Applying a physical understanding of variability mechanisms to climate change prediction over West Africa

Two modes of variability that are prominent in the present day climate can be related to simple conservation of absolute angular momentum (potential vorticity) arguments. One mode is the north/south dipole precipitation pattern that is often associated with dry years in the Sahel. When SSTs in the Gulf of Guinea are anomalously warm, for example, summer precipitation along the Guinean coast is high but the Sahel is dry. The northern (dry) part of this dipole anomaly is a result of sinking to conserve potential vorticity in the modified outflow from the Saharan high in the mid-troposphere. The second mode of variability concerns the strength and structure of the low-level westerly flow into West Africa, which has recently been identified as a jet. This westerly flow advects low relative vorticity air from the North Atlantic high over Sahelian Africa to help balance low-level stretching associated with mid-tropospheric condensational heating.

Physically understanding climate variability in such a deterministic way provides a good approach for evaluating the ability of climate models to capture a particular mode of variability, and to predict its role in the future. A number of recent coupled model integrations of the 20th are examined to choose models that capture these particular modes of variability properly. Then, 21st century integrations of the chosen models are examined to understand how these modes may change the precipitation distribution over West Africa in the CO2-enhanced climate of the future.