Biosphere-atmosphere interaction in a one-dimensional climate model of the tropics

Abstract

In this study, we develop a one-dimensional model of the tropics which includes two way interaction between the biosphere and the atmosphere. The model integrates a radiative-convective equilibrium model of the atmosphere, a land surface model including plant growth and competition and a monsoon circulation model which allows for the exchange of heat and moisture between the one-dimensional column and its surroundings. The model is applied to two domains in West Africa to test the sensitivity of the system's equilibrium to perturbations to initial vegetation. In the coastal domain, the model simulates a stable forest equilibrium. The equilibrium climate and vegetation show reasonable similarity to observations for the same region. The same equilibrium is reached in both our control simulation and our experimental simulation, in which deforestation is simulated by initializing the model with grassland. Modifications to parameters of the empirical monsoon circulation model show that the climate and vegetation in our model domain are sensitive to the strength of the monsoon circulation and also to climatic conditions in adjacent regions. In particular, changes in the monsoon which allowed hot and dry air to penetrate into the model domain from the north strongly affected the equilibrium climate and vegetation. These sensitivity studies indicated that 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 a threshold controlling competition between trees and grasses. In the inland domain, the model simulates a stable grassland equilibrium in both the control simulation and an afforestation experiment. While vegetation conditions in the inland domain strongly affected the energy balance, primarily through changes in surface albedo, they had little effect on precipitation and moisture availability.