The significance of large-scale land cover change on the Australian palaeomonsoon

We use the Regional Atmospheric Modeling System at a 50 km spatial resolution to explore the impact of large-scale vegetation changes on the Australian monsoon. We simulate multiple Januaries using vegetation cover representative of the present day, the last interglacial (LIG) (125,000 BP) and the last glacial maximum (20,000 BP), interpreted from palaeoecological data, to determine whether changes in vegetation can affect the Australian monsoon. We find that the large-scale replacement of current vegetation, to vegetation representing the LIG and the last glacial maximum has a substantial impact on the simulated latent heat flux and surface air temperature. Precipitation is affected, but only by approximately 5%. We show that the impact of vegetation change on precipitation is due to changes in the surface roughness length that affects the surface frictional drag, wind velocities and moisture convergence. The impact of large-scale vegetation changes on all quantities is restricted to the regions of land cover change. The perturbation induced by vegetation change interacts with the monsoon system by changing the local intensity of the atmospheric circulation causing relatively small intensification/moderation of the wind velocities. There is little evidence that the vegetation change induces a change in the large-scale structure of the meteorological system and there is no evidence that the vegetation changes induce a southward extension of the monsoon. We therefore find no evidence to support a hypothesis that vegetation feedbacks explain observed changes in lake levels in the Australian arid interior. We highlight some strengths and weaknesses of our approach and emphasise that the limitations implicit in our analytical methods means we cannot conclusively demonstrate that biospheric feedbacks can be ignored. Substantial additional work is therefore required to finally assess the role of biospheric feedbacks on the Australian palaeomonsoon. (c) 2006 Elsevier Ltd. All rights reserved.