Evaluating the National Center for Atmospheric Research climate system model over West Africa: Present-day and the 21st century A1 scenario

We present an analysis of observed and simulated June-July-August (JJA) West African climate during the last 2 decades of the 20th and 21st centuries. The National Center for Atmospheric Research coupled atmosphere-ocean climate system model (CSM) simulation is compared to long-term observations and National Centers for Environmental Prediction reanalysis with an emphasis on the wet season during the late 20th century. While there are significant improvements in the simulation of the African Easterly Jet (AEJ) and African easterly waves relative to the uncoupled Community Climate Model, version 3, biases still exist. These biases are related to a poor simulation of the Azores high, which extends into eastern Europe, allowing for cold air advection into North Africa. There is also little improvement in the upper troposphere Tropical Easterly Jet in the CSM, which is too weak and does not extend westward over the Atlantic Ocean. In the late 21st century, JJA summer temperatures in West Africa have warmed 1.5°-2.5°C relative to the 1951-1980 period. Moreover, the Sahelian region is wetter in the late 21st century in association with an increase in atmospheric moisture, stronger meridional winds from the Gulf of Guinea, and a slightly stronger AEJ.