African Easterly Jet Research Articles

African Easterly Jet South: control, maintenance mechanisms and link with Southern subtropical waves

The African Easterly Jet South (AEJ-S) is an important feature of the central African mid tropospheric circulation and has been identified as a key contributor to convection over the region. This study uses 21-year (1983–2003) Reanalysis data sets of ERA-Interim, NCEP2 and MERRA2 to establish mechanisms related to AEJ-S dynamics. Results demonstrate that AEJ-S is dominated by rotational circulation and is maintained by a mid-level high that forms over the Kalahari region during September to November. Because of the role played by this high pressure system in the AEJ-S dynamics, an effort is made to understand how this anticyclone develops. We show that this mid-tropospheric high over the Kalahari region is established in response to lower tropospheric dry convection over the region from September to November. At the core of this high, anticyclonic circulation is induced and maintains AEJ-S located at the northern flank of the high. A link between AEJ-S dynamics and southern subtropical westerly waves is also revealed. It is shown that, when waves amplify over the southern subtropics, they modify lower tropospheric heating. Depending on the phase of the wave, this modifies the cross latitude temperature gradient throughout equatorial regions, therefore modulating the intensity of the jet. By clarifying the mechanisms that govern the AEJ-S, this research work contributes to insight into central Africa climate mechanisms and also suggests a link between central Africa and Southern Africa climate systems. This research work also improves the foundation of new mechanisms that help to identify suitable metrics for the evaluation of Global models over the Central Africa region.

On the Structure of the Regional-Scale Circulation over Central Africa: Seasonal Evolution, Variability, and Mechanisms

AbstractAtmospheric circulation over central Africa is dominated by the tropical easterly jet, the African easterly jet, and the low-level westerly jet. In the lower troposphere, a zonal overturning cell occurs over central Africa, but the mechanisms driving its formation, seasonal evolution, and variability are still unclear. Here, using reanalyses (ERA-Interim, NCEP-2, and JRA-55) and the ECHAM5.3 atmospheric model forced by observed sea surface temperature, we highlight the existence, in the lower troposphere, of a separated single, closed, counterclockwise, and shallow zonal overturning cell, namely, the Congo basin cell. This Congo basin cell persists year round, with maximum intensity and width in August/September and minimum intensity and width in May. This shallow cell extracts heat from the warm central Africa landmass through latent and internal energies and transports it to the cold eastern equatorial Atlantic Ocean, reminiscent of the mixed Carnot–steam cycle. Indeed, the monsoon-like circulation triggered by the zonal surface pressure gradient between the warm central Africa landmass and surrounding cold oceans produces mass convergence at the Congo Air Boundary, providing necessary upward motion to air parcels to destabilize the atmosphere over central Africa. As result, convective updrafts depend on underlying moist static energy and the induced low-level westerly jet, controlled by the near-surface land–ocean thermal contrast through the zonal surface pressure gradient between the warm central African landmass and cold eastern equatorial Atlantic Ocean, rather than the midlevel easterly jet. This midlevel easterly jet is formed by the mechanical work that balances the convection associated with the saturation and rainfall. Furthermore, the efficiency of the Congo basin cell determines seasonality over central Africa.

Simulation and analysis of the moist vortex associated with the extreme rain event of Ouagadougou in 2009

AbstractThis study analyses the mechanisms involved in the formation of a moist meso‐vortex associated with an extreme rain event that occurred in Burkina Faso on 1 September 2009, on the basis of high‐resolution convection‐permitting simulations. After an evaluation of the 6‐day simulation skill to capture the main characteristics of this event, budgets of heat, moisture and relative vorticity are calculated. Results allow us to propose the following scenario for the occurrence of this extreme rain event, completing the large‐scale analysis of the same event performed in the companion article. Firstly, the arrival of a large‐scale wet spell is a key factor, creating a quite unusual wet environment over the Sahel, resulting in weak rain evaporation. As over oceanic regions, it favours intense heating at low levels and thus strong mean ascent and convergence. Secondly, the combination with the arrival of the trough of an African easterly wave provides a second key factor. The associated relative vorticity maximum on the southern flank of the African easterly jet (AEJ) core is boosted by the strong low‐level convergence through the stretching term. The vortex deepening is favoured by the tilting and by eddies at upper levels. Thirdly, in turn the quasi‐balanced vortex circulation brings warm and moist air from the north, favouring the convection triggering to the west of the vortex in the down‐shear direction, in agreement with Raymond and Jiang's theory for long‐lived mesoscale convective systems. This scenario exhibits a positive feedback that could explain its extreme character.

