Monsoon onset over Sudan-Sahel: Simulation by the regional scale model MM5.

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Regional scale model simulation for West Africa using a Mesoscale Model MM5

Dynamical downscaling is considered as a good tool for deriving regional climate in formation based on large scale climate conditions because they can represent surface conditions like topography and vegetation more realistic than GCM. The fifth generation Pennsylvania State University (PSU)-National Centre for Atmospheric Research (NCAR) Mesoscale Model MM5 is used to study the regional scale process that control West African rainfall, its associated circulations and other surface features. The simulations are carried out for seven months from March to September using 1.125X1.125 degree lat/lon version of ECMWF reanalysis data for defining the initial and laternal boundary conditions. The objective is to study the monsoon structure from the first rainy season in Guinea (5N) to the sahelian rainfall peak in August. The year 2000 is taken as a reference year in order to validate the model simulation for further study. The results are compared with reanalysis and observed data, then statistical methods are used to study temporal and spatial relationships among various field parameters with 60 km resolution. Preliminary results show that the model is able to reproduce a more realistic rainfall field than the reanalysis

Seasonal cycle and intraseasonal oscillations in the interannual variability over monsoon region

International audienceA characteristic seasonal cycle in the interannual variability (IAV) of dynamical fields over the Indian summer monsoon (ISM) region is revealed by an investigation of daily fields from a 40-year global data set from NCEP. The IAV, measured in terms of standard deviation, is remarkably suppressed over the monsoon region during June to September. Time-latitude distribution of IAV shows that a suppression of IAV in the summer months is a general feature, if not as pronounced as over ISM, of the northern hemisphere. Other new features revealed by our analysis are (a) a differential, nearly out of phase behaviour between zonal and meridional wind components north of 30N, and (b) strong intraseasonal oscillations in IAV, which are once again suppressed during the monsoon months. The low IAV implies a weaker contrast in these fields for different years; implications of this feature for long range forecasting of monsoon are discussed

WRF/ARPEGE-CLIMAT simulated climate trends over West Africa

International audienceThe Weather Regional Forecast (WRF) model is used in this study to downscale low-resolution data over West Africa. First, the performance of the regional model is estimated through contemporary period experiments (1981- 1990) forced by ARPEGE-CLIMAT GCM output (ARPEGE) and ERA-40 re-analyses. Key features of the West African monsoon circulation are reasonably well represented. WRF atmospheric dynamics and summer rainfall compare better to observations than ARPEGE forcing data. WRF simulated moisture transport over West Africa is also consistent in both structure and variability with re-analyses, emphasizing the substantial role played by the West African Monsoon (WAM) and African Easterly Jet (AEJ) flows. The statistical significance of potential climate changes for the A2 scenario between 2032-2041 is enhanced in the downscaling from ARPEGE by the regional experiments, with substantial rainfall increases over the Guinea Gulf and eastern Sahel. Future scenario WRF simulations are characterized by higher temperatures over the eastern Tropical Atlantic suggesting more evaporation available locally. This leads to increased moisture advection towards eastern regions of the Guinea Gulf where rainfall is enhanced through a strengthened WAM flow, supporting surface moisture convergence over West Africa. Warmer conditions over both the Mediterranean region and northeastern Sahel could also participate in enhancing moisture transport within the AEJ. The strengthening of the thermal gradient between the Sahara and Guinean regions, particularly pronounced north of 10◦N, would support an intensification of the AEJ northwards, given the dependance of the jet to the position/intensity of the meridional gradient. In turn, mid-tropospheric moisture divergence tends to be favored within the AEJ region supporting southwards ascents of moister air and deep convection over the Sahel where late summer rainfall regimes are sustained in the context of the A2 scenario regional projections. In conclusion, WRF proved to be a valuable and efficient tool to help downscaling GCM projections over West Africa, and thus assessing issues such as water ressources vulnerability locally

Impacts of the Mediterranean basins on the West African monsoon: observed connection patterns (1979

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