Atmosphere/vegetation feedbacks: A mechanism for abrupt climate change over northern Africa

Modeling studies and paleoclimate data of the mid- Holocene provide strong evidence of abrupt climate change over northern Africa. Regional climate model simulations are used to isolate and diagnose the role of interactions between vegetation and the dynamics of the West African summer monsoon in generating abrupt climate change. Simulations with both fixed and interactive vegetation distributions are included. The modeled climate is largely insensitive to the latitude of the desert border when the border is located south of 17.9 degrees N. But when the desert border is located only 180 km north of 17.9 degrees N, the climate is quite sensitive, with summer precipitation rates a factor of 5 greater over the Sahara desert and an African easterly jet that is 50% weaker. Prescribing vegetation north of the threshold latitude places moist soil beneath the thermal low, increasing the low-level moist static energy and convective instability. The atmospheric model is asynchronously coupled with a simple vegetation model that allows the vegetation to respond to and influence climate. When the model is initialized with the desert border at 10.0 degrees N, the resulting equilibrium vegetation distribution is similar to that of the present day. In contrast, when the model is initialized with the desert border at 20.9 degrees N, a new equilibrium vegetation distribution results in which the central Sahara is vegetated. The existence of these two stable climate and vegetation states and the threshold latitude indicates that interactions between vegetation (specifically through soil moisture) and the atmosphere (through the African easterly jet) can produce abrupt climate change.