Structural Changes in the African Easterly Jet and Its Role in Mediating the Effects of Saharan Dust on the Linear Dynamics of African Easterly Waves

Abstract Analytical and numerical analyses are used to examine how structural changes to the African easterly jet (AEJ) mediate the effects of Saharan mineral dust aerosols on the linear dynamics of African easterly waves (AEWs). An analytical expression for the generation of eddy available potential energy (APE) is derived that exposes how the AEJ and dust combine to affect the energetics of the AEWs. The expression is also used to interpret the numerical results, which are obtained by radiatively coupling a simplified version of the Weather Research and Forecasting Model to a conservation equation for dust. The WRF-Dust model is used to conduct linear simulations based on five observationally consistent zonal-mean AEJs: a reference AEJ and four other AEJs that are obtained by perturbing the maximum meridional and vertical shear. For a dust distribution consistent with summertime observations over North Africa, the numerical simulations show the following: (i) Irrespective of the AEJ structure or the zonal scale of the AEWs, the dust increases the growth rates of the AEWs. (ii) The growth rates of the AEWs are optimized when the ratio of baroclinic to barotropic energy conversions is largest. (iii) When the energy conversions are sufficiently large, the zonal scale of the fastest-growing AEW shortens. The numerical results confirm the analytical analysis, which shows that the dust effects, which are modulated by the Doppler-shifted frequency, are strongest north of the AEJ axis, a region where the dust augments the preexisting meridional temperature gradient.

Influence of Atmospheric Dynamic Factors on Dust Aerosol Mobilization Over West Africa: Simulations from WRF-Chem

Air pollution modeling has proved to be an effective tool in the description of atmospheric processes including pollutants source identification, transport, and dissipation on a regional scale where data are relatively sparse. This work employs the Weather Research Forecast with Chemistry option (WRF-Chem) coupled model to describe the dynamic atmospheric factors responsible for the dust diversion through very unusual path in the West African region on the 20th March, 2010. The simulation process was carried out between 15th and 25th March, 2010 to capture the dust activities during this unusual dust occurrence. Modeled aerosol optical depth (AOD) calculated by integration of aerosol extinction coefficient (Qext) on the vertical column was compared with observations from AERONET ground sun photometers at wavelength 440 nm. The results showed very strong correlation between the observed and the modeled AOD. Root-mean-square error (RMSE) of 0.46, 0.35, 0.37, and 3.09 over Banizoumbou, Dakar and IER_Cinzana, and Ilorin stations respectively, was obtained. Higher RMSE over Ilorin showed a restraint in capturing the AOD over Ilorin. A significant weakening of the Tropical Easterly Jet (TEJ) on the 19th March coupled with an abrupt wind regime change from easterly to westerly on the 20th day of the month at the African Easterly Jet (AEJ) level was seen to play an important role in the diversion of dust into the region. In addition, the development of stronger vertical wind shear within the troposphere atmospheric layer played a crucial role in the development and unusual transport of dust into the West African region during this period. This study provides concrete evidences of atmospheric processes influencing the transport of the Saharan dust, from the source region to other parts of West Africa.

