TEJ) is a characteristic upper-level feature of the West African monsoon (WAM) circulation. Moreover, the TEJ over West Africa is significantly correlated with summer Sahel rainfall on interannual and decadal time scales. In contrast, the relationship between Sahel rainfall and the regional TEJ on synoptic to intraseasonal time scales is unclear. Therefore, this relationship is investigated by means of multiple statistical analyses using temporally highly resolved measurement and reanalysis data. It is shown that average correlations between convective activity and regional TEJ intensity remain below 0.3 for all synoptic to intraseasonal time scales. Especially on the synoptic time scale, the TEJ significantly lags anomalies in convective activity by one or two days, which indicates that convection anomalies are more likely to drive changes in the regional TEJ than vice versa. To further shed light on the role of the TEJ for rainfall over West Africa, a previously proposed effect of TEJ-induced upper-level divergence on the development of mesoscale convective systems (MCSs) is examined more closely. An analysis of nearly 300 Sahelian MCSs shows that their initiation is generally not associated with significant TEJ anomalies or jet-induced upper-level divergence. Furthermore, no statistically significant evidence is found that preexisting TEJ-related upper-level divergence anomalies affect intensity, size, and lifetime of MCSs. A limiting factor of this study is the focus on TEJ-induced upper-level divergence. Therefore, a possible effect of the TEJ on Sahel rainfall via other mechanisms cannot be ruled out and should be subject to future studies.

The Influence of the MJO on Upstream Precursors to African Easterly Waves

2012 ◽  
Vol 25 (9) ◽  
pp. 3219-3236 ◽  
Author(s):  
Ghassan J. Alaka ◽  
Eric D. Maloney
Keyword(s):  
West Africa ◽  
West African Monsoon ◽  
West African ◽  
Temperature Fields ◽  
Boreal Summer ◽  
Tropospheric Temperature ◽  
The West ◽  
Lake Chad ◽  
African Monsoon ◽  
African Easterly Jet

The Madden–Julian oscillation (MJO) produces alternating periods of increased and reduced precipitation and African easterly wave (AEW) activity in West Africa. This study documents the influence of the MJO on the West African monsoon system during boreal summer using reanalysis and brightness temperature fields. MJO-related West African convective anomalies are likely induced by equatorial Kelvin and Rossby waves generated in the Indian Ocean and West Pacific by the MJO, which is consistent with previous studies. The initial modulation of tropical African convection occurs upstream of West Africa, near the entrance of the African easterly jet (AEJ). Previous studies have hypothesized that an area to the east of Lake Chad is an initiation region for AEWs. Called the “trigger region” in this study, this area exhibits significant intraseasonal convection and wave activity anomalies prior to the wet and dry MJO phases in the West African monsoon region. In the trigger region, cold tropospheric temperature anomalies and high precipitable water, as well as an eastward extension of the African easterly jet, appear to precede and contribute to the wet MJO phase in West Africa. An anomalous stratiform heating profile is observed in advance of the wet MJO phase with anomalous PV generation maximized at the jet level. The opposite behavior occurs in advance of the dry MJO phase. The moisture budget is examined to provide further insight as to how the MJO modulates and initiates precipitation and AEW variability in this region. In particular, meridional moisture advection anomalies foster moistening in the trigger region in advance of the wet MJO phase across West Africa.

The Coin of the African Realm

2017 ◽  
Author(s):  
Jeffrey Herbst
Keyword(s):  
Twentieth Century ◽  
West Africa ◽  
West African ◽  
Regional Economies ◽  
African Countries ◽  
New Strategies

This chapter examines the politics of the currency in West Africa from the beginning of the twentieth century. A public series of debates over the nature of the currency occurred in West Africa during both the colonial and independence periods. Since 1983, West African countries have been pioneers in Africa in developing new strategies to combat overvaluation of the currency and reduce the control of government over the currency supply. The chapter charts the evolution of West African currencies as boundaries and explores their relationship to state consolidation. It shows that leaders in African capitals managed to make the units they ruled increasingly distinct from the international and regional economies, but the greater salience of the currency did not end up promoting state consolidation. Rather, winning the ability to determine the value of the currency led to a series of disastrous decisions that severely weakened the states themselves.

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 Hybrid insolation forcing of Pliocene monsoon dynamics in West Africa

2018 ◽  
Vol 14 (1) ◽  
pp. 73-84 ◽  
Author(s):  
Rony R. Kuechler ◽  
Lydie M. Dupont ◽  
Enno Schefuß
Keyword(s):  
West Africa ◽  
Climate Changes ◽  
West African Monsoon ◽  
West African ◽  
Latitudinal Gradients ◽  
Glacial Cycle ◽  
Time Intervals ◽  
Continental Hydrology ◽  
Insolation Forcing ◽  
The Last Glacial

Abstract. The Pliocene is regarded as a potential analogue for future climate with conditions generally warmer-than-today and higher-than-preindustrial atmospheric CO2 levels. Here we present the first orbitally resolved records of continental hydrology and vegetation changes from West Africa for two Pliocene time intervals (5.0–4.6 Ma, 3.6–3.0 Ma), which we compare with records from the last glacial cycle (Kuechler et al., 2013). Our results indicate that changes in local insolation alone are insufficient to explain the full degree of hydrologic variations. Generally two modes of interacting insolation forcings are observed: during eccentricity maxima, when precession was strong, the West African monsoon was driven by summer insolation; during eccentricity minima, when precession-driven variations in local insolation were minimal, obliquity-driven changes in the summer latitudinal insolation gradient became dominant. This hybrid monsoonal forcing concept explains orbitally controlled tropical climate changes, incorporating the forcing mechanism of latitudinal gradients for the Pliocene, which probably increased in importance during subsequent Northern Hemisphere glaciations.

scholarly journals Promoting research to improve maternal, neonatal, infant and adolescent health in West Africa: the role of the West African Health Organisation

2017 ◽  
Vol 15 (S1) ◽  
Author(s):  
Issiaka Sombie ◽  
Aissa Bouwayé ◽  
Yves Mongbo ◽  
Namoudou Keita ◽  
Virgil Lokossou ◽  
...  
Keyword(s):  
West Africa ◽  
Adolescent Health ◽  
West African ◽  
The West ◽  
African Health
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