Influence of the Mediterranean Sea on the West African monsoon: Intraseasonal variability in numerical simulations

An idealized vertical–meridional zonally symmetric model is developed in order to recover a July typical monsoon regime over West Africa in response to surface conditions. The model includes a parameterization to account for heat and momentum fluxes associated with eddies. The sensitivity of the simulated West African monsoon equilibrium regime to some major processes is explored. It allows confirmation of the important role played by the sun’s latitudinal position, the aerosols, the albedo, and the SST’s magnitude in the Gulf of Guinea and in the Mediterranean Sea. The important role of aerosols in warming the Saharan lower layers and their effect on the whole monsoon is underlined. Model results also stress the importance of the Mediterranean Sea, which is needed to obtain the extreme dryness of the Sahara. The use of this idealized model is finally discussed for studying the scale interactions and coupling involved in the West African monsoon as explored in a companion paper.

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Coupled Land–Atmosphere Intraseasonal Variability of the West African Monsoon in a GCM

Journal of Climate ◽

10.1175/2010jcli3419.1 ◽

2010 ◽

Vol 23 (21) ◽

pp. 5557-5571 ◽

Cited By ~ 19

Author(s):

Sally L. Lavender ◽

Christopher M. Taylor ◽

Adrian J. Matthews

Keyword(s):

Soil Moisture ◽

Sensible Heat Flux ◽

Intraseasonal Variability ◽

West African Monsoon ◽

Atmospheric Model ◽

West African ◽

The West ◽

African Monsoon ◽

Surface Sensible Heat Flux ◽

Fully Coupled

Abstract Recent observational studies have suggested a role for soil moisture and land–atmosphere coupling in the 15-day westward-propagating mode of intraseasonal variability in the West African monsoon. This hypothesis is investigated with a set of three atmospheric general circulation model experiments. 1) When soil moisture is fully coupled with the atmospheric model, the 15-day mode of land–atmosphere variability is clearly identified. Precipitation anomalies lead soil moisture anomalies by 1–2 days, similar to the results from satellite observations. 2) To assess whether soil moisture is merely a passive response to the precipitation, or an active participant in this mode, the atmospheric model is forced with a 15-day westward-propagating cycle of regional soil moisture anomalies based on the fully coupled mode. Through a reduced surface sensible heat flux, the imposed wet soil anomalies induce negative low-level temperature anomalies and increased pressure (a cool high). An anticyclonic circulation then develops around the region of wet soil, enhancing northward moisture advection and precipitation to the west. Hence, in a coupled framework, this soil moisture–forced precipitation response would provide a self-consistent positive feedback on the westward-propagating soil moisture anomaly and implies an active role for soil moisture. 3) In a final sensitivity experiment, soil moisture is again externally prescribed but with all intraseasonal fluctuations suppressed. In the absence of soil moisture variability there are still pronounced surface sensible heat flux variations, likely due to cloud changes, and the 15-day westward-propagating precipitation signal is still present. However, it is not as coherent as in the previous experiments when interaction with soil moisture was permitted. Further examination of the soil moisture forcing experiment in GCM experiment 2 shows that this precipitation mode becomes phase locked to the imposed soil moisture anomalies. Hence, the 15-day westward-propagating mode in the West African monsoon can exist independently of soil moisture; however, soil moisture and land–atmosphere coupling act to feed back on the atmosphere and further enhance and organize it.

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Internal Intraseasonal Variability of the West African Monsoon in WRF

Journal of Climate ◽

10.1175/jcli-d-16-0750.1 ◽

2017 ◽

Vol 30 (15) ◽

pp. 5815-5833 ◽

Cited By ~ 2

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.