Saharan Dust Transport during the Incipient Growth Phase of African Easterly Waves

An analytical analysis is combined with numerical modeling simulations in order to expose the physical and dynamical processes that control the zonal-mean transport of Saharan mineral dust aerosols during the incipient growth phase of African easterly waves. The analytical analysis provides the theoretical basis for understanding and predicting how the waves and background flow combine to affect the zonal-mean eddy transports of dust. The analytically derived transport equations―which are valid for any wave field, irrespective of its spatial or temporal scale―predict that the eddy transports of dust are largest where the maximum in the background dust gradients coincide with a critical surface, i.e., where the Doppler-shifted frequency of the wave field vanishes. Linear simulations of the eddy dust transports are conducted using a mechanistic version of the Weather Research and Forecasting (WRF) model coupled to an interactive dust model. The simulations show that the eddy dust transports are directed down the background dust gradients and that the meridional transports of dust dominate over the vertical transports. The numerical simulations confirm the theoretical predictions. The predictions are used to explain recent statistical analyses of reanalysis data for dust-coupled African easterly waves.

On the Predictability of 30-Day Global Mesoscale Simulations of African Easterly Waves during Summer 2006: A View with the Generalized Lorenz Model

Recent advances in computational and global modeling technology have provided the potential to improve weather predictions at extended-range scales. In earlier studies by the author and his coauthors, realistic 30-day simulations of multiple African easterly waves (AEWs) and an averaged African easterly jet (AEJ) were obtained. The formation of hurricane Helene (2006) was also realistically simulated from Day 22 to Day 30. In this study, such extended predictability was further analyzed based on recent understandings of chaos and instability within Lorenz models and the generalized Lorenz model. The analysis suggested that a statement of the theoretical predictability of two weeks is not universal. New insight into chaotic and non-chaotic processes revealed by the generalized Lorenz model (GLM) indicated the potential for extending prediction lead times. Two major features within the GLM included: (1) three types of attractors (that also appeared in the original Lorenz model) and (2) two kinds of attractor coexistence. The features suggest a refined view on the nature of weather, as follows: The entirety of weather is a superset that consists of chaotic and non-chaotic processes. Better predictability can be obtained for stable, steady-state solutions and nonlinear periodic solutions that occur at small and large Rayleigh parameters, respectively. By comparison, chaotic solutions appear only at moderate Rayleigh parameters. Errors associated with dissipative small-scale processes do not necessarily contaminate the simulations of large scale processes. Based on the nonlinear periodic solutions (also known as limit cycle solutions), here, we propose a hypothetical mechanism for the recurrence (or periodicity) of successive AEWs. The insensitivity of limit cycles to initial conditions implies that AEW simulations with strong heating and balanced nonlinearity could be more predictable. Based on the hypothetical mechanism, the possibility of extending prediction lead times at extended range scales is discussed. Future work will include refining the model to better examine the validity of the mechanism to explain the recurrence of multiple AEWs.

Impacts of Local Convective Processes on Rain on the Caribbean Island of Puerto Rico

AbstractThis study aims to determine the impacts of tropical island processes on local convective storms. An analysis of rain events on the island of Puerto Rico between 1 June 2015 and 31 July 2016 showed that local island‐enhanced western storms accounted for 89 of 322 storms. This period is of particular importance for the Caribbean as 2015 was one of the driest years on record. While large‐scale influences such as the El Niño–Southern Oscillation, the North Atlantic Oscillation, African easterly waves, and Saharan dust transport modulate moisture conditions in the region, correlations between precipitation and El Niño–Southern Oscillation (−0.14), North Atlantic Oscillation (−0.42), and Saharan dust (0.1) for 1980–2016 ranged from weak to moderate. Local data for the island of Puerto Rico from weather stations, the Convection, Aerosol, and Synoptic‐Effects in the Tropics field campaign, and the North American Mesoscale model support the initiation or enhancement of convective rain events due to local island processes. In particular, analysis of surface wind speed/direction, convective available potential energy, lifted index, and the bulk Richardson number substantiate local instability due to surface heating, orographic uplift, and sea breeze trade‐wind convergence. These convective forcings along with available precipitable water in excess of 50 mm ultimately led to intense storms despite severe rainfall‐mitigating dust episodes for which aerosol optical thickness exceeded 0.4. These results may have major implications for considering the impacts of local air‐sea‐land interactions on rainfall over other tropical islands.

