Abstract
Within the equatorial zone, Western Equatorial Africa (WEA) has a record low sunshine duration during the June–September dry season due to the persistence of low clouds. This study examines the ability of two reanalysis products (ERA5 and MERRA-2) and eight CMIP6 models (both coupled and atmosphere-only historical simulations) to reproduce the climatology of these low clouds, by comparing it with ground observations and a satellite product. All datasets show a reasonable representation of the regional distribution of low clouds over the Tropical Atlantic and the neighbouring African continent. However, CMIP6 models tend to underestimate the low cloud fraction, especially over WEA in the coupled simulations. This underestimation is partly due to an insufficient seasonal sea-surface temperature (SST) cooling over the Eastern Equatorial Atlantic from April to July in most models, which reduces the lower-tropospheric stability (LTS). However, the inability to reproduce the JJAS low cloud fraction does not necessarily scale with the SST biases of the CMIP6 models. Observed interannual variations of WEA low-cloud fraction are strongly controlled by LTS, itself mostly related to Atlantic SST. The strong dependence of low clouds on interannual SST variations is captured by most, but not all the CMIP6 models. Additional drivers of interannual variations identified in this study, such as mid-tropospheric temperatures over WEA and Bight of Bonny surface winds, emerge inconsistently in CMIP6. Further analyses are needed to disentangle the roles played by SST and independent atmospheric forcings on WEA low cloud formation.
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Availability of data and material
All the data used in this work are publicly available. ISD cloud data are accessible from www.ncdc.noaa.gov/isd/data-access, and EECRA cloud data from NCAR at www.climateguide.ucar.edu/climate-data. The CALIOP satellite data come from the GCM-Oriented CALIPSO Cloud Product (GOCCP) at www.climserv.ipsl.polytechnique.fr/cfmip-obs/Calipso_goccp.html. ERA5 reanalyses are available from https://cds.climate.copernicus.eu and MERRA-2 from www.gmao.gsfc.nasa.gov/reanalysis/MERRA-2/data_access. CMIP6 data can be downloaded from www.esgf-node.llnl.gov/search/cmip6.
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Acknowledgements
This study is part of the project DYVALOCCA (https://dyvalocca.osug.fr/) funded by ANR and DFG under contracts ANR-19-CE01-0021 and DFG FI 786/5-1. Computations were performed using HPC resources from DNUM CCUB (Centre de Calcul de l’Université de Bourgogne), Dijon, France. The authors thank an anonymous reviewer whose very constructive remarks contributed to improve the manuscript.
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This work has been carried out as part of the DYVALOCCA project funded in France by ANR (Grant ANR-19-CE01-0021) and in Germany by DFG (Grant FI-786/5-1).
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Camberlin, P., Togbedji, C.F., Pergaud, J. et al. The representation of dry-season low-level clouds over Western Equatorial Africa in reanalyses and historical CMIP6 simulations. Clim Dyn 61, 2815–2837 (2023). https://doi.org/10.1007/s00382-023-06714-w
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DOI: https://doi.org/10.1007/s00382-023-06714-w