Abstract
Improving the simulation of the West African Monsoon (WAM) system is paramount to increasing confidence in the projections of the region’s monsoon rainfall change. This work aims to thoroughly analyze the representation of the WAM system in two state-of-the-art, high-resolution (~ 25 km) regional climate models (RCMs) in order to highlight the causes of models’ biases through a process-oriented evaluation approach. Model results generally feature a north (Sahel) - south (Guinea Coast) dipole-like rainfall bias, although, sometimes, positive or negative rainfall biases are evident almost over the whole of West Africa. Our analysis shows that biases in the sea- and land-surface temperature on the one hand, and biases in the sea-level and land-surface pressure, on the other hand, lead to biases in the simulated temperature and pressure contrasts between the west African landmass and the eastern Atlantic ocean. As a consequence, biases appear in the modeled monsoon flow strength, which, in turn, lead to errors in the amount of advected moisture in the interior of the continent via southwesterlies and the West African westerly jet (WAWJ) on the one hand, and the extent of deepening of the monsoon flux inland on the other hand. In addition, the African easterly jet (AEJ) is underestimated, inducing an underestimation of the African easterly waves (AEWs) activity and a weakening of the cyclonic convective circulation resulting from the AEWs’ troughs, leading to a decrease in the southwesterly flow feeding mesoscale convective systems (MCSs) embedded within the AEJ. The modeled equatorward or northward shifting of the AEJ is likewise found to contribute to the models' wet or dry biases over the Sahel. Finally, there is no consistency between models and reanalyses on the one hand, and between RCM experiments on the other hand, in the way, the simulated atmospheric instability/stability modulates the convection, especially over the Sahel.
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Data availability
REMO2015 and RegCM4-v7 output data are available through the Earth System Grid Federation (ESGF) website (https://esgf-data.dkrz.de/search/cordex-dkrz/).
The ERA‐Interim reanalysis is available from the European Centre for Medium‐Range Weather Forecast (ECMWF) and can be downloaded through the link https://apps.ecmwf.int/datasets/data/interim-full-daily/levtype=sfc/;
The ERA5 reanalysis is produced within the Copernicus Climate Change Service (C3S) by the ECMWF and is accessible via the link https://cds.climate.copernicus.eu/cdsapp#!/dataset/reanalysis-era5-pressure-levels-monthly-means? tab1/4form; the MERRA2 reanalysis, developed by NASA, is available online (at https://disc.gsfc.nasa.gov/datasets?keywords1⁄4%22MERRA-2%22&page1⁄41&source1⁄4Models%2FAnalyses%20MERRA-2). NCEP2 data set is provided by the NOAA/OAR/ESRL PSD and is available through the website https://psl.noaa.gov/data/gridded/data.ncep.reanalysis2.html.
The GPCC observational data set is available at https://opendata.dwd.de/climate_environment/GPCC/html/fulldata-monthly_v2020_doi_download.html. The GPCP‐v2.3 combined precipitation data set is available at https://www.esrl.noaa.gov/psd/data/gridded/data.gpcp.html; the CRU‐v4.04 dataset is available at https://data.ceda.ac.uk/badc/cru/data/cru_ts/cru_ts_4.04/data/pre (UEA, 2019); The UDel‐4.01 data set is available at http://climate.geog.udel.edu/~climate/html_pages/download.html (University of Delaware, 2019);
The CHIRPS2 data are available at https://data.chc.ucsb.edu/products/CHIRPS-2.0/global_daily/netcdf/.
The corresponding author is available for providing NIC131 data upon request.
ARC2 data are available at http://iridl.ldeo.columbia.edu/SOURCES/.NOAA/.NCEP/.CPC/.FEWS/.Africa/.DAILY/.ARC2/.daily/.
The UGDP is available at https://psl.noaa.gov/data/gridded/data.cpc.globalprecip.html.
The HadISST SST data are available at https://www.metoffice.gov.uk/hadobs/hadisst/data/download.html.
The ERSST-v5 SST data are available at https://psl.noaa.gov/data/gridded/data.noaa.ersst.v5.html.
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Acknowledgements
The research of this article was supported by DAAD within the framework of the ClimapAfrica programme with funds of the Federal Ministry of Education and Research (funding ID: 57556650 and Per. Reference No: 91795180). We also thank the Climate Service Center, Germany (GERICS), and the Earth System Physics (ESP) section of the International Centre for Theoretical Physics (ICTP) for performing respectively REMO2015 and RegCM4-v7 simulations. Thank you to all the reanalysis, satellite and observational data providers used in this study. The authors thank the three anonymous reviewers whose comments helped improve and clarify this manuscript.
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Bundesministerium für Bildung und Forschung,91795180,Alain T. Tamoffo
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Tamoffo, A.T., Dosio, A., Amekudzi, L.K. et al. Process-oriented evaluation of the West African Monsoon system in CORDEX-CORE regional climate models. Clim Dyn 60, 3187–3210 (2023). https://doi.org/10.1007/s00382-022-06502-y
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DOI: https://doi.org/10.1007/s00382-022-06502-y