The Plausibility of September–November Congo Basin Rainfall Change in Coupled Climate Models

AbstractAs one of three global hot spots of tropical convection, potential future changes to the Congo Basin climate system will have regional and tropics‐wide implications. However, the latest generation of climate models from the Coupled Model Intercomparison Project 5 disagree on the sign and magnitude of future change and diverge in their estimation of the historical rainfall climatology. This study assesses the plausibility of different signals of future rainfall change by examining the processes relating to rainfall projections in samples of historically wet or dry models during the September–November rainy season. In the west Congo Basin, there are no significant differences in rainfall change projections in models that are historically wet or dry. Both composites feature wetting in the north (up to 1.8 mm/day) and drying in the south, associated with enhanced tropical Atlantic sea surface temperatures, increased evaporation, and enhanced low‐level moisture flux into the basin. In the east Congo Basin, there is greater evidence that differences in model historical climatologies has an influence on the magnitude of future rainfall change. Historically wet models project significant wetting in the northeast (1.19 mm/day) associated with a weakened northern component of the African Easterly Jet (AEJ) and enhanced moisture convergence. Dry models do not capture the structure of the AEJs in the historical period, and so changes to the AEJs under warming do not produce the same wetting pattern. The analysis therefore casts doubt on the plausibility of the driest rainfall change signals in the east Congo Basin.

Two Contrasting African Easterly Wave Behaviors

AbstractThe dominant structural variability of African easterly waves (AEWs) is explored using an empirical orthogonal function (EOF) approach. The structure of AEWs is obtained by projecting the wind fields from reanalysis data and satellite-derived brightness temperature Tb onto the principal components associated with EOF patterns of filtered Tb (Tb EOF) and 700-hPa meridional wind (v700 EOF). The wave structure depicted by the Tb EOF has confined convection and circulation mostly south of the African easterly jet. It shares many characteristics with AEWs analyzed and discussed in the literature. In contrast, the v700 EOF exhibits less familiar characteristics and includes interactions with the equatorial and subtropical regions. The convective patterns are characterized by a “checkerboard” pattern of convection that has not been emphasized before. The most striking feature is the broad meridional extent, which depicts interactions with a mixed Rossby–gravity wave (MRG) in the equatorial region and interactions with the basic-state potential vorticity in the subtropics. The southern portion of the wave has a modified MRG structure, and this AEW–MRG hybrid cannot be separated using the EOF technique, indicating the prevalence of such structures. The subtropical interaction at mid- to lower levels establishes a vortex off the coast of Morocco that results in dry-air advection into the tropics in tandem with the northern vortex. At upper levels, a subtropical wave train is induced by the AEW-associated convective inflow and outflow. The contrasting AEW circulations are associated with differences in the precipitation rates and patterns over Africa. These results highlight the variability of AEW structures and their interactions with equatorial and subtropical waves.

Mesoscale Convective Systems: A Case Scenario of the ‘Heavy Rainfall’ Event of 15–20 January 2013 over Southern Africa

