The diurnal cycle of the West African monsoon circulation

2005 ◽  
Vol 131 (611) ◽  
pp. 2839-2860 ◽  
Author(s):  
D. J. Parker ◽  
R. R. Burton ◽  
A. Diongue-Niang ◽  
R. J. Ellis ◽  
M. Felton ◽  
...  
Keyword(s):  
Diurnal Cycle ◽  
West African Monsoon ◽  
West African ◽  
Monsoon Circulation ◽  
The West ◽  
African Monsoon

scholarly journals Characterization of the Diurnal Cycle of the West African Monsoon around the Monsoon Onset

2007 ◽  
Vol 20 (15) ◽  
pp. 4014-4032 ◽  
Author(s):  
Benjamin Sultan ◽  
Serge Janicot ◽  
Philippe Drobinski
Keyword(s):  
Diurnal Cycle ◽  
Deep Convection ◽  
West African Monsoon ◽  
Monsoon Onset ◽  
West African ◽  
Onset Date ◽  
The West ◽  
African Monsoon ◽  
Nocturnal Jet ◽  
Heat Low

Abstract This study investigates the diurnal cycle of the West African monsoon and its seasonal modulation with particular focus on the monsoon onset period. A composite analysis around the monsoon onset date is applied to the 1979–2000 NCEP–DOE reanalysis and 40-yr ECMWF Re-Analysis (ERA-40) at 0000, 0600, 1200, and 1800 UTC. This study points out two independent modes describing the space–time variability of the diurnal cycle of low-level wind and temperature. While the first mode appears to belong to a gradual and seasonal pattern linked with the northward migration of the whole monsoon system, the second mode is characterized by more rapid time variations with a peak of both temperature and wind anomalies around the monsoon onset date. This latter mode is connected with the time pattern of a nocturnal jet reaching its highest values around the onset date. The diurnal cycle of dry and deep convection is also investigated through the same method. A distinct diurnal cycle of deep convection in the ITCZ is evidenced with a peak at 1200 UTC before the monsoon onset, and at 1800 UTC after the monsoon onset. Strong ascending motions associated with deep convection may generate a gravity wave that propagates northward and reaches the Saharan heat low region 12 h later. The diurnal cycle of the dry convection in the Saharan heat low is similar during the preonset and the postonset periods with a peak at night (0000 UTC) consistent with the nocturnal jet intensification. This convection is localized at 15° and 20°N before and after the monsoon onset, respectively. Both during the first rainy season in spring and the monsoon season in summer, the nocturnal jet brings moisture in the boundary layer north of the ITCZ favoring humidification and initiation of new convective cells, helping the northward progression of the ITCZ. At the end of the summer the southward return of the ITCZ is associated with the disappearance of the core of the monsoon jet. Despite a lot of similarities between the results obtained using NCEP–DOE and ERA-40 reanalyses, giving confidence in the significance of these results, some differences are identified, especially in the diurnal cycle of deep convection, which limit the interpretation of some of these results and highlight discrepancies in the reanalyses.

The diurnal cycle of lower boundary-layer wind in the West African monsoon

2009 ◽  
Vol 136 (S1) ◽  
pp. 66-76 ◽  
Author(s):  
Kassimou Abdou ◽  
Douglas J. Parker ◽  
Barbara Brooks ◽  
Norbert Kalthoff ◽  
Thierry Lebel
Keyword(s):  
Boundary Layer ◽  
Diurnal Cycle ◽  
Lower Boundary ◽  
West African Monsoon ◽  
West African ◽  
The West ◽  
African Monsoon ◽  
Lower Boundary Layer

scholarly journals Some aspects of the West African monsoon circulation As deduced from a geostationary satellite

MAUSAM ◽  
2022 ◽  
Vol 44 (4) ◽  
pp. 359-364
Author(s):  
OLUWAGBEMIGA O. JEGEDE
Keyword(s):  
West African Monsoon ◽  
West African ◽  
Geostationary Satellite ◽  
Monsoon Circulation ◽  
Land Area ◽  
The West ◽  
African Monsoon ◽  
Low Level ◽  
Tropospheric Circulation ◽  
Line Of Discontinuity

This paper focusses on some aspects or the West African monsoonal circulation observed during the period 15 July-l0 August 1979 of the PGGE, as derived from the satellite cloud windvectors. Temporal averages of the computed winsfields reveal that the flow at the low level is southerly (monsoonal), and Its line of discontinuity with the continental northeasterly was found at approximately 16°-18°N, lying about 300 km south of the accepted mean position. At both the middle and upper tropospheres the flow is easterly with axis about 12o-14,N and, latitude 8 No respectively, such that it is a circulation south of the axis and northwards, it is anticyclonic. The satellite-observed tropospheric circulation IS then discussed in relation to the, weather manifestations over the sub-region typical of the July / August period.   The mass fields obtained from our gridded satellite-winds indicate that inflow into the land area occur mainly at the lowest layer (1000:850 hPa), whereas at the upper, levels (that is, above 850 hPa) it is predominantly an outflow, The tropospheric average gives a net mass for divergence from within the area, The significance of this result in relation to the observed weather phenomenology of a temporary cessation of the monsoon precipitations occurring about the peak of the season IS also discussed.

scholarly journals Development of a Coupled Regional Model and Its Application to the Study of Interactions between the West African Monsoon and the Eastern Tropical Atlantic Ocean

2009 ◽  
Vol 22 (10) ◽  
pp. 2591-2604 ◽  
Author(s):  
Samson M. Hagos ◽  
Kerry H. Cook
Keyword(s):  
West African Monsoon ◽  
Coupled Model ◽  
West African ◽  
Monsoon Circulation ◽  
Tropical Atlantic ◽  
Gulf Of Guinea ◽  
The West ◽  
African Monsoon ◽  
Wind Speeds ◽  
Westerly Flow

Abstract A regional ocean–atmosphere coupled model is developed for climate variability and change studies. The model allows dynamic and thermodynamic interactions between the atmospheric boundary layer and an ocean mixed layer with spatially and seasonally varying depth prescribed from observations. The model reproduces the West African monsoon circulation as well as aspects of observed seasonal SST variations in the tropical Atlantic. The model is used to identify various mechanisms that couple the West African monsoon circulation with eastern Atlantic SSTs. By reducing wind speeds and suppressing evaporation, the northward migration of the ITCZ off the west coast of Africa contributes to the modeled spring SST increases. During this period, the westerly monsoon flow is expanded farther westward and moisture transport on to the continent is enhanced. Near the end of the summer, upwelling associated with this enhanced westerly flow as well as the solar cycle lead to the seasonal cooling of the SSTs. Over the Gulf of Guinea, the acceleration of the southerly West African monsoon surface winds contributes to cooling of the Gulf of Guinea between April and July by increasing the entrainment of cool underlying water and enhancing evaporation.

scholarly journals Examining the West African Monsoon circulation response to atmospheric heating in a GCM dynamical core

2017 ◽  
Vol 9 (1) ◽  
pp. 149-167 ◽  
Author(s):  
R. Chadwick ◽  
G.M. Martin ◽  
D. Copsey ◽  
G. Bellon ◽  
M. Caian ◽  
...  
Keyword(s):  
West African Monsoon ◽  
West African ◽  
Monsoon Circulation ◽  
The West ◽  
Dynamical Core ◽  
African Monsoon ◽  
Atmospheric Heating

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 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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