Role of ecosystem dynamics in biosphere-atmosphere interaction over the coastal region of West Africa

In this study, we develop a one-dimensional model of the tropics which includes two-way interaction between the biosphere and the atmosphere, including ecosystem dynamics. The model integrates an atmospheric model, a biospheric model, and a monsoon circulation model and is applied to coastal West Africa to test the sensitivity of the coupled system to changes in vegetation cover. We perform three sets of simulations: one with a fixed monsoon circulation, one with an interactive monsoon circulation, and one with modified boundary conditions at the northern edge of the domain. Our control simulations show that the model is able to reasonably approximate observed conditions. The model simulates a single stable forest equilibrium in the first two sets of simulations, those which correspond most readily with present conditions in West Africa. These simulations indicate that the monsoon plays an important role in modulating the climate of the region and in shaping the response of the system to vegetation changes. Changes in the monsoon which allowed hot and dry air to penetrate into the model domain from the north strongly modified the equilibrium climate toward drier conditions. This finding motivated further testing of the system assuming degraded conditions to the north, which revealed the possibility of two different equilbria: one forest and one grassland. The existence of multiple equilibria in the biosphere-atmosphere system depends not only on the magnitude of the vegetation-induced climate perturbation but also on whether or not the perturbation extends across the threshold in moisture conditions controlling competition between trees and grasses.