Southern east Africa is prone to some extreme weather events and interannual variability of the hydrological cycle, including tropical cyclones and heavy rainfall events. Most of these events occur during austral summer and are linked to shifts in the intertropical convergence zone, changes in El Niño Southern Oscillation signatures, sea surface temperature and sea level pressure. A typical example include mesoscale convective systems (MCSs) that occur between October and March along the eastern part, adjacent to the warm waters of Mozambique Channel and Agulhas Current. In this study we discuss a heavy rainfall event over southern Africa, focusing particularly on the period 15–20 January 2013, the period during which MCSs were significant over the subcontinent. This event recorded one of the historic rainfalls due to extreme flooding and overflows, loss of lives and destruction of economic and social infrastructure. An active South Indian Convergence Zone was associated with the rainfall event sustained by a low-level trough linked to a Southern Hemisphere planetary wave pattern and an upper-level ridge over land. In addition, also noteworthy is a seemingly strong connection to the strength of the African Easterly Jet stream. Using rainfall data, satellite imagery and re-analysis (model processed data combined with observations) data, our analysis indicates that there was a substantial relation between rainfall totals recorded/observed and the presence of MCSs. The low-level trough and upper-level ridge contributed to moisture convergence, particularly from tropical South East Atlantic Ocean, which in turn contributed to the prolonged life span of the rainfall event. Positive temperature anomalies favored the substantial contribution of moisture fluxes from the Atlantic Ocean. This study provides a contextual assessment of rainfall processes and insight into the physical control mechanisms and feedback of large-scale convective interactions over tropical southern Africa.

A recurrence analysis of chaotic and non-chaotic solutions within a generalized nine-dimensional Lorenz model

Abstract Based on recent studies using high-dimensional Lorenz models (LMs), a revised view on the nature of weather has been proposed as follows: the entirety of weather is a superset that consists of both chaotic and non-chaotic processes. We suggest that better predictability may be obtained for non-chaotic processes if they can be identified in advance. In this study, to achieve the goal, we generate recurrence plots (RPs) for classifying various types of solutions obtained using simplified and full versions of the generalized Lorenz model (GLM) with various M modes, including M = 3 , 5 , 7 , and 9. We first perform recurrence analyses of the following solutions: (1) a periodic solution and quasi-periodic solutions containing multiple incommensurate frequencies; (2) a temporal transition from an unstable solution to a limit cycle solution, which is an isolated closed orbit; and (3) the coexistence of two types of solutions such as a steady-state and a chaotic solution or a steady-state and limit cycle solution. Various types of solutions that coexist depend only on the initial conditions (ICs). Additionally, to effectively detect the dependence of the coexistence on ICs, we further complete the following: (1) We derive a new system, referred to as version 2 (V2), by decomposing a total field into a basic state and a perturbation. The V2 system is capable of depicting the linear and nonlinear evolution of perturbations that deviate from the basic state represented by a non-trivial critical point solution. (2) We perform ensemble runs with various ICs distributed over a hypersphere centered at a non-trivial critical point. (3) We produce recurrence plots for the ensemble runs using various radii for the hypersphere. The feasibility of applying the above method for analyzing African Easterly Waves (AEWs) is discussed near the end.

The influence of global climate drivers on monsoon onset variability in Nigeria using S2S models

Rainfall onset has a lot of implications on the sustainability of the socio-economic activities in Nigeria. This study assesses the skills of CMA, ECMWF, and UKMO sub-seasonal-to-seasonal (S2S) models in predicting monsoon onset and its variability over Nigeria. It also investigates the global drivers modulating the variability and their teleconnections with rainfall onset anomaly. All the models, their ensemble members, and the observations were subjected to quantitative statistical analyses from 1998 to 2016. Results show that the three models are able to simulate the Northwards migration of the onset dates adequately with inherent biases and unique characteristics. They are also able to capture the evolution and variability of the global drivers modulating the monsoon onset. While CMA and the ECMWF models improve progressively towards the Sahel, the UKMO model performance is best over the Gulf of Guinea. In addition, despite the fairly poor performance of the models in predicting the variability of onset dates over the Gulf of Guinea and the Sahel, there is a considerable improvement in the correlation skill of the models over the Savannah. Furthermore, results show that only the ECMWF model was able to produce the strength of both the African Easterly Jets (AEJ) and the Tropical Easterly Jet (TEJ) in spatio-temporal mode. These are two of the crucial global drivers modulating the dynamics of West African monsoon. It was also found out that most global drivers, especially the Inter-tropical Discontinuity (ITD) and the Sea Surface Temperature (SST) over the Central Pacific, exhibit direct teleconnection with the onset anomaly. This direct relationship is shown to be strongest over both the Gulf of Guinea and the Sahel. Although the CMA model might have the least skill, it, however, showed that all the S2S models, despite the inherent biases, are able to predict rainfall onset over Nigeria, within the sub-seasonal timescale. Finally, the results show that improvements in multi-model ensembles are valuable added information able to significantly improve model performance.

