**Acknowledgements**

This work is supported by the African Institute for Mathematical Sciences, www. nexteinstein.org, with financial support from the Government of Canada, provided through Global Affairs Canada, www.international.gc.ca, and the International Development Research Centre, www.idrc.ca.

This research was carried out through the West African Science Service Centre on Climate Change and Adapted Land Use (WASCAL) initiative supported by the German Federal Ministry of Research and Education.

We acknowledge that the results of this research were achieved using computational resources at the German Climate Computing Center (DKRZ). The ERA-Interim dataset was downloaded from the European Centre for Medium – Range Weather Forecasts (ECMWF) (available online http://apps.ecmwf.int/datasets/data/interimfull-daily/levtype=sfc/). The CFSv2 dataset was downloaded from the Environmental Modeling Center at NCEP (available online ftp://nomads.ncdc.noaa.gov/modeldata/ cfsv2\_analysis\_flxf).
