Large-scale effect of aerosols on precipitation in the West African Monsoon region

2009 ◽  
Vol 135 (640) ◽  
pp. 581-594 ◽  
Author(s):  
J. Huang ◽  
C. Zhang ◽  
J. M. Prospero
Keyword(s):  
Scale Effect ◽  
Large Scale ◽  
West African Monsoon ◽  
West African ◽  
Monsoon Region ◽  
The West ◽  
African Monsoon

scholarly journals Internal Intraseasonal Variability of the West African Monsoon in WRF

2017 ◽  
Vol 30 (15) ◽  
pp. 5815-5833 ◽  
Author(s):  
Ghassan J. Alaka ◽  
Eric D. Maloney
Keyword(s):  
East Africa ◽  
Large Scale ◽  
Intraseasonal Variability ◽  
West African Monsoon ◽  
West African ◽  
Boreal Summer ◽  
Kelvin Wave ◽  
The West ◽  
African Monsoon ◽  
External Influences

The West African monsoon (WAM) and its landmark features, which include African easterly waves (AEWs) and the African easterly jet (AEJ), exhibit significant intraseasonal variability in boreal summer. However, the degree to which this variability is modulated by external large-scale phenomena, such as the Madden–Julian oscillation (MJO), remains unclear. The Weather Research and Forecasting (WRF) Model is employed to diagnose the importance of the MJO and other external influences for the intraseasonal variability of the WAM and associated AEW energetics by removing 30–90-day signals from initial and lateral boundary conditions in sensitivity tests. The WAM produces similar intraseasonal variability in the absence of external influences, indicating that the MJO is not critical to produce WAM variability. In control and sensitivity experiments, AEW precursor signals are similar near the AEJ entrance in East Africa. For example, an eastward extension of the AEJ increases barotropic and baroclinic energy conversions in East Africa prior to a 30–90-day maximum of perturbation kinetic energy in West Africa. The WAM appears to prefer a faster oscillation when MJO forcing is removed, suggesting that the MJO may serve as a pacemaker for intraseasonal oscillations in the WAM. WRF results show that eastward propagating intraseasonal signals (e.g., Kelvin wave fronts) are responsible for this pacing, while the role of westward propagating intraseasonal signals (e.g., MJO-induced Rossby waves) appears to be limited. Mean state biases across the simulations complicate the interpretation of results.

Agricultural impacts of large-scale variability of the West African monsoon

2005 ◽  
Vol 128 (1-2) ◽  
pp. 93-110 ◽  
Author(s):  
Benjamin Sultan ◽  
Christian Baron ◽  
Michael Dingkuhn ◽  
Benoît Sarr ◽  
Serge Janicot
Keyword(s):  
Large Scale ◽  
West African Monsoon ◽  
West African ◽  
The West ◽  
Agricultural Impacts ◽  
African Monsoon

scholarly journals Impact of Potential Large-Scale Irrigation on the West African Monsoon and Its Dependence on Location of Irrigated Area

2014 ◽  
Vol 27 (3) ◽  
pp. 994-1009 ◽  
Author(s):  
Eun-Soon Im ◽  
Marc P. Marcella ◽  
Elfatih A. B. Eltahir
Keyword(s):  
Large Scale ◽  
West African Monsoon ◽  
West African ◽  
The West ◽  
Rainfall Distribution ◽  
Irrigation Area ◽  
Anticyclonic Circulation ◽  
African Monsoon ◽  
Low Level ◽  
The Impact

Abstract This study investigates the impact of potential large-scale irrigation on the West African monsoon using the Massachusetts Institute of Technology regional climate model (MRCM). A new irrigation module is implemented to assess the impact of location and scheduling of irrigation on rainfall distribution over West Africa. A control simulation (without irrigation) and eight sensitivity experiments (with irrigation) are performed and compared to discern the effects of irrigation location and scheduling. It is found that the irrigation effect on soil moisture could force significant changes in spatial distribution and magnitude of rainfall, depending on the latitudinal location of irrigation. In general, the large irrigation-induced surface cooling owing to anomalously wet soil tends to suppress moist convection and rainfall, which in turn induces local subsidence and low-level anticyclonic circulation. These local effects are dominated by a consistent reduction of local rainfall over the irrigated land, irrespective of its location. However, the remote response of rainfall distribution to irrigation exhibits a significant sensitivity to the latitudinal position of irrigation and the intraseasonal variation of supplied irrigation water. The low-level northeasterly airflow associated with an anticyclonic circulation centered over the irrigation area, induced at optimal location and timing, would enhance the extent of low-level convergence areas through interaction with the prevailing monsoon flow, leading to a significant increase in rainfall. As the location of the irrigation area is moved from the coast northward, the regional rainfall change exhibits a significant decrease first, then increases gradually to a maximum corresponding to irrigation centered around 20°N, before it declines again.

scholarly journals The large-scale water cycle of the West African monsoon

2010 ◽  
Vol 12 (1) ◽  
pp. 51-57 ◽  
Author(s):  
O. Bock ◽  
F. Guichard ◽  
R. Meynadier ◽  
S. Gervois ◽  
A. Agustí-Panareda ◽  
...  
Keyword(s):  
Large Scale ◽  
Water Cycle ◽  
West African Monsoon ◽  
West African ◽  
The West ◽  
African Monsoon

scholarly journals An Idealized Two-Dimensional Framework to Study the West African Monsoon. Part II: Large-Scale Advection and the Diurnal Cycle

