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

2010 ◽  
Vol 23 (21) ◽  
pp. 5557-5571 ◽  
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.

scholarly journals Intraseasonal Land–Atmosphere Coupling in the West African Monsoon

2008 ◽  
Vol 21 (24) ◽  
pp. 6636-6648 ◽  
Author(s):  
Christopher M. Taylor
Keyword(s):  
Soil Moisture ◽  
Sensible Heat Flux ◽  
West African Monsoon ◽  
West African ◽  
Surface Fluxes ◽  
Heat Fluxes ◽  
The West ◽  
African Monsoon ◽  
Low Level ◽  
The Impact

Abstract Via its impact on surface fluxes, subseasonal variability in soil moisture has the potential to feed back on regional atmospheric circulations, and thereby rainfall. An understanding of this feedback mechanism in the climate system has been hindered by the lack of observations at an appropriate scale. In this study, passive microwave data at 10.65 GHz from the Tropical Rainfall Measuring Mission satellite are used to identify soil moisture variability during the West African monsoon. A simple model of surface sensible heat flux is developed from these data and is used, alongside atmospheric analyses from the European Centre for Medium-Range Weather Forecasting (ECMWF), to provide a new interpretation of monsoon variability on time scales of the order of 15 days. During active monsoon periods, the data indicate extensive areas of wet soil in the Sahel. The impact of the resulting weak surface heat fluxes is consistent in space and time with low-level variations in atmospheric heating and vorticity, as depicted in the ECMWF analyses. The surface-induced vorticity structure is similar to previously documented intraseasonal variations in the monsoon flow, notably a westward-propagating vortex at low levels. In those earlier studies, the variability in low-level flow was considered to be the critical factor in producing intraseasonal fluctuations in rainfall. The current analysis shows that this vortex can be regarded as an effect of the rainfall (via surface hydrology) as well as a cause.

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.

scholarly journals Intraseasonal variability of the West African monsoon

2011 ◽  
Vol 12 (1) ◽  
pp. 58-66 ◽  
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

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

A hydrological onset and withdrawal index for the West African monsoon

2008 ◽  
Vol 96 (1-2) ◽  
pp. 179-189 ◽  
Author(s):  
G. A. Dalu ◽  
M. Gaetani ◽  
M. Baldi
Keyword(s):  
West African Monsoon ◽  
West African ◽  
The West ◽  
African Monsoon
Sign in / Sign up

Export Citation Format

Share Document