Investigating the Factors That Contribute to African Easterly Wave Intensity Forecast Uncertainty in the ECMWF Ensemble Prediction System

Abstract Although there have been numerous studies documenting the processes/environments that lead to the intensification of African easterly waves (AEWs), only a few of these studies investigated the effect of those processes or the environment on the predictability of AEWs. Here, the large-scale modulation of AEW intensity predictability is evaluated using the 51-member ECMWF ensemble prediction system (EPS) during an active AEW period (July–September 2011–13). Forecasts are stratified based on the 72-h AEW intensity standard deviation (SD) to evaluate hypotheses for how different processes contribute to large forecast SD. While large and small SD forecasts are associated with similar baroclinic and barotropic energy conversions, forecasts with large SD are characterized by higher relative humidity values downstream of the AEW trough. These areas of higher humidity are also associated with higher precipitation and precipitation SD, suggesting that uncertainty associated with diabatic processes could be linked with large AEW intensity SD. Although water vapor is a strong function of longitude and phase of convectively coupled equatorial waves, the cases with large and small SD are characterized by similar longitude and wave phase, suggesting that AEWs occurring in certain locations or convectively coupled equatorial wave phases are not more or less predictable.

A Systematic Comparison of Tropical Waves over Northern Africa. Part II: Dynamics and Thermodynamics

AbstractThis study presents the first systematic comparison of the dynamics and thermodynamics associated with all major tropical wave types causing rainfall modulation over northern tropical Africa: the Madden–Julian oscillation (MJO), equatorial Rossby waves (ERs), tropical disturbances (TDs, including African easterly waves), Kelvin waves, mixed Rossby–gravity waves (MRGs), and eastward inertio-gravity waves (EIGs). Reanalysis and radiosonde data were analyzed for the period 1981–2013 based on space–time filtering of outgoing longwave radiation. The identified circulation patterns are largely consistent with theory. The slow modes, MJO and ER, mainly impact precipitable water, whereas the faster TDs, Kelvin waves, and MRGs primarily modulate moisture convergence. Monsoonal inflow intensifies during wet phases of the MJO, ERs, and MRGs, associated with a northward shift of the intertropical discontinuity for MJO and ERs. This study reveals that MRGs over Africa have a distinct dynamical structure that differs significantly from AEWs. During passages of vertically tilted imbalanced wave modes, such as the MJO, TDs, Kelvin waves, and partly MRG waves, increased vertical wind shear and improved conditions for up- and downdrafts facilitate the organization of mesoscale convective systems. The balanced ERs are not tilted, and rainfall is triggered by large-scale moistening and stratiform lifting. The MJO and ERs interact with intraseasonal variations of the Indian monsoon and extratropical Rossby wave trains. The latter causes a trough over the Atlas Mountains associated with a tropical plume and rainfall over the Sahara. The presented results unveil which dynamical processes need to be modeled realistically to represent the coupling between tropical waves and rainfall in northern tropical Africa.

Process-oriented assessment of RCA4 regional climate model projections over the Congo Basin under $$1.5 \ ^{\circ }{\text {C}}$$ 1.5 ∘ C and $$2 \ ^{\circ }{\text {C}}$$ 2 ∘ C global warming levels: influence of regional moisture fluxes

