Interactions Between North African Vegetation and the African Easterly Jet: A Mechanism for Abrupt Climate Change
Modeling studies and paleoclimate evidence of the mid-Holocene, 6,000 years ago, indicate that the potential for rapid climate change exists over northern Africa. A regional climate model that produces an excellent representation of today's climate over northern Africa is used to simulate the summer climate under present day solar forcing and SSTs and several prescribed static idealized zonal vegetation distributions. The purpose is to isolate and understand the role of interactions between vegetation and the dynamics of the West African summer monsoon in generating rapid climate change. Simulations with prescribed and interactive vegetation distributions are analyzed. In simulations with prescribed idealized vegetation, the regional model simulates only small differences in precipitation when the southern desert border is located between 10.0N and 17.9N. However, when the desert border is moved only 180 km, from 17.9N to 19.4N, summer precipitation increases by a factor of 5 over the Sahara, and the zonal structure observed in the present day climate is eliminated. These precipitation anomalies are associated with a 50% reduction in the magnitude of the African easterly jet, a 20% increase in the magnitude of the low-level westerly jet and a deepening and moistening of the thermal low. The model suggests that when the southern desert border is located north of a threshold latitude, the positive soil moisture anomalies beneath the region of maximum vertical velocity associated with the thermal low support positive low-level moist static energy anomalies, which are indicative of strengthened convection. Asynchronous coupling with a simple vegetation model reveals that when the initial desert border is located at 20.9N, a new equilibrium vegetation distribution results in which the central Sahara is vegetated, indicating that the interactions between vegetation and the atmosphere may produce rapid climate change. When the initial desert border is located at 10.0N, the equilibrium vegetation distribution closely resembles that of the present day, suggesting that the atmospheric conditions may allow regrowth of vegetation in a deforestation scenario under present day solar and SST forcings.
Committee members: Dr. Kerry Cook, Dr. Stephen Colucci, and Dr. Bryan Isacks
Africa; vegetation/atmosphere interactions; monsoon; climate change; African easterly jet; climate; soil moisture; Sahara
dissertation or thesis