Hierarchical modeling of tropical vegetation-atmosphere interactions

Interaction between vegetation and other components of the climate system are explored for current day and mid-Holocene climate conditions. Vegetation and atmospheric behavior are studied in an intermediate complexity Tropical climate model which includes components of land and vegetation, and in a simple model designed to capture the influence of vegetation in a climate system with large scale climate variability.; Tropical land climate sensitivities to surface conductance and albedo, linked through leaf area index, are studied in an intermediate complexity model. Sensitivity to changes in surface conductance and albedo is found to differ among local climatic regions, with high precipitation regions more influenced by surface conductance, while albedo has a larger influence in arid regions.; Although the global vegetation distribution is largely controlled by large-scale climate pattern, the observed vegetation-rainfall relationship is also influenced by vegetation feedback and climate variability. In a simplified coupled atmosphere-vegetation model, a positive feedback from interactive vegetation leads to a wetter and greener state. Climate variability is found to reduce vegetation and rainfall in higher rainfall regions, while enhancing them in lower rainfall regions, thus smoothing out the desert to forest gradient. The results suggest that vegetation plays an active role in determining the observed vegetation-rainfall distributions.; Paleo-evidence indicates generally wetter conditions existed in the Sahara during the mid-Holocene. Studies suggest that important factors in modeling this climate anomaly include mid-Holocene orbital parameters, land surface and vegetation feed backs, as well as the particular atmospheric dynamics of each model employed. Here, interactive vegetation and a dynamic process relevant to monsoon extent, the ventilation mechanism, are studied in an intermediate complexity atmospheric model coupled to simple land and vegetation components. Interactive vegetation is found to be effective at enhancing precipitation and vegetation amount in monsoon regions, yet not very effective in moving the monsoon boundary if ventilation is strong. The poleward extent of the mid-Holocene monsoon is quite sensitive to the strength of ventilation in the atmospheric model, with the largest monsoon advance when interactive vegetation and a reduced ventilation effect act simultaneously.