Understanding the processes responsible for precipitation and its future change is important to develop plausible and sustainable climate change adaptation strategies, especially in regions with few available observed data like Congo Basin (CB). This paper investigates the atmospheric circulation processes associated with climate model biases in CB rainfall, and explores drivers of projected rainfall changes. Here we use an ensemble of simulations from the Swedish Regional Climate Model (RCM) RCA4, driven by eight General Circulation Models (GCMs) from the Coupled Model Intercomparison Project Phase 5 (CMIP5), for the $$1.5 \ ^{\circ }{\text {C}}$$ and $$2\ ^{\circ }{\text {C}}$$ global warming levels (GWLs), and under the representative concentration pathways (RCPs) 4.5 and 8.5. RCA4 captures reasonably well the observed patterns of CB rainfall seasonality, but shows dry biases independent of seasons and large scale driving atmospheric conditions. While simulations mimic observed peaks in transition seasons (March–May and September–November), the rain-belt is misplaced southward (northward) in December–February (June–August), reducing the latitudinal extent of rainfall. Moreover, ERA-Interim reanalysis driven RCM simulation and RCM–GCM combinations show similar results, indicating the dominance of systematic biases. Modelled dry biases are associated with dry upper-tropospheric layers, resulting from a western outflow stronger than the eastern inflow and related to the northern component of African Easterly Jet. From the analysis of the climate change signal, we found that regional scale responses to anthropogenic forcings vary across GWLs and seasons. Changes of rainfall and moisture divergence are correlated, with values higher in March–May than in September–November, and larger for global warming of $$2.0 \ ^{\circ }{\text {C}}$$ than at $$1.5 \ ^{\circ }{\text {C}}$$ . There is an increase of zonal moisture divergence fluxes in upper atmospheric layers ( $$> 700\,{\text {hPa}}$$ ) under RCP8.5 compared to RCP4.5. Moreover, it is found that additional warming of $$0.5 \ ^{\circ }{\text {C}}$$ will change the hydrological cycle and water availability in the CB, with potential to cause challenges to water resource management, agriculture, hydro-power generation, sanitation and ecosystems.

Climatology of coastal wind regimes in Benin

Using surface and lower-tropospheric wind observations at Cotonou (Benin), a comprehensive ten-year (2006–2015) climatology of coastal wind regimes at the West African Guinea Coast is presented for the first time. Four wind regimes are objectively identified: the monsoon and Harmattan winds, the nocturnal low-level jet (NLLJ) and the land-sea breeze (LSB) system. Amongst other things, these regimes are important for the occurrence of coastal squall line systems and for the redistribution of pollutants emitted from coastal cities. The seasonal cycle is split into the long (December–February) and short (July–September) dry seasons, the long (May–June) and short rainy (October–November) seasons, and the beginning of the long rainy season (March–April). In terms of the LSB, the lack of a diurnal reversal in wind direction obscures sea breeze activity at the height of the southwesterly monsoon flow in boreal summer and during a period of stronger winds in March-April related to large land-sea thermal contrasts. During these periods, the sea breeze manifests itself in a relative increase of the monsoon background flow during daytime, while early morning land breezes are infrequent. The latter expectedly, peak during the long dry season in December with a secondary peak during the long rainy season in May–June. The large–scale monsoon and Harmattan flows, mutually exclusive by definition, occur respectively, 88.2–98.4 % and 1.6–11.8 % per year with Harmattan days restricted to the long dry season and deepest monsoon flows of about 2.5 km confined to the long rainy season. The NLLJ is most prominent during the little dry season with, however, a similar and hitherto less documented peak in occurrence frequency during the beginning of the long rainy season. At this time of the year, the African Easterly Jet passes northward over Cotonou, causing the largest 3000–600 m vertical wind shear.

The characteristics of African easterly waves coupled to Saharan mineral dust aerosols