2007 ◽  
Vol 64 (8) ◽  
pp. 2783-2803 ◽  
Author(s):  
Philippe Peyrillé ◽  
Jean-Philippe Lafore
Keyword(s):  
Diurnal Cycle ◽  
Large Scale ◽  
West African Monsoon ◽  
West African ◽  
The West ◽  
African Monsoon ◽  
Temperature And Humidity ◽  
Low Levels ◽  
Heat Low ◽  
2D Model

The idealized 2D model developed in Part I of this study is used to study the West African monsoon sensitivity to large-scale forcing. Using ECWMF reanalyses, a large-scale forcing is introduced in the 2D model in terms of temperature and humidity advection. A coherent structure of cooling–moistening near the surface and drying–warming in the 2–4-km layer is found in the Saharan heat low region. The effect of the advective forcing is to block the monsoon propagation by strengthening the northerly flux and by an increase of convective inhibition. The heat low thus appears to play a key role in the monsoon northward penetration through its temperature and humidity budget. Ultimately, warmer low levels and/or more moist midlevels in the heat low favor a more northerly position of the ITCZ. A detailed view of the continental diurnal cycle is also presented. Potential temperature and humidity budgets are performed in the deep convective and heat low area. The moistening process to sustain deep convection is made through nocturnal advection at low levels and daytime turbulence that redistributes humidity vertically. The same mechanism occurs in the heat low except that the vertical transfers by turbulence help maintain the dryness of the low levels. A possible mechanism of interaction between the deep convective zone and the Saharan heat low is also proposed that involves gravity waves in the upper troposphere.

scholarly journals Implication of the Madden–Julian Oscillation in the 40-Day Variability of the West African Monsoon

2009 ◽  
Vol 22 (13) ◽  
pp. 3769-3785 ◽  
Author(s):  
Benjamin Pohl ◽  
Serge Janicot ◽  
Bernard Fontaine ◽  
Romain Marteau
Keyword(s):  
Large Scale ◽  
West African Monsoon ◽  
Longwave Radiation ◽  
Rain Gauge ◽  
West African ◽  
Mode Analysis ◽  
Local Mode ◽  
The West ◽  
African Monsoon ◽  
Sahel Region

Abstract Madden–Julian oscillations (MJOs) are extracted over the Indo-Pacific basin using a local mode analysis. The convective perturbations are then projected over a larger domain to evaluate their remote consequences over the West African monsoon (WAM) intraseasonal variability. Rather weak (4–6 W m−2) convective fluctuations occurring in phase with those over the southern Indian basin are found over Africa, confirming the results of Matthews. In reverse, 40-day fluctuations in the WAM, similarly detected and projected over a widened area, demonstrate that a large majority of these events are embedded in the larger-scale patterns of the MJO. The regional amplitude of intraseasonal perturbations of the West African convection is not statistically associated with the amplitude of the MJO over the Indian basin but is instead closely related to background vertical velocity anomalies over Africa, possibly embedded in changes in the regional Walker-type circulation. Subsiding motion over Africa is recorded during the most energetic convective perturbations in the WAM. Composites analyses over the MJO life cycle, as depicted by the real-time daily indices developed by Wheeler and Hendon, show that positive outgoing longwave radiation (OLR) anomalies during the dry phase are of larger amplitude and spatially more coherent than negative anomalies during the wet phase, especially over the Sahel region. Over West Africa, the phase of suppressed convection is thus of greater importance for the region than the phase of enhanced convection. Rain gauge records fully confirm these results. The MJO appears to be significantly involved in the occurrences of dry spells during the monsoon over the Sahel, whereas large-scale convective clusters are only restricted to the equatorial latitudes and thus affect the Guinean belt, which experiences its short dry season at this time of the year.

scholarly journals On What Scale Can We Predict the Agronomic Onset of the West African Monsoon?

2016 ◽  
Vol 144 (4) ◽  
pp. 1571-1589 ◽  
Author(s):  
Rory G. J. Fitzpatrick ◽  
Caroline L. Bain ◽  
Peter Knippertz ◽  
John H. Marsham ◽  
Douglas J. Parker
Keyword(s):  
West Africa ◽  
Noise Analysis ◽  
West African Monsoon ◽  
West African ◽  
New Method ◽  
The West ◽  
African Monsoon ◽  
Spatial Consistency ◽  
Local Stakeholders ◽  
Local Rainfall

Abstract Accurate prediction of the commencement of local rainfall over West Africa can provide vital information for local stakeholders and regional planners. However, in comparison with analysis of the regional onset of the West African monsoon, the spatial variability of the local monsoon onset has not been extensively explored. One of the main reasons behind the lack of local onset forecast analysis is the spatial noisiness of local rainfall. A new method that evaluates the spatial scale at which local onsets are coherent across West Africa is presented. This new method can be thought of as analogous to a regional signal against local noise analysis of onset. This method highlights regions where local onsets exhibit a quantifiable degree of spatial consistency (denoted local onset regions or LORs). It is found that local onsets exhibit a useful amount of spatial agreement, with LORs apparent across the entire studied domain; this is in contrast to previously found results. Identifying local onset regions and understanding their variability can provide important insight into the spatial limit of monsoon predictability. While local onset regions can be found over West Africa, their size is much smaller than the scale found for seasonal rainfall homogeneity. A potential use of local onset regions is presented that shows the link between the annual intertropical front progression and local agronomic onset.

scholarly journals A Lagrangian perspective on stable water isotopes during the West African Monsoon

2021 ◽  
Author(s):  
Christopher Johannes Diekmann ◽  
Matthias Schneider ◽  
Peter Knippertz ◽  
Andries Jan de Vries ◽  
Stephan Pfahl ◽  
...  
Keyword(s):  
West African Monsoon ◽  
West African ◽  
Water Isotopes ◽  
Stable Water Isotopes ◽  
The West ◽  
African Monsoon
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