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The West African Monsoon Dynamics. Part I: Documentation of Intraseasonal Variability

Journal of Climate ◽

10.1175/1520-0442(2003)016<3389:twamdp>2.0.co;2 ◽

2003 ◽

Vol 16 (21) ◽

pp. 3389-3406 ◽

Cited By ~ 146

Author(s):

Benjamin Sultan ◽

Serge Janicot ◽

Arona Diedhiou

Keyword(s):

Intraseasonal Variability ◽

West African Monsoon ◽

West African ◽

The West ◽

African Monsoon ◽

Monsoon Dynamics

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Impacts of warm and cold situations in the Mediterranean basins on the West African monsoon: observed connection patterns (1979–2006) and climate simulations

Climate Dynamics ◽

10.1007/s00382-009-0599-3 ◽

2009 ◽

Vol 35 (1) ◽

pp. 95-114 ◽

Cited By ~ 48

Author(s):

Bernard Fontaine ◽

Javier Garcia-Serrano ◽

Pascal Roucou ◽

Belen Rodriguez-Fonseca ◽

Teresa Losada ◽

...

Keyword(s):

West African Monsoon ◽

West African ◽

The West ◽

African Monsoon ◽

Climate Simulations ◽

The Mediterranean

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Intraseasonal Variability of the Saharan Heat Low and Its Link with Midlatitudes

Journal of Climate ◽

10.1175/2010jcli3093.1 ◽

2010 ◽

Vol 23 (10) ◽

pp. 2544-2561 ◽

Cited By ~ 65

Author(s):

Fabrice Chauvin ◽

Romain Roehrig ◽

Jean-Philippe Lafore

Keyword(s):

North Africa ◽

Potential Temperature ◽

West African Monsoon ◽

West African ◽

Maximum Temperature ◽

Temperature Structure ◽

The West ◽

African Monsoon ◽

The Mediterranean ◽

Heat Low

Abstract The Saharan heat low (SHL) is thought to be a key feature of the West African monsoon, and its variations during the summer season have not yet been systematically assessed. To characterize the intraseasonal variations of the SHL, real and complex empirical orthogonal function analyses were applied to the 850-hPa potential temperature field over northern Africa and the Mediterranean, using NCEP–Department of Energy (DOE) Atmospheric Model Intercomparison Project (AMIP-II) reanalysis results. A robust quasi-propagative mode was highlighted over North Africa and the Mediterranean. This mode consists of two phases. The west phase corresponds to a maximum temperature over the coast of Morocco–Mauritania, propagating southwestward, and a minimum between Libya and Sicily, propagating southeastward. The east phase corresponds to the opposite temperature structure, which propagates as in the west phase. A lag-composite analysis revealed that this SHL mode was preceded by large-scale, midlatitude, intraseasonal fluctuations of the atmosphere. The southward penetration of a Rossby wave disturbance over Europe and North Africa generates modulations of the three-dimensional atmospheric structure. The low-level ventilations and harmattan-like circulation are particularly impacted, as are the subtropical westerlies and the polar jets in the upper troposphere. The west phase is concomitant with an enhanced convective signal over the Darfur region, which propagates westward, as far as the middle of the Atlantic, at a speed similar to that of the well-known African easterly waves. The SHL appears to be a bridge between the midlatitudes and the West African monsoon, which may offer promising sources of predictability over the Sahel on an intraseasonal time scale.

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Intraseasonal variability of the West African monsoon

Atmospheric Science Letters ◽

10.1002/asl.280 ◽

2011 ◽

Vol 12 (1) ◽

pp. 58-66 ◽

Cited By ~ 63

Author(s):

S. Janicot ◽

G. Caniaux ◽

F. Chauvin ◽

G. de Coëtlogon ◽

B. Fontaine ◽

...

Keyword(s):

Intraseasonal Variability ◽

West African Monsoon ◽

West African ◽

The West ◽

African Monsoon

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

Monthly Weather Review ◽

10.1175/mwr-d-15-0274.1 ◽

2016 ◽

Vol 144 (4) ◽

pp. 1571-1589 ◽

Cited By ~ 6

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.

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