The characteristics of African easterly waves (AEWs) and their relationship with synoptic‐scale plumes of Saharan mineral dust (SMD) are examined using 37 summers (July–September, 1980–2016) of 3‐D fields from the Modern‐Era Retrospective analysis for Research and Applications, version 2 (MERRA‐2). The AEWs and SMD plumes are obtained by projecting the kinematic, thermodynamic and dust fields onto the 2–6‐day time‐filtered aerosol optical depth (AOD) near the west coast of North Africa (15°N, 15°W). Results show that the kinematic fields of the AEWs have significant structure north of the African easterly jet (AEJ). For the SMD, signals occur in two regions relative to the AEW over Africa: in the northerlies ahead of the AEW trough in the Sahel (10–20°N), and in the southerlies behind the AEW trough in the Sahara (20–27°N). The SMD signals are emitted on the flanks of a low surface‐pressure system associated with the northern circulation, mixed vertically in ascent regions below 600 hPa and removed by dry and wet deposition processes. Over the eastern Atlantic Ocean, the kinematic fields of the AEWs weaken, which modifies the propagation of the SMD signals. Consequently, the SMD signals from the two sources merge in the Saharan air layer (SAL) over the Atlantic.The direct radiative effects of SMD on the AEWs are also investigated over Africa.The perturbation diabatic heating rate is combined with the perturbation temperature field to infer the structure of the diabatic generation term in the energetics of the AEWs. The implied energetics show that an SMD‐induced generation region of eddy potential energy occurs at mid‐levels (850–650 hPa) north of the AEJ, which replenishes energy that is converted by baroclinic processes.

A Systematic Comparison of Tropical Waves over Northern Africa. Part I: Influence on Rainfall

Abstract Low-latitude rainfall variability on the daily to intraseasonal time scale is often related to tropical waves, including convectively coupled equatorial waves, the Madden–Julian oscillation (MJO), and tropical disturbances (TDs). Despite the importance of rainfall variability for vulnerable societies in tropical Africa, the relative influence of tropical waves for this region is largely unknown. This article presents the first systematic comparison of the impact of six wave types on precipitation over northern tropical Africa during the transition and full monsoon seasons, using two satellite products and a dense rain gauge network. Composites of rainfall anomalies in the different datasets show comparable modulation intensities in the West Sahel and at the Guinea Coast, varying from less than 2 to above 7 mm day−1 depending on the wave type. African easterly waves (AEWs) and Kelvin waves dominate the 3-hourly to daily time scale and explain 10%–30% locally. On longer time scales (7–20 days), only the MJO and equatorial Rossby (ER) waves remain as modulating factors and explain about up to one-third of rainfall variability. Eastward inertio-gravity waves and mixed Rossby–gravity (MRG) waves are comparatively unimportant. An analysis of wave superposition shows that low-frequency waves (MJO, ER) in their wet phase amplify the activity of high-frequency waves (TD, MRG) and suppress them in the dry phase. The results stress that more attention should be paid to tropical waves when forecasting rainfall over northern tropical Africa.

Investigating West African Monsoon Features in Warm Years Using the Regional Climate Model RegCM4

This study investigates the changes in West African monsoon features during warm years using the Regional Climate Model version 4.5 (RegCM4.5). The analysis uses 30 years of datasets of rainfall, surface temperature and wind parameters (from 1980 to 2009). We performed a simulation at a spatial resolution of 50 km with the RegCM4.5 model driven by ERA-Interim reanalysis. The rainfall amount is weaker over the Sahel (western and central) and the Guinea region for the warmest years compared to the coldest ones. The analysis of heat fluxes show that the sensible (latent) heat flux is stronger (weaker) during the warmest (coldest) years. When considering the rainfall events, there is a decrease of the number of rainy days over the Guinea Coast (in the South of Cote d’Ivoire, of Ghana and of Benin) and the western and eastern Sahel during warm years. The maximum length of consecutive wet days decreases over the western and eastern Sahel, while the consecutive dry days increase mainly over the Sahel band during the warm years. The percentage of very warm days and warm nights increase mainly over the Sahel domain and the Guinea region. The model also simulates an increase of the warm spell duration index in the whole Sahel domain and over the Guinea Coast in warm years. The analysis of the wind dynamic exhibits during warm years a weakening of the monsoon flow in the lower levels, a strengthening in the magnitude of the African Easterly Jet (AEJ) in the mid-troposphere and a slight increase of the Tropical Easterly Jet (TEJ) in the upper levels of the atmosphere during warm years.